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using System;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Digests;
using Org.BouncyCastle.Pqc.Crypto.Ntru.Owcpa;
using Org.BouncyCastle.Pqc.Crypto.Ntru.Polynomials;
using Org.BouncyCastle.Security;
namespace Org.BouncyCastle.Pqc.Crypto.Ntru
{
/// <summary>
/// Encapsulate a secret using NTRU. Returns an <see cref="NtruEncapsulation"/> as encapsulation.
/// </summary>
///
/// <seealso cref="NtruKemExtractor"/>
/// <seealso href="https://ntru.org/">NTRU website</seealso>
public class NtruKemGenerator : IEncapsulatedSecretGenerator
{
private readonly SecureRandom _random;
public NtruKemGenerator(SecureRandom random)
{
_random = random;
}
public ISecretWithEncapsulation GenerateEncapsulated(AsymmetricKeyParameter recipientKey)
{
var parameterSet = ((NtruPublicKeyParameters)recipientKey).GetParameters().ParameterSet;
var sampling = new NtruSampling(parameterSet);
var owcpa = new NtruOwcpa(parameterSet);
Polynomial r;
Polynomial m;
var rm = new byte[parameterSet.OwcpaMsgBytes()];
var rmSeed = new byte[parameterSet.SampleRmBytes()];
_random.NextBytes(rmSeed);
var pair = sampling.SampleRm(rmSeed);
r = pair.R();
m = pair.M();
var rm1 = r.S3ToBytes(parameterSet.OwcpaMsgBytes());
Array.Copy(rm1, 0, rm, 0, rm1.Length);
var rm2 = m.S3ToBytes(rm.Length - parameterSet.PackTrinaryBytes());
Array.Copy(rm2, 0, rm, parameterSet.PackTrinaryBytes(), rm2.Length);
var sha3256 = new Sha3Digest(256);
sha3256.BlockUpdate(rm, 0, rm.Length);
var k = new byte[sha3256.GetDigestSize()];
sha3256.DoFinal(k, 0);
r.Z3ToZq();
var c = owcpa.Encrypt(r, m, ((NtruPublicKeyParameters)recipientKey).PublicKey);
var sharedKey = new byte[parameterSet.SharedKeyBytes];
Array.Copy(k, 0, sharedKey, 0, sharedKey.Length);
Array.Clear(k, 0, k.Length);
return new NtruEncapsulation(sharedKey, c);
}
}
}
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