sui_types/

nitro_attestation.rs

// Copyright (c) Mysten Labs, Inc.
// SPDX-License-Identifier: Apache-2.0

use serde::de::{Error, MapAccess, Visitor};
use serde::{Deserialize, Deserializer};
use std::collections::BTreeMap;
use std::fmt;
use x509_parser::public_key::PublicKey;
use x509_parser::time::ASN1Time;
use x509_parser::x509::SubjectPublicKeyInfo;

use crate::error::{SuiError, SuiErrorKind, SuiResult};

use ciborium::value::{Integer, Value};
use once_cell::sync::Lazy;
use p384::ecdsa::signature::Verifier;
use p384::ecdsa::{Signature, VerifyingKey};
use x509_parser::{certificate::X509Certificate, prelude::FromDer};

#[cfg(test)]
#[path = "unit_tests/nitro_attestation_tests.rs"]
mod nitro_attestation_tests;

/// Maximum length of the certificate chain. This is to limit the absolute upper bound on execution.
const MAX_CERT_CHAIN_LENGTH: usize = 10;
/// Max user data length from aws nitro spec.
const MAX_USER_DATA_LENGTH: usize = 512;
/// Max pk length from aws nitro spec.
const MAX_PK_LENGTH: usize = 1024;
/// Max pcrs length from aws nitro spec.
const MAX_PCRS_LENGTH: usize = 32;
/// Max certificate length from aws nitro spec.
const MAX_CERT_LENGTH: usize = 1024;

/// Root certificate for AWS Nitro Attestation.
static ROOT_CERTIFICATE: Lazy<Vec<u8>> = Lazy::new(|| {
    let pem_bytes = include_bytes!("./nitro_root_certificate.pem");
    let mut pem_cursor = std::io::Cursor::new(pem_bytes);
    let cert = rustls_pemfile::certs(&mut pem_cursor)
        .next()
        .expect("should have root cert")
        .expect("root cert should be valid");
    cert.to_vec()
});

/// Error type for Nitro attestation verification.
#[derive(Debug, PartialEq, Eq)]
pub enum NitroAttestationVerifyError {
    /// Invalid COSE_Sign1: {0}
    InvalidCoseSign1(String),
    /// Invalid signature
    InvalidSignature,
    /// Invalid public key
    InvalidPublicKey,
    /// Siganture failed to verify
    SignatureFailedToVerify,
    /// Invalid attestation document
    InvalidAttestationDoc(String),
    /// Invalid user data.
    InvalidUserData,
    /// Invalid certificate: {0}
    InvalidCertificate(String),
    /// Invalid PCRs
    InvalidPcrs,
}

impl fmt::Display for NitroAttestationVerifyError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            NitroAttestationVerifyError::InvalidCoseSign1(msg) => {
                write!(f, "InvalidCoseSign1: {}", msg)
            }
            NitroAttestationVerifyError::InvalidSignature => write!(f, "InvalidSignature"),
            NitroAttestationVerifyError::InvalidPublicKey => write!(f, "InvalidPublicKey"),
            NitroAttestationVerifyError::SignatureFailedToVerify => {
                write!(f, "SignatureFailedToVerify")
            }
            NitroAttestationVerifyError::InvalidAttestationDoc(msg) => {
                write!(f, "InvalidAttestationDoc: {}", msg)
            }
            NitroAttestationVerifyError::InvalidCertificate(msg) => {
                write!(f, "InvalidCertificate: {}", msg)
            }
            NitroAttestationVerifyError::InvalidPcrs => write!(f, "InvalidPcrs"),
            NitroAttestationVerifyError::InvalidUserData => write!(f, "InvalidUserData"),
        }
    }
}

impl From<NitroAttestationVerifyError> for SuiError {
    fn from(err: NitroAttestationVerifyError) -> Self {
        SuiErrorKind::NitroAttestationFailedToVerify(err.to_string()).into()
    }
}

/// Given an attestation in bytes, parse it into signature, signed message and a parsed payload.
pub fn parse_nitro_attestation(
    attestation_bytes: &[u8],
    is_upgraded_parsing: bool,
    include_all_nonzero_pcrs: bool,
    always_include_required_pcrs: bool,
) -> SuiResult<(Vec<u8>, Vec<u8>, AttestationDocument)> {
    let cose_sign1 = CoseSign1::parse_and_validate(attestation_bytes)?;
    let doc = AttestationDocument::parse_payload(
        &cose_sign1.payload,
        is_upgraded_parsing,
        include_all_nonzero_pcrs,
        always_include_required_pcrs,
    )?;
    let msg = cose_sign1.to_signed_message()?;
    let signature = cose_sign1.signature;
    Ok((signature, msg, doc))
}

