sshcerts 0.12.0

use std::collections::HashMap;
use std::fmt;
use std::fs::File;
use std::io::Read;
use std::path::Path;

use ring::{
    digest,
    rand::{SecureRandom, SystemRandom},
    signature::{
        RsaPublicKeyComponents, UnparsedPublicKey, ECDSA_P256_SHA256_FIXED,
        ECDSA_P384_SHA384_FIXED, ED25519, RSA_PKCS1_2048_8192_SHA1_FOR_LEGACY_USE_ONLY,
        RSA_PKCS1_2048_8192_SHA256, RSA_PKCS1_2048_8192_SHA512,
    },
};

use super::SSHCertificateSigner;
use super::{
    keytype::KeyType,
    pubkey::{PublicKey, PublicKeyKind},
    reader::Reader,
    writer::Writer,
};
use crate::{error::Error, Result};

use std::convert::TryFrom;

/// Represents the different types a certificate can be.
#[derive(Debug, PartialEq, Eq, Clone, Copy)]
pub enum CertType {
    /// Represents a user certificate.
    User = 1,

    /// Represents a host certificate.
    Host = 2,
}

impl TryFrom<&str> for CertType {
    type Error = &'static str;

    fn try_from(s: &str) -> std::result::Result<Self, Self::Error> {
        match s {
            "user" | "User" => Ok(CertType::User),
            "host" | "Host" => Ok(CertType::Host),
            _ => Err("Unknown certificate type"),
        }
    }
}

impl fmt::Display for CertType {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            CertType::User => write!(f, "user certificate"),
            CertType::Host => write!(f, "host certificate"),
        }
    }
}

/// These are the standard extensions used in an SSH certificate. If you don't
/// know what extensions you need, adding all of these is probably what you
/// want.
const STANDARD_EXTENSIONS: [(&str, &str); 5] = [
    ("permit-agent-forwarding", ""),
    ("permit-port-forwarding", ""),
    ("permit-pty", ""),
    ("permit-user-rc", ""),
    ("permit-X11-forwarding", ""),
];

/// A type which represents an OpenSSH certificate key.
/// Please refer to [PROTOCOL.certkeys] for more details about OpenSSH certificates.
/// [PROTOCOL.certkeys]: https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.certkeys?annotate=HEAD
#[derive(Debug, PartialEq, Eq)]
pub struct Certificate {
    /// Type of key.
    pub key_type: KeyType,

    /// Cryptographic nonce.
    pub nonce: Vec<u8>,

    /// Public key part of the certificate.
    pub key: PublicKey,

    /// Serial number of certificate.
    pub serial: u64,

    /// Represents the type of the certificate.
    pub cert_type: CertType,

    /// Key identity.
    pub key_id: String,

    /// The list of valid principals for the certificate.
    pub principals: Vec<String>,

    /// Time after which certificate is considered as valid.
    pub valid_after: u64,

    /// Time before which certificate is considered as valid.
    pub valid_before: u64,

    /// Critical options of the certificate. Generally used to
    /// control features which restrict access.
    pub critical_options: HashMap<String, String>,

    /// Certificate extensions. Extensions are usually used to
    /// enable features that grant access.
    pub extensions: HashMap<String, String>,

    /// The `reserved` field is currently unused and is ignored in this version of the protocol.
    pub reserved: Vec<u8>,

    /// Signature key contains the CA public key used to sign the certificate.
    pub signature_key: PublicKey,

    /// Signature of the certificate.
    pub signature: Vec<u8>,

    /// Associated comment, if any.
    pub comment: Option<String>,

    /// The entire serialized certificate, used for exporting
    pub serialized: Vec<u8>,
}

impl Certificate {
    /// Reads an OpenSSH certificate from a given path.
    ///
    /// # Example
    ///
    /// ```rust
    /// # use sshcerts::Certificate;
    /// # fn example() {
    ///     let cert = Certificate::from_path("/path/to/id_ed25519-cert.pub").unwrap();
    ///     println!("{}", cert);
    /// # }
    /// ```
    pub fn from_path<P: AsRef<Path>>(path: P) -> Result<Certificate> {
        let mut contents = String::new();
        File::open(path)?.read_to_string(&mut contents)?;

