ASN.1 / DER Encoding

ASN.1 is the schema language; DER is the binary encoding (Distinguished Encoding Rules).

DER encodes things as TLV — Type, Length, Value:

[TAG (1+ bytes)] [LENGTH (1+ bytes)] [VALUE (length bytes)]

Tags (single byte for simple types):

• 02 INTEGER
• 03 BIT STRING
• 04 OCTET STRING
• 05 NULL
• 06 OBJECT IDENTIFIER (OID)
• 0C UTF8String
• 13 PrintableString
• 17 UTCTime
• 18 GeneralizedTime
• 30 SEQUENCE (combined: 30 = constructed | tag 16)
• 31 SET (constructed | 17)

Length encoding:

• 0..127: single byte.
• 128+: first byte = 0x80 | num_length_bytes; following bytes = length big-endian.

INTEGER 100   = 02 01 64
INTEGER 1000  = 02 02 03 E8
SEQUENCE { INTEGER 1, INTEGER 2 } = 30 06 02 01 01 02 01 02

OIDs are dot-notation identifiers encoded as bytes:

• 1.2.840.113549.1.1.11 = sha256WithRSAEncryption
• 1.2.840.10045.2.1 = ecPublicKey
• 2.5.29.17 = subjectAltName

OID encoding:

• First two arcs combined: 40*X + Y.
• Subsequent: base-128 with high bit set on continuation bytes.
• 1.2.840.113549.1.1.11 → 2A 86 48 86 F7 0D 01 01 0B

DER quirks:

• INTEGER must be minimum bytes (no leading 0x00 unless to keep sign positive).
• BIT STRING includes "unused bits" count as first byte.
• SET OF is sorted lexicographically.

Parse with care: x.509 has been a wellspring of CVEs. Use battle-tested libraries.