RSA encryption from hex-encoded public key with phpseclib

Question

I am trying to encrypt a password to sent to through an API for authentication. From the API I can get the public key in this form:

 {   "result": {
 "keyId": "L5gslEaP921gEI34N5JRVRIEbbx78WJN",
 "key": {
   "n": "871db29fbb487b988f2d610d83a1e699c59473d73f6d38efa8d21645524b5fb549d5ffbc6d527b261ff2291cf3b3c81f25a4cb13c801d6f6eb3cae624d74724830f1cb45ec803d8836274f1ddea61ec25d5c44ad7ed0b8a56976291bd06abaa0beb9e1ecbbc59662d20f8ae71191e8cbe617acee2349a64aa20846d4d8910e53",
   "e": "10001"
 }   } }

I need to encrypt a user password and send it back to the API. I am using PHP and this is what I have so far, but am not getting a proper hex encrypted password to send back:

$modulus = "871db29fbb487b988f2d610d83a1e699c59473d73f6d38efa8d21645524b5fb549d5ffbc6d527b261ff2291cf3b3c81f25a4cb13c801d6f6eb3cae624d74724830f1cb45ec803d8836274f1ddea61ec25d5c44ad7ed0b8a56976291bd06abaa0beb9e1ecbbc59662d20f8ae71191e8cbe617acee2349a64aa20846d4d8910e53";
$exponent = "10001";
$plaintext = "********";

include('Crypt/RSA.php');

$rsa = new Crypt_RSA();

$modulus = new Math_BigInteger(base64_decode($modulus), 256);
$exponent = new Math_BigInteger(base64_decode($exponent), 256);

$rsa->loadKey(array('n' => $modulus, 'e' => $exponent));


$rsa->setEncryptionMode(CRYPT_RSA_ENCRYPTION_PKCS1);
$ciphertext = $rsa->encrypt($plaintext);

echo $ciphertext;

How do I properly encode the password given the modulus and exponent from the API?

Nate · Accepted Answer · 2015-10-12 23:33:21Z

1

It would be expected that the ciphertext is binary, not a hex string. A common way to convert it to a hex string is bin2hex().

Of course, this depends on what your API is expecting, but bin2hex() would be a typical way to do it.

echo bin2hex($ciphertext);

And there’s the reverse function, hex2bin().

answered Oct 12, 2015 at 23:33

Nate

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neubert · Accepted Answer · 2015-10-13 15:37:11Z

1

$modulus = "871db29fbb487b988f2d610d83a1e699c59473d73f6d38efa8d21645524b5fb549d5ffbc6d527b261ff2291cf3b3c81f25a4cb13c801d6f6eb3cae624d74724830f1cb45ec803d8836274f1ddea61ec25d5c44ad7ed0b8a56976291bd06abaa0beb9e1ecbbc59662d20f8ae71191e8cbe617acee2349a64aa20846d4d8910e53";
$exponent = "10001";

That's not base64 encoded. That's hex encoded. 10001 == 65537. Try to load your key like this:

$modulus = new Math_BigInteger(base64_decode($modulus), 16);
$exponent = new Math_BigInteger(base64_decode($exponent), 16);

eg. use 16 instead of 256 as the second parameter to the Math_BigInteger constructor.

answered Oct 13, 2015 at 15:37

neubert

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anton2g Over a year ago

Thanks for the suggestion. When I try that, I get an empty string as the output.

anton2g · Accepted Answer · 2015-10-13 20:06:16Z

0

I figured out the solution. It is two parts.

Encode the hex to binary correctly using these two lines:

$modulus = new Math_BigInteger($modulus, 16);
$exponent = new Math_BigInteger($exponent, 16);

And out put the result as a bin to hex as Nate stated.

echo bin2hex($ciphertext);

answered Oct 13, 2015 at 20:06

anton2g

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