        Certificate::from_string(&contents)
    }

    /// Reads an OpenSSH certificate from a given string.
    ///
    /// # Example
    ///
    /// ```rust
    /// use sshcerts::Certificate;
    ///
    /// let cert = Certificate::from_string(concat!(
    ///     "ssh-ed25519-cert-v01@openssh.com AAAAIHNzaC1lZDI1NTE5LWNlcnQtdjAxQG9wZW5zc2guY29tAAAAIGZlEWgv+aRvfJZiREMOKR0PVSTEstkuSeOyRgx",
    ///     "wI1v2AAAAIAwPJZIwmYs+W7WHNPneMUIAkQnBVw1LP0yQdfh7lT/S/v7+/v7+/v4AAAABAAAADG9iZWxpc2tAdGVzdAAAAAsAAAAHb2JlbGlzawAAAAAAAAAA///",
    ///     "///////8AAAAiAAAADWZvcmNlLWNvbW1hbmQAAAANAAAACS9iaW4vdHJ1ZQAAAIIAAAAVcGVybWl0LVgxMS1mb3J3YXJkaW5nAAAAAAAAABdwZXJtaXQtYWdlbnQ",
    ///     "tZm9yd2FyZGluZwAAAAAAAAAWcGVybWl0LXBvcnQtZm9yd2FyZGluZwAAAAAAAAAKcGVybWl0LXB0eQAAAAAAAAAOcGVybWl0LXVzZXItcmMAAAAAAAAAAAAAADM",
    ///     "AAAALc3NoLWVkMjU1MTkAAAAgXRsP8RFzML3wJDAqm2ENwOrRAHez5QqtcEpyBvwvniYAAABTAAAAC3NzaC1lZDI1NTE5AAAAQMo0Akv0eyr269StM2zBd0Alzjx",
    ///     "XAC6krgBQex2O31at8r550oCIelfgj8YwZIaXG9DmleP525LcseJ16Z8e5Aw= obelisk@exclave.lan"
    /// )).unwrap();
    /// println!("{:?}", cert);
    /// ```
    pub fn from_string(s: &str) -> Result<Certificate> {
        let mut iter = s.split_whitespace();

        let outer_kt = KeyType::from_name(iter.next().ok_or(Error::InvalidFormat)?)?;
        let data = iter.next().ok_or(Error::InvalidFormat)?;
        let comment = iter.next().map(String::from);
        let decoded = base64::decode(data)?;

        let mut cert = Certificate::from_bytes(&decoded)?;
        cert.comment = comment;

        if cert.key_type.kind != outer_kt.kind {
            return Err(Error::KeyTypeMismatch);
        }
        Ok(cert)
    }

    /// Reads an SSH certificate from a given byte sequence.
    ///
    /// The byte sequence is expected to be the base64 decoded body of the SSH certificate.
    ///
    pub fn from_bytes(data: &[u8]) -> Result<Certificate> {
        let mut reader = Reader::new(&data);

        // Validate key types before reading the rest of the data
        let kt_name = reader.read_string()?;

        let key_type = KeyType::from_name(&kt_name)?;
        if !key_type.is_cert {
            return Err(Error::NotCertificate);
        }

        let nonce = reader.read_bytes()?;
        let key = PublicKey::from_reader(&key_type.as_pubkey_name(), &mut reader)?;
        let serial = reader.read_u64()?;

        let cert_type = match reader.read_u32()? {
            1 => CertType::User,
            2 => CertType::Host,
            n => return Err(Error::InvalidCertType(n)),
        };

        let key_id = reader.read_string()?;
        let principals = reader.read_bytes().and_then(|v| read_principals(&v))?;
        let valid_after = reader.read_u64()?;
        let valid_before = reader.read_u64()?;
        let critical_options = reader.read_bytes().and_then(|v| read_options(&v))?;
        let extensions = reader.read_bytes().and_then(|v| read_options(&v))?;
        let reserved = reader.read_bytes()?;
        let signature_key = reader
            .read_bytes()
            .and_then(|v| PublicKey::from_bytes(&v))?;

        let signed_len = reader.get_offset();
        let signature = reader.read_bytes()?;

        reader.set_offset(0)?;
        let signed_bytes = reader.read_raw_bytes(signed_len)?;

        // Verify the certificate is properly signed
        verify_signature(&signature, &signed_bytes, &signature_key)?;

