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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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package sun.security.provider;

import java.io.IOException;
import java.security.MessageDigest;
import java.security.SecureRandomSpi;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;


This class provides a crytpographically strong pseudo-random number
generator based on the SHA-1 hash algorithm.

Note that if a seed is not provided, we attempt to provide sufficient
seed bytes to completely randomize the internal state of the generator
(20 bytes).  However, our seed generation algorithm has not been thoroughly
studied or widely deployed.

Also note that when a random object is deserialized,
engineNextBytes invoked on the
restored random object will yield the exact same (random) bytes as the
original object.  If this behaviour is not desired, the restored random
object should be seeded, using
engineSetSeed.

Author:Benjamin Renaud, Josh Bloch, Gadi Guy
/**
 * <p>This class provides a crytpographically strong pseudo-random number
 * generator based on the SHA-1 hash algorithm.
 *
 * <p>Note that if a seed is not provided, we attempt to provide sufficient
 * seed bytes to completely randomize the internal state of the generator
 * (20 bytes).  However, our seed generation algorithm has not been thoroughly
 * studied or widely deployed.
 *
 * <p>Also note that when a random object is deserialized,
 * <a href="#engineNextBytes(byte[])">engineNextBytes</a> invoked on the
 * restored random object will yield the exact same (random) bytes as the
 * original object.  If this behaviour is not desired, the restored random
 * object should be seeded, using
 * <a href="#engineSetSeed(byte[])">engineSetSeed</a>.
 *
 * @author Benjamin Renaud
 * @author Josh Bloch
 * @author Gadi Guy
 */

public final class SecureRandom extends SecureRandomSpi
implements java.io.Serializable {

    private static final long serialVersionUID = 3581829991155417889L;

    private static final int DIGEST_SIZE = 20;
    private transient MessageDigest digest;
    private byte[] state;
    private byte[] remainder;
    private int remCount;

    
This empty constructor automatically seeds the generator.  We attempt
to provide sufficient seed bytes to completely randomize the internal
state of the generator (20 bytes).  Note, however, that our seed
generation algorithm has not been thoroughly studied or widely deployed.

The first time this constructor is called in a given Virtual Machine,
it may take several seconds of CPU time to seed the generator, depending
on the underlying hardware.  Successive calls run quickly because they
rely on the same (internal) pseudo-random number generator for their
seed bits.

/**
     * This empty constructor automatically seeds the generator.  We attempt
     * to provide sufficient seed bytes to completely randomize the internal
     * state of the generator (20 bytes).  Note, however, that our seed
     * generation algorithm has not been thoroughly studied or widely deployed.
     *
     * <p>The first time this constructor is called in a given Virtual Machine,
     * it may take several seconds of CPU time to seed the generator, depending
     * on the underlying hardware.  Successive calls run quickly because they
     * rely on the same (internal) pseudo-random number generator for their
     * seed bits.
     */
    public SecureRandom() {
        init(null);
    }

    
This constructor is used to instantiate the private seeder object
with a given seed from the SeedGenerator.

Params:
  • seed – the seed.
/**
     * This constructor is used to instantiate the private seeder object
     * with a given seed from the SeedGenerator.
     *
     * @param seed the seed.
     */
    private SecureRandom(byte[] seed) {
        init(seed);
    }

    
This call, used by the constructors, instantiates the SHA digest
and sets the seed, if given.

/**
     * This call, used by the constructors, instantiates the SHA digest
     * and sets the seed, if given.
     */
    private void init(byte[] seed) {
        try {
            /*
             * Use the local SUN implementation to avoid native
             * performance overhead.
             */
            digest = MessageDigest.getInstance("SHA", "SUN");
        } catch (NoSuchProviderException | NoSuchAlgorithmException e) {
            // Fallback to any available.
            try {
                digest = MessageDigest.getInstance("SHA");
            } catch (NoSuchAlgorithmException exc) {
                throw new InternalError(
                    "internal error: SHA-1 not available.", exc);
            }
        }

        if (seed != null) {
           engineSetSeed(seed);
        }
    }

    
Returns the given number of seed bytes, computed using the seed
generation algorithm that this class uses to seed itself.  This
call may be used to seed other random number generators.  While
we attempt to return a "truly random" sequence of bytes, we do not
know exactly how random the bytes returned by this call are.  (See
the empty constructor SecureRandom
for a brief description of the underlying algorithm.)
The prudent user will err on the side of caution and get extra
seed bytes, although it should be noted that seed generation is
somewhat costly.

Params:
  • numBytes – the number of seed bytes to generate.
Returns:the seed bytes.
/**
     * Returns the given number of seed bytes, computed using the seed
     * generation algorithm that this class uses to seed itself.  This
     * call may be used to seed other random number generators.  While
     * we attempt to return a "truly random" sequence of bytes, we do not
     * know exactly how random the bytes returned by this call are.  (See
     * the empty constructor <a href = "#SecureRandom">SecureRandom</a>
     * for a brief description of the underlying algorithm.)
     * The prudent user will err on the side of caution and get extra
     * seed bytes, although it should be noted that seed generation is
     * somewhat costly.
     *
     * @param numBytes the number of seed bytes to generate.
     *
     * @return the seed bytes.
     */
    @Override
    public byte[] engineGenerateSeed(int numBytes) {
        // Neither of the SeedGenerator implementations require
        // locking, so no sync needed here.
        byte[] b = new byte[numBytes];
        SeedGenerator.generateSeed(b);
        return b;
    }

    
Reseeds this random object. The given seed supplements, rather than
replaces, the existing seed. Thus, repeated calls are guaranteed
never to reduce randomness.

