pdfcraft/crates/sign/src/revocation.rs

//! Revocation evidence (RFC 5280 CRLs, RFC 6960 OCSP): parsing, signature verification and
//! status lookup for evidence embedded in a DSS or supplied by the caller. Never fetches:
//! the transport and the choice of evidence stay with the caller.

use crate::der::{Tlv, tag};
use crate::keys::{self, DigestAlg, PublicKey};
use crate::x509::Certificate;
use crate::{SignError, Time};

/// A DER size cap: real CRLs and OCSP responses are far smaller.
const MAX_EVIDENCE: usize = 2 * 1024 * 1024;

const ID_PKIX_OCSP_BASIC: &str = "1.3.6.1.5.5.7.48.1.1";
const EKU_OCSP_SIGNING: &str = "1.3.6.1.5.5.7.3.9";

/// What a piece of revocation evidence says about a certificate at a point in time.
#[derive(Clone, Debug, PartialEq, Eq)]
pub enum RevocationStatus {
    /// The evidence covers the certificate and does not revoke it.
    Good,
    /// The evidence covers the certificate and revokes it.
    Revoked { at: Time },
    /// The evidence does not cover the certificate, is outside its window, or could not be
    /// verified.
    Unknown,
}

fn bad(what: impl Into<String>) -> SignError {
    SignError::Malformed(format!("revocation: {}", what.into()))
}

fn strip(b: &[u8]) -> &[u8] {
    match b {
        [0, rest @ ..] if !rest.is_empty() => rest,
        v => v,
    }
}

fn valid_in_window(this: Time, next: Option<Time>, at: Time) -> bool {
    at >= this && next.is_none_or(|n| at <= n)
}

/// Verify `signature` over the raw `signed` bytes with `key`, where the signature's algorithm
/// identifier names the digest.
fn verify_signature(signed: &[u8], alg_raw: &[u8], signature: &[u8], key: &PublicKey) -> bool {
    let Ok(alg) = Tlv::parse_all(alg_raw) else { return false };
    let Ok((scheme, Some(digest))) = keys::signature_algorithm(&alg) else { return false };
    let d = digest.digest(&[signed]);
    key.verify(scheme, digest, &d, signature).unwrap_or(false)
}

// ── X.509 CertificateList (CRL) ─────────────────────────────────────────────────────────────

/// A parsed CRL (RFC 5280 §5.1).
pub struct CertificateList {
    tbs: Vec<u8>,
    sig_alg: Vec<u8>,
    signature: Vec<u8>,
    pub issuer_raw: Vec<u8>,
    pub this_update: Time,
    pub next_update: Option<Time>,
    /// (serial, revocation date), top to bottom.
    pub revoked: Vec<(Vec<u8>, Time)>,
}

impl CertificateList {
    pub fn parse(bytes: &[u8]) -> Result<CertificateList, SignError> {
        if bytes.is_empty() || bytes.len() > MAX_EVIDENCE {
            return Err(bad("CRL is empty or exceeds the 2 MiB limit"));
        }
        let parts = Tlv::parse_all(bytes)?.expect(tag::SEQUENCE, "CertificateList")?.children()?;
        let [tbs, alg, sig] = parts.as_slice() else { return Err(bad("CertificateList")) };
        let mut f = tbs.children()?.into_iter().peekable();
        if f.peek().is_some_and(|t| t.tag == tag::INTEGER) {
            f.next(); // version
        }
        let _alg = f.next().ok_or_else(|| bad("tbsCertList"))?;
        let issuer = f.next().ok_or_else(|| bad("tbsCertList"))?.raw.to_vec();
        let this_update = f.next().ok_or_else(|| bad("tbsCertList"))?.time()?;
        let mut next_update = None;
        if f.peek().is_some_and(|t| t.tag == tag::UTC_TIME || t.tag == tag::GENERALIZED_TIME) {
            next_update = f.next().map(|t| t.time()).transpose()?;
        }
        let mut revoked = Vec::new();
        if f.peek().is_some_and(|t| t.tag == tag::SEQUENCE) {
            for entry in f.next().ok_or_else(|| bad("revokedCertificates"))?.children()? {
                let e = entry.children()?;
                let serial = e.first().ok_or_else(|| bad("revoked entry"))?.uint_bytes().to_vec();
                let at = e.get(1).ok_or_else(|| bad("revoked entry"))?.time()?;
                revoked.push((serial, at));
            }
        }
        Ok(CertificateList {
            tbs: tbs.raw.to_vec(),
            sig_alg: alg.raw.to_vec(),
            signature: sig.bits()?.to_vec(),
            issuer_raw: issuer,
            this_update,
            next_update,
            revoked,
        })
    }

