/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.commons.codec.digest;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import org.apache.commons.codec.Charsets;

/**
 * SHA2-based Unix crypt implementation.
 * <p>
 * Based on the C implementation released into the Public Domain by Ulrich Drepper &lt;drepper@redhat.com&gt;
 * http://www.akkadia.org/drepper/SHA-crypt.txt
 * <p>
 * Conversion to Kotlin and from there to Java in 2012 by Christian Hammers &lt;ch@lathspell.de&gt; and likewise put
 * into the Public Domain.
 * <p>
 * This class is immutable and thread-safe.
 * 
 * @version $Id: Sha2Crypt.html 891688 2013-12-24 20:49:46Z ggregory $
 * @since 1.7
 */
public class Sha2Crypt {

    /** Default number of rounds if not explicitly specified. */
    private static final int ROUNDS_DEFAULT = 5000;

    /** Maximum number of rounds. */
    private static final int ROUNDS_MAX = 999999999;

    /** Minimum number of rounds. */
    private static final int ROUNDS_MIN = 1000;

    /** Prefix for optional rounds specification. */
    private static final String ROUNDS_PREFIX = "rounds=";

    /** The number of bytes the final hash value will have (SHA-256 variant). */
    private static final int SHA256_BLOCKSIZE = 32;

    /** The prefixes that can be used to identify this crypt() variant (SHA-256). */
    static final String SHA256_PREFIX = "$5$";

    /** The number of bytes the final hash value will have (SHA-512 variant). */
    private static final int SHA512_BLOCKSIZE = 64;

    /** The prefixes that can be used to identify this crypt() variant (SHA-512). */
    static final String SHA512_PREFIX = "$6$";

    /** The pattern to match valid salt values. */
    private static final Pattern SALT_PATTERN = Pattern
            .compile("^\\$([56])\\$(rounds=(\\d+)\\$)?([\\.\\/a-zA-Z0-9]{1,16}).*");

    /**
     * Generates a libc crypt() compatible "$5$" hash value with random salt.
     * <p>
     * See {@link Crypt#crypt(String, String)} for details.
     * 
     * @param keyBytes
     *            plaintext to hash
     * @return complete hash value
     * @throws RuntimeException
     *             when a {@link java.security.NoSuchAlgorithmException} is caught.
     */
    public static String sha256Crypt(final byte[] keyBytes) {
        return sha256Crypt(keyBytes, null);
    }

    /**
     * Generates a libc6 crypt() compatible "$5$" hash value.
     * <p>
     * See {@link Crypt#crypt(String, String)} for details.
     * 
     * @param keyBytes
     *            plaintext to hash
     * @param salt
     *            real salt value without prefix or "rounds="
     * @return complete hash value including salt
     * @throws IllegalArgumentException
     *             if the salt does not match the allowed pattern
     * @throws RuntimeException
     *             when a {@link java.security.NoSuchAlgorithmException} is caught.
     */
    public static String sha256Crypt(final byte[] keyBytes, String salt) {
        if (salt == null) {
            salt = SHA256_PREFIX + B64.getRandomSalt(8);
        }
        return sha2Crypt(keyBytes, salt, SHA256_PREFIX, SHA256_BLOCKSIZE, MessageDigestAlgorithms.SHA_256);
    }

    /**
     * Generates a libc6 crypt() compatible "$5$" or "$6$" SHA2 based hash value.
     * <p>
     * This is a nearly line by line conversion of the original C function. The numbered comments are from the algorithm
     * description, the short C-style ones from the original C code and the ones with "Remark" from me.
     * <p>
     * See {@link Crypt#crypt(String, String)} for details.
     * 
     * @param keyBytes
     *            plaintext to hash
     * @param salt
     *            real salt value without prefix or "rounds="
     * @param saltPrefix
     *            either $5$ or $6$
     * @param blocksize
     *            a value that differs between $5$ and $6$
     * @param algorithm
     *            {@link MessageDigest} algorithm identifier string
     * @return complete hash value including prefix and salt
     * @throws IllegalArgumentException
     *             if the given salt is {@code null} or does not match the allowed pattern
     * @throws IllegalArgumentException
     *             when a {@link NoSuchAlgorithmException} is caught
     * @see MessageDigestAlgorithms
     */
    private static String sha2Crypt(final byte[] keyBytes, final String salt, final String saltPrefix,
            final int blocksize, final String algorithm) {

        final int keyLen = keyBytes.length;

