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The Bouncy Castle Java APIs for the OpenPGP Protocol. 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.gpg.keybox.bc;

import org.bouncycastle.crypto.digests.MD5Digest;
import org.bouncycastle.crypto.digests.SHA1Digest;
import org.bouncycastle.gpg.keybox.BlobVerifier;
import org.bouncycastle.util.Arrays;

public class BcBlobVerifier
    implements BlobVerifier
{
    private final SHA1Digest sha1Digest = new SHA1Digest();
    private final MD5Digest md5Digest = new MD5Digest();

    public boolean isMatched(byte[] blobData, byte[] blobDigest)
    {
        sha1Digest.update(blobData, 0, blobData.length);
        byte[] calculatedDigest = new byte[sha1Digest.getDigestSize()];
        sha1Digest.doFinal(calculatedDigest, 0);

        if (!Arrays.constantTimeAreEqual(calculatedDigest, blobDigest))
        {
            //
            // Special case for old key boxes that used MD5.
            //

          /*
           https://git.gnupg.org/cgi-bin/gitweb.cgi?p=gnupg.git;a=blob;f=kbx/keybox-blob.c;hb=HEAD#l129
           SHA-1 checksum (useful for KS syncronisation?)
           Note, that KBX versions before GnuPG 2.1 used an MD5
           checksum.  However it was only created but never checked.
           Thus we do not expect problems if we switch to SHA-1.  If
           the checksum fails and the first 4 bytes are zero, we can
           try again with MD5.  SHA-1 has the advantage that it is
           faster on CPUs with dedicated SHA-1 support.
          */

            if (blobDigest[0] == 0 && blobDigest[1] == 0 && blobDigest[2] == 0 && blobDigest[3] == 0)
            {
                md5Digest.update(blobData, 0, blobData.length);

                Arrays.fill(calculatedDigest, (byte)0);

                md5Digest.doFinal(calculatedDigest, 4);

                return Arrays.constantTimeAreEqual(calculatedDigest, blobDigest);
            }

            return false;
        }

        return true;
    }
}