Decrypt a signature using openssl_public_decrypt - php

Question

I'm trying to verify an external call to one of our endpoints, this endpoint is triggered by a third party, we receive a transaction data and a signature based on that transaction information, with that, we need to decrypt the signature and compare the result to verify the authenticity.
I'm trying to use openssl_public_decrypt to decrypt the signature using the provider's public key.
This is how I'm trying:
$signature = 'GcTtinhU0YgwGbZPtBwLdh+zdEe0w0W95TFPggeHMCjeDUBWgZfCZ6ZDRUk7DfT5BkKsbAi8/4o60Krcwz1JMdRjmsPf7vj33heVIB2PZJaf8eFR1jijLIsyl4vgH7BbbQ2I6kk6IcYXYWPVAHYRWxl1pJwOyNxZPr49fdW+hcw2zbpkEmj2114QBSiV6eHLowVYKLvpuiT8zLc6DN/wVzCYBuR/cg+CPHgYMeWFsuvu9J46hm6Hij00E68ldYAqVwImlmHPqfqvdEItg3Oi0ac4tXH2nCNgLPHcyU/H32NzTYC9iT1YZkoInqsU6Qv64vbU9lSMS91EQBEa5UQkUg==';
$pubKey = openssl_pkey_get_public('file://path/to/public.pem');
if( openssl_public_decrypt(base64_decode($signature), $data, $pubKey)){
echo $data;
}else{
echo 'Error';
}
I don't get any error but the $data value is not what I expect, is something like this
v_~�#&�W��q�&Ș�uQ���֔�
I'm sure I'm missing something but I can't find out what is it, due to the $data value looks like is encrypted.
The result that I expect from the decrypt is 167619085f7ed94026e357930b18dc011971f226c898ef7551cdf6ec9ad694cf this is the result of the following code
$canonical = 'c328e942-8be8-4104-abbe-048254f893dc|9687|2874.30|52409|BP1381|550bd8439cd1f41691671cdd4e8c6ae6';
$hashed = hash('sha256', $canonical);
That last part is how the provider generates the signature.
For the given example, canonic form is as follows:
cec4b9bf-5a39-4bd7-bc8b826ebc18208d|Internal_0005|12|39679|BP7610|947d589a40dece13c28f2b63c41ae451
We sign the response by hashing the canonic form with SHA-256 and encrypting the
resulting bytes with our private key.
RSA_ENCRYPT(SHA256(canonicForm), privkey.key)
To verify the payload, you must recalculate the canonic form and apply SHA-256 to the
result. The resulting value must be compared with the result of decrypting the signature
parameter with our public key.
Any hint would be appreciated.

Answer 1

perhaps post the public key and some valid test data so we can test ourselves?
anyway, v_~�#&�W��q�&Ș�uQ���֔� could be a valid signature, remember that SHA256 is 256 random bits, it's binary data, not ascii data, not hex, and not printable. SHA256 is also exactly 32 bytes long (256 bits, and 1 byte is 8 bits, and 256/8 is 32 bytes), so if you run var_dump(strlen($data)) after decryption, it should print 32, if it does not print 32, it implies they're using a padding scheme, try checking the strlen of both OPENSSL_PKCS1_PADDING and OPENSSL_NO_PADDING , when you get the correct padding scheme, strlen($data) after decryption should be int(32)
but my best guess is:
$signature = 'GcTtinhU0YgwGbZPtBwLdh+zdEe0w0W95TFPggeHMCjeDUBWgZfCZ6ZDRUk7DfT5BkKsbAi8/4o60Krcwz1JMdRjmsPf7vj33heVIB2PZJaf8eFR1jijLIsyl4vgH7BbbQ2I6kk6IcYXYWPVAHYRWxl1pJwOyNxZPr49fdW+hcw2zbpkEmj2114QBSiV6eHLowVYKLvpuiT8zLc6DN/wVzCYBuR/cg+CPHgYMeWFsuvu9J46hm6Hij00E68ldYAqVwImlmHPqfqvdEItg3Oi0ac4tXH2nCNgLPHcyU/H32NzTYC9iT1YZkoInqsU6Qv64vbU9lSMS91EQBEa5UQkUg==';
$canonical = 'c328e942-8be8-4104-abbe-048254f893dc|9687|2874.30|52409|BP1381|550bd8439cd1f41691671cdd4e8c6ae6';
$pubKey = openssl_pkey_get_public('file://path/to/public.pem');
if( openssl_public_decrypt(base64_decode($signature), $data, $pubKey)){
echo "signature decryption success! ";
if(hash_equals(hash("sha256",$canonical,true),$data)){
echo "checksum verification success!";
} else{
echo "checksum verification failed (after decryption was successful..)";
}
}else{
echo 'checksum decryption error';
}
but again, experiment with both
if( openssl_public_decrypt(base64_decode($signature), $data, $pubKey, OPENSSL_PKCS1_PADDING)){
and
if( openssl_public_decrypt(base64_decode($signature), $data, $pubKey, OPENSSL_NO_PADDING)){
1 of them is probably correct (and when it is correct, var_dump(strlen($data)) should print int(32) )