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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.signers;

import java.math.BigInteger;

import org.bouncycastle.util.Arrays;
import org.bouncycastle.util.BigIntegers;

public class PlainDSAEncoding
    implements DSAEncoding
{
    public static final PlainDSAEncoding INSTANCE = new PlainDSAEncoding();

    public byte[] encode(BigInteger n, BigInteger r, BigInteger s)
    {
        int valueLength = BigIntegers.getUnsignedByteLength(n);
        byte[] result = new byte[valueLength * 2];
        encodeValue(n, r, result, 0, valueLength);
        encodeValue(n, s, result, valueLength, valueLength);
        return result;
    }

    public BigInteger[] decode(BigInteger n, byte[] encoding)
    {
        int valueLength = BigIntegers.getUnsignedByteLength(n);
        if (encoding.length != valueLength * 2)
        {
            throw new IllegalArgumentException("Encoding has incorrect length");
        }

        return new BigInteger[] {
            decodeValue(n, encoding, 0, valueLength),
            decodeValue(n, encoding, valueLength, valueLength),
        };
    }

    protected BigInteger checkValue(BigInteger n, BigInteger x)
    {
        if (x.signum() < 0 || x.compareTo(n) >= 0)
        {
            throw new IllegalArgumentException("Value out of range");
        }

        return x;
    }

    protected BigInteger decodeValue(BigInteger n, byte[] buf, int off, int len)
    {
        byte[] bs = Arrays.copyOfRange(buf, off, off + len);
        return checkValue(n, new BigInteger(1, bs));
    }

    private void encodeValue(BigInteger n, BigInteger x, byte[] buf, int off, int len)
    {
        byte[] bs = checkValue(n, x).toByteArray();
        int bsOff = Math.max(0, bs.length - len);
        int bsLen = bs.length - bsOff;

        int pos = len - bsLen;
        Arrays.fill(buf, off, off + pos, (byte)0);
        System.arraycopy(bs, bsOff, buf, off + pos, bsLen);
    }
}