xml-sec 0.1.7

//! Parsing of XMLDSig `<Signature>` and `<SignedInfo>` elements.
//!
//! Implements strict child order enforcement per
//! [XMLDSig §4.1](https://www.w3.org/TR/xmldsig-core1/#sec-Signature):
//!
//! ```text
//! <Signature>
//!   <SignedInfo>
//!     <CanonicalizationMethod Algorithm="..."/>
//!     <SignatureMethod Algorithm="..."/>
//!     <Reference URI="..." Id="..." Type="...">+
//!   </SignedInfo>
//!   <SignatureValue>...</SignatureValue>
//!   <KeyInfo>?
//!   <Object>*
//! </Signature>
//! ```

use roxmltree::{Document, Node};
use x509_parser::extensions::ParsedExtension;
use x509_parser::prelude::FromDer;
use x509_parser::public_key::PublicKey;

use super::digest::DigestAlgorithm;
use super::transforms::{self, Transform};
use super::whitespace::{is_xml_whitespace_only, normalize_xml_base64_text};
use crate::c14n::C14nAlgorithm;

/// XMLDSig namespace URI.
pub(crate) const XMLDSIG_NS: &str = "http://www.w3.org/2000/09/xmldsig#";
/// XMLDSig 1.1 namespace URI.
pub(crate) const XMLDSIG11_NS: &str = "http://www.w3.org/2009/xmldsig11#";
const MAX_DER_ENCODED_KEY_VALUE_LEN: usize = 8192;
const MAX_DER_ENCODED_KEY_VALUE_TEXT_LEN: usize = 65_536;
const MAX_DER_ENCODED_KEY_VALUE_BASE64_LEN: usize = MAX_DER_ENCODED_KEY_VALUE_LEN.div_ceil(3) * 4;
const MAX_KEY_NAME_TEXT_LEN: usize = 4096;
const MAX_X509_BASE64_TEXT_LEN: usize = 262_144;
const MAX_X509_BASE64_NORMALIZED_LEN: usize = MAX_X509_BASE64_TEXT_LEN;
const MAX_X509_DECODED_BINARY_LEN: usize = MAX_X509_BASE64_NORMALIZED_LEN.div_ceil(4) * 3;
const MAX_X509_SUBJECT_NAME_TEXT_LEN: usize = 16_384;
const MAX_X509_ISSUER_NAME_TEXT_LEN: usize = 16_384;
const MAX_X509_SERIAL_NUMBER_TEXT_LEN: usize = 4096;
const MAX_X509_DATA_ENTRY_COUNT: usize = 64;
const MAX_X509_DATA_TOTAL_BINARY_LEN: usize = 1_048_576;
const MAX_X509_CHAIN_DEPTH: usize = 9;

/// Signature algorithms supported for signing and verification.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum SignatureAlgorithm {
    /// RSA with SHA-1. **Verify-only** — signing disabled.
    RsaSha1,
    /// RSA with SHA-256 (most common in SAML).
    RsaSha256,
    /// RSA with SHA-384.
    RsaSha384,
    /// RSA with SHA-512.
    RsaSha512,
    /// ECDSA P-256 with SHA-256.
    EcdsaP256Sha256,
    /// XMLDSig `ecdsa-sha384` URI.
    ///
    /// The variant name is historical.
    ///
    /// Verification currently accepts this XMLDSig URI for P-384 and for the
    /// donor P-521 interop case.
    EcdsaP384Sha384,
}

impl SignatureAlgorithm {
    /// Parse from an XML algorithm URI.
    #[must_use]
    pub fn from_uri(uri: &str) -> Option<Self> {
        match uri {
            "http://www.w3.org/2000/09/xmldsig#rsa-sha1" => Some(Self::RsaSha1),
            "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256" => Some(Self::RsaSha256),
            "http://www.w3.org/2001/04/xmldsig-more#rsa-sha384" => Some(Self::RsaSha384),
            "http://www.w3.org/2001/04/xmldsig-more#rsa-sha512" => Some(Self::RsaSha512),
            "http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256" => Some(Self::EcdsaP256Sha256),
            "http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha384" => Some(Self::EcdsaP384Sha384),
            _ => None,
        }
    }

    /// Return the XML namespace URI.
    #[must_use]
    pub fn uri(self) -> &'static str {
        match self {
            Self::RsaSha1 => "http://www.w3.org/2000/09/xmldsig#rsa-sha1",
            Self::RsaSha256 => "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256",
            Self::RsaSha384 => "http://www.w3.org/2001/04/xmldsig-more#rsa-sha384",
            Self::RsaSha512 => "http://www.w3.org/2001/04/xmldsig-more#rsa-sha512",
            Self::EcdsaP256Sha256 => "http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256",
            Self::EcdsaP384Sha384 => "http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha384",
        }
    }

    /// Whether this algorithm is allowed for signing (not just verification).
    #[must_use]
    pub fn signing_allowed(self) -> bool {
        !matches!(self, Self::RsaSha1)
    }
}

/// Parsed `<SignedInfo>` element.
#[derive(Debug)]
pub struct SignedInfo {
    /// Canonicalization method for SignedInfo itself.
    pub c14n_method: C14nAlgorithm,
    /// Signature algorithm.
    pub signature_method: SignatureAlgorithm,
    /// One or more `<Reference>` elements.
    pub references: Vec<Reference>,
}

/// Parsed `<Reference>` element.
#[derive(Debug)]
pub struct Reference {
    /// URI attribute (e.g., `""`, `"#_assert1"`).
    pub uri: Option<String>,
    /// Id attribute.
    pub id: Option<String>,
    /// Type attribute.
    pub ref_type: Option<String>,
    /// Transform chain.
    pub transforms: Vec<Transform>,
    /// Digest algorithm.
    pub digest_method: DigestAlgorithm,
    /// Raw digest value (base64-decoded).
    pub digest_value: Vec<u8>,
}

/// Parsed `<KeyInfo>` element.
#[derive(Debug, Default, Clone, PartialEq, Eq)]
#[non_exhaustive]
pub struct KeyInfo {
    /// Sources discovered under `<KeyInfo>` in document order.
    pub sources: Vec<KeyInfoSource>,
}

/// Top-level key material source parsed from `<KeyInfo>`.
#[derive(Debug, Clone, PartialEq, Eq)]
#[non_exhaustive]
pub enum KeyInfoSource {
    /// `<KeyName>` source.
    KeyName(String),
    /// `<KeyValue>` source.
    KeyValue(KeyValueInfo),
    /// `<X509Data>` source.
    X509Data(X509DataInfo),
    /// `dsig11:DEREncodedKeyValue` source (base64-decoded DER bytes).
    DerEncodedKeyValue(Vec<u8>),
}

/// Parsed `<KeyValue>` dispatch result.
#[derive(Debug, Clone, PartialEq, Eq)]
#[non_exhaustive]
pub enum KeyValueInfo {
    /// `<RSAKeyValue>`.
    RsaKeyValue,
    /// `dsig11:ECKeyValue` (the XMLDSig 1.1 namespace form).
    EcKeyValue,
    /// Any other `<KeyValue>` child not yet supported by this phase.
    Unsupported {
        /// Namespace URI of the unsupported child, when present.
        namespace: Option<String>,
        /// Local name of the unsupported child element.
        local_name: String,
    },
}

/// Parsed `<X509Data>` children.
#[derive(Debug, Default, Clone, PartialEq, Eq)]
#[non_exhaustive]
pub struct X509DataInfo {
    /// DER-encoded certificates from `<X509Certificate>`.
    ///
    /// This vector has a 1:1 index correspondence with `parsed_certificates`.
    pub certificates: Vec<Vec<u8>>,
    /// Text values from `<X509SubjectName>`.
    pub subject_names: Vec<String>,
    /// `(IssuerName, SerialNumber)` tuples from `<X509IssuerSerial>`.
    pub issuer_serials: Vec<(String, String)>,
    /// Raw bytes from `<X509SKI>`.
    pub skis: Vec<Vec<u8>>,
    /// DER-encoded CRLs from `<X509CRL>`.
    pub crls: Vec<Vec<u8>>,
    /// `(Algorithm URI, digest bytes)` tuples from `dsig11:X509Digest`.
    pub digests: Vec<(String, Vec<u8>)>,
    /// Parsed metadata for each `<X509Certificate>` entry.
    ///
    /// This vector has a 1:1 index correspondence with `certificates`.
    pub parsed_certificates: Vec<ParsedX509Certificate>,
    /// Ordered certificate indexes, starting with the signing certificate.
    pub certificate_chain: Vec<usize>,
}

/// Parsed X.509 certificate details extracted from DER.
#[derive(Debug, Clone, PartialEq, Eq)]
#[non_exhaustive]
pub struct ParsedX509Certificate {
    /// Subject distinguished name.
    pub subject_dn: String,
    /// Issuer distinguished name.
    pub issuer_dn: String,
    /// Certificate serial number bytes.
    pub serial_number: Vec<u8>,
    /// Uppercase hexadecimal certificate serial number without separators.
    pub serial_number_hex: String,
    /// Subject Key Identifier extension bytes (if present).
    pub subject_key_identifier: Option<Vec<u8>>,
    /// Parsed certificate public key material.
    pub public_key: X509PublicKeyInfo,
}

/// Public key material extracted from certificate SubjectPublicKeyInfo.
#[derive(Debug, Clone, PartialEq, Eq)]
#[non_exhaustive]
pub enum X509PublicKeyInfo {
    /// RSA public key (`modulus`, `exponent`).
    Rsa {
        /// Unsigned big-endian RSA modulus (`n`), normalized without leading zeroes.
        modulus: Vec<u8>,
        /// Unsigned big-endian RSA public exponent (`e`), normalized without leading zeroes.
        exponent: Vec<u8>,
    },
    /// EC public key (`curve_oid`, encoded point bytes).
    Ec {
        /// Named-curve OID from SubjectPublicKeyInfo parameters.
        curve_oid: String,
        /// Encoded EC point bytes from SubjectPublicKeyInfo.
        public_key: Vec<u8>,
    },
    /// Public key algorithm is present but not parsed into a concrete key type.
    Unsupported {
        /// SubjectPublicKeyInfo algorithm OID.
        algorithm_oid: String,
    },
}

/// Errors during XMLDSig element parsing.
#[derive(Debug, thiserror::Error)]
#[non_exhaustive]
pub enum ParseError {
    /// Missing required element.
    #[error("missing required element: <{element}>")]
    MissingElement {
        /// Name of the missing element.
        element: &'static str,
    },

    /// Invalid structure (wrong child order, unexpected element, etc.).
    #[error("invalid structure: {0}")]
    InvalidStructure(String),

