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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5 and up.
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package org.bouncycastle.crypto.agreement.srp;

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

import org.bouncycastle.crypto.CryptoException;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.util.BigIntegers;

public class SRP6Util
{
    private static BigInteger ZERO = BigInteger.valueOf(0);
    private static BigInteger ONE = BigInteger.valueOf(1);

    public static BigInteger calculateK(Digest digest, BigInteger N, BigInteger g)
    {
        return hashPaddedPair(digest, N, N, g);
    }

    public static BigInteger calculateU(Digest digest, BigInteger N, BigInteger A, BigInteger B)
    {
        return hashPaddedPair(digest, N, A, B);
    }

    public static BigInteger calculateX(Digest digest, BigInteger N, byte[] salt, byte[] identity, byte[] password)
    {
        byte[] output = new byte[digest.getDigestSize()];

        digest.update(identity, 0, identity.length);
        digest.update((byte)':');
        digest.update(password, 0, password.length);
        digest.doFinal(output, 0);

        digest.update(salt, 0, salt.length);
        digest.update(output, 0, output.length);
        digest.doFinal(output, 0);

        return new BigInteger(1, output);
    }

    public static BigInteger generatePrivateValue(Digest digest, BigInteger N, BigInteger g, SecureRandom random)
    {
        int minBits = Math.min(256, N.bitLength() / 2);
        BigInteger min = ONE.shiftLeft(minBits - 1);
        BigInteger max = N.subtract(ONE);

        return BigIntegers.createRandomInRange(min, max, random);
    }

    public static BigInteger validatePublicValue(BigInteger N, BigInteger val)
        throws CryptoException
    {
        val = val.mod(N);

        // Check that val % N != 0
        if (val.equals(ZERO))
        {
            throw new CryptoException("Invalid public value: 0");
        }

        return val;
    }

    /**
     * Computes the client evidence message (M1) according to the standard routine:
     * M1 = H( A | B | S )
     *
     * @param digest The Digest used as the hashing function H
     * @param N      Modulus used to get the pad length
     * @param A      The public client value
     * @param B      The public server value
     * @param S      The secret calculated by both sides
     * @return M1 The calculated client evidence message
     */
    public static BigInteger calculateM1(Digest digest, BigInteger N, BigInteger A, BigInteger B, BigInteger S)
    {
        BigInteger M1 = hashPaddedTriplet(digest, N, A, B, S);
        return M1;
    }

    /**
     * Computes the server evidence message (M2) according to the standard routine:
     * M2 = H( A | M1 | S )
     *
     * @param digest The Digest used as the hashing function H
     * @param N      Modulus used to get the pad length
     * @param A      The public client value
     * @param M1     The client evidence message
     * @param S      The secret calculated by both sides
     * @return M2 The calculated server evidence message
     */
    public static BigInteger calculateM2(Digest digest, BigInteger N, BigInteger A, BigInteger M1, BigInteger S)
    {
        BigInteger M2 = hashPaddedTriplet(digest, N, A, M1, S);
        return M2;
    }

    /**
     * Computes the final Key according to the standard routine: Key = H(S)
     *
     * @param digest The Digest used as the hashing function H
     * @param N      Modulus used to get the pad length
     * @param S      The secret calculated by both sides
     * @return the final Key value.
     */
    public static BigInteger calculateKey(Digest digest, BigInteger N, BigInteger S)
    {
        int padLength = (N.bitLength() + 7) / 8;
        byte[] _S = getPadded(S, padLength);
        digest.update(_S, 0, _S.length);

        byte[] output = new byte[digest.getDigestSize()];
        digest.doFinal(output, 0);
        return new BigInteger(1, output);
    }

    private static BigInteger hashPaddedTriplet(Digest digest, BigInteger N, BigInteger n1, BigInteger n2, BigInteger n3)
    {
        int padLength = (N.bitLength() + 7) / 8;

        byte[] n1_bytes = getPadded(n1, padLength);
        byte[] n2_bytes = getPadded(n2, padLength);
        byte[] n3_bytes = getPadded(n3, padLength);

        digest.update(n1_bytes, 0, n1_bytes.length);
        digest.update(n2_bytes, 0, n2_bytes.length);
        digest.update(n3_bytes, 0, n3_bytes.length);

        byte[] output = new byte[digest.getDigestSize()];
        digest.doFinal(output, 0);

        return new BigInteger(1, output);
    }

    private static BigInteger hashPaddedPair(Digest digest, BigInteger N, BigInteger n1, BigInteger n2)
    {
        int padLength = (N.bitLength() + 7) / 8;

        byte[] n1_bytes = getPadded(n1, padLength);
        byte[] n2_bytes = getPadded(n2, padLength);

        digest.update(n1_bytes, 0, n1_bytes.length);
        digest.update(n2_bytes, 0, n2_bytes.length);

        byte[] output = new byte[digest.getDigestSize()];
        digest.doFinal(output, 0);

        return new BigInteger(1, output);
    }

    private static byte[] getPadded(BigInteger n, int length)
    {
        byte[] bs = BigIntegers.asUnsignedByteArray(n);
        if (bs.length < length)
        {
            byte[] tmp = new byte[length];
            System.arraycopy(bs, 0, tmp, length - bs.length, bs.length);
            bs = tmp;
        }
        return bs;
    }
}