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I want to implement an ElGamal encryption from this link https://csharp.hotexamples.com/examples/Portable.Licensing.Security.Cryptography.ElGamal/ElGamalKeyStruct/-/php-elgamalkeystruct-class-examples.html

This is what I have tried so far and have no luck

string lic = "abcdefghijklmnopqrstuvwxyz";
byte[] licenseKey = Encoding.UTF8.GetBytes(lic);

ElGamalKeyStruct key = new ElGamalKeyStruct();
key.P = new BigInteger(123567890);
key.G = new BigInteger(1234567890);
key.Y = new BigInteger(1234567890);
key.X = new BigInteger(0); //zero

byte[] signature = CreateSignature(licenseKey, key);
bool verified = VerifySignature(licenseKey, signature, key);

The VerifySignature always returns false.

derodevil
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1 Answers1

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P must be prime. See here (that is the ebook from which the code you use was taken):

Create a random prime number, p. This number is the ElGamal "modulus." The number of bits required to represent p is the size of the public key, so that if p is represented using 1024 bits, then the key created by following the protocol is a 1024-bit key. We will select 607 as our example value.

If you look in the text then there is code to generate El Gamal keys (the CreateKeyPair() method)

xanatos
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  • I know that. The number 123456789 is just an example, I have ones that generated from `BigInteger`. I cannot create P, G, Y randomly because it's the main value of my license key – derodevil Jul 19 '18 at 15:34
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    @derodevil: Your example is not very meaningful because you picked values that don't meet the specifications of El Gamal. – President James K. Polk Jul 19 '18 at 15:41
  • @derodevil If you look at the [example](https://github.com/sven-s/Portable.Licensing) on the github of `Portable.Licensing` you'll see that they generate a completely random key (`Portable.Licensing.Security.Cryptography.KeyGenerator.Create()`) and only then they use it to sign the license (`CreateAndSignWithPrivateKey`) – xanatos Jul 19 '18 at 15:43