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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 FIPS provider. The APIs may also be used with other providers although if being used in a FIPS context it is the responsibility of the user to ensure that any other providers used are FIPS certified and used appropriately.
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package org.bouncycastle.openpgp;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;

import org.bouncycastle.bcpg.BCPGInputStream;
import org.bouncycastle.bcpg.BCPGOutputStream;
import org.bouncycastle.bcpg.HashUtils;
import org.bouncycastle.bcpg.OnePassSignaturePacket;
import org.bouncycastle.bcpg.Packet;
import org.bouncycastle.bcpg.SignaturePacket;
import org.bouncycastle.openpgp.operator.PGPContentVerifier;
import org.bouncycastle.openpgp.operator.PGPContentVerifierBuilder;
import org.bouncycastle.openpgp.operator.PGPContentVerifierBuilderProvider;
import org.bouncycastle.util.Arrays;

/**
 * A one pass signature object.
 */
public class PGPOnePassSignature
    extends PGPDefaultSignatureGenerator
{
    private OnePassSignaturePacket sigPack;
    private PGPContentVerifier verifier;

    private static OnePassSignaturePacket cast(Packet packet)
        throws IOException
    {
        if (!(packet instanceof OnePassSignaturePacket))
        {
            throw new IOException("unexpected packet in stream: " + packet);
        }
        return (OnePassSignaturePacket)packet;
    }

    public PGPOnePassSignature(
        BCPGInputStream    pIn)
        throws IOException, PGPException
    {
        this(cast(pIn.readPacket()));
    }
    
    PGPOnePassSignature(
        OnePassSignaturePacket sigPack)
    {
        // v3 OPSs are typically used with v4 sigs
        super(sigPack.getVersion() == OnePassSignaturePacket.VERSION_3 ? SignaturePacket.VERSION_4 : sigPack.getVersion());
        this.sigPack = sigPack;
        this.sigType = sigPack.getSignatureType();
    }

    /**
     * Initialise the signature object for verification.
     *
     * @param verifierBuilderProvider provider for a content verifier builder for the signature type of interest.
     * @param pubKey                  the public key to use for verification
     * @throws PGPException if there's an issue with creating the verifier.
     */
    public void init(PGPContentVerifierBuilderProvider verifierBuilderProvider, PGPPublicKey pubKey)
        throws PGPException
    {
        PGPContentVerifierBuilder verifierBuilder = verifierBuilderProvider.get(sigPack.getKeyAlgorithm(), sigPack.getHashAlgorithm());

        verifier = verifierBuilder.build(pubKey);

        lastb = 0;
        sigOut = verifier.getOutputStream();

        checkSaltSize();
        updateWithSalt();
    }

    private void checkSaltSize()
        throws PGPException
    {
        if (getVersion() != SignaturePacket.VERSION_6)
        {
            return;
        }

        int expectedSaltSize = HashUtils.getV6SignatureSaltSizeInBytes(getHashAlgorithm());
        if (expectedSaltSize != getSalt().length)
        {
            throw new PGPException("RFC9580 defines the salt size for " + PGPUtil.getDigestName(getHashAlgorithm()) +
                " as " + expectedSaltSize + " octets, but signature has " + getSalt().length + " octets.");
        }
    }

    private void updateWithSalt()
        throws PGPException
    {
        if (version == SignaturePacket.VERSION_6)
        {
            try
            {
                sigOut.write(getSalt());
            }
            catch (IOException e)
            {
                throw new PGPException("Cannot salt the signature.", e);
            }
        }
    }

    /**
     * Verify the calculated signature against the passed in PGPSignature.
     *
     * @param pgpSig
     * @return boolean
     * @throws PGPException
     */
    public boolean verify(
        PGPSignature pgpSig)
        throws PGPException
    {
        compareSalt(pgpSig);
        
        try
        {
            sigOut.write(pgpSig.getSignatureTrailer());

            sigOut.close();
        }
        catch (IOException e)
        {
            throw new PGPException("unable to add trailer: " + e.getMessage(), e);
        }

        return verifier.verify(pgpSig.getSignature());
    }

    private void compareSalt(PGPSignature signature)
        throws PGPException
    {
        if (version != SignaturePacket.VERSION_6)
        {
            return;
        }
        if (!Arrays.constantTimeAreEqual(getSalt(), signature.getSalt()))
        {
            throw new PGPException("Salt in OnePassSignaturePacket does not match salt in SignaturePacket.");
        }
    }

    /**
     * Return the packet version.
     *
     * @return packet version
     */
    public int getVersion()
    {
        return sigPack.getVersion();
    }

    /**
     * Return the key-ID of the issuer signing key.
     * For {@link OnePassSignaturePacket#VERSION_6} packets, the key-ID is derived from the fingerprint.
     *
     * @return key-ID
     */
    public long getKeyID()
    {
        return sigPack.getKeyID();
    }

    /**
     * Return the issuer key fingerprint.
     * Only for {@link OnePassSignaturePacket#VERSION_6} packets.
     * @return fingerprint
     */
    public byte[] getFingerprint()
    {
        return sigPack.getFingerprint();
    }

    /**
     * Return the salt used in the corresponding signature.
     * Only for {@link OnePassSignaturePacket#VERSION_6} packets.
     * @return salt
     */
    public byte[] getSalt()
    {
        return sigPack.getSalt();
    }

    public int getSignatureType()
    {
        return sigPack.getSignatureType();
    }

    public int getHashAlgorithm()
    {
        return sigPack.getHashAlgorithm();
    }

    public int getKeyAlgorithm()
    {
        return sigPack.getKeyAlgorithm();
    }

    /**
     * Return true, if the signature is contains any signatures that follow.
     * An bracketing OPS is followed by additional OPS packets and is calculated over all the data between itself
     * and its corresponding signature (it is an attestation for contained signatures).
     *
     * @return true if containing, false otherwise
     */
    public boolean isContaining()
    {
        return sigPack.isContaining();
    }

    public byte[] getEncoded()
        throws IOException
    {
        ByteArrayOutputStream bOut = new ByteArrayOutputStream();

        this.encode(bOut);

        return bOut.toByteArray();
    }

    public void encode(OutputStream outStream)
        throws IOException
    {
        BCPGOutputStream.wrap(outStream).writePacket(sigPack);
    }
}