DerParser reads TLV structures from DER-encoded byte slices. Supports all primitive types needed for X.509 certificates: BOOLEAN, INTEGER, BIT STRING, OCTET STRING, NULL, OID, UTF8String, PrintableString, IA5String, UTCTime, GeneralizedTime. Constructed types: SEQUENCE, SET. Context-specific tags with explicit/implicit tagging. Includes well-known OID constants for RSA, ECDSA, SHA-2, X.509 extensions, and X.500 attribute types. OID encode/decode with dotted notation. Rejects non-DER encodings (indefinite length, non-minimal integers/lengths). 45 tests covering all types, DER validation, real certificate fragments, and edge cases. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>