/*
 * Copyright (C) 2012 brjannc <brjannc at gmail.com>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package me.brjannc.plugins.sushi;

import java.math.BigInteger;
import java.security.KeyFactory;
import java.security.PublicKey;
import java.security.spec.DSAPublicKeySpec;
import java.security.spec.RSAPublicKeySpec;

import org.apache.commons.codec.binary.Base64;

// AuthorizedKeysDecoder was written by WhiteFang34, and was obtained from
// http://stackoverflow.com/questions/3531506/using-public-key-from-authorized-keys-with-java-security
//
// This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.
// http://creativecommons.org/licenses/by-sa/3.0/
public class AuthorizedKeysDecoder {

    private byte[] bytes;
    private int pos;

    public PublicKey decodePublicKey(String keyLine) throws Exception {
        bytes = null;
        pos = 0;

        // look for the Base64 encoded part of the line to decode
        // both ssh-rsa and ssh-dss begin with "AAAA" due to the length bytes
        for (String part : keyLine.split(" ")) {
            if (part.startsWith("AAAA")) {
                bytes = Base64.decodeBase64(part);
                break;
            }
        }
        if (bytes == null) {
            throw new IllegalArgumentException("no Base64 part to decode");
        }

        String type = decodeType();
        if (type.equals("ssh-rsa")) {
            BigInteger e = decodeBigInt();
            BigInteger m = decodeBigInt();
            RSAPublicKeySpec spec = new RSAPublicKeySpec(m, e);
            return KeyFactory.getInstance("RSA").generatePublic(spec);
        } else if (type.equals("ssh-dss")) {
            BigInteger p = decodeBigInt();
            BigInteger q = decodeBigInt();
            BigInteger g = decodeBigInt();
            BigInteger y = decodeBigInt();
            DSAPublicKeySpec spec = new DSAPublicKeySpec(y, p, q, g);
            return KeyFactory.getInstance("DSA").generatePublic(spec);
        } else {
            throw new IllegalArgumentException("unknown type " + type);
        }
    }

    private String decodeType() {
        int len = decodeInt();
        String type = new String(bytes, pos, len);
        pos += len;
        return type;
    }

    private int decodeInt() {
        return ((bytes[pos++] & 0xFF) << 24) | ((bytes[pos++] & 0xFF) << 16)
                | ((bytes[pos++] & 0xFF) << 8) | (bytes[pos++] & 0xFF);
    }

    private BigInteger decodeBigInt() {
        int len = decodeInt();
        byte[] bigIntBytes = new byte[len];
        System.arraycopy(bytes, pos, bigIntBytes, 0, len);
        pos += len;
        return new BigInteger(bigIntBytes);
    }
}