How to validate a signature

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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.
Nov 11 in Cyber Security & Ethical Hacking by Anupam
• 5,550 points
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1 answer to this question.

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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.");
        }
    }
}
answered Nov 12 by CaLLmeDaDDY
• 8,390 points

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