Some notes about key format.


“ASN.1 is a formal notation used for describing data transmitted by telecommunications protocols, regardless of language implementation and physical representation of these data, whatever the application, whether complex or very simple.”

Thare are two points:

1) The notation provides a certain number of pre-defined types such as INTEGER, BOOLEAN, and makes it possible to define constructed types such as SEQUENCE to describe a data protocol.

2) The encoding rules such as BER, DER, CER define the encoding and decoding rules for data transfer by telecommunications.

The official document


Key format

Public key format

Many tools use the format defined in x509

SubjectPublicKeyInfo  ::=  SEQUENCE  {
    algorithm            AlgorithmIdentifier,
    subjectPublicKey     BIT STRING

AlgorithmIdentifier  ::=  SEQUENCE  {
    algorithm               OBJECT IDENTIFIER,
    parameters              ANY DEFINED BY algorithm OPTIONAL

For the rsa public key, The AlgorithmIdentifier should be:

algorithm ::= rsaEncryption
parameters :: = NULL

and the subjectPublicKey should be BIT STRING of RSAPublicKey. See RFC3279

Private key format

Defined in pkcs#8

PrivateKeyInfo ::= SEQUENCE {
    version                   Version,
    privateKeyAlgorithm       PrivateKeyAlgorithmIdentifier,
    privateKey                PrivateKey,
    attributes           [0]  IMPLICIT Attributes OPTIONAL

Version ::= INTEGER
PrivateKeyAlgorithmIdentifier ::= AlgorithmIdentifier
PrivateKey ::= OCTET STRING
Attributes ::= SET OF Attribute

EncryptedPrivateKeyInfo ::= SEQUENCE {
    encryptionAlgorithm  EncryptionAlgorithmIdentifier,
    encryptedData        EncryptedData

EncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
EncryptedData ::= OCTET STRING

Rsa Public&Private key format

Defined in pkcs#1

RSAPublicKey ::= SEQUENCE {
    modulus           INTEGER,  -- n
    publicExponent    INTEGER   -- e

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

Key file format

DER key file is the asn.1 DER encoding of key or certificate.

PEM key file is the base64 encoding of asn.1 DER encoding of key or certificate with text HEADER, FOOTER and other information.