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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 1.8 and up. The APIs are designed primarily to be used in conjunction with the BC LTS provider but may also be used with other providers providing cryptographic services.

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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;
    }
}