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:
- 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
.
- 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)