package org.bouncycastle.crypto.generators;

import java.math.BigInteger;
import java.security.SecureRandom;

import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.digests.SHA1Digest;
import org.bouncycastle.crypto.params.DSAParameterGenerationParameters;
import org.bouncycastle.crypto.params.DSAParameters;
import org.bouncycastle.crypto.params.DSAValidationParameters;
import org.bouncycastle.util.Arrays;
import org.bouncycastle.util.BigIntegers;
import org.bouncycastle.util.encoders.Hex;

/**
 * Generate suitable parameters for DSA, in line with FIPS 186-2, or FIPS 186-3.
 */
public class DSAParametersGenerator
{
    private static final BigInteger ZERO = BigInteger.valueOf(0);
    private static final BigInteger ONE = BigInteger.valueOf(1);
    private static final BigInteger TWO = BigInteger.valueOf(2);

    private Digest          digest;
    private int             L, N;
    private int             certainty;
    private int             iterations;
    private SecureRandom    random;
    private boolean         use186_3;
    private int             usageIndex;

    public DSAParametersGenerator()
    {
        this(new SHA1Digest());
    }

    public DSAParametersGenerator(Digest digest)
    {
        this.digest = digest;
    }

    /**
     * initialise the key generator.
     *
     * @param size size of the key (range 2^512 -&gt; 2^1024 - 64 bit increments)
     * @param certainty measure of robustness of prime (for FIPS 186-2 compliance this should be at least 80).
     * @param random random byte source.
     */
    public void init(
        int             size,
        int             certainty,
        SecureRandom    random)
    {
        this.L = size;
        this.N = getDefaultN(size);
        this.certainty = certainty;
        this.iterations = Math.max(getMinimumIterations(L), (certainty + 1) / 2);
        this.random = random;
        this.use186_3 = false;
        this.usageIndex = -1;
    }

    /**
     * Initialise the key generator for DSA 2.
     * <p>
     *     Use this init method if you need to generate parameters for DSA 2 keys.
     * </p>
     *
     * @param params  DSA 2 key generation parameters.
     */
    public void init(
        DSAParameterGenerationParameters params)
    {
        int L = params.getL(), N = params.getN();

        if ((L < 1024 || L > 3072) || L % 1024 != 0)
        {
            throw new IllegalArgumentException("L values must be between 1024 and 3072 and a multiple of 1024");
        }
        else if (L == 1024 && N != 160)
        {
            throw new IllegalArgumentException("N must be 160 for L = 1024");
        }
        else if (L == 2048 && (N != 224 && N != 256))
        {
            throw new IllegalArgumentException("N must be 224 or 256 for L = 2048");
        }
        else if (L == 3072 && N != 256)
        {
            throw new IllegalArgumentException("N must be 256 for L = 3072");
        }

        if (digest.getDigestSize() * 8 < N)
        {
            throw new IllegalStateException("Digest output size too small for value of N");
        }

        this.L = L;
        this.N = N;
        this.certainty = params.getCertainty();
        this.iterations = Math.max(getMinimumIterations(L), (certainty + 1) / 2);
        this.random = params.getRandom();
        this.use186_3 = true;
        this.usageIndex = params.getUsageIndex();
    }

    /**
     * which generates the p and g values from the given parameters,
     * returning the DSAParameters object.
     * <p>
     * Note: can take a while...
     */
    public DSAParameters generateParameters()
    {
        return (use186_3)
            ? generateParameters_FIPS186_3()
            : generateParameters_FIPS186_2();
    }

    private DSAParameters generateParameters_FIPS186_2()
    {
        byte[]          seed = new byte[20];
        byte[]          part1 = new byte[20];
        byte[]          part2 = new byte[20];
        byte[]          u = new byte[20];
        int             n = (L - 1) / 160;
        byte[]          w = new byte[L / 8];

        if (!(digest instanceof SHA1Digest))
        {
            throw new IllegalStateException("can only use SHA-1 for generating FIPS 186-2 parameters");
        }

        for (;;)
        {
            random.nextBytes(seed);

            hash(digest, seed, part1, 0);
            System.arraycopy(seed, 0, part2, 0, seed.length);
            inc(part2);
            hash(digest, part2, part2, 0);

            for (int i = 0; i != u.length; i++)
            {
                u[i] = (byte)(part1[i] ^ part2[i]);
            }

            u[0] |= (byte)0x80;
            u[19] |= (byte)0x01;

