Computes shared secret for public value of remote and local DH or ECDH key

# openssl_pkey_derive

(PHP 7 >= 7.3.0, PHP 8)

openssl_pkey_derive — Computes shared secret for public value of remote and local DH or ECDH key

### Description

openssl_pkey_derive(OpenSSLAsymmetricKey|OpenSSLCertificate|array|string `$public_key`, #[\SensitiveParameter] OpenSSLAsymmetricKey|OpenSSLCertificate|array|string `$private_key`, int `$key_length` = 0): string|false

openssl_pkey_derive() takes a set of a `public_key` and `private_key` and derives a shared secret, for either DH or EC keys.

### Parameters

`public_key`

  `public_key` is the public key for the derivation. See Public/Private Key parameters for a list of valid values.

`private_key`

  `private_key` is the private key for the derivation. See Public/Private Key parameters for a list of valid values.

`key_length`

  If not zero, will attempt to set the desired length of the derived secret.

  Caution

  This parameter is deprecated and should not be used, as it does not work as expected. It never returns a secret longer than the size of the prime. If the desired length is smaller than the size of the prime, it truncates the length only for ECDH keys but fails for DH keys.

### Return Values

The derived secret on success or `false` on failure.

### Changelog

Version | Description
--- | ---
8.5.0 | The parameter `key_length` is now deprecated.

### Examples

Example #1 openssl_pkey_derive() example

`<?php
// Load in private key
$priv = openssl_pkey_get_private("-----BEGIN PRIVATE KEY-----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-----END PRIVATE KEY-----
");
// Load in public key
$pub = openssl_pkey_get_public("-----BEGIN PUBLIC KEY-----
MIICJDCCARcGCSqGSIb3DQEDATCCAQgCggEBAJLxRCaZ933uW+AXmabHFDDyupoj
BIRlbmQLJZfigDaSA1f9YOTsIv+WwVFTX/J1mtCyx9uBcz0Nt2kmVwxWuc2fVtCE
MPsmLsVXX7xRUFLpyX1Y1IYGBVXQOoOvLWYQjpZgnx47Pkh1Ok1+smffztfC0DCN
t4KorWrbsPcmqBejXHN79KvWFjZmXOksRiNu/Bn76RiqvofC4z8Ri3kHXQG2197J
GZzzFXHadGC3xbkg8UxsNbYhVMKbm0iANfafUH7/hoS9UjAVQYtvwe7YNiW/Hnyf
VCrKwcc7sadd8Iphh+3lf5P1AhaQEAMytanrzq9RDXKBxuvpSJifRYasZYsCAQID
ggEFAAKCAQAiCSBpxvGgsTorxAWtcAlSmzAJnJxFgSPef0g7OjhESytnc8G2QYmx
ovMt5KVergcitztWh08hZQUdAYm4rI+zMlAFDdN8LWwBT/mGKSzRkWeprd8E7mvy
ucqC1YXCMqmIwPySvLQUB/Dl8kgau7BLAnIJm8VP+MVrn8g9gghD0qRCgPgtEaDV
vocfgnOU43rhKnIgO0cHOKtw2qybSFB8QuZrYugq4j8Bwkrzh6rdMMeyMl/ej5Aj
c0wamOzuBDtXt0T9+Fx3khHaowjCc7xJZRgZCxg43SbqMWJ9lUg94I7+LTX61Gyv
dtlkbGbtoDOnxeNnN93gwQZngGYZYciu
-----END PUBLIC KEY-----
");
// Outputs the hex version of the derived key
echo bin2hex(openssl_pkey_derive($pub,$priv));`
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