    /// Unsupported algorithm URI.
    #[error("unsupported algorithm: {uri}")]
    UnsupportedAlgorithm {
        /// The unrecognized algorithm URI.
        uri: String,
    },

    /// Base64 decode error.
    #[error("base64 decode error: {0}")]
    Base64(String),

    /// DigestValue length did not match the declared DigestMethod.
    #[error(
        "digest length mismatch for {algorithm}: expected {expected} bytes, got {actual} bytes"
    )]
    DigestLengthMismatch {
        /// Digest algorithm URI/name used for diagnostics.
        algorithm: &'static str,
        /// Expected decoded digest length in bytes.
        expected: usize,
        /// Actual decoded digest length in bytes.
        actual: usize,
    },

    /// Transform parsing error.
    #[error("transform error: {0}")]
    Transform(#[from] super::types::TransformError),
}

/// Find the first `<ds:Signature>` element in the document.
#[must_use]
pub fn find_signature_node<'a>(doc: &'a Document<'a>) -> Option<Node<'a, 'a>> {
    doc.descendants().find(|n| {
        n.is_element()
            && n.tag_name().name() == "Signature"
            && n.tag_name().namespace() == Some(XMLDSIG_NS)
    })
}

/// Parse a `<ds:SignedInfo>` element.
///
/// Enforces strict child order per XMLDSig spec:
/// `<CanonicalizationMethod>` → `<SignatureMethod>` → `<Reference>`+
pub fn parse_signed_info(signed_info_node: Node) -> Result<SignedInfo, ParseError> {
    verify_ds_element(signed_info_node, "SignedInfo")?;

    let mut children = element_children(signed_info_node);

    // 1. CanonicalizationMethod (required, first)
    let c14n_node = children.next().ok_or(ParseError::MissingElement {
        element: "CanonicalizationMethod",
    })?;
    verify_ds_element(c14n_node, "CanonicalizationMethod")?;
    let c14n_uri = required_algorithm_attr(c14n_node, "CanonicalizationMethod")?;
    let mut c14n_method =
        C14nAlgorithm::from_uri(c14n_uri).ok_or_else(|| ParseError::UnsupportedAlgorithm {
            uri: c14n_uri.to_string(),
        })?;
    if let Some(prefix_list) = parse_inclusive_prefixes(c14n_node)? {
        if c14n_method.mode() == crate::c14n::C14nMode::Exclusive1_0 {
            c14n_method = c14n_method.with_prefix_list(&prefix_list);
        } else {
            return Err(ParseError::UnsupportedAlgorithm {
                uri: c14n_uri.to_string(),
            });
        }
    }

    // 2. SignatureMethod (required, second)
    let sig_method_node = children.next().ok_or(ParseError::MissingElement {
        element: "SignatureMethod",
    })?;
    verify_ds_element(sig_method_node, "SignatureMethod")?;
    let sig_uri = required_algorithm_attr(sig_method_node, "SignatureMethod")?;
    let signature_method =
        SignatureAlgorithm::from_uri(sig_uri).ok_or_else(|| ParseError::UnsupportedAlgorithm {
            uri: sig_uri.to_string(),
        })?;

    // 3. One or more Reference elements
    let mut references = Vec::new();
    for child in children {
        verify_ds_element(child, "Reference")?;
        references.push(parse_reference(child)?);
    }
    if references.is_empty() {
        return Err(ParseError::MissingElement {
            element: "Reference",
        });
    }

    Ok(SignedInfo {
        c14n_method,
        signature_method,
        references,
    })
}

/// Parse a single `<ds:Reference>` element.
///
/// Structure: `<Transforms>?` → `<DigestMethod>` → `<DigestValue>`
pub(crate) fn parse_reference(reference_node: Node) -> Result<Reference, ParseError> {
    let uri = reference_node.attribute("URI").map(String::from);
    let id = reference_node.attribute("Id").map(String::from);
    let ref_type = reference_node.attribute("Type").map(String::from);

    let mut children = element_children(reference_node);

    // Optional <Transforms>
    let mut transforms = Vec::new();
    let mut next = children.next().ok_or(ParseError::MissingElement {
        element: "DigestMethod",
    })?;

    if next.tag_name().name() == "Transforms" && next.tag_name().namespace() == Some(XMLDSIG_NS) {
        transforms = transforms::parse_transforms(next)?;
        next = children.next().ok_or(ParseError::MissingElement {
            element: "DigestMethod",
        })?;
    }

    // Required <DigestMethod>
    verify_ds_element(next, "DigestMethod")?;
    let digest_uri = required_algorithm_attr(next, "DigestMethod")?;
    let digest_method =
        DigestAlgorithm::from_uri(digest_uri).ok_or_else(|| ParseError::UnsupportedAlgorithm {
            uri: digest_uri.to_string(),
        })?;

    // Required <DigestValue>
    let digest_value_node = children.next().ok_or(ParseError::MissingElement {
        element: "DigestValue",
    })?;
    verify_ds_element(digest_value_node, "DigestValue")?;
    let digest_value = decode_digest_value_children(digest_value_node, digest_method)?;

    // No more children expected
    if let Some(unexpected) = children.next() {
        return Err(ParseError::InvalidStructure(format!(
            "unexpected element <{}> after <DigestValue> in <Reference>",
            unexpected.tag_name().name()
        )));
    }

    Ok(Reference {
        uri,
        id,
        ref_type,
        transforms,
        digest_method,
        digest_value,
    })
}

/// Parse `<ds:KeyInfo>` and dispatch supported child sources.
///
/// Supported source elements:
/// - `<ds:KeyName>`
/// - `<ds:KeyValue>` (dispatch by child QName; only `dsig11:ECKeyValue` is treated as supported EC)
/// - `<ds:X509Data>`
/// - `<dsig11:DEREncodedKeyValue>`
///
/// Unknown top-level `<KeyInfo>` children are ignored (lax processing), while
/// unknown XMLDSig-owned (`ds:*` / `dsig11:*`) children inside `<X509Data>` are
/// rejected fail-closed.
/// `<X509Data>` may still be empty or contain only non-XMLDSig extension children.
pub fn parse_key_info(key_info_node: Node) -> Result<KeyInfo, ParseError> {
    verify_ds_element(key_info_node, "KeyInfo")?;
    ensure_no_non_whitespace_text(key_info_node, "KeyInfo")?;

    let mut sources = Vec::new();
    for child in element_children(key_info_node) {
        match (child.tag_name().namespace(), child.tag_name().name()) {
            (Some(XMLDSIG_NS), "KeyName") => {
                ensure_no_element_children(child, "KeyName")?;
                let key_name =
                    collect_text_content_bounded(child, MAX_KEY_NAME_TEXT_LEN, "KeyName")?;
                sources.push(KeyInfoSource::KeyName(key_name));
            }
            (Some(XMLDSIG_NS), "KeyValue") => {
                let key_value = parse_key_value_dispatch(child)?;
                sources.push(KeyInfoSource::KeyValue(key_value));
            }
            (Some(XMLDSIG_NS), "X509Data") => {
                let x509 = parse_x509_data_dispatch(child)?;
                sources.push(KeyInfoSource::X509Data(x509));
            }
            (Some(XMLDSIG11_NS), "DEREncodedKeyValue") => {
                ensure_no_element_children(child, "DEREncodedKeyValue")?;
                let der = decode_der_encoded_key_value_base64(child)?;
                sources.push(KeyInfoSource::DerEncodedKeyValue(der));
            }
            _ => {}
        }
    }

    Ok(KeyInfo { sources })
}

// ── Helpers ──────────────────────────────────────────────────────────────────

/// Iterate only element children (skip text, comments, PIs).
fn element_children<'a>(node: Node<'a, 'a>) -> impl Iterator<Item = Node<'a, 'a>> {
    node.children().filter(|n| n.is_element())
}

/// Verify that a node is a `<ds:{expected_name}>` element.
fn verify_ds_element(node: Node, expected_name: &'static str) -> Result<(), ParseError> {
    if !node.is_element() {
        return Err(ParseError::InvalidStructure(format!(
            "expected element <{expected_name}>, got non-element node"
        )));
    }
    let tag = node.tag_name();
    if tag.name() != expected_name || tag.namespace() != Some(XMLDSIG_NS) {
        return Err(ParseError::InvalidStructure(format!(
            "expected <ds:{expected_name}>, got <{}{}>",
            tag.namespace()
                .map(|ns| format!("{{{ns}}}"))
                .unwrap_or_default(),
            tag.name()
        )));
    }
    Ok(())
}

/// Get the required `Algorithm` attribute from an element.
fn required_algorithm_attr<'a>(
    node: Node<'a, 'a>,
    element_name: &'static str,
) -> Result<&'a str, ParseError> {
    node.attribute("Algorithm").ok_or_else(|| {
        ParseError::InvalidStructure(format!("missing Algorithm attribute on <{element_name}>"))
    })
}

/// Parse the `PrefixList` attribute from an `<ec:InclusiveNamespaces>` child of
/// `<CanonicalizationMethod>`, if present.
///
/// This mirrors transform parsing for Exclusive C14N and keeps SignedInfo
/// canonicalization parameters lossless.
fn parse_inclusive_prefixes(node: Node) -> Result<Option<String>, ParseError> {
    const EXCLUSIVE_C14N_NS_URI: &str = "http://www.w3.org/2001/10/xml-exc-c14n#";

    for child in node.children() {
        if child.is_element() {
            let tag = child.tag_name();
            if tag.name() == "InclusiveNamespaces" && tag.namespace() == Some(EXCLUSIVE_C14N_NS_URI)
            {
                return child
                    .attribute("PrefixList")
                    .map(str::to_string)
                    .ok_or_else(|| {
                        ParseError::InvalidStructure(
                            "missing PrefixList attribute on <InclusiveNamespaces>".into(),
                        )
                    })
                    .map(Some);
            }
        }
    }

