Generating a symmetric key in php

Question

Im trying to generate a symmetric key, which i can use to both encrypt and decrypt a message with the same key, in php.

I've made my way through the php documentation below but can only find public/private key generations: http://www.php.net/manual/en/function.openssl-pkey-new.php

Any ideas on how i can use these key generating functions for making a symmetric key?

Thanks, Joe

There are special functions for generating public/private keypairs because the two keys have a special mathematical relationship to each other, but a symmetric key is ideally just a random number with no mathematical relationship to anything. So you want to look at functions for generating cryptographic-quality random numbers. — Wyzard
– Wyzard, Commented Feb 23, 2014 at 15:57

Community · Accepted Answer · 2023-11-21 15:10:29Z

1

A symetric key is basically a random set of bytes. If you want to use the openssl extension use openssl_random_pseudo_bytes():

$symetric_key = openssl_random_pseudo_bytes($length);

edited Nov 21, 2023 at 15:10

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answered Feb 23, 2014 at 15:51

hek2mgl

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avpaderno · Accepted Answer · 2024-03-03 23:10:51Z

0

To generate a symmetric key you can either use random_bytes() (starting from PHP 7) or openssl_random_pseudo_bytes(). Using the OpenSSL extension to crypt data with a symmetric key, you can use openssl_cipher_key_length() to know the required key length.

$cipher = "aes-256-gcm";
$plaintext = "Attack at dawn!";

if (in_array($cipher, openssl_get_cipher_methods())) {
  $iv = openssl_random_pseudo_bytes(openssl_cipher_iv_length($cipher));
  $key = openssl_random_pseudo_bytes(openssl_cipher_key_length($cipher));
  $ciphertext = openssl_encrypt($plaintext, $cipher, $key, OPENSSL_RAW_DATA, $iv, $tag);
}

answered Mar 3, 2024 at 23:10

avpaderno

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