/// Given the signature bytes, signed message and parsed payload, verify everything according to
/// <https://docs.aws.amazon.com/enclaves/latest/user/verify-root.html> and
/// <https://github.com/aws/aws-nitro-enclaves-nsm-api/blob/main/docs/attestation_process.md>.
pub fn verify_nitro_attestation(
    signature: &[u8],
    signed_message: &[u8],
    payload: &AttestationDocument,
    timestamp: u64,
) -> SuiResult<()> {
    // Extract public key from cert and signature as P384.
    let signature = Signature::from_slice(signature)
        .map_err(|_| NitroAttestationVerifyError::InvalidSignature)?;
    let cert = X509Certificate::from_der(payload.certificate.as_slice())
        .map_err(|e| NitroAttestationVerifyError::InvalidCertificate(e.to_string()))?;
    let pk_bytes = SubjectPublicKeyInfo::parsed(cert.1.public_key())
        .map_err(|err| NitroAttestationVerifyError::InvalidCertificate(err.to_string()))?;

    // Verify the signature against the public key and the message.
    match pk_bytes {
        PublicKey::EC(ec) => {
            let verifying_key = VerifyingKey::from_sec1_bytes(ec.data())
                .map_err(|_| NitroAttestationVerifyError::InvalidPublicKey)?;
            verifying_key
                .verify(signed_message, &signature)
                .map_err(|_| NitroAttestationVerifyError::SignatureFailedToVerify)?;
        }
        _ => {
            return Err(NitroAttestationVerifyError::InvalidPublicKey.into());
        }
    }

    payload.verify_cert(timestamp)?;
    Ok(())
}

///  Implementation of the COSE_Sign1 structure as defined in [RFC8152](https://tools.ietf.org/html/rfc8152).
///  protected_header: See Section 3 (Note: AWS Nitro does not have unprotected header.)
///  payload: See Section 4.2.
///  signature: See Section 4.2.
///  Class and trait impl adapted from <https://github.com/awslabs/aws-nitro-enclaves-cose/blob/main/src/sign.rs>
#[derive(Debug, Clone)]
pub struct CoseSign1 {
    /// protected: empty_or_serialized_map,
    protected: Vec<u8>,
    /// unprotected: HeaderMap
    #[allow(dead_code)]
    unprotected: HeaderMap,
    /// payload: bstr
    /// The spec allows payload to be nil and transported separately, but it's not useful at the
    /// moment, so this is just a Bytes for simplicity.
    payload: Vec<u8>,
    /// signature: bstr
    signature: Vec<u8>,
}

/// Empty map wrapper for COSE headers.
#[derive(Clone, Debug, Default)]
pub struct HeaderMap;

impl<'de> Deserialize<'de> for HeaderMap {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        struct MapVisitor;

        impl<'de> Visitor<'de> for MapVisitor {
            type Value = HeaderMap;

            fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
                formatter.write_str("a map")
            }

            fn visit_map<A>(self, mut access: A) -> Result<Self::Value, A::Error>
            where
                A: MapAccess<'de>,
            {
                let mut seen_keys = Vec::new();

                // Check for duplicate keys while consuming entries
                while let Some((key, _value)) = access.next_entry::<Value, Value>()? {
                    if seen_keys.contains(&key) {
                        return Err(Error::custom("duplicate key found in CBOR map"));
                    }
                    seen_keys.push(key);
                }
                Ok(HeaderMap)
            }
        }

        deserializer.deserialize_map(MapVisitor)
    }
}

impl<'de> Deserialize<'de> for CoseSign1 {
    fn deserialize<D>(deserializer: D) -> Result<CoseSign1, D::Error>
    where
        D: Deserializer<'de>,
    {
        use serde::de::{Error, SeqAccess, Visitor};
        use std::fmt;

        struct CoseSign1Visitor;

        impl<'de> Visitor<'de> for CoseSign1Visitor {
            type Value = CoseSign1;

            fn expecting(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                f.write_str("a possibly tagged CoseSign1 structure")
            }

            fn visit_seq<A>(self, mut seq: A) -> Result<CoseSign1, A::Error>
            where
                A: SeqAccess<'de>,
            {
                fn extract_bytes(value: Value) -> Option<Vec<u8>> {
                    match value {
                        Value::Bytes(bytes) => Some(bytes),
                        _ => None,
                    }
                }