    /// Check `cert` against this CRL at `at`. The CRL must be issued by `cert`'s issuer, its
    /// signature must verify with the issuer's key, and `at` must fall inside its window.
    pub fn check(&self, cert: &Certificate, issuer: &Certificate, at: Time) -> RevocationStatus {
        if self.issuer_raw != issuer.subject.raw || issuer.subject.raw != cert.issuer.raw {
            return RevocationStatus::Unknown;
        }
        if !verify_signature(&self.tbs, &self.sig_alg, &self.signature, &issuer.public_key) {
            return RevocationStatus::Unknown;
        }
        if !valid_in_window(self.this_update, self.next_update, at) {
            return RevocationStatus::Unknown;
        }
        match self.revoked.iter().find(|(serial, _)| strip(serial) == strip(&cert.serial)) {
            Some((_, t)) => RevocationStatus::Revoked { at: *t },
            None => RevocationStatus::Good,
        }
    }
}

// ── OCSP BasicOCSPResponse ──────────────────────────────────────────────────────────────────

/// One `CertID` (RFC 6960 §4.1.1): hashes of the issuer's name and key, and the serial.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct CertId {
    pub hash_alg: DigestAlg,
    pub issuer_name_hash: Vec<u8>,
    pub issuer_key_hash: Vec<u8>,
    pub serial: Vec<u8>,
}

impl CertId {
    /// The `CertID` naming `cert` as issued by `issuer`, using `hash_alg`.
    pub fn build(cert: &Certificate, issuer: &Certificate, hash_alg: DigestAlg) -> CertId {
        CertId {
            hash_alg,
            issuer_name_hash: hash_alg.digest(&[&cert.issuer.raw]),
            issuer_key_hash: hash_alg.digest(&[&issuer.public_key.key_bits()]),
            serial: cert.serial.clone(),
        }
    }
}

#[derive(Clone, Debug, PartialEq, Eq)]
enum CertStatus {
    Good,
    Revoked(Time),
    Unknown,
}

/// One SingleResponse: the status of one certificate at a time.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct SingleResponse {
    pub cert_id: CertId,
    status: CertStatus,
    pub this_update: Time,
    pub next_update: Option<Time>,
}

enum ResponderId {
    ByName(Vec<u8>),
    ByKey(Vec<u8>),
}

/// A parsed OCSP response (RFC 6960 §4.2.1): the responder's identity, per-certificate
/// responses, and the delegated responder's certificate when it is embedded.
pub struct OcspResponse {
    tbs: Vec<u8>,
    sig_alg: Vec<u8>,
    signature: Vec<u8>,
    responder_id: ResponderId,
    pub responses: Vec<SingleResponse>,
    responder_cert: Option<Certificate>,
}

impl OcspResponse {
    pub fn parse(bytes: &[u8]) -> Result<OcspResponse, SignError> {
        if bytes.is_empty() || bytes.len() > MAX_EVIDENCE {
            return Err(bad("OCSP response is empty or exceeds the 2 MiB limit"));
        }
        let resp = Tlv::parse_all(bytes)?.expect(tag::SEQUENCE, "OCSPResponse")?.children()?;
        let status = resp.first().ok_or_else(|| bad("missing responseStatus"))?.u64()?;
        if status != 0 {
            return Err(bad(format!("responder reported an error (status {status})")));
        }
        let response_bytes =
            resp.get(1).ok_or_else(|| bad("successful response has no responseBytes"))?.expect(tag::ctx(0), "responseBytes")?.inner()?.children()?;
        let [type_oid, wrapped] = response_bytes.as_slice() else { return Err(bad("responseBytes")) };
        if type_oid.oid()? != ID_PKIX_OCSP_BASIC {
            return Err(bad("unsupported response type"));
        }
        let basic = Tlv::parse_all(wrapped.value)?.expect(tag::SEQUENCE, "BasicOCSPResponse")?.children()?;
        let mut it = basic.into_iter();
        let tbs = it.next().ok_or_else(|| bad("BasicOCSPResponse"))?;
        let alg = it.next().ok_or_else(|| bad("BasicOCSPResponse"))?;
        let signature = it.next().ok_or_else(|| bad("BasicOCSPResponse"))?.expect(tag::BIT_STRING, "signature")?.bits()?.to_vec();
        let mut responder_cert = None;
        for t in it {
            if t.tag == tag::ctx(0) {
                for c in t.children()? {
                    if c.tag == tag::SEQUENCE
                        && let Ok(cert) = Certificate::parse(c.raw)
                    {
                        responder_cert.get_or_insert(cert);
                    }
                }
            }
        }
        let fields = tbs.children()?;
        let mut f = fields.into_iter().peekable();
        if f.peek().is_some_and(|t| t.tag == tag::ctx(0)) {
            f.next(); // version
        }
        let rid = f.next().ok_or_else(|| bad("ResponseData"))?;
        let responder_id = match rid.tag {
            t if t == tag::ctx(1) => ResponderId::ByName(rid.value.to_vec()),
            t if t == tag::ctx(2) => ResponderId::ByKey(Tlv::parse_all(rid.value)?.expect(tag::OCTET_STRING, "byKey responderID")?.value.to_vec()),
            _ => return Err(bad("responderID")),
        };
        let _produced_at = f.next().ok_or_else(|| bad("ResponseData"))?.time()?;
        let mut responses = Vec::new();
        for single in f.next().ok_or_else(|| bad("ResponseData"))?.children()? {
            let s = single.children()?;
            let cert_id_fields = s.first().ok_or_else(|| bad("SingleResponse"))?.children()?;
            let [hash_alg, name_hash, key_hash, serial] = cert_id_fields.as_slice() else { return Err(bad("CertID")) };
            let alg_oid = hash_alg.children()?.first().ok_or_else(|| bad("CertID"))?.oid()?;
            let Some(hash_alg) = DigestAlg::from_oid(&alg_oid) else { return Err(bad("unsupported CertID hash")) };
            let cert_id = CertId {
                hash_alg,
                issuer_name_hash: name_hash.value.to_vec(),
                issuer_key_hash: key_hash.value.to_vec(),
                serial: serial.uint_bytes().to_vec(),
            };
            let status = match s.get(1).map(|t| t.tag) {
                // RFC 6960 §4.2.1: CertStatus is CHOICE with IMPLICIT tags — good [0] NULL,
                // revoked [1] RevokedInfo (the value is the RevokedInfo's own contents),
                // unknown [2] UnknownInfo.
                Some(t) if t == tag::ctx_prim(0) => CertStatus::Good,
                Some(t) if t == tag::ctx(1) => {
                    let info = s.get(1).unwrap_or(&single);
                    let (date, _) = Tlv::parse(info.value)?;
                    CertStatus::Revoked(date.time()?)
                }
                Some(t) if t == tag::ctx_prim(2) => CertStatus::Unknown,
                _ => return Err(bad("certStatus")),
            };
            let this_update = s.get(2).ok_or_else(|| bad("SingleResponse"))?.time()?;
            let next_update = s.get(3).filter(|t| t.tag == tag::ctx(0)).map(|t| t.inner().and_then(|g| g.time())).transpose()?;
            responses.push(SingleResponse { cert_id, status, this_update, next_update });
        }
        Ok(OcspResponse { tbs: tbs.raw.to_vec(), sig_alg: alg.raw.to_vec(), signature, responder_id, responses, responder_cert })
    }