        Ok(Certificate {
            key_type,
            nonce,
            key,
            serial,
            cert_type,
            key_id,
            principals,
            valid_after,
            valid_before,
            critical_options,
            extensions,
            reserved,
            signature_key,
            signature,
            comment: None,
            serialized: data.to_vec(),
        })
    }

    /// Returns the set of standard extensions used for SSH certificates. If you're
    /// unsure about what you need, using the standard extensions is probably what
    /// you want.
    pub fn standard_extensions() -> HashMap<String, String> {
        let mut hm = HashMap::new();
        for extension in &STANDARD_EXTENSIONS {
            hm.insert(String::from(extension.0), String::from(extension.1));
        }
        hm
    }

    /// Create a new empty SSH certificate. Values must then be filled in using
    /// the mutator methods below.
    ///
    /// # Example
    ///
    /// ```rust
    /// # use sshcerts::{Certificate, PublicKey, PrivateKey};
    /// # use sshcerts::ssh::CertType;
    /// # fn example() {
    ///     let private_key = PrivateKey::from_string(concat!(
    ///         "-----BEGIN OPENSSH PRIVATE KEY-----",
    ///         "b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW",
    ///         "QyNTUxOQAAACBBvD18M5xE6toNtTkIwVwl7xkJb9DBUSgHfKaKbeTW3gAAAKj3njlq9545",
    ///         "agAAAAtzc2gtZWQyNTUxOQAAACBBvD18M5xE6toNtTkIwVwl7xkJb9DBUSgHfKaKbeTW3g",
    ///         "AAAEBLyc6RR+xrjQFV9hhmW9z5TYEA4IMVG7+xBq0WHjdnNkG8PXwznETq2g21OQjBXCXv",
    ///         "GQlv0MFRKAd8popt5NbeAAAAIW9iZWxpc2tATWl0Y2hlbGxzLU1CUC5sb2NhbGRvbWFpbg",
    ///         "ECAwQ=",
    ///         "-----END OPENSSH PRIVATE KEY-----",
    ///     )).unwrap();
    ///     let ssh_pubkey = PublicKey::from_string("ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIHk1jR7i5Ao85pfz0X6xAWT3N+Wicm17v3UnYw3ZEGnH").unwrap();
    ///     let cert = Certificate::builder(&ssh_pubkey, CertType::User, &private_key.pubkey).unwrap()
    ///        .serial(0xFEFEFEFEFEFEFEFE)
    ///        .key_id("key_id")
    ///        .principal("obelisk")
    ///        .valid_after(0)
    ///        .valid_before(0xFFFFFFFFFFFFFFFF)
    ///        .set_extensions(Certificate::standard_extensions())
    ///        .sign(&private_key);
    ///
    ///     match cert {
    ///       Ok(cert) => println!("{}", cert),
    ///       Err(e) => println!("Encountered an error while creating certificate: {}", e),
    ///     }
    /// # }
    /// ```
    pub fn builder(
        pubkey: &PublicKey,
        cert_type: CertType,
        signing_key: &PublicKey,
    ) -> Result<Certificate> {
        let kt_name = pubkey.key_type.as_cert_name();
        let key_type = KeyType::from_name(kt_name.as_str())?;
        let rng = SystemRandom::new();

        let mut nonce = [0x0u8; 32];
        match SecureRandom::fill(&rng, &mut nonce) {
            Ok(()) => (),
            Err(_) => return Err(Error::UnexpectedEof),
        };

        let mut serial = [0x0u8; 8];
        match SecureRandom::fill(&rng, &mut serial) {
            Ok(()) => (),
            Err(_) => return Err(Error::UnexpectedEof),
        };
        let serial = u64::from_be_bytes(serial);

        Ok(Certificate {
            nonce: nonce.to_vec(),
            key: pubkey.clone(),
            key_type,
            serial,
            cert_type,
            key_id: String::new(),
            principals: vec![],
            valid_after: 0,
            valid_before: 0,
            critical_options: HashMap::new(),
            extensions: HashMap::new(),
            reserved: vec![0, 0, 0, 0, 0, 0, 0, 0],
            signature_key: signing_key.clone(),
            signature: vec![],
            comment: None,
            serialized: vec![],
        })
    }