        // Extracts effective salt and the number of rounds from the given salt.
        int rounds = ROUNDS_DEFAULT;
        boolean roundsCustom = false;
        if (salt == null) {
            throw new IllegalArgumentException("Salt must not be null");
        }

        final Matcher m = SALT_PATTERN.matcher(salt);
        if (m == null || !m.find()) {
            throw new IllegalArgumentException("Invalid salt value: " + salt);
        }
        if (m.group(3) != null) {
            rounds = Integer.parseInt(m.group(3));
            rounds = Math.max(ROUNDS_MIN, Math.min(ROUNDS_MAX, rounds));
            roundsCustom = true;
        }
        final String saltString = m.group(4);
        final byte[] saltBytes = saltString.getBytes(Charsets.UTF_8);
        final int saltLen = saltBytes.length;

        // 1. start digest A
        // Prepare for the real work.
        MessageDigest ctx = DigestUtils.getDigest(algorithm);

        // 2. the password string is added to digest A
        /*
         * Add the key string.
         */
        ctx.update(keyBytes);

        // 3. the salt string is added to digest A. This is just the salt string
        // itself without the enclosing '$', without the magic salt_prefix $5$ and
        // $6$ respectively and without the rounds=<N> specification.
        //
        // NB: the MD5 algorithm did add the $1$ salt_prefix. This is not deemed
        // necessary since it is a constant string and does not add security
        // and /possibly/ allows a plain text attack. Since the rounds=<N>
        // specification should never be added this would also create an
        // inconsistency.
        /*
         * The last part is the salt string. This must be at most 16 characters and it ends at the first `$' character
         * (for compatibility with existing implementations).
         */
        ctx.update(saltBytes);

        // 4. start digest B
        /*
         * Compute alternate sha512 sum with input KEY, SALT, and KEY. The final result will be added to the first
         * context.
         */
        MessageDigest altCtx = DigestUtils.getDigest(algorithm);

        // 5. add the password to digest B
        /*
         * Add key.
         */
        altCtx.update(keyBytes);

        // 6. add the salt string to digest B
        /*
         * Add salt.
         */
        altCtx.update(saltBytes);

        // 7. add the password again to digest B
        /*
         * Add key again.
         */
        altCtx.update(keyBytes);

        // 8. finish digest B
        /*
         * Now get result of this (32 bytes) and add it to the other context.
         */
        byte[] altResult = altCtx.digest();

        // 9. For each block of 32 or 64 bytes in the password string (excluding
        // the terminating NUL in the C representation), add digest B to digest A
        /*
         * Add for any character in the key one byte of the alternate sum.
         */
        /*
         * (Remark: the C code comment seems wrong for key length > 32!)
         */
        int cnt = keyBytes.length;
        while (cnt > blocksize) {
            ctx.update(altResult, 0, blocksize);
            cnt -= blocksize;
        }

        // 10. For the remaining N bytes of the password string add the first
        // N bytes of digest B to digest A
        ctx.update(altResult, 0, cnt);

        // 11. For each bit of the binary representation of the length of the
        // password string up to and including the highest 1-digit, starting
        // from to lowest bit position (numeric value 1):
        //
        // a) for a 1-digit add digest B to digest A
        //
        // b) for a 0-digit add the password string
        //
        // NB: this step differs significantly from the MD5 algorithm. It
        // adds more randomness.
        /*
         * Take the binary representation of the length of the key and for every 1 add the alternate sum, for every 0
         * the key.
         */
        cnt = keyBytes.length;
        while (cnt > 0) {
            if ((cnt & 1) != 0) {
                ctx.update(altResult, 0, blocksize);
            } else {
                ctx.update(keyBytes);
            }
            cnt >>= 1;
        }

        // 12. finish digest A
        /*
         * Create intermediate result.
         */
        altResult = ctx.digest();

        // 13. start digest DP
        /*
         * Start computation of P byte sequence.
         */
        altCtx = DigestUtils.getDigest(algorithm);

        // 14. for every byte in the password (excluding the terminating NUL byte
        // in the C representation of the string)
        //
        // add the password to digest DP
        /*
         * For every character in the password add the entire password.
         */
        for (int i = 1; i <= keyLen; i++) {
            altCtx.update(keyBytes);
        }

        // 15. finish digest DP
        /*
         * Finish the digest.
         */
        byte[] tempResult = altCtx.digest();

        // 16. produce byte sequence P of the same length as the password where
        //
        // a) for each block of 32 or 64 bytes of length of the password string
        // the entire digest DP is used
        //
        // b) for the remaining N (up to 31 or 63) bytes use the first N
        // bytes of digest DP
        /*
         * Create byte sequence P.
         */
        final byte[] pBytes = new byte[keyLen];
        int cp = 0;
        while (cp < keyLen - blocksize) {
            System.arraycopy(tempResult, 0, pBytes, cp, blocksize);
            cp += blocksize;
        }
        System.arraycopy(tempResult, 0, pBytes, cp, keyLen - cp);

        // 17. start digest DS
        /*
         * Start computation of S byte sequence.
         */
        altCtx = DigestUtils.getDigest(algorithm);

