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I wrote my own implementation of the RSA digital signature. I use SHA-512 hashing. I sign the picture in png format. At the output, I get a signature of 512 bytes, is this normal or a bug?

It turns out that it is due to the fact that our hash has a length of 64 bytes, the maximum number generated by me can have a weight of 8 bytes, it turns out 1 byte of the file = 8 bytes of digital signature. The hashToInt function represents an array of bytes as an array of ints (add 256 to negative bytes). Made to adequately perform the ModPow operation

Random random = new Random();

    byte[] byteArray = new byte[8];

    MessageDigest sha512 = MessageDigest.getInstance("SHA-512");
    byte[] array = Files.readAllBytes(Path.of("C:\\Users\\User\\IdeaProjects\\CryptoLab1\\src\\crypto\\dino.png"));
    byte[] hash = sha512.digest(array);
    var hashInt = hashToIntArray(hash);

    FileOutputStream out = new FileOutputStream("C:\\Users\\User\\IdeaProjects\\CryptoLab1\\src\\crypto\\DigitalSignatureRSA.png");

    BigInteger P = BigInteger.probablePrime(32, random);
    BigInteger Q = BigInteger.probablePrime(32, random);

    BigInteger N = P.multiply(Q);
    BigInteger f = (P.subtract(BigInteger.ONE)).multiply(Q.subtract(BigInteger.ONE));

    BigInteger d;
    BigInteger c;
    BigInteger signature;
    BigInteger checkSignature;

    do {
        d = new BigInteger(32, random);
    } while (!(d.gcd(f).equals(BigInteger.ONE)) && d.compareTo(f) < 0);


    c = d.modInverse(f);

    List<BigInteger> digitalSignature = new ArrayList<>();
    List<BigInteger> deDigitalSignature = new ArrayList<>();
    List<byte[]> byteList = new ArrayList<>();

    //create a digital signature
    for (int i = 0; i < hash.length; i++) {
        BigInteger y = BigInteger.valueOf(hashInt[i]);

        signature = cryptoLab1.ModPow(y, c, N);
        digitalSignature.add(signature);
        byteArray = toHH(signature); //toHH - representation of a number in 8 byte format
        byteList.add(byteArray);

        out.write(byteList.get(i));
    }

    // numeric signature verification

    var fileData = Files.readAllBytes(Path.of("C:\\Users\\User\\IdeaProjects\\CryptoLab1\\src\\crypto\\dino.png"));
    var hashFileData = hashToIntArray(sha512.digest(fileData));

    for (int i = 0; i < hash.length; i++) {

        checkSignature = cryptoLab1.ModPow(digitalSignature.get(i), d, N);
        deDigitalSignature.add(checkSignature);
    }

    for(int i = 0; i < hash.length; i++) {
        if(BigInteger.valueOf(hashFileData[i]).equals(deDigitalSignature.get(i))) {
            return true;
        }
    }
    return false;
}
Rume Diablo
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  • Welcome to Stack Overflow. Please take the [tour] to learn how Stack Overflow works and read [ask] on how to improve the quality of your question. Then [edit] your question to include your source code as a working [mcve], which can be compiled and tested by others. – Progman Nov 15 '22 at 11:09
  • You might want to look at other questions like https://crypto.stackexchange.com/questions/3505/what-is-the-length-of-an-rsa-signature and https://stackoverflow.com/questions/6658728/rsa-signature-size – Progman Nov 15 '22 at 11:15
  • *I wrote my own implementation of the RSA digital signature.* Why? – g00se Nov 15 '22 at 12:48
  • Homework at the university. Have I implemented the RSA digital signature correctly? – Rume Diablo Nov 15 '22 at 15:31
  • I can't tell you but there's one easy way to find out: just run a sha512 utility and compare your results ;) – g00se Nov 15 '22 at 19:58

0 Answers0