                // Get protected header bytes
                let protected = match seq.next_element::<Value>()? {
                    Some(v) => extract_bytes(v)
                        .ok_or_else(|| A::Error::custom("protected header must be bytes"))?,
                    None => return Err(A::Error::missing_field("protected")),
                };

                let unprotected = match seq.next_element()? {
                    Some(v) => v,
                    None => return Err(A::Error::missing_field("unprotected")),
                };
                // Get payload bytes
                let payload = match seq.next_element::<Value>()? {
                    Some(v) => {
                        extract_bytes(v).ok_or_else(|| A::Error::custom("payload must be bytes"))?
                    }
                    None => return Err(A::Error::missing_field("payload")),
                };

                // Get signature bytes
                let signature = match seq.next_element::<Value>()? {
                    Some(v) => extract_bytes(v)
                        .ok_or_else(|| A::Error::custom("signature must be bytes"))?,
                    None => return Err(A::Error::missing_field("signature")),
                };

                Ok(CoseSign1 {
                    protected,
                    unprotected,
                    payload,
                    signature,
                })
            }

            fn visit_newtype_struct<D>(self, deserializer: D) -> Result<CoseSign1, D::Error>
            where
                D: Deserializer<'de>,
            {
                // This is the tagged version: we ignore the tag part, and just go into it
                deserializer.deserialize_seq(CoseSign1Visitor)
            }
        }

        deserializer.deserialize_any(CoseSign1Visitor)
    }
}

impl CoseSign1 {
    /// Parse CBOR bytes into struct. Adapted from <https://github.com/awslabs/aws-nitro-enclaves-cose/blob/main/src/sign.rs>
    pub fn parse_and_validate(bytes: &[u8]) -> Result<Self, NitroAttestationVerifyError> {
        let tagged_value: ciborium::value::Value = ciborium::de::from_reader(bytes)
            .map_err(|e| NitroAttestationVerifyError::InvalidCoseSign1(e.to_string()))?;

        let (tag, value) = match tagged_value {
            ciborium::value::Value::Tag(tag, box_value) => (Some(tag), *box_value),
            other => (None, other),
        };

        // Validate tag (18 is the COSE_Sign1 tag)
        match tag {
            None | Some(18) => (),
            Some(_) => {
                return Err(NitroAttestationVerifyError::InvalidCoseSign1(
                    "invalid tag".to_string(),
                ));
            }
        }

        // Create a buffer for serialization
        let mut buf = Vec::new();

        // Serialize the value into the buffer
        ciborium::ser::into_writer(&value, &mut buf)
            .map_err(|e| NitroAttestationVerifyError::InvalidCoseSign1(e.to_string()))?;

        // Deserialize the COSE_Sign1 structure from the buffer
        let cosesign1: Self = ciborium::de::from_reader(&buf[..])
            .map_err(|e| NitroAttestationVerifyError::InvalidCoseSign1(e.to_string()))?;

        // Validate protected header
        let _: HeaderMap = ciborium::de::from_reader(cosesign1.protected.as_slice())
            .map_err(|e| NitroAttestationVerifyError::InvalidCoseSign1(e.to_string()))?;

        cosesign1.validate_header()?;
        Ok(cosesign1)
    }

    /// Validate protected header, payload and signature length.
    pub fn validate_header(&self) -> Result<(), NitroAttestationVerifyError> {
        if !(Self::is_valid_protected_header(self.protected.as_slice())
            && (1..16384).contains(&self.payload.len())
            && self.signature.len() == 96)
        {
            return Err(NitroAttestationVerifyError::InvalidCoseSign1(
                "invalid cbor header".to_string(),
            ));
        }
        Ok(())
    }