    /// Check `cert` (issued by `issuer`) at `at`. The responder must be the issuer itself, or
    /// a delegated responder with the OCSP-signing EKU whose certificate is issued by the
    /// issuer; the response signature and CertID must match, and `at` must fall inside the
    /// response window.
    pub fn check(&self, cert: &Certificate, issuer: &Certificate, at: Time) -> RevocationStatus {
        // Who signed the response? The issuer identified by responderID, or a delegated,
        // embedded responder identified the same way.
        let responder: Option<&Certificate> = match &self.responder_id {
            ResponderId::ByName(name) => {
                if *name == issuer.subject.raw {
                    Some(issuer)
                } else {
                    self.responder_cert.as_ref().filter(|c| *name == c.subject.raw)
                }
            }
            ResponderId::ByKey(hash) => {
                if *hash == DigestAlg::Sha1.digest(&[&issuer.public_key.key_bits()]) {
                    Some(issuer)
                } else {
                    self.responder_cert.as_ref().filter(|c| *hash == DigestAlg::Sha1.digest(&[&c.public_key.key_bits()]))
                }
            }
        };
        let Some(responder) = responder else { return RevocationStatus::Unknown };
        // A delegated responder must carry the OCSP-signing EKU and be issued by the issuer.
        if responder.subject.raw != issuer.subject.raw {
            if responder.issuer.raw != issuer.subject.raw || !responder.signed_by(&issuer.public_key) {
                return RevocationStatus::Unknown;
            }
            if !responder.extended_key_usage.as_ref().is_some_and(|e| e.iter().any(|o| o == EKU_OCSP_SIGNING)) {
                return RevocationStatus::Unknown;
            }
        }
        if !verify_signature(&self.tbs, &self.sig_alg, &self.signature, &responder.public_key) {
            return RevocationStatus::Unknown;
        }
        // Find the SingleResponse for this certificate: the CertID hashes must recompute.
        let single = self.responses.iter().find(|r| {
            let name_hash = r.cert_id.hash_alg.digest(&[&cert.issuer.raw]);
            let key_hash = r.cert_id.hash_alg.digest(&[&issuer.public_key.key_bits()]);
            name_hash == r.cert_id.issuer_name_hash && key_hash == r.cert_id.issuer_key_hash && strip(&r.cert_id.serial) == strip(&cert.serial)
        });
        let Some(single) = single else { return RevocationStatus::Unknown };
        if !valid_in_window(single.this_update, single.next_update, at) {
            return RevocationStatus::Unknown;
        }
        match single.status {
            CertStatus::Good => RevocationStatus::Good,
            CertStatus::Revoked(at) => RevocationStatus::Revoked { at },
            CertStatus::Unknown => RevocationStatus::Unknown,
        }
    }
}