ASN.1 DER parser #52

Phase 5: Pure Rust Crypto#

Implement an ASN.1 DER (Distinguished Encoding Rules) parser in the crypto crate. This is needed for X.509 certificate parsing and RSA/ECDSA key parsing.

Requirements#

• Parse DER-encoded TLV (Tag-Length-Value) structures
• Support primitive types: INTEGER, BIT STRING, OCTET STRING, NULL, OID, UTF8String, PrintableString, IA5String, UTCTime, GeneralizedTime, BOOLEAN
• Support constructed types: SEQUENCE, SET
• Support context-specific tags (implicit and explicit)
• OID encoding/decoding (dotted notation and binary)
• Nested structure traversal

Acceptance Criteria#

• DerParser that reads TLV items from a byte slice
• Parse all ASN.1 types needed for X.509 and PKCS#1
• OID comparison (e.g., oid == OID_SHA256_WITH_RSA)
• Parse nested SEQUENCE/SET structures
• Handle context-specific tagged values [0], [1], etc.
• Reject non-DER encodings (indefinite length, non-minimal integers)
• Test with real DER-encoded certificate fragments
• cargo test -p we-crypto passes
• cargo clippy -p we-crypto -- -D warnings clean