build public key PEM file with existing modulus and exponent in php with openssl library

Question

I have public's key's modulus and exponent. Can I generate the public key with openssl api in php ? I've read the http://php.net/manual/en/ref.openssl.php but there seems no function that archive that.

If openssl cannot do it(build public key with modulus and exponent), can i use any any option in php to do that?

I am not a PHP expert. But this approach is not good. It is generally advisable to change the public key and private key after certain period of the time. Generally certificate has validity period. If you key's modulus and exponent, you have public key (this is what RSA public key means). If you need a certificate, you need private key to generate CSR. — doptimusprime, Apr 01 '13 at 05:47

neubert · Answer 1 · 2013-04-05T15:48:38.973

Using phpseclib, a pure PHP RSA implementation:

<?php
include('Crypt/RSA.php');

$rsa = new Crypt_RSA();

$rsa->loadKey( array ('e'=>new Math_BigInteger('65537'), 'n'=>new Math_BigInteger('11944573237954459805614520005393273287786384679965238498958830373752732874397055'.
'98832111464872863171681422024469555089029360351247093972211786644957214280299847'.
'26868375359168203283442617134197706515425366188396513684446494070223079865755643'.
'116690165578452542158755074958452695530623055205290232290667934914919')) );

echo $rsa;

The output is as follows:

score 1 · Answer 2 · edited Oct 07 '21 at 11:39

If your modulus and exponent came from a RSA key in XML format or you are retrieving the key from a JWKS endpoint you can use the following code:

use phpseclib\Crypt\RSA;

$n = "6R-qAvAPQNrx8I_Q7GmQE4zpKFBpKMM38yeFE797v9eLBdXr9JIW5PBuBqjkmzMug3MUvl4FM6_nU0SCMLDMSY_zGIt5xlmFF5iqRb8elTZ_bLe6bp_1cZncadl4vzRdi6jQE_aIZX_9CWypO4su-kNj6DVCh5wldbGZej0c-1sKCgkJF12hiAqQPxaIA4hDgDCFiCRWSPzxHRuS-aXV0h3DIhn84lCkzrBqWea-nS03z0k2ohmuNsTd0yIPz_s-R2TkE-lUaKVtKGqsRPL3GKzQJO4TUzYmqHkN8XyNkmvBaitWreiZjyX79qTJ46SNX55gzVAfWqevpJc0RezVsQ==";
$e = "AQAB";

// Convert Base64 uri-safe variant to default (needed for JWKS)
$n = strtr($n, '-_', '+/');

$rsa = new RSA();

$key = "<RSAKeyPair>"
        . "<Modulus>" . $n . "</Modulus>"
        . "<Exponent>" . $e . "</Exponent>"
        . "</RSAKeyPair>";

$rsa->loadKey($key, RSA::PUBLIC_FORMAT_XML);

echo $rsa;

build public key PEM file with existing modulus and exponent in php with openssl library

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