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The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 to JDK 1.8. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.

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package org.bouncycastle.cert.dane;

import java.io.OutputStream;

import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
import org.bouncycastle.operator.DigestCalculator;

/**
 * A calculator which produces a truncated digest from a regular one, with the truncation
 * achieved by dropping off the right most octets.
 */
public class TruncatingDigestCalculator
    implements DigestCalculator
{
    private final DigestCalculator baseCalculator;
    private final int length;

    /**
     * Default constructor - truncate to 28.
     *
     * @param baseCalculator actual calculator for working out the digest.
     */
    public TruncatingDigestCalculator(DigestCalculator baseCalculator)
    {
       this(baseCalculator, 28);
    }

    /**
     * Constructor specifying a length.
     *
     * @param baseCalculator actual calculator for working out the digest.
     * @param length length in bytes of the final result.
     */
    public TruncatingDigestCalculator(DigestCalculator baseCalculator, int length)
    {
        this.baseCalculator = baseCalculator;
        this.length = length;
    }

    public AlgorithmIdentifier getAlgorithmIdentifier()
    {
        return baseCalculator.getAlgorithmIdentifier();
    }

    public OutputStream getOutputStream()
    {
        return baseCalculator.getOutputStream();
    }

    public byte[] getDigest()
    {
        byte[] rv = new byte[length];

        byte[] dig = baseCalculator.getDigest();

        System.arraycopy(dig, 0, rv, 0, rv.length);

        return rv;
    }
}