Converting an OpenSSL generated RSA public key to OpenSSH format (PHP)

Question

I've been trying for a while to generate an RSA keypair using PHP's openssl extension and save the result as an OpenSSH compatible keypair - meaning the private key is PEM encoded (which is easy) and the public key is stored in an OpenSSH specific format of the following form:

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABA...more base64 encoded stuff...

As far as I could gather this format consists of:

• the key type in clear text, followed by space (i.e. "openssh-rsa ")
• A base64 encoded string representing the following data:
  • the length of the algorithm name in bytes (in this case 7) encoded as 32 bit unsigned long big endian
  • the algorithm name, in this case 'ssh-rsa'
  • the length of the RSA 'e' number in bytes, encoded as 32 bit unsigned long big endian
  • the RSA 'e' number
  • the length of the RSA 'n' number in bytes, encoded as 32 bit unsigned long big endian
  • the RSA 'n' number

I've tried implementing this using PHP's pack() function, but no matter what I try the result is never equivalent to what I get from using the ssh-keygen -y -f command on the same RSA private key generated by openssl.

Here is a simplified version of my code:

<?php

// generate private key
$privKey = openssl_pkey_new(array(
    'private_key_bits' => 1024,
    'private_key_type' => OPENSSL_KEYTYPE_RSA
));

// convert public key to OpenSSH format
$keyInfo = openssl_pkey_get_details($privKey);
$data = pack("Na*", 7, 'ssh-rsa');
$data .= pack("Na*", strlen($keyInfo['rsa']['e']), $keyInfo['rsa']['e']);
$data .= pack("Na*", strlen($keyInfo['rsa']['n']), $keyInfo['rsa']['n']);

$pubKey = "ssh-rsa " . base64_encode($data);

echo "PHP generated RSA public key:\n$pubKey\n\n";

// For comparison, generate public key using ssh-keygen
openssl_pkey_export($privKey, $pem);
$umask = umask(0066); // this is needed for ssh-keygen to work properly
file_put_contents('/tmp/ssh-keygen-test', $pem);
umask($umask); 

exec('ssh-keygen -y -f /tmp/ssh-keygen-test', $out, $ret);
$otherPubKey = $out[0];
echo "ssh-keygen generated RSA public key:\n$otherPubKey\n\n";

echo ($pubKey == $otherPubKey ? "yes! they are the same\n" : "FAIL! they are different\n");

?>

Any tips on how I can do this without relying on ssh-keygen?

Answer 1

Ok, I have just solved my own problem by taking a closer look at the C implementation references from Convert pem key to ssh-rsa format (which I did before but apparently I missed some important stuff). I needed to AND the first character of N and e with 0x80 and if it matches add another NULL character at the beginning of the number and increase the size by 1 respectively.

I am not sure why this is done (I didn't find any reference to this in on-line searching I did) but it works.

I've only done basic tests on this but it seems to work well, and here my code:

<?php

$privKey = openssl_pkey_get_private($rsaKey);
$pubKey = sshEncodePublicKey($privKey);

echo "PHP generated RSA public key:\n$pubKey\n\n";

function sshEncodePublicKey($privKey)
{
    $keyInfo = openssl_pkey_get_details($privKey);

    $buffer  = pack("N", 7) . "ssh-rsa" . 
               sshEncodeBuffer($keyInfo['rsa']['e']) . 
               sshEncodeBuffer($keyInfo['rsa']['n']);

    return "ssh-rsa " . base64_encode($buffer); 
}

function sshEncodeBuffer($buffer)
{
    $len = strlen($buffer);
    if (ord($buffer[0]) & 0x80) {
        $len++;
        $buffer = "\x00" . $buffer;
    }

    return pack("Na*", $len, $buffer);
}
?>