无法通过Python验证RSASSA-PSS签名-> 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-----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-----END PUBLIC KEY-----"
    //privPem := "-----BEGIN RSA PRIVATE KEY-----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-----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.