X.509 certificate parsing and chain validation #55

Phase 5: Pure Rust Crypto#

Implement X.509 certificate parsing and certificate chain validation in the crypto crate.

Dependencies#

• Requires ASN.1 DER parser
• Requires SHA-2 (for certificate fingerprints and signature hash)
• Requires RSA PKCS#1 v1.5 signature verification
• Requires ECDSA signature verification

Requirements#

X.509 certificate parsing (RFC 5280):

• Parse DER-encoded X.509v3 certificates
• Extract: version, serial number, issuer, subject, validity (notBefore/notAfter), subject public key info, extensions
• Parse common extensions: Basic Constraints, Key Usage, Subject Alternative Names (SAN), Authority Key Identifier, Subject Key Identifier
• PEM decoding (Base64 between -----BEGIN CERTIFICATE----- markers)

Certificate chain validation:

• Build chain from leaf to root
• Verify each certificate's signature against its issuer's public key
• Check validity period (not expired, not yet valid)
• Check Basic Constraints (CA:TRUE for intermediate certs)
• Verify subject/issuer chain linkage
• Root CA trust store: embed Mozilla's root certificates or load from system

Signature algorithms to support:

• sha256WithRSAEncryption
• sha384WithRSAEncryption
• ecdsa-with-SHA256
• ecdsa-with-SHA384

Acceptance Criteria#

• Parse real-world X.509 certificates (e.g., from Let's Encrypt, DigiCert)
• Extract subject, issuer, SAN, validity period
• PEM decoding
• Verify certificate chain for a real HTTPS site
• Reject expired certificates
• Reject certificates with invalid signatures
• Reject certificates where issuer doesn't match
• Embedded root CA store (at minimum: Let's Encrypt, DigiCert, GlobalSign roots)
• cargo test -p we-crypto passes
• cargo clippy -p we-crypto -- -D warnings clean