            BigInteger q = new BigInteger(1, u);

            if (!isProbablePrime(q))
            {
                continue;
            }

            byte[] offset = Arrays.clone(seed);
            inc(offset);

            for (int counter = 0; counter < 4096; ++counter)
            {
                {
                    for (int k = 1; k <= n; k++)
                    {
                        inc(offset);
                        hash(digest, offset, w, w.length - k * part1.length);
                    }

                    int remaining = w.length - (n * part1.length);
                    inc(offset);
                    hash(digest, offset, part1, 0);
                    System.arraycopy(part1, part1.length - remaining, w, 0, remaining);

                    w[0] |= (byte)0x80;
                }

                BigInteger x = new BigInteger(1, w);

                BigInteger c = x.mod(q.shiftLeft(1));

                BigInteger p = x.subtract(c.subtract(ONE));

                if (p.bitLength() != L)
                {
                    continue;
                }

                if (isProbablePrime(p))
                {
                    BigInteger g = calculateGenerator_FIPS186_2(p, q, random);

                    return new DSAParameters(p, q, g, new DSAValidationParameters(seed, counter));
                }
            }
        }
    }

    private static BigInteger calculateGenerator_FIPS186_2(BigInteger p, BigInteger q, SecureRandom r)
    {
        BigInteger e = p.subtract(ONE).divide(q);
        BigInteger pSub2 = p.subtract(TWO);

        for (;;)
        {
            BigInteger h = BigIntegers.createRandomInRange(TWO, pSub2, r);
            BigInteger g = h.modPow(e, p);
            if (g.bitLength() > 1)
            {
                return g;
            }
        }
    }

    /**
     * generate suitable parameters for DSA, in line with
     * <i>FIPS 186-3 A.1 Generation of the FFC Primes p and q</i>.
     */
    private DSAParameters generateParameters_FIPS186_3()
    {
// A.1.1.2 Generation of the Probable Primes p and q Using an Approved Hash Function
        // FIXME This should be configurable (digest size in bits must be >= N)
        Digest d = digest;
        int outlen = d.getDigestSize() * 8;

// 1. Check that the (L, N) pair is in the list of acceptable (L, N pairs) (see Section 4.2). If
//    the pair is not in the list, then return INVALID.
        // Note: checked at initialisation

// 2. If (seedlen < N), then return INVALID.
        // FIXME This should be configurable (must be >= N)
        int seedlen = N;
        byte[] seed = new byte[seedlen / 8];

// 3. n = ceiling(L / outlen) - 1.
        int n = (L - 1) / outlen;

// 4. b = L - 1 - (n * outlen).
        int b = (L - 1) % outlen;

        byte[] w = new byte[L / 8];
        byte[] output = new byte[d.getDigestSize()];
        for (;;)
        {
// 5. Get an arbitrary sequence of seedlen bits as the domain_parameter_seed.
            random.nextBytes(seed);

// 6. U = Hash (domain_parameter_seed) mod 2^(N–1).
            hash(d, seed, output, 0);

            BigInteger U = new BigInteger(1, output).mod(ONE.shiftLeft(N - 1));

// 7. q = 2^(N–1) + U + 1 – ( U mod 2).
            BigInteger q = U.setBit(0).setBit(N - 1);

// 8. Test whether or not q is prime as specified in Appendix C.3.
            if (!isProbablePrime(q))
            {
// 9. If q is not a prime, then go to step 5.
                continue;
            }

// 10. offset = 1.
            // Note: 'offset' value managed incrementally
            byte[] offset = Arrays.clone(seed);

// 11. For counter = 0 to (4L – 1) do
            int counterLimit = 4 * L;
            for (int counter = 0; counter < counterLimit; ++counter)
            {
// 11.1 For j = 0 to n do
//      Vj = Hash ((domain_parameter_seed + offset + j) mod 2^seedlen).
// 11.2 W = V0 + (V1 ∗ 2^outlen) + ... + (V^(n–1) ∗ 2^((n–1) ∗ outlen)) + ((Vn mod 2^b) ∗ 2^(n ∗ outlen)).
                {
                    for (int j = 1; j <= n; ++j)
                    {
                        inc(offset);
                        hash(d, offset, w, w.length - j * output.length);
                    }

                    int remaining = w.length - (n * output.length);
                    inc(offset);
                    hash(d, offset, output, 0);
                    System.arraycopy(output, output.length - remaining, w, 0, remaining);

