How to generate ssh compatible id_rsa(.pub) from Java

The key format used by ssh is defined in the RFC #4253. The format for RSA public key is the following :

  string    "ssh-rsa"
  mpint     e /* key public exponent */
  mpint     n /* key modulus */

All data type encoding is defined in the section #5 of RFC #4251. string and mpint (multiple precision integer) types are encoded this way :

  4-bytes word: data length (unsigned big-endian 32 bits integer)
  n bytes     : binary representation of the data

for instance, the encoding of the string “ssh-rsa” is:

  byte[] data = new byte[] {0, 0, 0, 7, 's', 's', 'h', '-', 'r', 's', 'a'};

To encode the public :

   public byte[] encodePublicKey(RSAPublicKey key) throws IOException
   {
       ByteArrayOutputStream out = new ByteArrayOutputStream();
       /* encode the "ssh-rsa" string */
       byte[] sshrsa = new byte[] {0, 0, 0, 7, 's', 's', 'h', '-', 'r', 's', 'a'};
       out.write(sshrsa);
       /* Encode the public exponent */
       BigInteger e = key.getPublicExponent();
       byte[] data = e.toByteArray();
       encodeUInt32(data.length, out);
       out.write(data);
       /* Encode the modulus */
       BigInteger m = key.getModulus();
       data = m.toByteArray();
       encodeUInt32(data.length, out);
       out.write(data);
       return out.toByteArray();
   }

   public void encodeUInt32(int value, OutputStream out) throws IOException
   {
       byte[] tmp = new byte[4];
       tmp[0] = (byte)((value >>> 24) & 0xff);
       tmp[1] = (byte)((value >>> 16) & 0xff);
       tmp[2] = (byte)((value >>> 8) & 0xff);
       tmp[3] = (byte)(value & 0xff);
       out.write(tmp);
   }

To have a string représentation of the key just encode the returned byte array in Base64.

For the private key encoding there is two cases:

1. the private key is not protected by a password. In that case the private key is encoded according to the PKCS#8 standard and then encoded with Base64. It is possible to get the PKCS8 encoding of the private key by calling getEncoded on RSAPrivateKey.
2. the private key is protected by a password. In that case the key encoding is an OpenSSH dedicated format. I don’t know if there is any documentation on this format (except the OpenSSH source code of course)