PasswordEncoderUtils.java

/*
 * Copyright 2002-2016 the original author or authors.
 *
 * Licensed 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.springframework.security.authentication.encoding;

import org.springframework.security.crypto.codec.Utf8;

/**
 * Utility for constant time comparison to prevent against timing attacks.
 *
 * @author Rob Winch
 */
class PasswordEncoderUtils {

	/**
	 * Constant time comparison to prevent against timing attacks.
	 * @param expected
	 * @param actual
	 * @return
	 */
	static boolean equals(String expected, String actual) {
		byte[] expectedBytes = bytesUtf8(expected);
		byte[] actualBytes = bytesUtf8(actual);
		int expectedLength = expectedBytes == null ? -1 : expectedBytes.length;
		int actualLength = actualBytes == null ? -1 : actualBytes.length;

		int result = expectedLength == actualLength ? 0 : 1;
		for (int i = 0; i < actualLength; i++) {
			byte expectedByte = expectedLength <= 0 ? 0 : expectedBytes[i % expectedLength];
			byte actualByte = actualBytes[i % actualLength];
			result |= expectedByte ^ actualByte;
		}
		return result == 0;
	}

	private static byte[] bytesUtf8(String s) {
		if (s == null) {
			return null;
		}

		return Utf8.encode(s); // need to check if Utf8.encode() runs in constant time (probably not). This may leak length of string.
	}

	private PasswordEncoderUtils() {
	}
}