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// Copyright 2008 Google Inc.
//
// 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 com.google.gerrit.server.mail;

import com.google.common.io.BaseEncoding;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Arrays;
import javax.crypto.Mac;
import javax.crypto.ShortBufferException;
import javax.crypto.spec.SecretKeySpec;
import org.apache.commons.codec.binary.Base64;

/**
 * Utility function to compute and verify XSRF tokens.
 *
 * 

{@link SignedTokenEmailTokenVerifier} uses this class to verify tokens appearing in the custom * xsrfKey * JSON request property. The tokens protect against cross-site request forgery by depending * upon the browser's security model. The classic browser security model prohibits a script from * site A from reading any data received from site B. By sending unforgeable tokens from the server * and asking the client to return them to us, the client script must have had read access to the * token at some point and is therefore also from our server. */ public class SignedToken { private static final int INT_SZ = 4; private static final String MAC_ALG = "HmacSHA1"; /** * Generate a random key for use with the XSRF library. * * @return a new private key, base 64 encoded. */ public static String generateRandomKey() { final byte[] r = new byte[26]; new SecureRandom().nextBytes(r); return encodeBase64PrivateKey(r); } private final int maxAge; private final SecretKeySpec key; private final SecureRandom rng; private final int tokenLength; /** * Create a new utility, using the specific key. * * @param age the number of seconds a token may remain valid. * @param keyBase64 base 64 encoded representation of the key. * @throws XsrfException the JVM doesn't support the necessary algorithms. */ public SignedToken(final int age, final String keyBase64) throws XsrfException { maxAge = age > 5 ? age / 5 : age; key = new SecretKeySpec(decodeBase64PrivateKey(keyBase64), MAC_ALG); rng = new SecureRandom(); tokenLength = 2 * INT_SZ + newMac().getMacLength(); } /** * Generate a new signed token. * * @param text the text string to sign. Typically this should be some user-specific string, to * prevent replay attacks. The text must be safe to appear in whatever context the token * itself will appear, as the text is included on the end of the token. * @return the signed token. The text passed in text will appear after the first ',' * in the returned token string. * @throws XsrfException the JVM doesn't support the necessary algorithms. */ String newToken(final String text) throws XsrfException { final int q = rng.nextInt(); final byte[] buf = new byte[tokenLength]; encodeInt(buf, 0, q); encodeInt(buf, INT_SZ, now() ^ q); computeToken(buf, text); return encodeBase64(buf) + '$' + text; } /** * Validate a returned token. If the token is valid then return a {@link ValidToken}, else will * throw {@link XsrfException} when it's an unexpected token overflow or {@link * CheckTokenException} when it's an illegal token string format. * * @param tokenString a token string previously created by this class. * @param text text that must have been used during {@link #newToken(String)} in order for the * token to be valid. If null the text will be taken from the token string itself. * @return the token which is valid. * @throws XsrfException the JVM doesn't support the necessary algorithms to generate a token. * XSRF services are simply not available. * @throws CheckTokenException throws when token is null, the empty string, has expired, does not * match the text supplied, or is a forged token. */ public ValidToken checkToken(final String tokenString, final String text) throws XsrfException, CheckTokenException { if (tokenString == null || tokenString.length() == 0) { throw new CheckTokenException("Empty token"); } final int s = tokenString.indexOf('$'); if (s <= 0) { throw new CheckTokenException("Token does not contain character '$'"); } final String recvText = tokenString.substring(s + 1); final byte[] in; try { in = decodeBase64(tokenString.substring(0, s)); } catch (RuntimeException e) { throw new CheckTokenException("Base64 decoding failed", e); } if (in.length != tokenLength) { throw new CheckTokenException("Token length mismatch"); } final int q = decodeInt(in, 0); final int c = decodeInt(in, INT_SZ) ^ q; final int n = now(); if (maxAge > 0 && Math.abs(c - n) > maxAge) { throw new CheckTokenException("Token is expired"); } final byte[] gen = new byte[tokenLength]; System.arraycopy(in, 0, gen, 0, 2 * INT_SZ); computeToken(gen, text != null ? text : recvText); if (!Arrays.equals(gen, in)) { throw new CheckTokenException("Token text mismatch"); } return new ValidToken(maxAge > 0 && c + (maxAge >> 1) <= n, recvText); } private void computeToken(final byte[] buf, final String text) throws XsrfException { final Mac m = newMac(); m.update(buf, 0, 2 * INT_SZ); m.update(toBytes(text)); try { m.doFinal(buf, 2 * INT_SZ); } catch (ShortBufferException e) { throw new XsrfException("Unexpected token overflow", e); } } private Mac newMac() throws XsrfException { try { final Mac m = Mac.getInstance(MAC_ALG); m.init(key); return m; } catch (NoSuchAlgorithmException e) { throw new XsrfException(MAC_ALG + " not supported", e); } catch (InvalidKeyException e) { throw new XsrfException("Invalid private key", e); } } private static int now() { return (int) (System.currentTimeMillis() / 5000L); } private static byte[] decodeBase64PrivateKey(final String privateKeyBase64String) { return Base64.decodeBase64(toBytes(privateKeyBase64String)); } private static String encodeBase64PrivateKey(final byte[] buf) { return toString(Base64.encodeBase64(buf)); } private static byte[] decodeBase64(final String s) { return BaseEncoding.base64Url().decode(s); } private static String encodeBase64(final byte[] buf) { return BaseEncoding.base64Url().encode(buf); } private static void encodeInt(final byte[] buf, final int o, final int v) { int _v = v; buf[o + 3] = (byte) _v; _v >>>= 8; buf[o + 2] = (byte) _v; _v >>>= 8; buf[o + 1] = (byte) _v; _v >>>= 8; buf[o] = (byte) _v; } private static int decodeInt(final byte[] buf, final int o) { int r = buf[o] << 8; r |= buf[o + 1] & 0xff; r <<= 8; r |= buf[o + 2] & 0xff; return (r << 8) | (buf[o + 3] & 0xff); } private static byte[] toBytes(final String s) { final byte[] r = new byte[s.length()]; for (int k = r.length - 1; k >= 0; k--) { r[k] = (byte) s.charAt(k); } return r; } private static String toString(final byte[] b) { final StringBuilder r = new StringBuilder(b.length); for (int i = 0; i < b.length; i++) { r.append((char) b[i]); } return r.toString(); } }





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