    // Check protected header: https://docs.aws.amazon.com/enclaves/latest/user/verify-root.html#COSE-CBOR
    // 18(/* COSE_Sign1 CBOR tag is 18 */
    //     {1: -35}, /* This is equivalent with {algorithm: ECDS 384} */
    //     {}, /* We have nothing in unprotected */
    //     $ATTESTATION_DOCUMENT_CONTENT /* Attestation Document */,
    //     signature /* This is the signature */
    // )
    fn is_valid_protected_header(bytes: &[u8]) -> bool {
        let expected_key: Integer = Integer::from(1);
        let expected_val: Integer = Integer::from(-35);
        let value: Value = match ciborium::de::from_reader(bytes) {
            Ok(v) => v,
            Err(_) => return false,
        };
        match value {
            Value::Map(vec) => match &vec[..] {
                [(Value::Integer(key), Value::Integer(val))] => {
                    key == &expected_key && val == &expected_val
                }
                _ => false,
            },
            _ => false,
        }
    }

    /// This is the content that the signature is committed over.
    fn to_signed_message(&self) -> SuiResult<Vec<u8>> {
        let value = Value::Array(vec![
            Value::Text("Signature1".to_string()),
            Value::Bytes(self.protected.as_slice().to_vec()),
            Value::Bytes(vec![]),
            Value::Bytes(self.payload.as_slice().to_vec()),
        ]);
        // 17 for extra metadata bytes
        let mut bytes = Vec::with_capacity(self.protected.len() + self.payload.len() + 17);
        ciborium::ser::into_writer(&value, &mut bytes).map_err(|_| {
            SuiErrorKind::NitroAttestationFailedToVerify("cannot parse message".to_string())
        })?;
        Ok(bytes)
    }
}

/// The AWS Nitro Attestation Document, see <https://docs.aws.amazon.com/enclaves/latest/user/verify-root.html#doc-def>
#[allow(dead_code)]
#[derive(Debug, Clone)]
pub struct AttestationDocument {
    pub module_id: String,
    pub timestamp: u64,
    pub digest: String,
    pub pcr_vec: Vec<Vec<u8>>,
    pub pcr_map: BTreeMap<u8, Vec<u8>>,
    certificate: Vec<u8>,
    cabundle: Vec<Vec<u8>>,
    pub public_key: Option<Vec<u8>>,
    pub user_data: Option<Vec<u8>>,
    pub nonce: Option<Vec<u8>>,
}

impl AttestationDocument {
    /// Validate and parse the payload of the attestation document.
    /// Adapted from <https://github.com/EternisAI/remote-attestation-verifier/blob/main/src/lib.rs>
    pub fn parse_payload(
        payload: &[u8],
        is_upgraded_parsing: bool,
        include_all_nonzero_pcrs: bool,
        always_include_required_pcrs: bool,
    ) -> Result<AttestationDocument, NitroAttestationVerifyError> {
        let document_map = Self::to_map(payload, is_upgraded_parsing)?;
        Self::validate_document_map(
            &document_map,
            is_upgraded_parsing,
            include_all_nonzero_pcrs,
            always_include_required_pcrs,
        )
    }

    fn to_map(
        payload: &[u8],
        is_upgraded_parsing: bool,
    ) -> Result<BTreeMap<String, Value>, NitroAttestationVerifyError> {
        let document_data: ciborium::value::Value =
            ciborium::de::from_reader(payload).map_err(|err| {
                NitroAttestationVerifyError::InvalidAttestationDoc(format!(
                    "cannot parse payload CBOR: {}",
                    err
                ))
            })?;

        let document_map: BTreeMap<String, Value> = match document_data {
            ciborium::value::Value::Map(map) => {
                let map_size = map.len();
                let result = map
                    .into_iter()
                    .map(|(k, v)| {
                        let k = k.as_text().ok_or(
                            NitroAttestationVerifyError::InvalidAttestationDoc(format!(
                                "invalid key type: {:?}",
                                k
                            )),
                        )?;
                        Ok((k.to_string(), v))
                    })
                    .collect::<Result<BTreeMap<String, Value>, NitroAttestationVerifyError>>()?;

                if is_upgraded_parsing && result.len() != map_size {
                    return Err(NitroAttestationVerifyError::InvalidAttestationDoc(
                        "duplicate keys found in attestation document".to_string(),
                    ));
                }
                result
            }
            _ => {
                return Err(NitroAttestationVerifyError::InvalidAttestationDoc(format!(
                    "expected map, got {:?}",
                    document_data
                )));
            }
        };
        Ok(document_map)
    }

