attestation-doc-validation 0.10.0

//! Module for parsing and validating attestation documents from AWS Nitro Enclaves.
use super::{
    error::{AttestationError, AttestationResult},
    nsm::{CryptoClient, Hash, SigningPublicKey},
    true_or_invalid,
};
pub(super) use aws_nitro_enclaves_cose::CoseSign1;
use aws_nitro_enclaves_nsm_api::api::{AttestationDoc, Digest};
use base64::Engine;
use std::collections::BTreeMap;

// Helper macros to get, write and compare PCRs
macro_rules! extract_pcr {
    ($measurements:expr, $idx:literal) => {{
        $measurements
            .get(&$idx)
            .ok_or(AttestationError::MissingPCR($idx))?
            .to_string()
    }};
}

macro_rules! compare_pcrs {
    ($lhs:ident, $rhs:ident, $pcr:ident) => {
        let $pcr = $lhs.$pcr().map(|lhs_pcr| {
            $rhs.$pcr()
                .map_or_else(|| false, |rhs_pcr| lhs_pcr == rhs_pcr)
        });
        if !$pcr.unwrap_or_else(|| true) {
            return false;
        }
    };
}

/// Trait to allow custom implementations of PCR-like types. This helps to make the per language bindings more idiomatic.
pub trait PCRProvider {
    fn pcr_0(&self) -> Option<&str>;
    fn pcr_1(&self) -> Option<&str>;
    fn pcr_2(&self) -> Option<&str>;
    fn pcr_8(&self) -> Option<&str>;

    fn to_string(&self) -> String {
        format!(
            "PCRS {{ PCR0: {:?}, PCR1: {:?}, PCR2: {:?}, PCR8: {:?} }}",
            self.pcr_0(),
            self.pcr_1(),
            self.pcr_2(),
            self.pcr_8()
        )
    }

    fn eq<T: PCRProvider>(&self, rhs: &T) -> bool {
        compare_pcrs!(self, rhs, pcr_0);
        compare_pcrs!(self, rhs, pcr_1);
        compare_pcrs!(self, rhs, pcr_2);
        compare_pcrs!(self, rhs, pcr_8);
        true
    }
}

/// Reference implementation of the AWS attestation doc's PCRs exposed at build time.
#[derive(Debug, PartialEq, Eq, Clone)]
pub struct PCRs {
    pub pcr_0: String,
    pub pcr_1: String,
    pub pcr_2: String,
    pub pcr_8: String,
}

impl PCRProvider for PCRs {
    fn pcr_0(&self) -> Option<&str> {
        Some(self.pcr_0.as_str())
    }
    fn pcr_1(&self) -> Option<&str> {
        Some(self.pcr_1.as_str())
    }
    fn pcr_2(&self) -> Option<&str> {
        Some(self.pcr_2.as_str())
    }
    fn pcr_8(&self) -> Option<&str> {
        Some(self.pcr_8.as_str())
    }
}

/// Parses the PCRs from the attestation doc and compares against the expected values
///
/// # Errors
///
/// Returns an error if any of the expected PCRs are missing from the attestation document, or if the expected PCRs don't match the values embedded in the doc
pub fn validate_expected_pcrs<T: PCRProvider>(
    attestation_doc: &AttestationDoc,
    expected_pcrs: &T,
) -> AttestationResult<()> {
    let received_pcrs = get_pcrs(attestation_doc)?;

    let same_pcrs = expected_pcrs.eq(&received_pcrs);
    true_or_invalid(
        same_pcrs,
        AttestationError::UnexpectedPCRs(expected_pcrs.to_string(), received_pcrs.to_string()),
    )
}

/// Parses `PCRs` from an attestation doc
///
/// # Errors
///
/// Returns an error if any of the expected PCRs are missing from the attestation document
pub fn get_pcrs(attestation_doc: &AttestationDoc) -> AttestationResult<PCRs> {
    let encoded_measurements = attestation_doc
        .pcrs
        .iter()
        .map(|(&index, buf)| (index, hex::encode(&buf[..])))
        .collect::<BTreeMap<_, _>>();

    Ok(PCRs {
        pcr_0: extract_pcr!(encoded_measurements, 0),
        pcr_1: extract_pcr!(encoded_measurements, 1),
        pcr_2: extract_pcr!(encoded_measurements, 2),
        pcr_8: extract_pcr!(encoded_measurements, 8),
    })
}

