/*

 * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code 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

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package sun.security.rsa;

import java.security.*;

/**

 * This class implements the MGF1 mask generation function defined in PKCS#1

 * v2.2 B.2.1. A mask generation function takes an octet string of variable

 * length and a desired output length as input and outputs an octet string of

 * the desired length. MGF1 is a mask generation function based on a hash

 * function, i.e. message digest algorithm.

 *

 * @since   11

 */

public final class MGF1 {

    private final MessageDigest md;

    /**

     * Construct an instance of MGF1 based on the specified digest algorithm.

     */

    MGF1(String mdAlgo) throws NoSuchAlgorithmException {

        this.md = MessageDigest.getInstance(mdAlgo);

    }

    /**

     * Using the specified seed bytes, generate the mask, xor the mask

     * with the specified output buffer and store the result into the

     * output buffer (essentially replaced in place).

     *

     * @param seed the buffer holding the seed bytes

     * @param seedOfs the index of the seed bytes

     * @param seedLen the length of the seed bytes to be used by MGF1

     * @param maskLen the intended length of the generated mask

     * @param out the output buffer holding the mask

     * @param outOfs the index of the output buffer for the mask

     */

    void generateAndXor(byte[] seed, int seedOfs, int seedLen, int maskLen,

            byte[] out, int outOfs) throws RuntimeException {

        byte[] C = new byte[4]; // 32 bit counter

        byte[] digest = new byte[md.getDigestLength()];

        while (maskLen > 0) {

            md.update(seed, seedOfs, seedLen);

            md.update(C);

            try {

                md.digest(digest, 0, digest.length);

            } catch (DigestException e) {

                // should never happen

                throw new RuntimeException(e.toString());

            }

            for (int i = 0; (i < digest.length) && (maskLen > 0); maskLen--) {

                out[outOfs++] ^= digest[i++];

            }

            if (maskLen > 0) {

                // increment counter

                for (int i = C.length - 1; (++C[i] == 0) && (i > 0); i--) {

                    // empty

                }

            }

        }

    }

    /**

     * Returns the name of this MGF1 instance, i.e. "MGF1" followed by the

     * digest algorithm it based on.

     */

    String getName() {

        return "MGF1" + md.getAlgorithm();

    }

}