    /// Set the serial of a certificate builder
    pub fn serial(mut self, serial: u64) -> Self {
        self.serial = serial;
        self
    }

    /// Set the Key ID of a certificate builder
    pub fn key_id<S: AsRef<str>>(mut self, key_id: S) -> Self {
        self.key_id = key_id.as_ref().to_owned();
        self
    }

    /// Add a principal to the certificate
    pub fn principal<S: AsRef<str>>(mut self, principal: S) -> Self {
        self.principals.push(principal.as_ref().to_owned());
        self
    }

    /// Set the principals of the certificate
    pub fn set_principals(mut self, principals: &[String]) -> Self {
        self.principals = principals.to_vec();
        self
    }

    /// Set the initial validity time of the certificate
    pub fn valid_after(mut self, valid_after: u64) -> Self {
        self.valid_after = valid_after;
        self
    }

    /// Set the expiry of the certificate
    pub fn valid_before(mut self, valid_before: u64) -> Self {
        self.valid_before = valid_before;
        self
    }

    /// Add a critical option to the certificate
    pub fn critical_option<S: AsRef<str>>(mut self, option: S, value: S) -> Self {
        self.critical_options
            .insert(option.as_ref().to_owned(), value.as_ref().to_owned());
        self
    }

    /// Set the critical options of the certificate
    pub fn set_critical_options(mut self, critical_options: HashMap<String, String>) -> Self {
        self.critical_options = critical_options;
        self
    }

    /// Add an extension to the certificate
    pub fn extension<S: AsRef<str>>(mut self, option: S, value: S) -> Self {
        self.extensions
            .insert(option.as_ref().to_owned(), value.as_ref().to_owned());
        self
    }

    /// Set the extensions of the certificate
    pub fn set_extensions(mut self, extensions: HashMap<String, String>) -> Self {
        self.extensions = extensions;
        self
    }

    /// Set the comment of the certificate
    pub fn comment<S: AsRef<str>>(mut self, comment: S) -> Self {
        self.comment = Some(comment.as_ref().to_owned());
        self
    }

    /// Get the certificate data without the signature field at the end.
    pub fn tbs_certificate(&self) -> Vec<u8> {
        let mut writer = Writer::new();
        let kt_name = self.key_type.as_cert_name();
        // Write the cert type
        writer.write_string(kt_name.as_str());

        // Write the nonce
        writer.write_bytes(&self.nonce);

        // Write the user public key
        writer.write_pub_key_data(&self.key);

        // Write the serial number
        writer.write_u64(self.serial);

        // Write what kind of cert this is
        writer.write_u32(self.cert_type as u32);

        // Write the key id
        writer.write_string(&self.key_id);

        // Write the principals
        writer.write_string_vec(&self.principals);

        // Write valid after
        writer.write_u64(self.valid_after);

        // Write valid before
        writer.write_u64(self.valid_before);

        // Write the critical options
        writer.write_string_map(&self.critical_options);

        // Write the extensions
        writer.write_string_map(&self.extensions);

        // Write the unused reserved bytes
        writer.write_u32(0x0);

        // Write the CA public key
        writer.write_bytes(&self.signature_key.encode());

        // Return the tbs certificate data
        writer.as_bytes().to_vec()
    }

    /// Attempts to add the given signature to the certificate. This function
    /// returns an error if the signature provided is not valid for the
    /// certificate under the set CA key.
    pub fn add_signature(mut self, signature: &[u8]) -> Result<Self> {
        let mut tbs = self.tbs_certificate();
        verify_signature(signature, &tbs, &self.signature_key)?;

        let mut wrapped_writer = Writer::new();
        wrapped_writer.write_bytes(signature);

        // After this it's no longer "tbs"
        tbs.extend_from_slice(&wrapped_writer.into_bytes());

        self.signature = signature.to_vec();
        self.serialized = tbs;

        Ok(self)
    }

    /// Take the certificate settings and generate a valid signature using the provided signer function
    pub fn sign<T: SSHCertificateSigner>(self, signer: &T) -> Result<Self> {
        let tbs_certificate = self.tbs_certificate();

        // Sign the data and write it to the cert
        let signature = signer.sign(&tbs_certificate).ok_or(Error::SigningError)?;
        self.add_signature(&signature)
    }
}

