PHP Manual
OpenSSL Functions
Computes shared secret for public value of remote DH public key and local DH key

openssl_dh_compute_key

(PHP 5 >= 5.3.0, PHP 7, PHP 8)

openssl_dh_compute_key — Computes shared secret for public value of remote DH public key and local DH key

Description

openssl_dh_compute_key(string $public_key, OpenSSLAsymmetricKey $private_key): string|false

The shared secret returned by openssl_dh_compute_key() is often used as an encryption key to secretly communicate with a remote party. This is known as the Diffie-Hellman key exchange.

Note: ECDH is only supported as of PHP 8.1.0 and OpenSSL 3.0.0.

Parameters

public_key: DH Public key of the remote party.
private_key: A local DH private key, corresponding to the public key to be shared with the remote party.

Return Values

Returns shared secret on success or false on failure.

Changelog

Version	Description
8.0.0	`private_key` accepts an OpenSSLAsymmetricKey now; previously, a resource of type `OpenSSL key` was accepted.

Examples

Example #1 Compute a shared secret

First generate a public/private DH keypair locally, and have the remote party do the same. We need to use the openssl command-line utility.

# generate private/public key keypair
openssl dhparam -out dhparam.pem 2048
openssl genpkey -paramfile dhparam.pem -out privatekey.pem

# extract public key only
openssl pkey -in privatekey.pem -pubout -out publickey.pem

Next, send your public key to the remote party. Use the openssl pkey command to view the public key you will be sent from the remote party.

openssl pkey -pubin -in remotepublickey.pem -text -noout

The above example will output something similar to:

PKCS#3 DH Public-Key: (2048 bit)
    public-key:
        67:e5:e5:fa:e0:7b:0f:96:2c:dc:96:44:5f:50:02:
        9e:8d:c2:6c:04:68:b0:d1:1d:75:66:fc:63:f5:e3:
        42:30:b8:96:c1:45:cc:08:60:b4:21:3b:dd:ee:66:
        88:db:77:d9:1e:11:89:d4:5c:f2:7a:f2:f1:fe:1c:
        77:9d:6f:13:b8:b2:56:00:ef:cb:3b:60:79:74:02:
        98:f5:f9:8e:3e:b5:62:08:de:ca:8c:c3:40:4a:80:
        79:d5:43:06:17:a8:19:56:af:cc:95:5e:e2:32:2d:
        d2:14:7b:76:5a:9a:f1:3c:76:76:35:cc:7b:c1:a5:
        f4:39:e5:b6:ca:71:3f:7c:3f:97:e5:ab:86:c1:cd:
        0e:e6:ee:04:c9:e6:2d:80:7e:59:c0:49:eb:b6:64:
        4f:a8:f9:bb:a3:87:b3:3d:76:01:9e:2b:16:94:a4:
        37:30:fb:35:e2:63:be:23:90:b9:ef:3f:46:46:04:
        94:8f:60:79:7a:51:55:d6:1a:1d:f5:d9:7f:4a:3e:
        aa:ac:b0:d0:82:cc:c2:e0:94:e0:54:c1:17:83:0b:
        74:08:4d:5a:79:ae:ff:7f:1c:04:ab:23:39:4a:ae:
        87:83:55:43:ab:7a:7c:04:9d:20:80:bb:af:5f:16:
        a3:e3:20:b9:21:47:8c:f8:7f:a8:60:80:9e:61:77:
        36
 [...abbreviated...]

Use this public key as a parameter to openssl_dh_compute_key() in order to compute the shared secret.


<?php
$remote_public_key = '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';

$local_priv_key = openssl_pkey_get_private('file://privatekey.pem');

$shared_secret = openssl_dh_compute_key(hex2bin($remote_public_key), $local_priv_key);
echo bin2hex($shared_secret)."\n";
?>

Example #2 Generate a DH public/private keypair in php

First, generate the DH prime number

openssl dhparam -out dhparam.pem 2048
openssl dh -in dhparam.pem -noout  -text

The above example will output something similar to:

    PKCS#3 DH Parameters: (2048 bit)
        prime:
            00:a3:25:1e:73:3f:44:b9:2b:ee:f4:9d:9f:37:6a:
            4b:fd:1d:bd:f4:af:da:c8:10:77:59:41:c6:5f:73:
            d2:88:29:39:cd:1c:5f:c3:9f:0f:22:d2:9c:20:c1:
            e4:c0:18:03:b8:b6:d8:da:ad:3b:39:a6:da:8e:fe:
            12:30:e9:03:5d:22:ba:ef:18:d2:7b:69:f9:5b:cb:
            78:c6:0c:8c:6b:f2:49:92:c2:49:e0:45:77:72:b3:
            55:36:30:f2:40:17:89:18:50:03:fa:2d:54:7a:7f:
            34:4c:73:32:b6:88:14:51:14:be:80:57:95:e6:a3:
            f6:51:ff:17:47:4f:15:d6:0e:6c:47:53:72:2c:2a:
            4c:21:cb:7d:f3:49:97:c9:47:5e:40:33:7b:99:52:
            7e:7a:f3:52:27:80:de:1b:26:6b:40:bb:14:11:0b:
            fb:e6:d8:2f:cf:a0:06:2f:96:b9:1c:0b:b4:cb:d3:
            a6:62:9c:48:67:f6:81:f2:c6:ff:45:03:0a:9d:67:
            9d:ce:27:d9:6b:48:5d:ca:fb:c2:5d:84:9b:8b:cb:
            40:c7:a4:0c:8a:6e:f4:ab:ba:b6:10:c3:b8:25:4d:
            cf:60:96:f4:db:e8:00:1c:58:47:7a:fb:51:86:d1:
            22:d7:4e:94:31:7a:d5:da:3d:53:de:da:bb:64:8d:
            62:6b
        generator: 2 (0x2)

Prime and generator values ares passed as p and g into openssl_pkey_new()


<?php
$configargs = array();
$configargs['p'] = hex2bin('00a3251e733f44b92beef49d9f376a4bfd1dbdf4afdac810775941c65f73d2882939cd1c5fc39f0f22d29c20c1e4c01803b8b6d8daad3b39a6da8efe1230e9035d22baef18d27b69f95bcb78c60c8c6bf24992c249e0457772b3553630f2401789185003fa2d547a7f344c7332b688145114be805795e6a3f651ff17474f15d60e6c4753722c2a4c21cb7df34997c9475e40337b99527e7af3522780de1b266b40bb14110bfbe6d82fcfa0062f96b91c0bb4cbd3a6629c4867f681f2c6ff45030a9d679dce27d96b485dcafbc25d849b8bcb40c7a40c8a6ef4abbab610c3b8254dcf6096f4dbe8001c58477afb5186d122d74e94317ad5da3d53dedabb648d626b');
$configargs['g'] = hex2bin('02');
$private_key = openssl_pkey_new(array('dh' => $configargs));
openssl_pkey_export_to_file($private_key,'privatekey.pem',$passphrase='y0urp@s5phr@se');

$details = openssl_pkey_get_details($private_key);
$local_pub_key = $details['dh']['pub_key'];
echo bin2hex($local_pub_key)."\n";//you can send your public key to the remote party

$details = openssl_pkey_get_details(openssl_pkey_get_public("file://remotepublickey.pem"));
$remote_public_key = $details['dh']['pub_key'];
$shared_secret = openssl_dh_compute_key($remote_public_key, $private_key);
echo bin2hex($shared_secret)."\n";
?>