What is ASN.1?

Crypto is full of obscure data formats. .pem files, .p8 files, .p12 files, .key files, .cert files. And lots of these are just wrappers for yet more formats. One of these data formats is ASN.1, “Abstract Syntax Notation One”. ASN.1 is a bit like JSON, XML, or protocol buffers: it can encode numbers, strings, sequences, sets and more.

For example, if I generate an RSA private key with openssl genrsa -out private_key.pem 512, the private key file is quite unreadable:

Generating RSA private key, 512 bit long modulus
e is 65537 (0x10001)
$ cat private_key.pem

But there is structure here. A .pem file can contain many blocks, and this file contains one RSA PRIVATE KEY block. Each block has some bytes of data as the body, which are encoded in base64. This masks the real structure of the body, which is a sequence of nine integers encoded in the ASN.1 format. We can reveal this structure with openssl asn1parse:

$ openssl asn1parse -i -in private_key.pem
    0:d=0  hl=4 l= 315 cons: SEQUENCE
    4:d=1  hl=2 l=   1 prim:  INTEGER           :00
    7:d=1  hl=2 l=  65 prim:  INTEGER           :C3E448D29FCDB2F7E52ABD17712AC76E4ABD66D54F2EF182DC4562B3FA240E3FF76658E7324E441E2C16628C703FF9DEFC76006278B35E21D890E5C2225BCD5B
   74:d=1  hl=2 l=   3 prim:  INTEGER           :010001
   79:d=1  hl=2 l=  65 prim:  INTEGER           :80E6125525FA40AD860A5D6AB378A6CA183C1112838C2C3723A23494A120608EE811B630B11891DE0A8D9813A16C1C579A4D2509ABF836D9AE3B0B382B030981
  146:d=1  hl=2 l=  33 prim:  INTEGER           :E0C060BF7272A88349CE42BC7C6ACB75899E81E2D7D0F8F8ED37132E0E213F4B
  181:d=1  hl=2 l=  33 prim:  INTEGER           :DF20B242104CCF05B046A2ED758E4A62C2AB6D2BDF46BA0FFAEC713A05DE1031
  216:d=1  hl=2 l=  33 prim:  INTEGER           :CDB8C9D5F398680EE47BDC3253601DA414B7F60401AFD275680CC58D9E796849
  251:d=1  hl=2 l=  32 prim:  INTEGER           :3A92A6B8D8F690D1F9785184EC7B31E3C31EA5460312D9FD1FEDB9BB7596D9C1
  285:d=1  hl=2 l=  32 prim:  INTEGER           :41CB15A4ED3F9170A758932C9B1B13F60911561EEE3B474457E9FAB4C72C164A

This integer sequence structure is an RSA private key in PKCS#1 format as described by RFC 3447:

RSAPrivateKey ::= SEQUENCE {
    version           Version,
    modulus           INTEGER,  -- n
    publicExponent    INTEGER,  -- e
    privateExponent   INTEGER,  -- d
    prime1            INTEGER,  -- p
    prime2            INTEGER,  -- q
    exponent1         INTEGER,  -- d mod (p-1)
    exponent2         INTEGER,  -- d mod (q-1)
    coefficient       INTEGER,  -- (inverse of q) mod p
    otherPrimeInfos   OtherPrimeInfos OPTIONAL

The above is an “ASN.1 modules”, a text file which describes a set of ASN.1 values. This is much like protocol buffer schema, XML schema, or JSON schema.

Lots of SSL files are PEM blocks containing a body encoded in ASN.1. As another example, Certificate Signing Requests, .csr files, look like this:

$ openssl asn1parse -i -in cert_request.csr
    0:d=0  hl=4 l= 392 cons: SEQUENCE
    4:d=1  hl=4 l= 306 cons:  SEQUENCE
    8:d=2  hl=2 l=   1 prim:   INTEGER           :00
   11:d=2  hl=3 l= 173 cons:   SEQUENCE
   14:d=3  hl=2 l=  11 cons:    SET
   16:d=4  hl=2 l=   9 cons:     SEQUENCE
   18:d=5  hl=2 l=   3 prim:      OBJECT            :countryName
   23:d=5  hl=2 l=   2 prim:      PRINTABLESTRING   :GB
   27:d=3  hl=2 l=  15 cons:    SET
   29:d=4  hl=2 l=  13 cons:     SEQUENCE
   31:d=5  hl=2 l=   3 prim:      OBJECT            :stateOrProvinceName
   36:d=5  hl=2 l=   6 prim:      UTF8STRING        :London
   44:d=3  hl=2 l=  15 cons:    SET
   46:d=4  hl=2 l=  13 cons:     SEQUENCE
   48:d=5  hl=2 l=   3 prim:      OBJECT            :localityName
   53:d=5  hl=2 l=   6 prim:      UTF8STRING        :London
   61:d=3  hl=2 l=  29 cons:    SET
   63:d=4  hl=2 l=  27 cons:     SEQUENCE
   65:d=5  hl=2 l=   3 prim:      OBJECT            :organizationName
   70:d=5  hl=2 l=  20 prim:      UTF8STRING        :Jameshfisher Limited
   92:d=3  hl=2 l=  24 cons:    SET
   94:d=4  hl=2 l=  22 cons:     SEQUENCE
   96:d=5  hl=2 l=   3 prim:      OBJECT            :organizationalUnitName
  101:d=5  hl=2 l=  15 prim:      UTF8STRING        :Human Resources
  118:d=3  hl=2 l=  28 cons:    SET
  120:d=4  hl=2 l=  26 cons:     SEQUENCE
  122:d=5  hl=2 l=   3 prim:      OBJECT            :commonName
  127:d=5  hl=2 l=  19 prim:      UTF8STRING        :hr.jameshfisher.com
  148:d=3  hl=2 l=  37 cons:    SET
  150:d=4  hl=2 l=  35 cons:     SEQUENCE
  152:d=5  hl=2 l=   9 prim:      OBJECT            :emailAddress
  163:d=5  hl=2 l=  22 prim:      IA5STRING         :jameshfisher@gmail.com
  187:d=2  hl=2 l=  92 cons:   SEQUENCE
  189:d=3  hl=2 l=  13 cons:    SEQUENCE
  191:d=4  hl=2 l=   9 prim:     OBJECT            :rsaEncryption
  202:d=4  hl=2 l=   0 prim:     NULL
  204:d=3  hl=2 l=  75 prim:    BIT STRING
  281:d=2  hl=2 l=  31 cons:   cont [ 0 ]
  283:d=3  hl=2 l=  29 cons:    SEQUENCE
  285:d=4  hl=2 l=   9 prim:     OBJECT            :challengePassword
  296:d=4  hl=2 l=  16 cons:     SET
  298:d=5  hl=2 l=  14 prim:      UTF8STRING        :securepassword
  314:d=1  hl=2 l=  13 cons:  SEQUENCE
  316:d=2  hl=2 l=   9 prim:   OBJECT            :sha256WithRSAEncryption
  327:d=2  hl=2 l=   0 prim:   NULL
  329:d=1  hl=2 l=  65 prim:  BIT STRING
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