    Ok(None)
}

fn parse_key_value_dispatch(node: Node) -> Result<KeyValueInfo, ParseError> {
    verify_ds_element(node, "KeyValue")?;
    ensure_no_non_whitespace_text(node, "KeyValue")?;

    let mut children = element_children(node);
    let Some(first_child) = children.next() else {
        return Err(ParseError::InvalidStructure(
            "KeyValue must contain exactly one key-value child".into(),
        ));
    };
    if children.next().is_some() {
        return Err(ParseError::InvalidStructure(
            "KeyValue must contain exactly one key-value child".into(),
        ));
    }

    match (
        first_child.tag_name().namespace(),
        first_child.tag_name().name(),
    ) {
        (Some(XMLDSIG_NS), "RSAKeyValue") => Ok(KeyValueInfo::RsaKeyValue),
        (Some(XMLDSIG11_NS), "ECKeyValue") => Ok(KeyValueInfo::EcKeyValue),
        (namespace, child_name) => Ok(KeyValueInfo::Unsupported {
            namespace: namespace.map(str::to_string),
            local_name: child_name.to_string(),
        }),
    }
}

fn parse_x509_data_dispatch(node: Node) -> Result<X509DataInfo, ParseError> {
    verify_ds_element(node, "X509Data")?;
    ensure_no_non_whitespace_text(node, "X509Data")?;

    let mut info = X509DataInfo::default();
    let mut total_binary_len = 0usize;
    for child in element_children(node) {
        match (child.tag_name().namespace(), child.tag_name().name()) {
            (Some(XMLDSIG_NS), "X509Certificate") => {
                ensure_no_element_children(child, "X509Certificate")?;
                ensure_x509_data_entry_budget(&info)?;
                let cert = decode_x509_base64(child, "X509Certificate")?;
                add_x509_data_usage(&mut total_binary_len, cert.len())?;
                let parsed_cert = parse_x509_certificate(cert.as_slice())?;
                info.parsed_certificates.push(parsed_cert);
                info.certificates.push(cert);
            }
            (Some(XMLDSIG_NS), "X509SubjectName") => {
                ensure_no_element_children(child, "X509SubjectName")?;
                ensure_x509_data_entry_budget(&info)?;
                let subject_name = collect_text_content_bounded(
                    child,
                    MAX_X509_SUBJECT_NAME_TEXT_LEN,
                    "X509SubjectName",
                )?;
                info.subject_names.push(subject_name);
            }
            (Some(XMLDSIG_NS), "X509IssuerSerial") => {
                ensure_x509_data_entry_budget(&info)?;
                let issuer_serial = parse_x509_issuer_serial(child)?;
                info.issuer_serials.push(issuer_serial);
            }
            (Some(XMLDSIG_NS), "X509SKI") => {
                ensure_no_element_children(child, "X509SKI")?;
                ensure_x509_data_entry_budget(&info)?;
                let ski = decode_x509_base64(child, "X509SKI")?;
                add_x509_data_usage(&mut total_binary_len, ski.len())?;
                info.skis.push(ski);
            }
            (Some(XMLDSIG_NS), "X509CRL") => {
                ensure_no_element_children(child, "X509CRL")?;
                ensure_x509_data_entry_budget(&info)?;
                let crl = decode_x509_base64(child, "X509CRL")?;
                add_x509_data_usage(&mut total_binary_len, crl.len())?;
                info.crls.push(crl);
            }
            (Some(XMLDSIG11_NS), "X509Digest") => {
                ensure_no_element_children(child, "X509Digest")?;
                ensure_x509_data_entry_budget(&info)?;
                let algorithm = required_algorithm_attr(child, "X509Digest")?;
                let digest = decode_x509_base64(child, "X509Digest")?;
                add_x509_data_usage(&mut total_binary_len, digest.len())?;
                info.digests.push((algorithm.to_string(), digest));
            }
            (Some(XMLDSIG_NS), child_name) | (Some(XMLDSIG11_NS), child_name) => {
                return Err(ParseError::InvalidStructure(format!(
                    "X509Data contains unsupported XMLDSig child element <{child_name}>"
                )));
            }
            _ => {}
        }
    }

    info.certificate_chain = build_x509_certificate_chain(&info)?;
    Ok(info)
}

fn build_x509_certificate_chain(info: &X509DataInfo) -> Result<Vec<usize>, ParseError> {
    if info.parsed_certificates.is_empty() {
        return Ok(Vec::new());
    }

    let signing_idx = select_x509_signing_certificate(info)?;
    let mut chain = vec![signing_idx];

    loop {
        if chain.len() > MAX_X509_CHAIN_DEPTH {
            return Err(ParseError::InvalidStructure(
                "X509Data certificate chain exceeds maximum depth".into(),
            ));
        }

        let current_idx = *chain
            .last()
            .expect("chain starts with signing certificate index");
        let current = &info.parsed_certificates[current_idx];
        if current.subject_dn == current.issuer_dn {
            break;
        }

        let candidates = info
            .parsed_certificates
            .iter()
            .enumerate()
            .filter(|(idx, cert)| *idx != current_idx && cert.subject_dn == current.issuer_dn)
            .map(|(idx, _)| idx)
            .collect::<Vec<_>>();

        match candidates.as_slice() {
            [] => break,
            [issuer_idx] => {
                if chain.contains(issuer_idx) {
                    return Err(ParseError::InvalidStructure(
                        "X509Data certificate chain contains a cycle".into(),
                    ));
                }
                if chain.len() == MAX_X509_CHAIN_DEPTH {
                    return Err(ParseError::InvalidStructure(
                        "X509Data certificate chain exceeds maximum depth".into(),
                    ));
                }
                chain.push(*issuer_idx);
            }
            _ => {
                return Err(ParseError::InvalidStructure(
                    "X509Data certificate chain contains ambiguous issuer certificates".into(),
                ));
            }
        }
    }

    Ok(chain)
}

fn select_x509_signing_certificate(info: &X509DataInfo) -> Result<usize, ParseError> {
    let mut candidates = Vec::new();
    let has_lookup_identifiers =
        !info.subject_names.is_empty() || !info.issuer_serials.is_empty() || !info.skis.is_empty();

    for subject_name in &info.subject_names {
        let subject_name = subject_name.trim();
        let matches = info
            .parsed_certificates
            .iter()
            .enumerate()
            .filter(|(_, cert)| subject_name == cert.subject_dn)
            .map(|(idx, _)| idx)
            .collect::<Vec<_>>();
        if matches.is_empty() {
            return Err(ParseError::InvalidStructure(
                "X509Data lookup identifiers do not match any embedded certificate".into(),
            ));
        }
        candidates.extend(matches);
    }

    for (issuer_name, serial) in &info.issuer_serials {
        let serial_hex = x509_serial_decimal_to_hex(serial).ok_or_else(|| {
            ParseError::InvalidStructure(
                "X509Data lookup identifiers do not match any embedded certificate".into(),
            )
        })?;
        let issuer_name = issuer_name.trim();
        let matches = info
            .parsed_certificates
            .iter()
            .enumerate()
            .filter(|(_, cert)| {
                issuer_name == cert.issuer_dn && serial_hex == cert.serial_number_hex
            })
            .map(|(idx, _)| idx)
            .collect::<Vec<_>>();
        if matches.is_empty() {
            return Err(ParseError::InvalidStructure(
                "X509Data lookup identifiers do not match any embedded certificate".into(),
            ));
        }
        candidates.extend(matches);
    }

    for ski in &info.skis {
        let matches = info
            .parsed_certificates
            .iter()
            .enumerate()
            .filter(|(_, cert)| {
                cert.subject_key_identifier
                    .as_ref()
                    .is_some_and(|subject_key_identifier| subject_key_identifier == ski)
            })
            .map(|(idx, _)| idx)
            .collect::<Vec<_>>();
        if matches.is_empty() {
            return Err(ParseError::InvalidStructure(
                "X509Data lookup identifiers do not match any embedded certificate".into(),
            ));
        }
        candidates.extend(matches);
    }

    if has_lookup_identifiers {
        candidates.sort_unstable();
        candidates.dedup();
    }

    match candidates.as_slice() {
        [idx] => return Ok(*idx),
        [] if has_lookup_identifiers => {
            return Err(ParseError::InvalidStructure(
                "X509Data lookup identifiers do not match any embedded certificate".into(),
            ));
        }
        [] => {}
        _ => {
            return Err(ParseError::InvalidStructure(
                "X509Data lookup identifiers match multiple certificates".into(),
            ));
        }
    }

    let leaf_candidates = info
        .parsed_certificates
        .iter()
        .enumerate()
        .filter(|(_, cert)| {
            cert.subject_dn != cert.issuer_dn
                && !info
                    .parsed_certificates
                    .iter()
                    .any(|other| other.issuer_dn == cert.subject_dn)
        })
        .map(|(idx, _)| idx)
        .collect::<Vec<_>>();

    match leaf_candidates.as_slice() {
        [idx] => Ok(*idx),
        [] => Ok(0),
        _ => Err(ParseError::InvalidStructure(
            "X509Data contains multiple possible signing certificates".into(),
        )),
    }
}

fn ensure_x509_data_entry_budget(info: &X509DataInfo) -> Result<(), ParseError> {
    let total_entries = info.certificates.len()
        + info.subject_names.len()
        + info.issuer_serials.len()
        + info.skis.len()
        + info.crls.len()
        + info.digests.len();
    if total_entries >= MAX_X509_DATA_ENTRY_COUNT {
        return Err(ParseError::InvalidStructure(
            "X509Data contains too many entries".into(),
        ));
    }
    Ok(())
}

fn add_x509_data_usage(total_binary_len: &mut usize, delta: usize) -> Result<(), ParseError> {
    *total_binary_len = total_binary_len.checked_add(delta).ok_or_else(|| {
        ParseError::InvalidStructure("X509Data exceeds maximum allowed total binary length".into())
    })?;
    if *total_binary_len > MAX_X509_DATA_TOTAL_BINARY_LEN {
        return Err(ParseError::InvalidStructure(
            "X509Data exceeds maximum allowed total binary length".into(),
        ));
    }
    Ok(())
}

fn decode_x509_base64(
    node: Node<'_, '_>,
    element_name: &'static str,
) -> Result<Vec<u8>, ParseError> {
    use base64::Engine;
    use base64::engine::general_purpose::STANDARD;

    let mut cleaned = String::new();
    let mut raw_text_len = 0usize;
    for text in node
        .children()
        .filter(|child| child.is_text())
        .filter_map(|child| child.text())
    {
        if raw_text_len.saturating_add(text.len()) > MAX_X509_BASE64_TEXT_LEN {
            return Err(ParseError::InvalidStructure(format!(
                "{element_name} exceeds maximum allowed text length"
            )));
        }
        raw_text_len = raw_text_len.saturating_add(text.len());
        normalize_xml_base64_text(text, &mut cleaned).map_err(|err| {
            ParseError::Base64(format!(
                "invalid XML whitespace U+{:04X} in {element_name}",
                err.invalid_byte
            ))
        })?;
        if cleaned.len() > MAX_X509_BASE64_NORMALIZED_LEN {
            return Err(ParseError::InvalidStructure(format!(
                "{element_name} exceeds maximum allowed base64 length"
            )));
        }
    }