    fn validate_document_map(
        document_map: &BTreeMap<String, Value>,
        is_upgraded_parsing: bool,
        include_all_nonzero_pcrs: bool,
        always_include_required_pcrs: bool,
    ) -> Result<AttestationDocument, NitroAttestationVerifyError> {
        let module_id = document_map
            .get("module_id")
            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                "module id not found".to_string(),
            ))?
            .as_text()
            .filter(|s| !s.is_empty())
            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                "invalid module id".to_string(),
            ))?
            .to_string();

        let digest = document_map
            .get("digest")
            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                "digest not found".to_string(),
            ))?
            .as_text()
            .filter(|s| s == &"SHA384")
            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                "invalid digest".to_string(),
            ))?
            .to_string();

        let certificate = document_map
            .get("certificate")
            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                "certificate not found".to_string(),
            ))?
            .as_bytes()
            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                "invalid certificate".to_string(),
            ))?
            .to_vec();

        if certificate.is_empty() || certificate.len() > MAX_CERT_LENGTH {
            return Err(NitroAttestationVerifyError::InvalidAttestationDoc(
                "invalid certificate".to_string(),
            ));
        }

        let timestamp = document_map
            .get("timestamp")
            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                "timestamp not found".to_string(),
            ))?
            .as_integer()
            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                "timestamp is not an integer".to_string(),
            ))
            .and_then(|integer| {
                u64::try_from(integer).map_err(|_| {
                    NitroAttestationVerifyError::InvalidAttestationDoc(
                        "timestamp not u64".to_string(),
                    )
                })
            })?;

        let public_key = document_map
            .get("public_key")
            .and_then(|v| v.as_bytes())
            .map(|bytes| bytes.to_vec());

        if let Some(data) = &public_key {
            if is_upgraded_parsing {
                if data.len() > MAX_PK_LENGTH {
                    return Err(NitroAttestationVerifyError::InvalidAttestationDoc(
                        "invalid public key".to_string(),
                    ));
                }
            } else if data.is_empty() || data.len() > MAX_PK_LENGTH {
                return Err(NitroAttestationVerifyError::InvalidAttestationDoc(
                    "invalid public key".to_string(),
                ));
            }
        }

        let user_data = document_map
            .get("user_data")
            .and_then(|v| v.as_bytes())
            .map(|bytes| bytes.to_vec());

        if let Some(data) = &user_data
            && data.len() > MAX_USER_DATA_LENGTH
        {
            return Err(NitroAttestationVerifyError::InvalidAttestationDoc(
                "invalid user data".to_string(),
            ));
        }

        let nonce = document_map
            .get("nonce")
            .and_then(|v| v.as_bytes())
            .map(|bytes| bytes.to_vec());

        if let Some(data) = &nonce
            && data.len() > MAX_USER_DATA_LENGTH
        {
            return Err(NitroAttestationVerifyError::InvalidAttestationDoc(
                "invalid nonce".to_string(),
            ));
        }

        let (pcr_vec, pcr_map) = document_map
            .get("pcrs")
            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                "pcrs not found".to_string(),
            ))?
            .as_map()
            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                "invalid pcrs format".to_string(),
            ))
            .and_then(|pairs| {
                if pairs.len() > MAX_PCRS_LENGTH {
                    return Err(NitroAttestationVerifyError::InvalidAttestationDoc(
                        "invalid PCRs length".to_string(),
                    ));
                }
                let mut pcr_vec = Vec::with_capacity(pairs.len());
                let mut pcr_map = BTreeMap::new();
                for (k, v) in pairs.iter() {
                    let key = k.as_integer().ok_or(
                        NitroAttestationVerifyError::InvalidAttestationDoc(
                            "invalid PCR key format".to_string(),
                        ),
                    )?;
                    let value =
                        v.as_bytes()
                            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                                "invalid PCR value format".to_string(),
                            ))?;

                    if value.len() != 32 && value.len() != 48 && value.len() != 64 {
                        return Err(NitroAttestationVerifyError::InvalidAttestationDoc(
                            "invalid PCR value length".to_string(),
                        ));
                    }