/// Extracts the nonce embedded in the attestation doc, encodes it to base64 and compares it to the base64 encoded nonce given
///
/// # Errors
///
/// Returns a `NonceMismatch` error if the attestation document contains an unexpected nonce, or does not contain a nonce
pub fn validate_expected_nonce(
    attestation_doc: &AttestationDoc,
    expected_nonce: &str,
) -> AttestationResult<()> {
    let matching_nonce = attestation_doc
        .nonce
        .as_ref()
        .map(|existing_nonce| base64::prelude::BASE64_STANDARD.encode(existing_nonce))
        .ok_or_else(|| AttestationError::NonceMismatch {
            expected: expected_nonce.to_string(),
            received: None,
        })?;

    true_or_invalid(
        matching_nonce == expected_nonce,
        AttestationError::NonceMismatch {
            expected: expected_nonce.to_string(),
            received: Some(matching_nonce),
        },
    )
}

/// Takes a public key and attestation doc in `CoseSign1` form and returns a result based on it's validity
///
/// # Errors
///
/// Returns a `InvalidCoseSignature` error if signature is invalid
pub fn validate_cose_signature<H: Hash>(
    signing_cert_public_key: &dyn SigningPublicKey,
    cose_sign_1_decoded: &CoseSign1,
) -> AttestationResult<()> {
    true_or_invalid(
        cose_sign_1_decoded
            .verify_signature::<H>(signing_cert_public_key)
            .map_err(|err| AttestationError::InvalidCose(err.to_string()))?,
        AttestationError::InvalidCoseSignature,
    )
}

/// Takes an `AttestationDoc` and expected challenge and compares them
///
/// # Errors
///
/// Returns a `MissingUserData` error if user data is not present in attestation doc
/// Returns a `UserDataMismatch` error if the challenges do not match
pub fn validate_expected_challenge(
    attestation_doc: &AttestationDoc,
    expected_challenge: &[u8],
) -> AttestationResult<()> {
    let embedded_challenge = attestation_doc
        .user_data
        .as_ref()
        .ok_or(AttestationError::MissingUserData)?;
    true_or_invalid(
        embedded_challenge == expected_challenge,
        AttestationError::UserDataMismatch,
    )
}

/// Takes a byte array and parses is as an `AttestationDoc` and `CoseSign1`
///
/// # Errors
///
/// Returns a `InvalidCose` if the byte array can't be parsed as a `CoseSign1`
/// Returns a `DocStructureInvalid` if the attestation doc doesn't follow the [AWS criteria](https://github.com/aws/aws-nitro-enclaves-nsm-api/blob/main/docs/attestation_process.md)
pub fn decode_attestation_document(
    cose_sign_1_bytes: &[u8],
) -> AttestationResult<(CoseSign1, AttestationDoc)> {
    let cose_sign_1_decoded: CoseSign1 = serde_cbor::from_slice(cose_sign_1_bytes)?;
    let cbor = cose_sign_1_decoded
        .get_payload::<CryptoClient>(None)
        .map_err(|err| AttestationError::InvalidCose(err.to_string()))?;
    let attestation_doc: AttestationDoc = serde_cbor::from_slice(&cbor)?;
    validate_attestation_document_structure(&attestation_doc)?;
    Ok((cose_sign_1_decoded, attestation_doc))
}

pub(super) fn validate_attestation_document_structure(
    attestation_document: &AttestationDoc,
) -> AttestationResult<()> {
    let module_id_present = !attestation_document.module_id.is_empty();
    true_or_invalid(module_id_present, AttestationError::MissingModuleId)?;

    let digest_valid = attestation_document.digest == Digest::SHA384;
    true_or_invalid(digest_valid, AttestationError::DigestAlgorithmInvalid)?;

    let pcrs_valid = !attestation_document.pcrs.is_empty()
        && attestation_document.pcrs.len() <= 32
        && attestation_document
            .pcrs
            .keys()
            .all(|&pcr_index| pcr_index < 32)
        && attestation_document
            .pcrs
            .values()
            .all(|pcr| [32, 48, 64].contains(&pcr.len()));

    true_or_invalid(pcrs_valid, AttestationError::InvalidCABundle)?;

    let valid_ca_bundle = attestation_document
        .cabundle
        .iter()
        .all(|cert| cert.len() > 0 && cert.len() <= 1024);
    true_or_invalid(valid_ca_bundle, AttestationError::InvalidCABundle)?;

    let valid_public_key = attestation_document
        .public_key
        .as_ref()
        .map_or(true, |key| key.len() > 0 && key.len() <= 1024); // these default to true if not present
    true_or_invalid(valid_public_key, AttestationError::InvalidPublicKey)?;

