Portecle, DER and PEM files, and PEM encodings for private key files

29-07-2024

DER (Distinguished Encoding Rules) files:

  • results in a truly binary representation of the encoded data

PEM (Privacy Enhanced Mai) files:

  • essentially base64 encoded versions of the DER encoded data (a format to send these in an encoding of printable characters)
  • usually generated by OpenSSL and composed of a header, the base64 encoded data, and a footer
  • format
    • a line consisting of 5 hyphens, the word BEGIN, one or a few (space-separated) words defining the type of data, and 5 hyphens
    • an optional (and rare) rfc822-style header, terminated by an empty line
    • base64 of the data, broken into lines of 64 characters (except the last); some programs instead use the (slightly newer) MIME limit of 76 characters
    • a line like the BEGIN line but with END instead

RSA Private Key file (PKCS#1):

  • starts with -----BEGIN RSA PRIVATE KEY-----
  • essentially just the key object from PKCS#8, but without the version or algorithm identifier in front
  • contains CRT parameters
RSAPrivateKey ::= SEQUENCE {
  version           Version,
  modulus           INTEGER,  -- n
  publicExponent    INTEGER,  -- e
  privateExponent   INTEGER,  -- d
    "prime1NTEGER,  --",
    "prime2NTEGER,  --",
  exponent1         INTEGER,  -- d mod (p-1)
  exponent2         INTEGER,  -- d mod (q-1)
  coefficient       INTEGER,  -- (inverse of q) mod p
  otherPrimeInfos   OtherPrimeInfos OPTIONAL
}

Private Key file (PKCS#8):

  • starts with -----BEGIN PRIVATE KEY -----
  • this is a more generic key format that identifies the type of public key and contains the relevant data
    • useful because RSA is not used exclusively inside X509 and SSL/TLS
  • does not contain CRT parameters (more headers, but smaller size)
  • OpenSSL version 3.0.0 and later use this format by default
PrivateKeyInfo ::= SEQUENCE {
  version         Version,
  algorithm       AlgorithmIdentifier,
  PrivateKey      BIT STRING
}
 
AlgorithmIdentifier ::= SEQUENCE {
  algorithm       OBJECT IDENTIFIER,
  parameters      ANY DEFINED BY algorithm OPTIONAL
}

References: