salt_length (int)– The length of the salt. It is recommended that this be set to
In RFC8017, which specifies PKCS#1 and thus also PSS, the default value of the salt length is defined as the output length of the hash (s. A.2.3. RSASSA-PSS):
For a given
hashAlgorithm, the default value of
saltLengthis the octet length of the hash value.
Most libraries, e.g. PHPSECLIB, apply for the default value of the salt length the default defined in RFC8017, i.e. the output length of the hash (s. here). Therefore the maximum salt length must be set explicitly. The maximum salt length is given by (s. here):
signature length (bytes) - digest output length (bytes) - 2 = 256 - 32 - 2 = 222
for a 2048 bits key and SHA256.
Thus, the verification in the PHP code must be changed as follows:
$verified = $key-> withPadding(RSA::SIGNATURE_PSS)-> //withHash('sha256')-> // default //withMGFHash('sha256')-> // default withSaltLength(256-32-2)-> // set maximum salt length verify($code, pack('H*', $signature)); // alternatively hex2bin()
Note that in the posted code of the question
h (hex string, low nibble first) is specified in the format string of
. I' ve chosen the more common
H (hex string, high nibble first) in my code snippet which is also compatible with Python's
hex(). Ultimately, the format string to choose depends on the encoding applied in the Python code.
Using this change, on my machine, the signature generated with the Python code can be successfully verified with the PHP code.
Alternatively, of course, the salt length of the Python code can be adapted to the output length of the digest (32 bytes in this case).
By the way, a verification with
openssl_verify() is not possible, because PSS is not supported.
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