    let decoded = STANDARD
        .decode(&cleaned)
        .map_err(|e| ParseError::Base64(format!("{element_name}: {e}")))?;
    if decoded.is_empty() {
        return Err(ParseError::InvalidStructure(format!(
            "{element_name} must not be empty"
        )));
    }
    if decoded.len() > MAX_X509_DECODED_BINARY_LEN {
        return Err(ParseError::InvalidStructure(format!(
            "{element_name} exceeds maximum allowed binary length"
        )));
    }
    Ok(decoded)
}

fn parse_x509_certificate(cert_der: &[u8]) -> Result<ParsedX509Certificate, ParseError> {
    let (rest, cert) =
        x509_parser::certificate::X509Certificate::from_der(cert_der).map_err(|err| {
            ParseError::InvalidStructure(format!("X509Certificate is not valid DER X.509: {err}"))
        })?;
    if !rest.is_empty() {
        return Err(ParseError::InvalidStructure(
            "X509Certificate contains trailing bytes after DER certificate".into(),
        ));
    }

    let subject_dn = cert.subject().to_string();
    let issuer_dn = cert.issuer().to_string();
    let serial_number = cert.tbs_certificate.raw_serial().to_vec();
    let serial_number_hex = format_x509_serial_value_hex(&serial_number);

    let subject_key_identifier = cert.extensions().iter().find_map(|ext| {
        if let ParsedExtension::SubjectKeyIdentifier(ski) = ext.parsed_extension() {
            Some(ski.0.to_vec())
        } else {
            None
        }
    });

    let spki = cert.public_key();
    let public_key = match spki.parsed().map_err(|err| {
        ParseError::InvalidStructure(format!("X509Certificate public key parse error: {err}"))
    })? {
        PublicKey::RSA(rsa) => {
            let modulus = trim_leading_zeroes(rsa.modulus);
            let exponent = trim_leading_zeroes(rsa.exponent);
            if modulus.is_empty() || exponent.is_empty() {
                return Err(ParseError::InvalidStructure(
                    "X509Certificate RSA key contains empty modulus or exponent".into(),
                ));
            }
            X509PublicKeyInfo::Rsa { modulus, exponent }
        }
        PublicKey::EC(ec_point) => {
            let Some(params) = spki.algorithm.parameters.as_ref() else {
                return Err(ParseError::InvalidStructure(
                    "X509Certificate EC key is missing curve parameters".into(),
                ));
            };

            match params.as_oid() {
                Ok(oid) => X509PublicKeyInfo::Ec {
                    curve_oid: oid.to_id_string(),
                    public_key: ec_point.data().to_vec(),
                },
                Err(_) => X509PublicKeyInfo::Unsupported {
                    algorithm_oid: spki.algorithm.algorithm.to_id_string(),
                },
            }
        }
        _ => X509PublicKeyInfo::Unsupported {
            algorithm_oid: spki.algorithm.algorithm.to_id_string(),
        },
    };

    Ok(ParsedX509Certificate {
        subject_dn,
        issuer_dn,
        serial_number,
        serial_number_hex,
        subject_key_identifier,
        public_key,
    })
}

fn format_x509_serial_hex(serial: &[u8]) -> String {
    serial
        .iter()
        .map(|byte| format!("{byte:02X}"))
        .collect::<String>()
}

fn format_x509_serial_value_hex(serial: &[u8]) -> String {
    let first_non_zero = serial
        .iter()
        .position(|byte| *byte != 0)
        .unwrap_or(serial.len());
    let canonical = if first_non_zero == serial.len() {
        &[0]
    } else {
        &serial[first_non_zero..]
    };
    format_x509_serial_hex(canonical)
}

fn x509_serial_decimal_to_hex(serial: &str) -> Option<String> {
    let serial = serial.trim();
    let serial = serial.strip_prefix('+').unwrap_or(serial);
    if serial.is_empty() || !serial.bytes().all(|byte| byte.is_ascii_digit()) {
        return None;
    }

    let mut bytes = Vec::<u8>::new();
    for digit in serial.bytes().map(|byte| byte - b'0') {
        let mut carry = u16::from(digit);
        for byte in bytes.iter_mut().rev() {
            let value = u16::from(*byte) * 10 + carry;
            *byte = value as u8;
            carry = value >> 8;
        }
        while carry > 0 {
            bytes.insert(0, carry as u8);
            carry >>= 8;
        }
    }

    Some(format_x509_serial_value_hex(&bytes))
}

fn trim_leading_zeroes(bytes: &[u8]) -> Vec<u8> {
    let first_non_zero = bytes
        .iter()
        .position(|byte| *byte != 0)
        .unwrap_or(bytes.len());
    bytes[first_non_zero..].to_vec()
}

fn parse_x509_issuer_serial(node: Node<'_, '_>) -> Result<(String, String), ParseError> {
    verify_ds_element(node, "X509IssuerSerial")?;
    ensure_no_non_whitespace_text(node, "X509IssuerSerial")?;

    let children = element_children(node).collect::<Vec<_>>();
    if children.len() != 2 {
        return Err(ParseError::InvalidStructure(
            "X509IssuerSerial must contain exactly X509IssuerName then X509SerialNumber".into(),
        ));
    }
    if !matches!(
        (
            children[0].tag_name().namespace(),
            children[0].tag_name().name()
        ),
        (Some(XMLDSIG_NS), "X509IssuerName")
    ) {
        return Err(ParseError::InvalidStructure(
            "X509IssuerSerial must contain X509IssuerName as the first child element".into(),
        ));
    }
    if !matches!(
        (
            children[1].tag_name().namespace(),
            children[1].tag_name().name()
        ),
        (Some(XMLDSIG_NS), "X509SerialNumber")
    ) {
        return Err(ParseError::InvalidStructure(
            "X509IssuerSerial must contain X509SerialNumber as the second child element".into(),
        ));
    }

    let issuer_node = children[0];
    ensure_no_element_children(issuer_node, "X509IssuerName")?;
    let issuer_name =
        collect_text_content_bounded(issuer_node, MAX_X509_ISSUER_NAME_TEXT_LEN, "X509IssuerName")?;

    let serial_node = children[1];
    ensure_no_element_children(serial_node, "X509SerialNumber")?;
    let serial_number = collect_text_content_bounded(
        serial_node,
        MAX_X509_SERIAL_NUMBER_TEXT_LEN,
        "X509SerialNumber",
    )?;
    if issuer_name.trim().is_empty() || serial_number.trim().is_empty() {
        return Err(ParseError::InvalidStructure(
            "X509IssuerSerial requires non-empty X509IssuerName and X509SerialNumber".into(),
        ));
    }

    Ok((issuer_name, serial_number))
}

/// Base64-decode a digest value string, stripping whitespace.
///
/// XMLDSig allows whitespace within base64 content (line-wrapped encodings).
fn base64_decode_digest(b64: &str, digest_method: DigestAlgorithm) -> Result<Vec<u8>, ParseError> {
    use base64::Engine;
    use base64::engine::general_purpose::STANDARD;

    let expected = digest_method.output_len();
    let max_base64_len = expected.div_ceil(3) * 4;
    let mut cleaned = String::with_capacity(b64.len().min(max_base64_len));
    normalize_xml_base64_text(b64, &mut cleaned).map_err(|err| {
        ParseError::Base64(format!(
            "invalid XML whitespace U+{:04X} in DigestValue",
            err.invalid_byte
        ))
    })?;
    if cleaned.len() > max_base64_len {
        return Err(ParseError::Base64(
            "DigestValue exceeds maximum allowed base64 length".into(),
        ));
    }
    let digest = STANDARD
        .decode(&cleaned)
        .map_err(|e| ParseError::Base64(e.to_string()))?;
    let actual = digest.len();
    if actual != expected {
        return Err(ParseError::DigestLengthMismatch {
            algorithm: digest_method.uri(),
            expected,
            actual,
        });
    }
    Ok(digest)
}

fn decode_digest_value_children(
    digest_value_node: Node<'_, '_>,
    digest_method: DigestAlgorithm,
) -> Result<Vec<u8>, ParseError> {
    let max_base64_len = digest_method.output_len().div_ceil(3) * 4;
    let mut cleaned = String::with_capacity(max_base64_len);

    for child in digest_value_node.children() {
        if child.is_element() {
            return Err(ParseError::InvalidStructure(
                "DigestValue must not contain element children".into(),
            ));
        }
        if let Some(text) = child.text() {
            normalize_xml_base64_text(text, &mut cleaned).map_err(|err| {
                ParseError::Base64(format!(
                    "invalid XML whitespace U+{:04X} in DigestValue",
                    err.invalid_byte
                ))
            })?;
            if cleaned.len() > max_base64_len {
                return Err(ParseError::Base64(
                    "DigestValue exceeds maximum allowed base64 length".into(),
                ));
            }
        }
    }

    base64_decode_digest(&cleaned, digest_method)
}

fn decode_der_encoded_key_value_base64(node: Node<'_, '_>) -> Result<Vec<u8>, ParseError> {
    use base64::Engine;
    use base64::engine::general_purpose::STANDARD;

    let mut cleaned = String::new();
    let mut raw_text_len = 0usize;
    for text in node
        .children()
        .filter(|child| child.is_text())
        .filter_map(|child| child.text())
    {
        if raw_text_len.saturating_add(text.len()) > MAX_DER_ENCODED_KEY_VALUE_TEXT_LEN {
            return Err(ParseError::InvalidStructure(
                "DEREncodedKeyValue exceeds maximum allowed text length".into(),
            ));
        }
        raw_text_len = raw_text_len.saturating_add(text.len());
        normalize_xml_base64_text(text, &mut cleaned).map_err(|err| {
            ParseError::Base64(format!(
                "invalid XML whitespace U+{:04X} in base64 text",
                err.invalid_byte
            ))
        })?;
        if cleaned.len() > MAX_DER_ENCODED_KEY_VALUE_BASE64_LEN {
            return Err(ParseError::InvalidStructure(
                "DEREncodedKeyValue exceeds maximum allowed length".into(),
            ));
        }
    }

    let der = STANDARD
        .decode(&cleaned)
        .map_err(|e| ParseError::Base64(e.to_string()))?;
    if der.is_empty() {
        return Err(ParseError::InvalidStructure(
            "DEREncodedKeyValue must not be empty".into(),
        ));
    }
    if der.len() > MAX_DER_ENCODED_KEY_VALUE_LEN {
        return Err(ParseError::InvalidStructure(
            "DEREncodedKeyValue exceeds maximum allowed length".into(),
        ));
    }
    Ok(der)
}

fn collect_text_content_bounded(
    node: Node<'_, '_>,
    max_len: usize,
    element_name: &'static str,
) -> Result<String, ParseError> {
    let mut text = String::new();
    for chunk in node
        .children()
        .filter_map(|child| child.is_text().then(|| child.text()).flatten())
    {
        if text.len().saturating_add(chunk.len()) > max_len {
            return Err(ParseError::InvalidStructure(format!(
                "{element_name} exceeds maximum allowed text length"
            )));
        }
        text.push_str(chunk);
    }
    Ok(text)
}

fn ensure_no_element_children(node: Node<'_, '_>, element_name: &str) -> Result<(), ParseError> {
    if node.children().any(|child| child.is_element()) {
        return Err(ParseError::InvalidStructure(format!(
            "{element_name} must not contain child elements"
        )));
    }
    Ok(())
}

fn ensure_no_non_whitespace_text(node: Node<'_, '_>, element_name: &str) -> Result<(), ParseError> {
    for child in node.children().filter(|child| child.is_text()) {
        if let Some(text) = child.text()
            && !is_xml_whitespace_only(text)
        {
            return Err(ParseError::InvalidStructure(format!(
                "{element_name} must not contain non-whitespace mixed content"
            )));
        }
    }
    Ok(())
}

