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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.io.IOException;
import java.math.BigInteger;

import org.bouncycastle.asn1.ASN1EncodableVector;
import org.bouncycastle.asn1.ASN1Encoding;
import org.bouncycastle.asn1.ASN1Integer;
import org.bouncycastle.asn1.ASN1Primitive;
import org.bouncycastle.asn1.ASN1Sequence;
import org.bouncycastle.asn1.DERSequence;
import org.bouncycastle.util.Arrays;

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

    public byte[] encode(BigInteger n, BigInteger r, BigInteger s) throws IOException
    {
        ASN1EncodableVector v = new ASN1EncodableVector();
        encodeValue(n, v, r);
        encodeValue(n, v, s);
        return new DERSequence(v).getEncoded(ASN1Encoding.DER);
    }

    public BigInteger[] decode(BigInteger n, byte[] encoding) throws IOException
    {
        ASN1Sequence seq = (ASN1Sequence)ASN1Primitive.fromByteArray(encoding);
        if (seq.size() == 2)
        {
            BigInteger r = decodeValue(n, seq, 0);
            BigInteger s = decodeValue(n, seq, 1);

            byte[] expectedEncoding = encode(n, r, s);
            if (Arrays.areEqual(expectedEncoding,  encoding))
            {
                return new BigInteger[]{ r, s };
            }
        }

        throw new IllegalArgumentException("Malformed signature");
    }

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

        return x;
    }

    protected BigInteger decodeValue(BigInteger n, ASN1Sequence s, int pos)
    {
        return checkValue(n, ((ASN1Integer)s.getObjectAt(pos)).getValue());
    }

    protected void encodeValue(BigInteger n, ASN1EncodableVector v, BigInteger x)
    {
        v.add(new ASN1Integer(checkValue(n, x)));
    }
}