        // 18. repeast the following 16+A[0] times, where A[0] represents the first
        // byte in digest A interpreted as an 8-bit unsigned value
        //
        // add the salt to digest DS
        /*
         * For every character in the password add the entire password.
         */
        for (int i = 1; i <= 16 + (altResult[0] & 0xff); i++) {
            altCtx.update(saltBytes);
        }

        // 19. finish digest DS
        /*
         * Finish the digest.
         */
        tempResult = altCtx.digest();

        // 20. produce byte sequence S of the same length as the salt string where
        //
        // a) for each block of 32 or 64 bytes of length of the salt string
        // the entire digest DS is used
        //
        // b) for the remaining N (up to 31 or 63) bytes use the first N
        // bytes of digest DS
        /*
         * Create byte sequence S.
         */
        // Remark: The salt is limited to 16 chars, how does this make sense?
        final byte[] sBytes = new byte[saltLen];
        cp = 0;
        while (cp < saltLen - blocksize) {
            System.arraycopy(tempResult, 0, sBytes, cp, blocksize);
            cp += blocksize;
        }
        System.arraycopy(tempResult, 0, sBytes, cp, saltLen - cp);

        // 21. repeat a loop according to the number specified in the rounds=<N>
        // specification in the salt (or the default value if none is
        // present). Each round is numbered, starting with 0 and up to N-1.
        //
        // The loop uses a digest as input. In the first round it is the
        // digest produced in step 12. In the latter steps it is the digest
        // produced in step 21.h. The following text uses the notation
        // "digest A/C" to describe this behavior.
        /*
         * Repeatedly run the collected hash value through sha512 to burn CPU cycles.
         */
        for (int i = 0; i <= rounds - 1; i++) {
            // a) start digest C
            /*
             * New context.
             */
            ctx = DigestUtils.getDigest(algorithm);

            // b) for odd round numbers add the byte sequense P to digest C
            // c) for even round numbers add digest A/C
            /*
             * Add key or last result.
             */
            if ((i & 1) != 0) {
                ctx.update(pBytes, 0, keyLen);
            } else {
                ctx.update(altResult, 0, blocksize);
            }

            // d) for all round numbers not divisible by 3 add the byte sequence S
            /*
             * Add salt for numbers not divisible by 3.
             */
            if (i % 3 != 0) {
                ctx.update(sBytes, 0, saltLen);
            }

            // e) for all round numbers not divisible by 7 add the byte sequence P
            /*
             * Add key for numbers not divisible by 7.
             */
            if (i % 7 != 0) {
                ctx.update(pBytes, 0, keyLen);
            }

            // f) for odd round numbers add digest A/C
            // g) for even round numbers add the byte sequence P
            /*
             * Add key or last result.
             */
            if ((i & 1) != 0) {
                ctx.update(altResult, 0, blocksize);
            } else {
                ctx.update(pBytes, 0, keyLen);
            }

            // h) finish digest C.
            /*
             * Create intermediate result.
             */
            altResult = ctx.digest();
        }

        // 22. Produce the output string. This is an ASCII string of the maximum
        // size specified above, consisting of multiple pieces:
        //
        // a) the salt salt_prefix, $5$ or $6$ respectively
        //
        // b) the rounds=<N> specification, if one was present in the input
        // salt string. A trailing '$' is added in this case to separate
        // the rounds specification from the following text.
        //
        // c) the salt string truncated to 16 characters
        //
        // d) a '$' character
        /*
         * Now we can construct the result string. It consists of three parts.
         */
        final StringBuilder buffer = new StringBuilder(saltPrefix);
        if (roundsCustom) {
            buffer.append(ROUNDS_PREFIX);
            buffer.append(rounds);
            buffer.append("$");
        }
        buffer.append(saltString);
        buffer.append("$");

        // e) the base-64 encoded final C digest. The encoding used is as
        // follows:
        // [...]
        //
        // Each group of three bytes from the digest produces four
        // characters as output:
        //
        // 1. character: the six low bits of the first byte
        // 2. character: the two high bits of the first byte and the
        // four low bytes from the second byte
        // 3. character: the four high bytes from the second byte and
        // the two low bits from the third byte
        // 4. character: the six high bits from the third byte
        //
        // The groups of three bytes are as follows (in this sequence).
        // These are the indices into the byte array containing the
        // digest, starting with index 0. For the last group there are
        // not enough bytes left in the digest and the value zero is used
        // in its place. This group also produces only three or two
        // characters as output for SHA-512 and SHA-512 respectively.