#[cfg(test)]
#[expect(clippy::unwrap_used, reason = "tests use trusted XML fixtures")]
mod tests {
    use super::*;
    use base64::Engine;

    fn fixture_rsa_cert_base64() -> String {
        fixture_cert_base64("../../tests/fixtures/keys/rsa/rsa-2048-cert.pem")
    }

    fn fixture_cert_base64(path: &str) -> String {
        match path {
            "../../tests/fixtures/keys/rsa/rsa-2048-cert.pem" => {
                include_str!("../../tests/fixtures/keys/rsa/rsa-2048-cert.pem")
            }
            "../../tests/fixtures/keys/rsa/rsa-4096-cert.pem" => {
                include_str!("../../tests/fixtures/keys/rsa/rsa-4096-cert.pem")
            }
            "../../tests/fixtures/keys/ca2cert.pem" => {
                include_str!("../../tests/fixtures/keys/ca2cert.pem")
            }
            "../../tests/fixtures/keys/cacert.pem" => {
                include_str!("../../tests/fixtures/keys/cacert.pem")
            }
            _ => unreachable!("unknown certificate fixture"),
        }
        .lines()
        .skip_while(|line| *line != "-----BEGIN CERTIFICATE-----")
        .skip(1)
        .take_while(|line| *line != "-----END CERTIFICATE-----")
        .collect::<String>()
    }

    // ── SignatureAlgorithm ───────────────────────────────────────────

    #[test]
    fn signature_algorithm_from_uri_rsa_sha256() {
        assert_eq!(
            SignatureAlgorithm::from_uri("http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"),
            Some(SignatureAlgorithm::RsaSha256)
        );
    }

    #[test]
    fn signature_algorithm_from_uri_rsa_sha1() {
        assert_eq!(
            SignatureAlgorithm::from_uri("http://www.w3.org/2000/09/xmldsig#rsa-sha1"),
            Some(SignatureAlgorithm::RsaSha1)
        );
    }

    #[test]
    fn signature_algorithm_from_uri_ecdsa_sha256() {
        assert_eq!(
            SignatureAlgorithm::from_uri("http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256"),
            Some(SignatureAlgorithm::EcdsaP256Sha256)
        );
    }

    #[test]
    fn signature_algorithm_from_uri_unknown() {
        assert_eq!(
            SignatureAlgorithm::from_uri("http://example.com/unknown"),
            None
        );
    }

    #[test]
    fn signature_algorithm_uri_round_trip() {
        for algo in [
            SignatureAlgorithm::RsaSha1,
            SignatureAlgorithm::RsaSha256,
            SignatureAlgorithm::RsaSha384,
            SignatureAlgorithm::RsaSha512,
            SignatureAlgorithm::EcdsaP256Sha256,
            SignatureAlgorithm::EcdsaP384Sha384,
        ] {
            assert_eq!(
                SignatureAlgorithm::from_uri(algo.uri()),
                Some(algo),
                "round-trip failed for {algo:?}"
            );
        }
    }

    #[test]
    fn rsa_sha1_verify_only() {
        assert!(!SignatureAlgorithm::RsaSha1.signing_allowed());
        assert!(SignatureAlgorithm::RsaSha256.signing_allowed());
        assert!(SignatureAlgorithm::EcdsaP256Sha256.signing_allowed());
    }

    // ── find_signature_node ──────────────────────────────────────────

    #[test]
    fn find_signature_in_saml() {
        let xml = r#"<samlp:Response xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol">
            <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
                <ds:SignedInfo/>
            </ds:Signature>
        </samlp:Response>"#;
        let doc = Document::parse(xml).unwrap();
        let sig = find_signature_node(&doc);
        assert!(sig.is_some());
        assert_eq!(sig.unwrap().tag_name().name(), "Signature");
    }

    #[test]
    fn find_signature_missing() {
        let xml = "<root><child/></root>";
        let doc = Document::parse(xml).unwrap();
        assert!(find_signature_node(&doc).is_none());
    }

    #[test]
    fn find_signature_ignores_wrong_namespace() {
        let xml = r#"<root><Signature xmlns="http://example.com/fake"/></root>"#;
        let doc = Document::parse(xml).unwrap();
        assert!(find_signature_node(&doc).is_none());
    }

    // ── parse_key_info: dispatch parsing ──────────────────────────────

    #[test]
    fn parse_key_info_dispatches_supported_children() {
        let cert_base64 = fixture_rsa_cert_base64();
        let xml = format!(
            r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#"
                              xmlns:dsig11="http://www.w3.org/2009/xmldsig11#">
            <KeyName>idp-signing-key</KeyName>
            <KeyValue>
                <RSAKeyValue>
                    <Modulus>AQAB</Modulus>
                    <Exponent>AQAB</Exponent>
                </RSAKeyValue>
            </KeyValue>
            <X509Data>
                <X509Certificate>{cert_base64}</X509Certificate>
                <X509SubjectName>C=US, ST=California, O=XML Security Library (http://www.aleksey.com/xmlsec), CN=Test Key rsa-2048</X509SubjectName>
                <X509IssuerSerial>
                    <X509IssuerName>C=US, ST=California, O=XML Security Library (http://www.aleksey.com/xmlsec), OU=Second level CA, CN=Aleksey Sanin, Email=xmlsec@aleksey.com</X509IssuerName>
                    <X509SerialNumber>680572598617295163017172295025714171905498632019</X509SerialNumber>
                </X509IssuerSerial>
                <X509SKI>bcOXN/nsVl8GatRbcKrPbzIbw0Y=</X509SKI>
                <X509CRL>BAUGBw==</X509CRL>
                <dsig11:X509Digest Algorithm="http://www.w3.org/2001/04/xmlenc#sha256">CAkK</dsig11:X509Digest>
            </X509Data>
            <dsig11:DEREncodedKeyValue>AQIDBA==</dsig11:DEREncodedKeyValue>
        </KeyInfo>"#
        );
        let doc = Document::parse(&xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        assert_eq!(key_info.sources.len(), 4);

        assert_eq!(
            key_info.sources[0],
            KeyInfoSource::KeyName("idp-signing-key".to_string())
        );
        assert_eq!(
            key_info.sources[1],
            KeyInfoSource::KeyValue(KeyValueInfo::RsaKeyValue)
        );
        let x509_info = match &key_info.sources[2] {
            KeyInfoSource::X509Data(x509) => x509,
            other => panic!("expected X509Data source, got {other:?}"),
        };
        let expected_cert = base64::engine::general_purpose::STANDARD
            .decode(&cert_base64)
            .expect("fixture PEM must contain valid base64");
        assert_eq!(x509_info.certificates, vec![expected_cert]);
        assert_eq!(
            x509_info.subject_names,
            vec![
                "C=US, ST=California, O=XML Security Library (http://www.aleksey.com/xmlsec), CN=Test Key rsa-2048"
                    .to_string()
            ]
        );
        assert_eq!(
            x509_info.issuer_serials,
            vec![(
                "C=US, ST=California, O=XML Security Library (http://www.aleksey.com/xmlsec), OU=Second level CA, CN=Aleksey Sanin, Email=xmlsec@aleksey.com".to_string(),
                "680572598617295163017172295025714171905498632019".to_string()
            )]
        );
        assert_eq!(
            x509_info.skis,
            vec![vec![
                109, 195, 151, 55, 249, 236, 86, 95, 6, 106, 212, 91, 112, 170, 207, 111, 50, 27,
                195, 70
            ]]
        );
        assert_eq!(x509_info.crls, vec![vec![4, 5, 6, 7]]);
        assert_eq!(
            x509_info.digests,
            vec![(
                "http://www.w3.org/2001/04/xmlenc#sha256".to_string(),
                vec![8, 9, 10]
            )]
        );
        assert_eq!(x509_info.parsed_certificates.len(), 1);
        assert_eq!(x509_info.certificate_chain, vec![0]);
        let parsed_cert = &x509_info.parsed_certificates[0];
        assert!(!parsed_cert.subject_dn.is_empty());
        assert!(!parsed_cert.issuer_dn.is_empty());
        assert_eq!(
            parsed_cert.serial_number_hex,
            "7735EE487F6862DAF1B3956D961CCB0FA6F34F53"
        );
        assert!(parsed_cert.subject_key_identifier.is_some());
        assert!(matches!(
            parsed_cert.public_key,
            X509PublicKeyInfo::Rsa { .. }
        ));

        assert_eq!(
            key_info.sources[3],
            KeyInfoSource::DerEncodedKeyValue(vec![1, 2, 3, 4])
        );
    }

    #[test]
    fn parse_key_info_ignores_unknown_children() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <Foo>bar</Foo>
            <KeyName>ok</KeyName>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        assert_eq!(key_info.sources, vec![KeyInfoSource::KeyName("ok".into())]);
    }

    #[test]
    fn parse_key_info_keyvalue_requires_single_child() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <KeyValue/>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_accepts_empty_x509data() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <X509Data/>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        assert_eq!(
            key_info.sources,
            vec![KeyInfoSource::X509Data(X509DataInfo::default())]
        );
    }

