Fail to verify RSASSA-PSS signature across Python -> Go

Question

I've boiled this down to the simplest test case I can. I need to take RSASSA-PSS signatures that were generated in Python and validate them in Go. The Python code to create the RSA keypair and sign with it is as follows:

>>> from tuf import pycrypto_keys as k
>>> pub, priv = k.generate_rsa_public_and_private()
>>> sig, method = k.create_rsa_signature(priv, "The quick brown fox jumps over the lazy dog.")
>>> sig.encode("hex")
'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'

The pycrypto_keys library referenced there can be found here for reference of the specific implementation of the functions generate_rsa_public_and_private and create_rsa_signature.

My Go test consists of 2 simple files that only rely on core packages. First the verification function, in verify.go:

package example

import (
    "crypto"
    "crypto/rsa"
    "crypto/sha256"
    "crypto/x509"
    "fmt"
)

func Verify(key []byte, sig []byte, msg []byte) error {
    digest := sha256.Sum256(msg)

    pub, err := x509.ParsePKIXPublicKey(key)
    if err != nil {
        return fmt.Errorf("Failed to parse key")
    }

    rsaPub, ok := pub.(*rsa.PublicKey)
    if !ok {
        return fmt.Errorf("Invalid value returned from ParsePKIXPublicKey")
    }

    opts := rsa.PSSOptions{SaltLength: 16, Hash: crypto.SHA256}
    if err = rsa.VerifyPSS(rsaPub, crypto.SHA256, digest[:], sig, &opts); err != nil {
        return fmt.Errorf("Failed Verification")
    }
    return nil
}

And second, a test case. The key pair and signature were generated using the Python commands at the top and copied in here to create a static set of parameters to use for compatibility testing.

package example

import (
    "encoding/hex"
    "encoding/pem"
    "testing"
)

func TestPyCryptoRSACompatVerify(t *testing.T) {
    pubPem := "-----BEGIN PUBLIC KEY-----\nMIIBojANBgkqhkiG9w0BAQEFAAOCAY8AMIIBigKCAYEAnKuXZeefa2LmgxaL5NsM\nzKOHNe+x/nL6ik+lDBCTV6OdcwAhHQS+PONGhrChIUVR6Vth3hUCrreLzPO73Oo5\nVSCuRJ53UronENl6lsa5mFKP8StYLvIDITNvkoT3j52BJIjyNUK9UKY9As2TNqDf\nBEPIRp28ev/NViwGOEkBu2UAbwCIdnDXm8JQErCZA0Ydm7PKGgjLbFsFGrVzqXHK\n6pdzJXlhr9yap3UpgQ/iO9JtoEYB2EXsnSrPc9JRjR30bNHHtnVql3fvinXrAEwq\n3xmN4p+R4VGzfdQN+8Kl/IPjqWB535twhFYEG/B7Ze8IwbygBjK3co/KnOPqMUrM\nBI8ztvPiogz+MvXb8WvarZ6TMTh8ifZI96r7zzqyzjR1hJulEy3IsMGvz8XS2J0X\n7sXoaqszEtXdq5ef5zKVxkiyIQZcbPgmpHLq4MgfdryuVVc/RPASoRIXG4lKaTJj\n1ANMFPxDQpHudCLxwCzjCb+sVa20HBRPTnzo8LSZkI6jAgMBAAE=\n-----END PUBLIC KEY-----"
    //privPem := "-----BEGIN RSA PRIVATE KEY-----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\n/qNP5JetY0eH6C+KRc8JqDB0nvbqZNOgYXOfYXo/5Gk8XIHTFihm\n-----END RSA PRIVATE KEY-----"
    testStr := "The quick brown fox jumps over the lazy dog."
    sigHex := "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"
    testKey, _ := pem.Decode([]byte(pubPem))

    sigBytes, err := hex.DecodeString(sigHex)
    if err != nil {
        t.Fatal(err)
    }
    err = Verify(testKey.Bytes, sigBytes, []byte(testStr))
    if err != nil {
        t.Fatal(err)
    }
}

The code fails in the final check of the Verify function, outputting the error Failed verification. I've had a look at the Go PSS verification code and if anything goes wrong, a generic verification error is returned so it's not that helpful. Even if it was more specific, either there is a compatibility problem here, a bug in one of the language implementations, or my code is wrong.

Much thanks for any help.

Answer 1

So it appears I misinterpreted _SALT_SIZE in the Python code. With some help from the TUF developers (i.e. they told me) I discovered PyCrypto's RSA PSS code infers the salt size to be equal to the size of the hash digest of the hashing algorithm in use. In this case it was 32 bytes as the hashing algorithm was Sha256. With just the change of opts := rsa.PSSOptions{SaltLength: 32, Hash: crypto.SHA256} the code works.