        // This was just a safeguard in the C implementation:
        // int buflen = salt_prefix.length() - 1 + ROUNDS_PREFIX.length() + 9 + 1 + salt_string.length() + 1 + 86 + 1;

        if (blocksize == 32) {
            B64.b64from24bit(altResult[0], altResult[10], altResult[20], 4, buffer);
            B64.b64from24bit(altResult[21], altResult[1], altResult[11], 4, buffer);
            B64.b64from24bit(altResult[12], altResult[22], altResult[2], 4, buffer);
            B64.b64from24bit(altResult[3], altResult[13], altResult[23], 4, buffer);
            B64.b64from24bit(altResult[24], altResult[4], altResult[14], 4, buffer);
            B64.b64from24bit(altResult[15], altResult[25], altResult[5], 4, buffer);
            B64.b64from24bit(altResult[6], altResult[16], altResult[26], 4, buffer);
            B64.b64from24bit(altResult[27], altResult[7], altResult[17], 4, buffer);
            B64.b64from24bit(altResult[18], altResult[28], altResult[8], 4, buffer);
            B64.b64from24bit(altResult[9], altResult[19], altResult[29], 4, buffer);
            B64.b64from24bit((byte) 0, altResult[31], altResult[30], 3, buffer);
        } else {
            B64.b64from24bit(altResult[0], altResult[21], altResult[42], 4, buffer);
            B64.b64from24bit(altResult[22], altResult[43], altResult[1], 4, buffer);
            B64.b64from24bit(altResult[44], altResult[2], altResult[23], 4, buffer);
            B64.b64from24bit(altResult[3], altResult[24], altResult[45], 4, buffer);
            B64.b64from24bit(altResult[25], altResult[46], altResult[4], 4, buffer);
            B64.b64from24bit(altResult[47], altResult[5], altResult[26], 4, buffer);
            B64.b64from24bit(altResult[6], altResult[27], altResult[48], 4, buffer);
            B64.b64from24bit(altResult[28], altResult[49], altResult[7], 4, buffer);
            B64.b64from24bit(altResult[50], altResult[8], altResult[29], 4, buffer);
            B64.b64from24bit(altResult[9], altResult[30], altResult[51], 4, buffer);
            B64.b64from24bit(altResult[31], altResult[52], altResult[10], 4, buffer);
            B64.b64from24bit(altResult[53], altResult[11], altResult[32], 4, buffer);
            B64.b64from24bit(altResult[12], altResult[33], altResult[54], 4, buffer);
            B64.b64from24bit(altResult[34], altResult[55], altResult[13], 4, buffer);
            B64.b64from24bit(altResult[56], altResult[14], altResult[35], 4, buffer);
            B64.b64from24bit(altResult[15], altResult[36], altResult[57], 4, buffer);
            B64.b64from24bit(altResult[37], altResult[58], altResult[16], 4, buffer);
            B64.b64from24bit(altResult[59], altResult[17], altResult[38], 4, buffer);
            B64.b64from24bit(altResult[18], altResult[39], altResult[60], 4, buffer);
            B64.b64from24bit(altResult[40], altResult[61], altResult[19], 4, buffer);
            B64.b64from24bit(altResult[62], altResult[20], altResult[41], 4, buffer);
            B64.b64from24bit((byte) 0, (byte) 0, altResult[63], 2, buffer);
        }

        /*
         * Clear the buffer for the intermediate result so that people attaching to processes or reading core dumps
         * cannot get any information.
         */
        // Is there a better way to do this with the JVM?
        Arrays.fill(tempResult, (byte) 0);
        Arrays.fill(pBytes, (byte) 0);
        Arrays.fill(sBytes, (byte) 0);
        ctx.reset();
        altCtx.reset();
        Arrays.fill(keyBytes, (byte) 0);
        Arrays.fill(saltBytes, (byte) 0);

        return buffer.toString();
    }

    /**
     * Generates a libc crypt() compatible "$6$" hash value with random salt.
     * <p>
     * See {@link Crypt#crypt(String, String)} for details.
     * 
     * @param keyBytes
     *            plaintext to hash
     * @return complete hash value
     * @throws RuntimeException
     *             when a {@link java.security.NoSuchAlgorithmException} is caught.
     */
    public static String sha512Crypt(final byte[] keyBytes) {
        return sha512Crypt(keyBytes, null);
    }

    /**
     * Generates a libc6 crypt() compatible "$6$" hash value.
     * <p>
     * See {@link Crypt#crypt(String, String)} for details.
     * 
     * @param keyBytes
     *            plaintext to hash
     * @param salt
     *            real salt value without prefix or "rounds="
     * @return complete hash value including salt
     * @throws IllegalArgumentException
     *             if the salt does not match the allowed pattern
     * @throws RuntimeException
     *             when a {@link java.security.NoSuchAlgorithmException} is caught.
     */
    public static String sha512Crypt(final byte[] keyBytes, String salt) {
        if (salt == null) {
            salt = SHA512_PREFIX + B64.getRandomSalt(8);
        }
        return sha2Crypt(keyBytes, salt, SHA512_PREFIX, SHA512_BLOCKSIZE, MessageDigestAlgorithms.SHA_512);
    }
}