    #[test]
    fn parse_key_info_rejects_unknown_xmlsig_child_in_x509data() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <X509Data>
                <Foo/>
            </X509Data>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_unknown_xmlsig11_child_in_x509data() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#"
                              xmlns:dsig11="http://www.w3.org/2009/xmldsig11#">
            <X509Data>
                <dsig11:Foo/>
            </X509Data>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_x509_issuer_serial_without_required_children() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <X509Data>
                <X509IssuerSerial>
                    <X509IssuerName>CN=CA</X509IssuerName>
                </X509IssuerSerial>
            </X509Data>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_x509_issuer_serial_with_duplicate_issuer_name() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <X509Data>
                <X509IssuerSerial>
                    <X509IssuerName>CN=CA-1</X509IssuerName>
                    <X509IssuerName>CN=CA-2</X509IssuerName>
                    <X509SerialNumber>42</X509SerialNumber>
                </X509IssuerSerial>
            </X509Data>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_x509_issuer_serial_with_duplicate_serial_number() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <X509Data>
                <X509IssuerSerial>
                    <X509IssuerName>CN=CA</X509IssuerName>
                    <X509SerialNumber>1</X509SerialNumber>
                    <X509SerialNumber>2</X509SerialNumber>
                </X509IssuerSerial>
            </X509Data>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_x509_issuer_serial_with_whitespace_only_values() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <X509Data>
                <X509IssuerSerial>
                    <X509IssuerName>   </X509IssuerName>
                    <X509SerialNumber>
                        
                    </X509SerialNumber>
                </X509IssuerSerial>
            </X509Data>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_x509_issuer_serial_with_wrong_child_order() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <X509Data>
                <X509IssuerSerial>
                    <X509SerialNumber>42</X509SerialNumber>
                    <X509IssuerName>CN=CA</X509IssuerName>
                </X509IssuerSerial>
            </X509Data>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_x509_issuer_serial_with_extra_child_element() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#"
                              xmlns:foo="urn:example:foo">
            <X509Data>
                <X509IssuerSerial>
                    <X509IssuerName>CN=CA</X509IssuerName>
                    <X509SerialNumber>42</X509SerialNumber>
                    <foo:Extra/>
                </X509IssuerSerial>
            </X509Data>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_x509_digest_without_algorithm() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#"
                              xmlns:dsig11="http://www.w3.org/2009/xmldsig11#">
            <X509Data>
                <dsig11:X509Digest>AQID</dsig11:X509Digest>
            </X509Data>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_invalid_x509_certificate_base64() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <X509Data>
                <X509Certificate>%%%invalid%%%</X509Certificate>
            </X509Data>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::Base64(_)));
    }

    #[test]
    fn parse_key_info_rejects_x509_data_exceeding_entry_budget() {
        let subjects = (0..(MAX_X509_DATA_ENTRY_COUNT + 1))
            .map(|idx| format!("<X509SubjectName>CN={idx}</X509SubjectName>"))
            .collect::<Vec<_>>()
            .join("");
        let xml = format!(
            "<KeyInfo xmlns=\"http://www.w3.org/2000/09/xmldsig#\"><X509Data>{subjects}</X509Data></KeyInfo>"
        );
        let doc = Document::parse(&xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_x509_data_exceeding_total_binary_budget() {
        let payload = base64::engine::general_purpose::STANDARD.encode(vec![0u8; 190_000]);
        let entries = (0..6)
            .map(|_| format!("<X509SKI>{payload}</X509SKI>"))
            .collect::<Vec<_>>()
            .join("");
        let xml = format!(
            "<KeyInfo xmlns=\"http://www.w3.org/2000/09/xmldsig#\"><X509Data>{entries}</X509Data></KeyInfo>"
        );
        let doc = Document::parse(&xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_x509_certificate_with_invalid_der() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <X509Data>
                <X509Certificate>AQID</X509Certificate>
            </X509Data>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_x509_certificate_with_trailing_der_bytes() {
        let mut cert = base64::engine::general_purpose::STANDARD
            .decode(fixture_rsa_cert_base64())
            .unwrap();
        cert.extend_from_slice(&[0x00, 0x01]);
        let cert_base64 = base64::engine::general_purpose::STANDARD.encode(cert);
        let xml = format!(
            r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
                <X509Data>
                    <X509Certificate>{cert_base64}</X509Certificate>
                </X509Data>
            </KeyInfo>"#
        );
        let doc = Document::parse(&xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_marks_unsupported_spki_algorithm_as_unsupported() {
        let xml = include_str!(
            "../../tests/fixtures/xmldsig/merlin-xmldsig-twenty-three/signature-x509-crt.xml"
        );
        let doc = Document::parse(xml).unwrap();
        let key_info_node = doc
            .descendants()
            .find(|node| {
                node.is_element()
                    && node.tag_name().namespace() == Some(XMLDSIG_NS)
                    && node.tag_name().name() == "KeyInfo"
            })
            .expect("fixture must contain ds:KeyInfo");

        let key_info = parse_key_info(key_info_node).expect("KeyInfo parse should succeed");
        let x509_info = match &key_info.sources[0] {
            KeyInfoSource::X509Data(x509) => x509,
            other => panic!("expected X509Data source, got {other:?}"),
        };
        assert_eq!(x509_info.certificates.len(), 1);
        assert_eq!(x509_info.parsed_certificates.len(), 1);
        assert_eq!(x509_info.certificate_chain, vec![0]);
        let parsed_cert = &x509_info.parsed_certificates[0];
        assert!(!parsed_cert.subject_dn.is_empty());
        assert!(!parsed_cert.issuer_dn.is_empty());
        assert!(parsed_cert.subject_key_identifier.is_some());
        assert!(matches!(
            parsed_cert.public_key,
            X509PublicKeyInfo::Unsupported { .. }
        ));
    }

    #[test]
    fn parse_key_info_orders_x509_certificate_chain_from_signing_cert() {
        let root = fixture_cert_base64("../../tests/fixtures/keys/cacert.pem");
        let intermediate = fixture_cert_base64("../../tests/fixtures/keys/ca2cert.pem");
        let leaf = fixture_cert_base64("../../tests/fixtures/keys/rsa/rsa-2048-cert.pem");
        let xml = format!(
            r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
                <X509Data>
                    <X509Certificate>{root}</X509Certificate>
                    <X509Certificate>{intermediate}</X509Certificate>
                    <X509Certificate>{leaf}</X509Certificate>
                </X509Data>
            </KeyInfo>"#
        );
        let doc = Document::parse(&xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        let x509_info = match &key_info.sources[0] {
            KeyInfoSource::X509Data(x509) => x509,
            other => panic!("expected X509Data source, got {other:?}"),
        };

        assert_eq!(x509_info.certificate_chain, vec![2, 1, 0]);
    }

    #[test]
    fn parse_key_info_uses_issuer_serial_to_select_x509_signing_certificate() {
        let root = fixture_cert_base64("../../tests/fixtures/keys/cacert.pem");
        let intermediate = fixture_cert_base64("../../tests/fixtures/keys/ca2cert.pem");
        let leaf = fixture_cert_base64("../../tests/fixtures/keys/rsa/rsa-2048-cert.pem");
        let xml = format!(
            r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
                <X509Data>
                    <X509IssuerSerial>
                        <X509IssuerName>C=US, ST=California, O=XML Security Library (http://www.aleksey.com/xmlsec), OU=Second level CA, CN=Aleksey Sanin, Email=xmlsec@aleksey.com</X509IssuerName>
                        <X509SerialNumber>680572598617295163017172295025714171905498632019</X509SerialNumber>
                    </X509IssuerSerial>
                    <X509Certificate>{root}</X509Certificate>
                    <X509Certificate>{intermediate}</X509Certificate>
                    <X509Certificate>{leaf}</X509Certificate>
                </X509Data>
            </KeyInfo>"#
        );
        let doc = Document::parse(&xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        let x509_info = match &key_info.sources[0] {
            KeyInfoSource::X509Data(x509) => x509,
            other => panic!("expected X509Data source, got {other:?}"),
        };

        assert_eq!(x509_info.certificate_chain, vec![2, 1, 0]);
    }

    #[test]
    fn parse_key_info_uses_decimal_issuer_serial_to_select_x509_signing_certificate() {
        assert_eq!(
            x509_serial_decimal_to_hex("680572598617295163017172295025714171905498632019")
                .as_deref(),
            Some("7735EE487F6862DAF1B3956D961CCB0FA6F34F53")
        );
        let root = fixture_cert_base64("../../tests/fixtures/keys/cacert.pem");
        let intermediate = fixture_cert_base64("../../tests/fixtures/keys/ca2cert.pem");
        let leaf = fixture_cert_base64("../../tests/fixtures/keys/rsa/rsa-2048-cert.pem");
        let other_leaf = fixture_cert_base64("../../tests/fixtures/keys/rsa/rsa-4096-cert.pem");
        let xml = format!(
            r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
                <X509Data>
                    <X509IssuerSerial>
                        <X509IssuerName>C=US, ST=California, O=XML Security Library (http://www.aleksey.com/xmlsec), OU=Second level CA, CN=Aleksey Sanin, Email=xmlsec@aleksey.com</X509IssuerName>
                        <X509SerialNumber>680572598617295163017172295025714171905498632019</X509SerialNumber>
                    </X509IssuerSerial>
                    <X509Certificate>{root}</X509Certificate>
                    <X509Certificate>{intermediate}</X509Certificate>
                    <X509Certificate>{leaf}</X509Certificate>
                    <X509Certificate>{other_leaf}</X509Certificate>
                </X509Data>
            </KeyInfo>"#
        );
        let doc = Document::parse(&xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        let x509_info = match &key_info.sources[0] {
            KeyInfoSource::X509Data(x509) => x509,
            other => panic!("expected X509Data source, got {other:?}"),
        };

        assert_eq!(x509_info.certificate_chain, vec![2, 1, 0]);
    }

    #[test]
    fn parse_key_info_rejects_ambiguous_x509_signing_certificate_candidates() {
        let first_leaf = fixture_cert_base64("../../tests/fixtures/keys/rsa/rsa-2048-cert.pem");
        let second_leaf = fixture_cert_base64("../../tests/fixtures/keys/rsa/rsa-4096-cert.pem");
        let xml = format!(
            r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
                <X509Data>
                    <X509Certificate>{first_leaf}</X509Certificate>
                    <X509Certificate>{second_leaf}</X509Certificate>
                </X509Data>
            </KeyInfo>"#
        );
        let doc = Document::parse(&xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_unmatched_x509_lookup_identifier() {
        let cert = fixture_cert_base64("../../tests/fixtures/keys/rsa/rsa-2048-cert.pem");
        let xml = format!(
            r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
                <X509Data>
                    <X509SubjectName>CN=Not The Embedded Certificate</X509SubjectName>
                    <X509Certificate>{cert}</X509Certificate>
                </X509Data>
            </KeyInfo>"#
        );
        let doc = Document::parse(&xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(
            matches!(err, ParseError::InvalidStructure(message) if message.contains("lookup identifiers"))
        );
    }