// Reads `option` values from a byte sequence.
// The `option` values are used to represent the `critical options` and
// `extensions` in an OpenSSH certificate key, which are represented as tuples
// containing the `name` and `data` values of type `string`.
// Some `options` are `flags` only (e.g. the certificate extensions) and the
// associated value with them is the empty string (""), while others are `string`
// options and have an associated value, which is a `string`.
// The `critical options` of a certificate are always `string` options, since they
// have an associated `string` value, which is embedded in a separate buffer, so
// in order to extract the associated value we need to read the buffer first and then
// read the `string` value itself.
fn read_options(buf: &[u8]) -> Result<HashMap<String, String>> {
    let mut reader = Reader::new(&buf);
    let mut options = HashMap::new();

    // Use a `Reader` and loop until EOF is reached, so that we can
    // read all options from the provided byte slice.
    loop {
        let name = match reader.read_string() {
            Ok(v) => v,
            Err(e) => match e {
                Error::UnexpectedEof => break,
                _ => return Err(e),
            },
        };

        // If we have a `string` option extract the value from the buffer,
        // otherwise we have a `flag` option which is the `empty` string.
        let value_buf = reader.read_bytes()?;
        let value = if !value_buf.is_empty() {
            Reader::new(&value_buf).read_string()?
        } else {
            "".to_string()
        };

        options.insert(name, value);
    }

    Ok(options)
}

// Reads the `principals` field of a certificate key.
// The `principals` are represented as a sequence of `string` values
// embedded in a buffer.
// This function reads the whole byte slice until EOF is reached in order to
// ensure all principals are read from the byte slice.
fn read_principals(buf: &[u8]) -> Result<Vec<String>> {
    let mut reader = Reader::new(&buf);
    let mut items = Vec::new();

    loop {
        let principal = match reader.read_string() {
            Ok(v) => v,
            Err(e) => match e {
                Error::UnexpectedEof => break,
                _ => return Err(e),
            },
        };
        items.push(principal);
    }
    Ok(items)
}

/// Verifies the certificate's signature is valid.
fn verify_signature(
    signature_buf: &[u8],
    signed_bytes: &[u8],
    public_key: &PublicKey,
) -> Result<Vec<u8>> {
    let mut reader = Reader::new(&signature_buf);
    let sig_type = reader.read_string().and_then(|v| KeyType::from_name(&v))?;

    if public_key.key_type.kind != sig_type.kind {
        return Err(Error::KeyTypeMismatch); 
    }

    match &public_key.kind {
        PublicKeyKind::Ecdsa(key) => {
            let sig_reader = reader.read_bytes()?;
            let mut sig_reader = Reader::new(&sig_reader);

            let (alg, len) = match sig_type.name {
                "ecdsa-sha2-nistp256" | "sk-ecdsa-sha2-nistp256@openssh.com" => {
                    (&ECDSA_P256_SHA256_FIXED, 32)
                }
                "ecdsa-sha2-nistp384" => (&ECDSA_P384_SHA384_FIXED, 48),
                _ => return Err(Error::KeyTypeMismatch),
            };

            // Read the R value
            let r_bytes = sig_reader.read_positive_mpint()?;
            // Read the S value
            let s_bytes = sig_reader.read_positive_mpint()?;

            // (r/s)_bytes are user controlled so ensure maliciously signatures
            // can't cause integer underflow.
            if r_bytes.len() > len || s_bytes.len() > len {
                return Err(Error::InvalidFormat);
            }

            // Determine and create the padding required
            let mut r = vec![0; len - r_bytes.len()];
            let mut s = vec![0; len - s_bytes.len()];

            // Pad *_bytes
            r.extend(r_bytes);
            s.extend(s_bytes);

            // Build a properly padded signature
            let mut sig = r;
            sig.extend(s);

            if let Some(sk_application) = &key.sk_application {
                let flags = reader.read_raw_bytes(1)?[0];
                let signature_counter = reader.read_u32()?;

                let mut app_hash = digest::digest(&digest::SHA256, sk_application.as_bytes())
                    .as_ref()
                    .to_vec();
                let mut data_hash = digest::digest(&digest::SHA256, signed_bytes)
                    .as_ref()
                    .to_vec();

                app_hash.push(flags);
                app_hash.extend_from_slice(&signature_counter.to_be_bytes());
                app_hash.append(&mut data_hash);