                    // legacy parsing that populates a vector.
                    let key_u64 = u64::try_from(key).map_err(|_| {
                        NitroAttestationVerifyError::InvalidAttestationDoc(
                            "invalid PCR index".to_string(),
                        )
                    })?;
                    for i in [0, 1, 2, 3, 4, 8] {
                        if key_u64 == i {
                            pcr_vec.push(value.to_vec());
                        }
                    }

                    // when upgraded parsing is enabled, use btreemap to avoid dup.
                    if is_upgraded_parsing {
                        // valid key is 0..31, can parse with u8.
                        let key_u8 = u8::try_from(key).map_err(|_| {
                            NitroAttestationVerifyError::InvalidAttestationDoc(
                                "invalid PCR index".to_string(),
                            )
                        })?;

                        if pcr_map.contains_key(&key_u8) {
                            return Err(NitroAttestationVerifyError::InvalidAttestationDoc(
                                format!("duplicate PCR index {}", key_u8),
                            ));
                        }

                        if include_all_nonzero_pcrs {
                            // If include_all_nonzero_pcrs = true, parse all 0..31 PCRs, but
                            // only include nonzero values.
                            // See: <https://github.com/aws/aws-nitro-enclaves-nsm-api/issues/18#issuecomment-970172662>
                            // Also: <https://github.com/aws/aws-nitro-enclaves-nsm-api/blob/main/nsm-test/src/bin/nsm-check.rs#L193-L199>
                            let is_required_pcr = matches!(key_u8, 0 | 1 | 2 | 3 | 4 | 8);
                            let is_nonzero = !value.iter().all(|&b| b == 0);

                            // If always_include_required_pcrs = true, always include required PCRs,
                            // regardless if they are zero or not.
                            if key_u8 <= 31
                                && (is_nonzero || (is_required_pcr && always_include_required_pcrs))
                            {
                                pcr_map.insert(key_u8, value.to_vec());
                            }
                        } else {
                            // Legacy mode (include_all_nonzero_pcrs=false): Parse only
                            // required PCRs (0, 1, 2, 3, 4, 8), regardless if they are zero or not.
                            // See: <https://docs.aws.amazon.com/enclaves/latest/user/set-up-attestation.html#where>
                            if matches!(key_u8, 0 | 1 | 2 | 3 | 4 | 8) {
                                pcr_map.insert(key_u8, value.to_vec());
                            }
                        }
                    }
                }
                Ok((pcr_vec, pcr_map))
            })?;

        let cabundle = document_map
            .get("cabundle")
            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                "cabundle not found".to_string(),
            ))?
            .as_array()
            .map(|arr| {
                if arr.is_empty() || arr.len() > MAX_CERT_CHAIN_LENGTH {
                    return Err(NitroAttestationVerifyError::InvalidAttestationDoc(
                        "invalid ca chain length".to_string(),
                    ));
                }
                let mut certs = Vec::with_capacity(arr.len());
                for cert in arr.iter() {
                    let cert_bytes = cert.as_bytes().ok_or(
                        NitroAttestationVerifyError::InvalidAttestationDoc(
                            "invalid certificate bytes".to_string(),
                        ),
                    )?;
                    if cert_bytes.is_empty() || cert_bytes.len() > MAX_CERT_LENGTH {
                        return Err(NitroAttestationVerifyError::InvalidAttestationDoc(
                            "invalid ca length".to_string(),
                        ));
                    }
                    certs.push(cert_bytes.to_vec());
                }
                Ok(certs)
            })
            .ok_or(NitroAttestationVerifyError::InvalidAttestationDoc(
                "invalid cabundle".to_string(),
            ))??;

        let doc = AttestationDocument {
            module_id,
            timestamp,
            digest,
            pcr_vec,
            pcr_map,
            certificate,
            cabundle,
            public_key,
            user_data,
            nonce,
        };
        Ok(doc)
    }

    /// Verify the certificate against AWS Nitro root of trust and checks expiry.
    /// Assume the cabundle is in order.
    fn verify_cert(&self, now: u64) -> Result<(), NitroAttestationVerifyError> {
        // Create chain starting with leaf cert all the way to root.
        let mut chain = Vec::with_capacity(1 + self.cabundle.len());
        chain.push(self.certificate.as_slice());
        chain.extend(self.cabundle.iter().rev().map(|cert| cert.as_slice()));
        verify_cert_chain(&chain, now)
    }