    #[test]
    fn parse_key_info_rejects_malformed_issuer_serial_even_with_matching_subject() {
        let cert = fixture_cert_base64("../../tests/fixtures/keys/rsa/rsa-2048-cert.pem");
        let xml = format!(
            r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
                <X509Data>
                    <X509SubjectName>C=US, ST=California, O=XML Security Library (http://www.aleksey.com/xmlsec), CN=Test Key rsa-2048</X509SubjectName>
                    <X509IssuerSerial>
                        <X509IssuerName>C=US, ST=California, O=XML Security Library (http://www.aleksey.com/xmlsec), OU=Second level CA, CN=Aleksey Sanin, Email=xmlsec@aleksey.com</X509IssuerName>
                        <X509SerialNumber>not-a-decimal-serial</X509SerialNumber>
                    </X509IssuerSerial>
                    <X509Certificate>{cert}</X509Certificate>
                </X509Data>
            </KeyInfo>"#
        );
        let doc = Document::parse(&xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(
            matches!(err, ParseError::InvalidStructure(message) if message.contains("lookup identifiers"))
        );
    }

    #[test]
    fn parse_key_info_rejects_unmatched_ski_even_with_matching_subject() {
        let cert = fixture_cert_base64("../../tests/fixtures/keys/rsa/rsa-2048-cert.pem");
        let xml = format!(
            r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
                <X509Data>
                    <X509SubjectName>C=US, ST=California, O=XML Security Library (http://www.aleksey.com/xmlsec), CN=Test Key rsa-2048</X509SubjectName>
                    <X509SKI>AQIDBA==</X509SKI>
                    <X509Certificate>{cert}</X509Certificate>
                </X509Data>
            </KeyInfo>"#
        );
        let doc = Document::parse(&xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(
            matches!(err, ParseError::InvalidStructure(message) if message.contains("lookup identifiers"))
        );
    }

    #[test]
    fn parse_key_info_rejects_lookup_hints_for_different_certificates() {
        let first_cert = fixture_cert_base64("../../tests/fixtures/keys/rsa/rsa-2048-cert.pem");
        let second_cert = fixture_cert_base64("../../tests/fixtures/keys/rsa/rsa-4096-cert.pem");
        let xml = format!(
            r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
                <X509Data>
                    <X509SubjectName>C=US, ST=California, O=XML Security Library (http://www.aleksey.com/xmlsec), CN=Test Key rsa-2048</X509SubjectName>
                    <X509SKI>60zMLKCfzQ3qnXAzABzRNpdgQ8Q=</X509SKI>
                    <X509Certificate>{first_cert}</X509Certificate>
                    <X509Certificate>{second_cert}</X509Certificate>
                </X509Data>
            </KeyInfo>"#
        );
        let doc = Document::parse(&xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(
            matches!(err, ParseError::InvalidStructure(message) if message.contains("lookup identifiers match multiple certificates"))
        );
    }

    #[test]
    fn build_x509_certificate_chain_rejects_chain_exceeding_max_depth() {
        let parsed_certificates = (0..=MAX_X509_CHAIN_DEPTH)
            .map(|idx| ParsedX509Certificate {
                subject_dn: format!("CN=cert-{idx}"),
                issuer_dn: if idx == MAX_X509_CHAIN_DEPTH {
                    format!("CN=cert-{idx}")
                } else {
                    format!("CN=cert-{}", idx + 1)
                },
                serial_number: vec![u8::try_from(idx).unwrap()],
                serial_number_hex: format!("{idx:02X}"),
                subject_key_identifier: None,
                public_key: X509PublicKeyInfo::Unsupported {
                    algorithm_oid: "1.2.3.4".into(),
                },
            })
            .collect();
        let info = X509DataInfo {
            parsed_certificates,
            ..X509DataInfo::default()
        };

        let err = build_x509_certificate_chain(&info).unwrap_err();
        assert!(
            matches!(err, ParseError::InvalidStructure(message) if message.contains("maximum depth"))
        );
    }

    #[test]
    fn x509_serial_hex_strips_der_sign_extension_zeroes() {
        assert_eq!(format_x509_serial_value_hex(&[0x00, 0xFF]), "FF");
        assert_eq!(format_x509_serial_value_hex(&[0x00, 0x7F]), "7F");
        assert_eq!(format_x509_serial_value_hex(&[0x00, 0x00]), "00");
    }

    #[test]
    fn parse_key_info_accepts_large_textual_x509_entries_within_entry_budget() {
        let issuer_name = "C".repeat(MAX_X509_ISSUER_NAME_TEXT_LEN);
        let serial_number = "7".repeat(MAX_X509_SERIAL_NUMBER_TEXT_LEN);
        let issuer_serials = (0..52)
            .map(|_| {
                format!(
                    "<X509IssuerSerial><X509IssuerName>{issuer_name}</X509IssuerName><X509SerialNumber>{serial_number}</X509SerialNumber></X509IssuerSerial>"
                )
            })
            .collect::<Vec<_>>()
            .join("");
        let xml = format!(
            "<KeyInfo xmlns=\"http://www.w3.org/2000/09/xmldsig#\"><X509Data>{issuer_serials}</X509Data></KeyInfo>"
        );
        let doc = Document::parse(&xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        let parsed = match &key_info.sources[0] {
            KeyInfoSource::X509Data(x509) => x509,
            _ => panic!("expected X509Data source"),
        };
        assert_eq!(parsed.issuer_serials.len(), 52);
    }

    #[test]
    fn parse_key_info_accepts_x509data_with_only_foreign_namespace_children() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#"
                              xmlns:foo="urn:example:foo">
            <X509Data>
                <foo:Bar/>
            </X509Data>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        assert_eq!(
            key_info.sources,
            vec![KeyInfoSource::X509Data(X509DataInfo::default())]
        );
    }

    #[test]
    fn parse_key_info_der_encoded_key_value_rejects_invalid_base64() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#"
                              xmlns:dsig11="http://www.w3.org/2009/xmldsig11#">
            <dsig11:DEREncodedKeyValue>%%%invalid%%%</dsig11:DEREncodedKeyValue>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::Base64(_)));
    }

    #[test]
    fn parse_key_info_der_encoded_key_value_accepts_xml_whitespace() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#"
                              xmlns:dsig11="http://www.w3.org/2009/xmldsig11#">
            <dsig11:DEREncodedKeyValue>
                AQID
                BA==
            </dsig11:DEREncodedKeyValue>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        assert_eq!(
            key_info.sources,
            vec![KeyInfoSource::DerEncodedKeyValue(vec![1, 2, 3, 4])]
        );
    }

    #[test]
    fn parse_key_info_dispatches_dsig11_ec_keyvalue() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#"
                              xmlns:dsig11="http://www.w3.org/2009/xmldsig11#">
            <KeyValue>
                <dsig11:ECKeyValue/>
            </KeyValue>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        assert_eq!(
            key_info.sources,
            vec![KeyInfoSource::KeyValue(KeyValueInfo::EcKeyValue)]
        );
    }

    #[test]
    fn parse_key_info_marks_ds_namespace_ec_keyvalue_as_unsupported() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <KeyValue>
                <ECKeyValue/>
            </KeyValue>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        assert_eq!(
            key_info.sources,
            vec![KeyInfoSource::KeyValue(KeyValueInfo::Unsupported {
                namespace: Some(XMLDSIG_NS.to_string()),
                local_name: "ECKeyValue".into(),
            })]
        );
    }

    #[test]
    fn parse_key_info_keeps_unsupported_keyvalue_child_as_marker() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <KeyValue>
                <DSAKeyValue/>
            </KeyValue>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        assert_eq!(
            key_info.sources,
            vec![KeyInfoSource::KeyValue(KeyValueInfo::Unsupported {
                namespace: Some(XMLDSIG_NS.to_string()),
                local_name: "DSAKeyValue".into(),
            })]
        );
    }

    #[test]
    fn parse_key_info_rejects_keyname_with_child_elements() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <KeyName>ok<foo/></KeyName>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_preserves_keyname_text_without_trimming() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <KeyName>  signing key  </KeyName>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let key_info = parse_key_info(doc.root_element()).unwrap();
        assert_eq!(
            key_info.sources,
            vec![KeyInfoSource::KeyName("  signing key  ".into())]
        );
    }

    #[test]
    fn parse_key_info_rejects_oversized_keyname_text() {
        let oversized = "A".repeat(4097);
        let xml = format!(
            "<KeyInfo xmlns=\"http://www.w3.org/2000/09/xmldsig#\"><KeyName>{oversized}</KeyName></KeyInfo>"
        );
        let doc = Document::parse(&xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_non_whitespace_mixed_content() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">oops<KeyName>k</KeyName></KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_rejects_nbsp_as_non_xml_whitespace_mixed_content() {
        let xml = "<KeyInfo xmlns=\"http://www.w3.org/2000/09/xmldsig#\">\u{00A0}<KeyName>k</KeyName></KeyInfo>";
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_der_encoded_key_value_rejects_oversized_payload() {
        let oversized =
            base64::engine::general_purpose::STANDARD
                .encode(vec![0u8; MAX_DER_ENCODED_KEY_VALUE_LEN + 1]);
        let xml = format!(
            "<KeyInfo xmlns=\"http://www.w3.org/2000/09/xmldsig#\" xmlns:dsig11=\"http://www.w3.org/2009/xmldsig11#\"><dsig11:DEREncodedKeyValue>{oversized}</dsig11:DEREncodedKeyValue></KeyInfo>"
        );
        let doc = Document::parse(&xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_der_encoded_key_value_rejects_empty_payload() {
        let xml = r#"<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#"
                              xmlns:dsig11="http://www.w3.org/2009/xmldsig11#">
            <dsig11:DEREncodedKeyValue>
                
            </dsig11:DEREncodedKeyValue>
        </KeyInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let err = parse_key_info(doc.root_element()).unwrap_err();
        assert!(matches!(err, ParseError::InvalidStructure(_)));
    }

    #[test]
    fn parse_key_info_der_encoded_key_value_non_xml_ascii_whitespace_is_not_parseable_xml() {
        let xml = "<KeyInfo xmlns=\"http://www.w3.org/2000/09/xmldsig#\" xmlns:dsig11=\"http://www.w3.org/2009/xmldsig11#\"><dsig11:DEREncodedKeyValue>\u{000C}</dsig11:DEREncodedKeyValue></KeyInfo>";
        assert!(Document::parse(xml).is_err());
    }

