How to validate a signature

Question

I'm implementing digital signatures to verify data integrity and authenticity. How can I validate a digital signature in a secure way, especially if using a public/private key pair? Are there libraries or tools that simplify this process?

Guidance on validating signatures in popular languages or frameworks would be beneficial for my project.

Answer 1

To guarantee the validity and integrity of data, it is essential to validate a digital signature. Here is a explanation on safely validating a digital signature that includes suggestions for widely used frameworks and languages.

Understanding Digital Signatures

Before diving into validation, let's quickly review the digital signature process using a public/private key pair:

1. Signing:

• Private Key: The sender uses their private key to generate a digital signature from the hashed data (message digest).
• Data + Signature: The sender shares the original data and the digital signature with the recipient.

2. Validation (our focus):

• Public Key: The recipient uses the sender's public key to verify the digital signature.

Secure Validation Steps

1. Obtain the Sender's Public Key:

• Ensure the public key is authentic and trusted (e.g., via a trusted certificate authority or pre-shared knowledge).

2. Recreate the Message Digest:

• Use the same hashing algorithm (e.g., SHA-256, SHA-384) as the sender to hash the received data.

3. Verify the Digital Signature:

• Use the sender's public key and the recreated message digest to verify the digital signature.
• If the verification succeeds, the data is authentic and intact.

Examples

1. Python with cryptography Library

from cryptography.hazmat.primitives import serialization, hashes
from cryptography.hazmat.primitives.asymmetric import padding, rsa
from cryptography.exceptions import InvalidSignature

# Assuming 'public_key' is a loaded RSAPublicKey object
# and 'signature' and 'data' are bytes

def validate_signature(public_key, signature, data):
    try:
        public_key.verify(
            signature,
            data,
            padding.PSS(
                mgf=padding.MGF1(hashes.SHA256()),
                salt_length=padding.PSS.MAX_LENGTH
            ),
            hashes.SHA256()
        )
        print("Signature is valid.")
    except InvalidSignature:
        print("Signature is invalid.")

# Load public key from PEM
with open("path/to/public/key.pem", "rb") as key_file:
    public_key = serialization.load_pem_public_key(
        key_file.read(),
    )

# Example usage
validate_signature(public_key, signature_bytes, data_bytes)

2. Java with java.security Package

import java.security.*;
import java.security.spec.X509EncodedKeySpec;
import java.nio.charset.StandardCharsets;

public class SignatureValidator {

    public static boolean validateSignature(PublicKey publicKey, byte[] signature, String data) {
        try {
            Signature sig = Signature.getInstance("SHA256withRSA");
            sig.initVerify(publicKey);
            sig.update(data.getBytes(StandardCharsets.UTF_8));
            return sig.verify(signature);
        } catch (GeneralSecurityException e) {
            return false;
        }
    }

    public static void main(String[] args) throws Exception {
        // Load public key from X.509 encoded key
        byte[] publicKeyBytes = /* bytes from public key file or source */;
        X509EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(publicKeyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        PublicKey publicKey = keyFactory.generatePublic(publicKeySpec);

        byte[] signatureBytes = /* bytes of the signature */;
        String data = /* original data as string */;

        if (validateSignature(publicKey, signatureBytes, data)) {
            System.out.println("Signature is valid.");
        } else {
            System.out.println("Signature is invalid.");
        }
    }
}