Add a method OpenSSL::PKey#size

[herwinw]

I was trying to migrate an old Perl script to Ruby, which receives a string consisting of a signature and the contents concatenated. The core of the script is like this:

my $sign_key = Crypt::OpenSSL::RSA->new_public_key($key_string);
my $signature = substr($data, 0, $sign_key->size, ''); # Cut off signature of $data
die "data wasn't signed correctly!\n" unless $sign_key->verify($data, $signature);

There does not appear a way to get the size of the signature from Ruby. It should just be a call to EVP_PKEY_get_size

[rhenium]

Currently you can get a PKCS#v1.5 signature size via rsa_pkey.n.num_bytes

I don't think EVP_PKEY provides a common way to determine the exact signature size in a way that would be suitable for such use cases. RSA signatures are straightforward and EVP_PKEY_get_size() returns the exact size, but ECDSA signatures (for example) are variable-length and you'd need to decode the DER encoding to know where to cut off.

The man page https://docs.openssl.org/master/man3/EVP_PKEY_get_size/ says:

EVP_PKEY_get_size() returns the maximum suitable size for the output buffers for almost all operations that can be done with pkey. This corresponds to the provider parameter OSSL_PKEY_PARAM_MAX_SIZE. The primary documented use is with EVP_SignFinal(3) and EVP_SealInit(3), but it isn't limited there. The returned size is also large enough for the output buffer of EVP_PKEY_sign(3), EVP_PKEY_encrypt(3), EVP_PKEY_decrypt(3), EVP_PKEY_derive(3).

It must be stressed that, unless the documentation for the operation that's being performed says otherwise, the size returned by EVP_PKEY_get_size() is only preliminary and not exact, so the final contents of the target buffer may be smaller. It is therefore crucial to take note of the size given back by the function that performs the operation, such as EVP_PKEY_sign(3) (the siglen argument will receive that length), to avoid bugs.

I feel this may be one of the unfortunately named OpenSSL functions which don't behave the way people might expect, but haven't been removed for compatibility reasons.