PseudoRandomFunction Class — netty Architecture

Source Code

handler/src/main/java/io/netty/handler/ssl/PseudoRandomFunction.java lines 36–94

final class PseudoRandomFunction {

    /**
     * Constructor never to be called.
     */
    private PseudoRandomFunction() {
    }

    /**
     * Use a single hash function to expand a secret and seed into an
     * arbitrary quantity of output.
     *
     * P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) +
     *                        HMAC_hash(secret, A(2) + seed) +
     *                        HMAC_hash(secret, A(3) + seed) + ...
     * where + indicates concatenation.
     * A() is defined as:
     *       A(0) = seed
     *       A(i) = HMAC_hash(secret, A(i-1))
     * @param secret The starting secret to use for expansion
     * @param label An ascii string without a length byte or trailing null character.
     * @param seed The seed of the hash
     * @param length The number of bytes to return
     * @param algo the hmac algorithm to use
     * @return The expanded secrets
     * @throws IllegalArgumentException if the algo could not be found.
     */
    static byte[] hash(byte[] secret, byte[] label, byte[] seed, int length, String algo) {
        checkPositiveOrZero(length, "length");
        try {
            Mac hmac = Mac.getInstance(algo);
            hmac.init(new SecretKeySpec(secret, algo));
            /*
             * P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) +
             * HMAC_hash(secret, A(2) + seed) + HMAC_hash(secret, A(3) + seed) + ...
             * where + indicates concatenation. A() is defined as: A(0) = seed, A(i)
             * = HMAC_hash(secret, A(i-1))
             */

            int iterations = (int) Math.ceil(length / (double) hmac.getMacLength());
            byte[] expansion = EmptyArrays.EMPTY_BYTES;
            byte[] data = concat(label, seed);
            byte[] A = data;
            for (int i = 0; i < iterations; i++) {
                A = hmac.doFinal(A);
                expansion = concat(expansion, hmac.doFinal(concat(A, data)));
            }
            return Arrays.copyOf(expansion, length);
        } catch (GeneralSecurityException e) {
            throw new IllegalArgumentException("Could not find algo: " + algo, e);
        }
    }

    private static byte[] concat(byte[] first, byte[] second) {
        byte[] result = Arrays.copyOf(first, first.length + second.length);
        System.arraycopy(second, 0, result, first.length, second.length);
        return result;
    }
}