Params:
  • seed – the seed.
/**
     * Reseeds this random object. The given seed supplements, rather than
     * replaces, the existing seed. Thus, repeated calls are guaranteed
     * never to reduce randomness.
     *
     * @param seed the seed.
     */
    @Override
    public synchronized void engineSetSeed(byte[] seed) {
        if (state != null) {
            digest.update(state);
            for (int i = 0; i < state.length; i++) {
                state[i] = 0;
            }
        }
        state = digest.digest(seed);
        remCount = 0;
    }

    private static void updateState(byte[] state, byte[] output) {
        int last = 1;
        int v;
        byte t;
        boolean zf = false;

        // state(n + 1) = (state(n) + output(n) + 1) % 2^160;
        for (int i = 0; i < state.length; i++) {
            // Add two bytes
            v = (int)state[i] + (int)output[i] + last;
            // Result is lower 8 bits
            t = (byte)v;
            // Store result. Check for state collision.
            zf = zf | (state[i] != t);
            state[i] = t;
            // High 8 bits are carry. Store for next iteration.
            last = v >> 8;
        }

        // Make sure at least one bit changes!
        if (!zf) {
           state[0]++;
        }
    }

    
This static object will be seeded by SeedGenerator, and used
to seed future instances of SHA1PRNG SecureRandoms.
Bloch, Effective Java Second Edition: Item 71

/**
     * This static object will be seeded by SeedGenerator, and used
     * to seed future instances of SHA1PRNG SecureRandoms.
     *
     * Bloch, Effective Java Second Edition: Item 71
     */
    private static class SeederHolder {

        private static final SecureRandom seeder;

        static {
            /*
             * Call to SeedGenerator.generateSeed() to add additional
             * seed material (likely from the Native implementation).
             */
            seeder = new SecureRandom(SeedGenerator.getSystemEntropy());
            byte [] b = new byte[DIGEST_SIZE];
            SeedGenerator.generateSeed(b);
            seeder.engineSetSeed(b);
        }
    }

    
Generates a user-specified number of random bytes.

Params:
  • result – the array to be filled in with random bytes.
/**
     * Generates a user-specified number of random bytes.
     *
     * @param result the array to be filled in with random bytes.
     */
    @Override
    public synchronized void engineNextBytes(byte[] result) {
        int index = 0;
        int todo;
        byte[] output = remainder;

        if (state == null) {
            byte[] seed = new byte[DIGEST_SIZE];
            SeederHolder.seeder.engineNextBytes(seed);
            state = digest.digest(seed);
        }

        // Use remainder from last time
        int r = remCount;
        if (r > 0) {
            // How many bytes?
            todo = (result.length - index) < (DIGEST_SIZE - r) ?
                        (result.length - index) : (DIGEST_SIZE - r);
            // Copy the bytes, zero the buffer
            for (int i = 0; i < todo; i++) {
                result[i] = output[r];
                output[r++] = 0;
            }
            remCount += todo;
            index += todo;
        }

        // If we need more bytes, make them.
        while (index < result.length) {
            // Step the state
            digest.update(state);
            output = digest.digest();
            updateState(state, output);

            // How many bytes?
            todo = (result.length - index) > DIGEST_SIZE ?
                DIGEST_SIZE : result.length - index;
            // Copy the bytes, zero the buffer
            for (int i = 0; i < todo; i++) {
                result[index++] = output[i];
                output[i] = 0;
            }
            remCount += todo;
        }

        // Store remainder for next time
        remainder = output;
        remCount %= DIGEST_SIZE;
    }

    /*
     * readObject is called to restore the state of the random object from
     * a stream.  We have to create a new instance of MessageDigest, because
     * it is not included in the stream (it is marked "transient").
     *
     * Note that the engineNextBytes() method invoked on the restored random
     * object will yield the exact same (random) bytes as the original.
     * If you do not want this behaviour, you should re-seed the restored
     * random object, using engineSetSeed().
     */
    private void readObject(java.io.ObjectInputStream s)
        throws IOException, ClassNotFoundException {

        s.defaultReadObject ();

        try {
            /*
             * Use the local SUN implementation to avoid native
             * performance overhead.
             */
            digest = MessageDigest.getInstance("SHA", "SUN");
        } catch (NoSuchProviderException | NoSuchAlgorithmException e) {
            // Fallback to any available.
            try {
                digest = MessageDigest.getInstance("SHA");
            } catch (NoSuchAlgorithmException exc) {
                throw new InternalError(
                    "internal error: SHA-1 not available.", exc);
            }
        }
    }
}