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/**
 * Copyright 2009 sshj contributors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package net.schmizz.sshj.signature;

import java.security.SignatureException;

import net.schmizz.sshj.common.KeyType;
import net.schmizz.sshj.common.SSHRuntimeException;

/** DSA {@link Signature} */
public class SignatureDSA
        extends AbstractSignature {

    /** A named factory for DSA signature */
    public static class Factory
            implements net.schmizz.sshj.common.Factory.Named {

        @Override
        public Signature create() {
            return new SignatureDSA();
        }

        @Override
        public String getName() {
            return KeyType.DSA.toString();
        }

    }

    public SignatureDSA() {
        super("SHA1withDSA");
    }

    @Override
    public byte[] encode(byte[] sig) {
        // sig is in ASN.1
        // SEQUENCE::={ r INTEGER, s INTEGER }

        int rIndex = 3;
        int rLen = sig[rIndex++] & 0xff;
        byte[] r = new byte[rLen];
        System.arraycopy(sig, rIndex, r, 0, r.length);

        int sIndex = rIndex + rLen + 1;
        int sLen = sig[sIndex++] & 0xff;
        byte[] s = new byte[sLen];
        System.arraycopy(sig, sIndex, s, 0, s.length);

        byte[] result = new byte[40];

        // result must be 40 bytes, but length of r and s may not be 20 bytes

        int r_copylen = (r.length < 20) ? r.length : 20;
        int s_copylen = (s.length < 20) ? s.length : 20;

        System.arraycopy(r, r.length - r_copylen, result, 20 - r_copylen, r_copylen);
        System.arraycopy(s, s.length - s_copylen, result, 40 - s_copylen, s_copylen);

        return result;
    }

    @Override
    public boolean verify(byte[] sig) {
        sig = extractSig(sig);

        // ASN.1
        int frst = (sig[0] & 0x80) != 0 ? 1 : 0;
        int scnd = (sig[20] & 0x80) != 0 ? 1 : 0;

        int length = sig.length + 6 + frst + scnd;
        byte[] tmp = new byte[length];
        tmp[0] = (byte) 0x30;
        tmp[1] = (byte) 0x2c;
        tmp[1] += frst;
        tmp[1] += scnd;
        tmp[2] = (byte) 0x02;
        tmp[3] = (byte) 0x14;
        tmp[3] += frst;
        System.arraycopy(sig, 0, tmp, 4 + frst, 20);
        tmp[4 + tmp[3]] = (byte) 0x02;
        tmp[5 + tmp[3]] = (byte) 0x14;
        tmp[5 + tmp[3]] += scnd;
        System.arraycopy(sig, 20, tmp, 6 + tmp[3] + scnd, 20);
        sig = tmp;

        try {
            return signature.verify(sig);
        } catch (SignatureException e) {
            throw new SSHRuntimeException(e);
        }
    }

}