                UnparsedPublicKey::new(alg, &key.key).verify(&app_hash, &sig)?;
            } else {
                UnparsedPublicKey::new(alg, &key.key).verify(signed_bytes, &sig)?;
            }

            Ok(signature_buf.to_vec())
        }
        PublicKeyKind::Rsa(key) => {
            let alg = match sig_type.name {
                "rsa-sha2-256" => &RSA_PKCS1_2048_8192_SHA256,
                "rsa-sha2-512" => &RSA_PKCS1_2048_8192_SHA512,
                "ssh-rsa" => &RSA_PKCS1_2048_8192_SHA1_FOR_LEGACY_USE_ONLY,
                _ => return Err(Error::KeyTypeMismatch),
            };
            let signature = reader.read_bytes()?;
            let public_key = RsaPublicKeyComponents {
                n: &key.n,
                e: &key.e,
            };
            public_key.verify(alg, signed_bytes, &signature)?;
            Ok(signature_buf.to_vec())
        }
        PublicKeyKind::Ed25519(key) => {
            match sig_type.name {
                "ssh-ed25519" => (),
                "sk-ssh-ed25519@openssh.com" => (),
                _ => return Err(Error::KeyTypeMismatch),
            };

            let alg = &ED25519;
            let signature = reader.read_bytes()?;
            let peer_public_key = UnparsedPublicKey::new(alg, &key.key);

            if let Some(sk_application) = &key.sk_application {
                let flags = reader.read_raw_bytes(1)?[0];
                let signature_counter = reader.read_u32()?;

                let mut app_hash = digest::digest(&digest::SHA256, sk_application.as_bytes())
                    .as_ref()
                    .to_vec();
                let mut data_hash = digest::digest(&digest::SHA256, signed_bytes)
                    .as_ref()
                    .to_vec();

                app_hash.push(flags);
                app_hash.extend_from_slice(&signature_counter.to_be_bytes());
                app_hash.append(&mut data_hash);

                peer_public_key.verify(&app_hash, &signature)?;
            } else {
                peer_public_key.verify(signed_bytes, &signature)?;
            }

            Ok(signature_buf.to_vec())
        }
    }
}

impl fmt::Display for Certificate {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        if !f.alternate() {
            write!(
                f,
                "{} {} {}",
                &self.key_type.name,
                base64::encode(&self.serialized),
                &self.key_id
            )
        } else {
            let mut pretty: String = format!("Type: {} {}\n", self.key_type, self.cert_type);
            pretty.push_str(&format!(
                "Public Key: {} {}:{}\n",
                self.key_type.short_name,
                self.key.fingerprint().kind,
                self.key.fingerprint().hash
            ));
            pretty.push_str(&format!(
                "Signing CA: {} {}:{} (using {})\n",
                self.signature_key.key_type.short_name,
                self.signature_key.fingerprint().kind,
                self.signature_key.fingerprint().hash,
                self.signature_key.key_type
            ));
            pretty.push_str(&format!("Key ID: \"{}\"\n", self.key_id));
            pretty.push_str(&format!("Serial: {}\n", self.serial));
            if self.valid_before == 0xFFFFFFFFFFFFFFFF && self.valid_after == 0x0 {
                pretty.push_str("Valid: forever\n");
            } else {
                pretty.push_str(&format!(
                    "Valid between: {} and {}\n",
                    self.valid_after, self.valid_before
                ));
            }

            if self.principals.is_empty() {
                pretty.push_str("Principals: (none)\n");
            } else {
                pretty.push_str("Principals\n");
                for principal in &self.principals {
                    pretty.push_str(&format!("\t{}\n", principal));
                }
            }

            if self.critical_options.is_empty() {
                pretty.push_str("Critical Options: (none)\n");
            } else {
                pretty.push_str("Critical Options:\n");
                for (name, value) in &self.critical_options {
                    pretty.push_str(&format!("\t{} {}\n", name, value));
                }
            }

            if self.extensions.is_empty() {
                pretty.push_str("Extensions: (none)\n");
            } else {
                pretty.push_str("Extensions:\n");
                for name in self.extensions.keys() {
                    pretty.push_str(&format!("\t{}\n", &name));
                }
            }

            write!(f, "{}", pretty)
        }
    }
}