    // ── parse_signed_info: happy path ────────────────────────────────

    #[test]
    fn parse_signed_info_rsa_sha256_with_reference() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="">
                <Transforms>
                    <Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
                    <Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
                </Transforms>
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let si = parse_signed_info(doc.root_element()).unwrap();

        assert_eq!(si.signature_method, SignatureAlgorithm::RsaSha256);
        assert_eq!(si.references.len(), 1);

        let r = &si.references[0];
        assert_eq!(r.uri.as_deref(), Some(""));
        assert_eq!(r.digest_method, DigestAlgorithm::Sha256);
        assert_eq!(r.digest_value, vec![0u8; 32]);
        assert_eq!(r.transforms.len(), 2);
    }

    #[test]
    fn parse_signed_info_multiple_references() {
        let xml = r##"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256"/>
            <Reference URI="#a">
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=</DigestValue>
            </Reference>
            <Reference URI="#b">
                <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
                <DigestValue>AAAAAAAAAAAAAAAAAAAAAAAAAAA=</DigestValue>
            </Reference>
        </SignedInfo>"##;
        let doc = Document::parse(xml).unwrap();
        let si = parse_signed_info(doc.root_element()).unwrap();

        assert_eq!(si.signature_method, SignatureAlgorithm::EcdsaP256Sha256);
        assert_eq!(si.references.len(), 2);
        assert_eq!(si.references[0].uri.as_deref(), Some("#a"));
        assert_eq!(si.references[0].digest_method, DigestAlgorithm::Sha256);
        assert_eq!(si.references[1].uri.as_deref(), Some("#b"));
        assert_eq!(si.references[1].digest_method, DigestAlgorithm::Sha1);
    }

    #[test]
    fn parse_reference_without_transforms() {
        // Transforms element is optional
        let xml = r##"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="#obj">
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=</DigestValue>
            </Reference>
        </SignedInfo>"##;
        let doc = Document::parse(xml).unwrap();
        let si = parse_signed_info(doc.root_element()).unwrap();

        assert!(si.references[0].transforms.is_empty());
    }

    #[test]
    fn parse_reference_with_all_attributes() {
        let xml = r##"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="#data" Id="ref1" Type="http://www.w3.org/2000/09/xmldsig#Object">
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=</DigestValue>
            </Reference>
        </SignedInfo>"##;
        let doc = Document::parse(xml).unwrap();
        let si = parse_signed_info(doc.root_element()).unwrap();
        let r = &si.references[0];

        assert_eq!(r.uri.as_deref(), Some("#data"));
        assert_eq!(r.id.as_deref(), Some("ref1"));
        assert_eq!(
            r.ref_type.as_deref(),
            Some("http://www.w3.org/2000/09/xmldsig#Object")
        );
    }

    #[test]
    fn parse_reference_absent_uri() {
        // URI attribute is optional per spec
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference>
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let si = parse_signed_info(doc.root_element()).unwrap();
        assert!(si.references[0].uri.is_none());
    }

    #[test]
    fn parse_signed_info_preserves_inclusive_prefixes() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#"
                                 xmlns:ec="http://www.w3.org/2001/10/xml-exc-c14n#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#">
                <ec:InclusiveNamespaces PrefixList="ds saml #default"/>
            </CanonicalizationMethod>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let si = parse_signed_info(doc.root_element()).unwrap();
        assert!(si.c14n_method.inclusive_prefixes().contains("ds"));
        assert!(si.c14n_method.inclusive_prefixes().contains("saml"));
        assert!(si.c14n_method.inclusive_prefixes().contains(""));
    }

    // ── parse_signed_info: error cases ───────────────────────────────

    #[test]
    fn missing_canonicalization_method() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>dGVzdA==</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let result = parse_signed_info(doc.root_element());
        assert!(result.is_err());
        // SignatureMethod is first child but expected CanonicalizationMethod
        assert!(matches!(
            result.unwrap_err(),
            ParseError::InvalidStructure(_)
        ));
    }

    #[test]
    fn missing_signature_method() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>dGVzdA==</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let result = parse_signed_info(doc.root_element());
        assert!(result.is_err());
        // Reference is second child but expected SignatureMethod
        assert!(matches!(
            result.unwrap_err(),
            ParseError::InvalidStructure(_)
        ));
    }

    #[test]
    fn no_references() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let result = parse_signed_info(doc.root_element());
        assert!(matches!(
            result.unwrap_err(),
            ParseError::MissingElement {
                element: "Reference"
            }
        ));
    }

    #[test]
    fn unsupported_c14n_algorithm() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://example.com/bogus-c14n"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>dGVzdA==</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let result = parse_signed_info(doc.root_element());
        assert!(matches!(
            result.unwrap_err(),
            ParseError::UnsupportedAlgorithm { .. }
        ));
    }

    #[test]
    fn unsupported_signature_algorithm() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://example.com/bogus-sign"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>dGVzdA==</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let result = parse_signed_info(doc.root_element());
        assert!(matches!(
            result.unwrap_err(),
            ParseError::UnsupportedAlgorithm { .. }
        ));
    }

    #[test]
    fn unsupported_digest_algorithm() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://example.com/bogus-digest"/>
                <DigestValue>dGVzdA==</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let result = parse_signed_info(doc.root_element());
        assert!(matches!(
            result.unwrap_err(),
            ParseError::UnsupportedAlgorithm { .. }
        ));
    }

    #[test]
    fn missing_digest_method() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="">
                <DigestValue>dGVzdA==</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let result = parse_signed_info(doc.root_element());
        // DigestValue is not DigestMethod
        assert!(result.is_err());
    }

    #[test]
    fn missing_digest_value() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let result = parse_signed_info(doc.root_element());
        assert!(matches!(
            result.unwrap_err(),
            ParseError::MissingElement {
                element: "DigestValue"
            }
        ));
    }

    #[test]
    fn invalid_base64_digest_value() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>!!!not-base64!!!</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let result = parse_signed_info(doc.root_element());
        assert!(matches!(result.unwrap_err(), ParseError::Base64(_)));
    }

    #[test]
    fn digest_value_length_must_match_digest_method() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>dGVzdA==</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let result = parse_signed_info(doc.root_element());
        assert!(matches!(
            result.unwrap_err(),
            ParseError::DigestLengthMismatch {
                algorithm: "http://www.w3.org/2001/04/xmlenc#sha256",
                expected: 32,
                actual: 4,
            }
        ));
    }

    #[test]
    fn inclusive_prefixes_on_inclusive_c14n_is_rejected() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#"
                                 xmlns:ec="http://www.w3.org/2001/10/xml-exc-c14n#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315">
                <ec:InclusiveNamespaces PrefixList="ds"/>
            </CanonicalizationMethod>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
                <DigestValue>AAAAAAAAAAAAAAAAAAAAAAAAAAA=</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let result = parse_signed_info(doc.root_element());
        assert!(matches!(
            result.unwrap_err(),
            ParseError::UnsupportedAlgorithm { .. }
        ));
    }

    #[test]
    fn extra_element_after_digest_value() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                <DigestValue>AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=</DigestValue>
                <Unexpected/>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let result = parse_signed_info(doc.root_element());
        assert!(matches!(
            result.unwrap_err(),
            ParseError::InvalidStructure(_)
        ));
    }

    #[test]
    fn digest_value_with_element_child_is_rejected() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
                <DigestValue>AAAAAAAAAAAAAAAAAAAAAAAAAAA=<Junk/>AAAA</DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();

        let result = parse_signed_info(doc.root_element());
        assert!(matches!(
            result.unwrap_err(),
            ParseError::InvalidStructure(_)
        ));
    }

    #[test]
    fn wrong_namespace_on_signed_info() {
        let xml = r#"<SignedInfo xmlns="http://example.com/fake">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let result = parse_signed_info(doc.root_element());
        assert!(matches!(
            result.unwrap_err(),
            ParseError::InvalidStructure(_)
        ));
    }

    // ── Whitespace-wrapped base64 ────────────────────────────────────

    #[test]
    fn base64_with_whitespace() {
        let xml = r#"<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
            <Reference URI="">
                <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
                <DigestValue>
                    AAAAAAAA
                    AAAAAAAAAAAAAAAAAAA=
                </DigestValue>
            </Reference>
        </SignedInfo>"#;
        let doc = Document::parse(xml).unwrap();
        let si = parse_signed_info(doc.root_element()).unwrap();
        assert_eq!(si.references[0].digest_value, vec![0u8; 20]);
    }

    #[test]
    fn base64_decode_digest_accepts_xml_whitespace_chars() {
        let digest =
            base64_decode_digest("AAAA\tAAAA\rAAAA\nAAAA AAAAAAAAAAA=", DigestAlgorithm::Sha1)
                .expect("XML whitespace in DigestValue must be accepted");
        assert_eq!(digest, vec![0u8; 20]);
    }

    #[test]
    fn base64_decode_digest_rejects_non_xml_ascii_whitespace() {
        let err = base64_decode_digest(
            "AAAA\u{000C}AAAAAAAAAAAAAAAAAAAAAAA=",
            DigestAlgorithm::Sha1,
        )
        .expect_err("form-feed/vertical-tab in DigestValue must be rejected");
        assert!(matches!(err, ParseError::Base64(_)));
    }

    #[test]
    fn base64_decode_digest_rejects_oversized_base64_before_decode() {
        let err = base64_decode_digest("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA", DigestAlgorithm::Sha1)
            .expect_err("oversized DigestValue base64 must fail before decode");
        match err {
            ParseError::Base64(message) => {
                assert!(
                    message.contains("DigestValue exceeds maximum allowed base64 length"),
                    "unexpected message: {message}"
                );
            }
            other => panic!("expected ParseError::Base64, got {other:?}"),
        }
    }

    // ── Real-world SAML structure ────────────────────────────────────

    #[test]
    fn saml_response_signed_info() {
        let xml = r##"<ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
            <ds:SignedInfo>
                <ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
                <ds:SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
                <ds:Reference URI="#_resp1">
                    <ds:Transforms>
                    <ds:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
                    <ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
                    </ds:Transforms>
                    <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
                    <ds:DigestValue>AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=</ds:DigestValue>
                </ds:Reference>
            </ds:SignedInfo>
            <ds:SignatureValue>ZmFrZQ==</ds:SignatureValue>
        </ds:Signature>"##;
        let doc = Document::parse(xml).unwrap();

        // Find SignedInfo within Signature
        let sig_node = doc.root_element();
        let signed_info_node = sig_node
            .children()
            .find(|n| n.is_element() && n.tag_name().name() == "SignedInfo")
            .unwrap();

        let si = parse_signed_info(signed_info_node).unwrap();
        assert_eq!(si.signature_method, SignatureAlgorithm::RsaSha256);
        assert_eq!(si.references.len(), 1);
        assert_eq!(si.references[0].uri.as_deref(), Some("#_resp1"));
        assert_eq!(si.references[0].transforms.len(), 2);
        assert_eq!(si.references[0].digest_value, vec![0u8; 32]);
    }
}