    let valid_nonce = attestation_document
        .nonce
        .as_ref()
        .map_or(true, |nonce| nonce.len() > 0 && nonce.len() <= 512);
    true_or_invalid(valid_nonce, AttestationError::InvalidNonce)?;
    let valid_user_data = attestation_document
        .user_data
        .as_ref()
        .map_or(true, |user_data| {
            user_data.len() > 0 && user_data.len() <= 512
        });
    true_or_invalid(valid_user_data, AttestationError::InvalidUserData)
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn validate_valid_attestation_doc_structure() {
        // this test only validates the structure of the AD, but not the validity of the Nitro signature over it
        // so it will pass despite the AD being expired.
        let sample_cose_sign_1_bytes = std::fs::read(std::path::Path::new(
            "../test-data/beta/valid-attestation-doc-bytes",
        ))
        .unwrap();
        let expected_pcrs = PCRs {
          pcr_0: "f4d48b81a460c9916d1e685119074bf24660afd3e34fae9fca0a0d28d9d5599936332687e6f66fc890ac8cf150142d8b".to_string(),
          pcr_1: "bcdf05fefccaa8e55bf2c8d6dee9e79bbff31e34bf28a99aa19e6b29c37ee80b214a414b7607236edf26fcb78654e63f".to_string(),
          pcr_2: "d8f114da658de5481f8d9ec73907feb553560787522f705c92d7d96beed8e15e2aa611984e098c576832c292e8dc469a".to_string(),
          pcr_8: "8790eb3cce6c83d07e84b126dc61ca923333d6f66615c4a79157de48c5ab2418bdc60746ea7b7afbff03a1c6210201cb".to_string(),
        };
        let (_, decoded_ad) = decode_attestation_document(&sample_cose_sign_1_bytes).unwrap();
        let is_valid_ad = validate_attestation_document_structure(&decoded_ad).is_ok();
        assert!(is_valid_ad);
        let pcrs_match = validate_expected_pcrs(&decoded_ad, &expected_pcrs).is_ok();
        assert!(pcrs_match);
    }

    #[test]
    fn validate_valid_attestation_doc_structure_with_mismatched_pcrs() {
        let sample_cose_sign_1_bytes = std::fs::read(std::path::Path::new(
            "../test-data/beta/valid-attestation-doc-bytes",
        ))
        .unwrap();
        let expected_pcrs = PCRs {
          pcr_0: "f4d48b81a460c---wrong-value---5119074bf24660afd3e34fae9fca0a0d28d9d5599936332687e6f66ff150142d8b".to_string(),
          pcr_1: "bcdf05fefccaa8e55bf2c8d---fail-test---f31e34bf28a99aa19e6b29c37ee80b214a414b7607236edf26fcb7863f".to_string(),
          pcr_2: "d8f114da658de5481f8d9ec73907feb553560787522f705c92d7d96beed8e15e---incorrect---2c292e8dc46ffff9a".to_string(),
          pcr_8: "8790eb3cce6c83d07e84b126dc61ca923333d6f66615c4a79157de48c5ab2418bdc60746ea7b7afbff03a1c6210201cb".to_string(),
        };
        let (_, decoded_ad) = decode_attestation_document(&sample_cose_sign_1_bytes).unwrap();
        let is_valid_ad = validate_attestation_document_structure(&decoded_ad).is_ok();
        assert!(is_valid_ad);
        let pcrs_match = validate_expected_pcrs(&decoded_ad, &expected_pcrs).is_ok();
        assert!(!pcrs_match);
    }

    #[test]
    fn validate_get_pcrs() {
        let sample_cose_sign_1_bytes = std::fs::read(std::path::Path::new(
            "../test-data/beta/valid-attestation-doc-bytes",
        ))
        .unwrap();
        let expected_pcrs = PCRs {
          pcr_0: "f4d48b81a460c9916d1e685119074bf24660afd3e34fae9fca0a0d28d9d5599936332687e6f66fc890ac8cf150142d8b".to_string(),
          pcr_1: "bcdf05fefccaa8e55bf2c8d6dee9e79bbff31e34bf28a99aa19e6b29c37ee80b214a414b7607236edf26fcb78654e63f".to_string(),
          pcr_2: "d8f114da658de5481f8d9ec73907feb553560787522f705c92d7d96beed8e15e2aa611984e098c576832c292e8dc469a".to_string(),
          pcr_8: "8790eb3cce6c83d07e84b126dc61ca923333d6f66615c4a79157de48c5ab2418bdc60746ea7b7afbff03a1c6210201cb".to_string(),
        };
        let (_, decoded_ad) = decode_attestation_document(&sample_cose_sign_1_bytes).unwrap();
        let pcrs = get_pcrs(&decoded_ad).unwrap();
        assert_eq!(pcrs, expected_pcrs);
    }
}