Trying to decrypt with aes-256-gcm with php and key [closed] - php

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I wondered whether anyone can help,
I am using encryption method aes-256-gcm, I receive encrypt data, but cannot decrypt.
Below is my code and the issue encountered, can anyone see where I'm going wrong.
Fatal error: Uncaught Exception: OpenSSL error: in /var/www/vhosts/mvisatest.weblogy.net/httpdocs/visad/pushpayment/transactions/receive/p2m/index.php:106 Stack trace: #0 {main} thrown in /var/www/vhosts/mvisatest.weblogy.net/httpdocs/visad/pushpayment/transactions/receive/p2m/index.php on line 106
//$secret_key = '-1FwMVo7r{}B17iqly51zM3zvOAh0/r9636-NOXS'; //Shared Secret
//$secret_iv = 'f0f0b6f6-972e-d338-56a5-1ffbccc77001'; //Key ID
$textToDecrypt = "eyJhbGciOiJBMjU2R0NNS1ciLCJpdiI6IjFrQ1dpTXZKdFNIX3c1ZEwiLCJ0YWciOiJjT3k2TVhTenhaT3lCYi1WdXBfY1lnIiwiZW5jIjoiQTI1NkdDTSIsInR5cCI6IkpPU0UiLCJraWQiOiJmMGYwYjZmNi05NzJlLWQzMzgtNTZhNS0xZmZiY2NjNzcwMDEiLCJjaGFubmVsU2VjdXJpdHlDb250ZXh0IjoiU0hBUkVEX1NFQ1JFVCIsImlhdCI6IjE1ODc1Nzg4OTkifQ.O0y-pu5knc9IUAfAENoMwT_3LwW-oZzzItVxQD8UhHk.uFmZHB7BU9JlKRWC.7ZoJ0oTI8MWXIr4GtzziTw.6qdmvDbnZdi6zf34nhcnoQ";
$encrypted = base64_decode($textToDecrypt);
$password = "-1FwMVo7r{}B17iqly51zM3zvOAh0/r9636-NOXS";
$key = substr(hash('sha256', $password, true), 0, 32);
//$key = hash('sha256', $password, true);
$cipher = "aes-256-gcm";
$iv_len = openssl_cipher_iv_length($cipher);
$tag_length = 16;
//$iv = substr($encrypted, 0, $iv_len);
//$iv = openssl_random_pseudo_bytes($iv_len);
$iv = random_bytes($iv_len);
//$tag = substr($encrypted, $iv_len, $tag_length);
//$ciphertext = substr($encrypted, $iv_len + $tag_length);
$ciphertext = substr($encrypted, $iv_len);
//$decrypted = openssl_decrypt($ciphertext, $cipher, $key, OPENSSL_RAW_DATA, $iv, $tag); //php 7
$decrypted = openssl_decrypt($ciphertext, $cipher, $key, OPENSSL_RAW_DATA, $iv); //php < 7
$data = 4957030013948007;
if (false === $decrypted) {
throw new Exception(sprintf(
"OpenSSL error: %s", openssl_error_string()
));
}
printf ("Decryption %s\n", $data === $decrypted ? 'Ok' : 'Failed');

Answer 1

The data are encrypted with JWE as specified in RFC 7516. They consist of five Base64url encoded parts, each separated by a dot. The first part is the Base64url encoded header, which is Base64url decoded:
{"alg":"A256GCMKW","iv":"1kCWiMvJtSH_w5dL","tag":"cOy6MXSzxZOyBb-Vup_cYg","enc":"A256GCM","typ":"JOSE","kid":"f0f0b6f6-972e-d338-56a5-1ffbccc77001","channelSecurityContext":"SHARED_SECRET","iat":"1587578899"}
alg describes the algorithm used to encrypt the key and is A256GCMKW, which means Key wrapping with AES GCM using 256-bit key. iv and tag contain the IV and tag, respectively.
The second part is the Base64url encoded ciphertext, which contains the encrypted key and which must be decrypted in the first step. As in the posted code, the key for this decryption is derived from the password. The following code shows this first step:
// base64url_decode from: https://www.php.net/manual/de/function.base64-encode.php#121767
function base64url_decode( $data ){
return base64_decode(strtr( $data, '-_', '+/') . str_repeat('=', 3 - ( 3 + strlen( $data )) % 4 ));
}
$textToDecrypt = "eyJhbGciOiJBMjU2R0NNS1ciLCJpdiI6IjFrQ1dpTXZKdFNIX3c1ZEwiLCJ0YWciOiJjT3k2TVhTenhaT3lCYi1WdXBfY1lnIiwiZW5jIjoiQTI1NkdDTSIsInR5cCI6IkpPU0UiLCJraWQiOiJmMGYwYjZmNi05NzJlLWQzMzgtNTZhNS0xZmZiY2NjNzcwMDEiLCJjaGFubmVsU2VjdXJpdHlDb250ZXh0IjoiU0hBUkVEX1NFQ1JFVCIsImlhdCI6IjE1ODc1Nzg4OTkifQ.O0y-pu5knc9IUAfAENoMwT_3LwW-oZzzItVxQD8UhHk.uFmZHB7BU9JlKRWC.7ZoJ0oTI8MWXIr4GtzziTw.6qdmvDbnZdi6zf34nhcnoQ";
$parts = explode(".",$textToDecrypt);
// Header, IV, tag
$headerB64 = $parts[0];
$header = base64url_decode($headerB64);
$headerJSON = json_decode($header);
$ivKW = base64url_decode($headerJSON->iv);
$tagKW = base64url_decode($headerJSON->tag);
// Ciphertext
$ciphertextKW = base64url_decode($parts[1]);
// Key derivation via password (as already supplied in the posted code)
$password = "-1FwMVo7r{}B17iqly51zM3zvOAh0/r9636-NOXS";
$keyKW = substr(hash('sha256', $password, true), 0, 32);
// Decrypt key
$decryptedKey = openssl_decrypt($ciphertextKW, "aes-256-gcm", $keyKW, OPENSSL_RAW_DATA, $ivKW, $tagKW);
print("Decrypted key (hex): " . bin2hex($decryptedKey) . "\n");
Now, in the second step, the actual ciphertext can be decrypted using the decrypted key. The third, fourth and fifth part are, each Base64url encoded, the IV, the actual ciphertext and the tag, respectively. The encryption algorithm is specified in the parameter enc of the header and is A256GCM, which means AES GCM using 256-bit key. In this step the Base64url encoded header must be used as AAD:
// IV, ciphertext, tag
$ivCT = base64url_decode($parts[2]);
$ciphertextCT = base64url_decode($parts[3]);
$tagCT = base64url_decode($parts[4]);
// Decrypt ciphertext
$decryptedText = openssl_decrypt($ciphertextCT, "aes-256-gcm", $decryptedKey, OPENSSL_RAW_DATA, $ivCT, $tagCT, $headerB64);
print("Decrypted ciphertext: " . $decryptedText . "\n");
This gives 4957030013948007 as plaintext, matching the expected result.