// 11.3 X = W + 2^(L–1). Comment: 0 ≤ W < 2^(L–1); hence, 2^(L–1) ≤ X < 2^L.
                    w[0] |= (byte)0x80;
                }

                BigInteger X = new BigInteger(1, w);
 
// 11.4 c = X mod 2q.
                BigInteger c = X.mod(q.shiftLeft(1));

// 11.5 p = X - (c - 1). Comment: p ≡ 1 (mod 2q).
                BigInteger p = X.subtract(c.subtract(ONE));

// 11.6 If (p < 2^(L-1)), then go to step 11.9
                if (p.bitLength() != L)
                {
                    continue;
                }

// 11.7 Test whether or not p is prime as specified in Appendix C.3.
                if (isProbablePrime(p))
                {
// 11.8 If p is determined to be prime, then return VALID and the values of p, q and
//      (optionally) the values of domain_parameter_seed and counter.
                    if (usageIndex >= 0)
                    {
                        BigInteger g = calculateGenerator_FIPS186_3_Verifiable(d, p, q, seed, usageIndex);
                        if (g != null)
                        {
                           return new DSAParameters(p, q, g, new DSAValidationParameters(seed, counter, usageIndex));
                        }
                    }

                    BigInteger g = calculateGenerator_FIPS186_3_Unverifiable(p, q, random);

                    return new DSAParameters(p, q, g, new DSAValidationParameters(seed, counter));
                }

// 11.9 offset = offset + n + 1.      Comment: Increment offset; then, as part of
//                                    the loop in step 11, increment counter; if
//                                    counter < 4L, repeat steps 11.1 through 11.8.
                // Note: 'offset' value already incremented in inner loop
            }
// 12. Go to step 5.
        }
    }

    private boolean isProbablePrime(BigInteger x)
    {
        /*
         * TODO Use Primes class for FIPS 186-4 C.3 primality checking - but it breaks existing
         * tests using FixedSecureRandom
         */
//        return !Primes.hasAnySmallFactors(x) && Primes.isMRProbablePrime(x, random, iterations);
        return x.isProbablePrime(certainty);
    }

    private static BigInteger calculateGenerator_FIPS186_3_Unverifiable(BigInteger p, BigInteger q,
        SecureRandom r)
    {
        return calculateGenerator_FIPS186_2(p, q, r);
    }

    private static BigInteger calculateGenerator_FIPS186_3_Verifiable(Digest d, BigInteger p, BigInteger q,
        byte[] seed, int index)
    {
// A.2.3 Verifiable Canonical Generation of the Generator g
        BigInteger e = p.subtract(ONE).divide(q);
        byte[] ggen = Hex.decode("6767656E");

        // 7. U = domain_parameter_seed || "ggen" || index || count.
        byte[] U = new byte[seed.length + ggen.length + 1 + 2];
        System.arraycopy(seed, 0, U, 0, seed.length);
        System.arraycopy(ggen, 0, U, seed.length, ggen.length);
        U[U.length - 3] = (byte)index;

        byte[] w = new byte[d.getDigestSize()];
        for (int count = 1; count < (1 << 16); ++count)
        {
            inc(U);
            hash(d, U, w, 0);
            BigInteger W = new BigInteger(1, w);
            BigInteger g = W.modPow(e, p);
            if (g.compareTo(TWO) >= 0)
            {
                return g;
            }
        }

        return null;
    }

    private static void hash(Digest d, byte[] input, byte[] output, int outputPos)
    {
        d.update(input, 0, input.length);
        d.doFinal(output, outputPos);
    }

    private static int getDefaultN(int L)
    {
        return L > 1024 ? 256 : 160;
    }

    private static int getMinimumIterations(int L)
    {
        // Values based on FIPS 186-4 C.3 Table C.1
        return L <= 1024 ? 40 : (48 + 8 * ((L - 1) / 1024)); 
    }

    private static void inc(byte[] buf)
    {
        for (int i = buf.length - 1; i >= 0; --i)
        {
            byte b = (byte)((buf[i] + 1) & 0xff);
            buf[i] = b;

            if (b != 0)
            {
                break;
            }
        }
    }
}