    /// Get the length of the certificate chain.
    pub fn get_cert_chain_length(&self) -> usize {
        self.cabundle.len()
    }
}
/// Verify the certificate chain against the root of trust.
fn verify_cert_chain(cert_chain: &[&[u8]], now_ms: u64) -> Result<(), NitroAttestationVerifyError> {
    let root_cert = X509Certificate::from_der(ROOT_CERTIFICATE.as_slice())
        .map_err(|e| NitroAttestationVerifyError::InvalidCertificate(e.to_string()))?
        .1;

    let now_secs =
        ASN1Time::from_timestamp((now_ms as i64).checked_div(1000).ok_or_else(|| {
            NitroAttestationVerifyError::InvalidAttestationDoc("timestamp overflow".to_string())
        })?)
        .map_err(|e| NitroAttestationVerifyError::InvalidCertificate(e.to_string()))?;

    // Validate the chain starting from the leaf
    for i in 0..cert_chain.len() {
        let cert = X509Certificate::from_der(cert_chain[i])
            .map_err(|e| NitroAttestationVerifyError::InvalidCertificate(e.to_string()))?
            .1;

        // Check key usage for all certificates
        if let Ok(Some(key_usage)) = cert.key_usage() {
            if i == 0 {
                // Target certificate must have digitalSignature
                if !key_usage.value.digital_signature() {
                    return Err(NitroAttestationVerifyError::InvalidCertificate(
                        "Target certificate missing digitalSignature key usage".to_string(),
                    ));
                }
            } else {
                // CA certificates must have keyCertSign
                if !key_usage.value.key_cert_sign() {
                    return Err(NitroAttestationVerifyError::InvalidCertificate(
                        "CA certificate missing keyCertSign key usage".to_string(),
                    ));
                }
            }
        } else {
            return Err(NitroAttestationVerifyError::InvalidCertificate(
                "Missing key usage extension".to_string(),
            ));
        }

        if i != 0 {
            // all ca certs must have basic contraint, must be critical, ca flag must be true,
            // pathLenConstraint is optional but if present must be checked.
            if let Ok(Some(bc)) = cert.basic_constraints() {
                if !bc.critical || !bc.value.ca {
                    return Err(NitroAttestationVerifyError::InvalidCertificate(
                        "CA certificate invalid".to_string(),
                    ));
                }
                if let Some(path_len) = bc.value.path_len_constraint {
                    // path_len_constraint is the maximum number of CA certificates
                    // that may follow this certificate. A value of zero indicates
                    // that only an end-entity certificate may follow in the path.
                    if i - 1 > path_len as usize {
                        return Err(NitroAttestationVerifyError::InvalidCertificate(
                            "Cert chain exceeds pathLenConstraint".to_string(),
                        ));
                    }
                }
            } else {
                return Err(NitroAttestationVerifyError::InvalidCertificate(
                    "missing basic constraint".to_string(),
                ));
            }
        } else if let Ok(Some(bc)) = cert.basic_constraints() {
            // end entity can have basic constraint optionally, if present, ca must be false and
            // pathLenConstraint is undefined.
            if bc.value.path_len_constraint.is_some() || bc.value.ca {
                return Err(NitroAttestationVerifyError::InvalidCertificate(
                    "Cert chain exceeds pathLenConstraint".to_string(),
                ));
            }
        }

        // Check timestamp validity
        if !cert.validity().is_valid_at(now_secs) {
            return Err(NitroAttestationVerifyError::InvalidCertificate(
                "Certificate timestamp not valid".to_string(),
            ));
        }

        // Get issuer cert from either next in chain or root
        let issuer_cert = if i < cert_chain.len() - 1 {
            X509Certificate::from_der(cert_chain[i + 1])
                .map_err(|e| NitroAttestationVerifyError::InvalidCertificate(e.to_string()))?
                .1
        } else {
            root_cert.clone()
        };

        // Verify issuer/subject chaining
        if cert.issuer() != issuer_cert.subject() {
            return Err(NitroAttestationVerifyError::InvalidCertificate(
                "certificate chain issuer mismatch".to_string(),
            ));
        }

        // Verify signature
        cert.verify_signature(Some(issuer_cert.public_key()))
            .map_err(|_| {
                NitroAttestationVerifyError::InvalidCertificate(
                    "certificate fails to verify".to_string(),
                )
            })?;
    }

    Ok(())
}