JSF “Error Mac did not verify!” [duplicate]

ByteArrayGuard class randomly outputs "ERROR: MAC did not verify!" messages to the error console. This only occurs when client side viewstate encryption is enabled and that multiple clients are running against the server.
So you must change that class in your Java Server Faces references with...
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package com.sun.faces.renderkit;

import com.sun.faces.util.FacesLogger;
import java.security.SecureRandom;
import java.util.Arrays;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.Mac;
import javax.crypto.SecretKey;
import javax.crypto.spec.IvParameterSpec;
import javax.faces.FacesException;

/**
 * <p>This utility class is to provide both encryption and
 * decryption <code>Ciphers</code> to <code>ResponseStateManager</code>
 * implementations wishing to provide encryption support.</p>
 * 
 * <p>The algorithm used to encrypt byte array is AES with CBC.</p>
 *  
 * <p>Original author Inderjeet Singh, J2EE Blue Prints Team. Modified to suit JSF
 * needs.</p> 
 */
public final class ByteArrayGuard {


     // Log instance for this class
    private static final Logger LOGGER = FacesLogger.RENDERKIT.getLogger();

    private static final int MAC_LENGTH = 32;
    private static final int KEY_LENGTH = 128;
    private static final int IV_LENGTH = 16;

    private static final String KEY_ALGORITHM = "AES";
    private static final String CIPHER_CODE = "AES/CBC/PKCS5Padding";
    private static final String MAC_CODE = "HmacSHA256";
    private SecretKey sk;

    // ------------------------------------------------------------ Constructors

    public ByteArrayGuard() {

        try {
            setupKeyAndMac();
        } catch (Exception e) {
            if (LOGGER.isLoggable(Level.SEVERE)) { 
                LOGGER.log(Level.SEVERE,
                           "Unexpected exception initializing encryption."
                           + "  No encryption will be performed.",
                           e);
            }
            System.err.println("ERROR: Initializing Ciphers");
        }
    }

    // ---------------------------------------------------------- Public Methods    


    /**
     * This method:
     *    Encrypts bytes using a cipher.  
     *    Generates MAC for intialization vector of the cipher
     *    Generates MAC for encrypted data
     *    Returns a byte array consisting of the following concatenated together:
     *       |MAC for cnrypted Data | MAC for Init Vector | Encrypted Data |
     * @param bytes The byte array to be encrypted.
     * @return the encrypted byte array.
     */
    public byte[] encrypt(byte[] bytes) {
        byte[] securedata = null;
        try {
            // Generate IV
            SecureRandom rand = new SecureRandom();
            byte[] iv = new byte[16];
            rand.nextBytes(iv);
            IvParameterSpec ivspec = new IvParameterSpec(iv);
            Cipher encryptCipher = Cipher.getInstance(CIPHER_CODE);
            encryptCipher.init(Cipher.ENCRYPT_MODE, sk, ivspec);
            Mac encryptMac = Mac.getInstance(MAC_CODE);
            encryptMac.init(sk);
            encryptMac.update(iv);
            // encrypt the plaintext
            byte[] encdata = encryptCipher.doFinal(bytes);
            byte[] macBytes = encryptMac.doFinal(encdata);
            byte[] tmp = concatBytes(macBytes, iv);
            securedata = concatBytes(tmp, encdata);
        } catch (Exception e) {
            if (LOGGER.isLoggable(Level.SEVERE)) {
                LOGGER.log(Level.SEVERE,
                           "Unexpected exception initializing encryption."
                           + "  No encryption will be performed.",
                           e);
            }
            return null;
        }
        return securedata;
    }

    /**
     * This method decrypts the provided byte array.
     * The decryption is only performed if the regenerated MAC
     * is the same as the MAC for the received value.
     * @param bytes Encrypted byte array to be decrypted.
     * @return Decrypted byte array.
     */
    public byte[] decrypt(byte[] bytes) {
        try {
            // Extract MAC
            byte[] macBytes = new byte[MAC_LENGTH];
            System.arraycopy(bytes, 0, macBytes, 0, macBytes.length);

            // Extract IV
            byte[] iv = new byte[IV_LENGTH];
            System.arraycopy(bytes, macBytes.length, iv, 0, iv.length);

            // Extract encrypted data
            byte[] encdata = new byte[bytes.length - macBytes.length - iv.length];
            System.arraycopy(bytes, macBytes.length + iv.length, encdata, 0, encdata.length);

            IvParameterSpec ivspec = new IvParameterSpec(iv);
            Cipher decryptCipher = Cipher.getInstance(CIPHER_CODE);
            decryptCipher.init(Cipher.DECRYPT_MODE, sk, ivspec);

            // verify MAC by regenerating it and comparing it with the received value
            Mac decryptMac = Mac.getInstance(MAC_CODE);
            decryptMac.init(sk);
            decryptMac.update(iv);
            decryptMac.update(encdata);
            byte[] macBytesCalculated = decryptMac.doFinal();
            if (Arrays.equals(macBytes, macBytesCalculated)) {
                // continue only if the MAC was valid
                // System.out.println("Valid MAC found!");
                byte[] plaindata = decryptCipher.doFinal(encdata);
                return plaindata;
            } else {
                System.err.println("ERROR: MAC did not verify!");
                return null;
            }
        } catch (Exception e) {
            System.err.println("ERROR: Decrypting:"+e.getCause());
            return null; // Signal to JSF runtime
        }
    }

    // --------------------------------------------------------- Private Methods

    /**
     * Generates secret key.
     * Initializes MAC(s).
     */
    private void setupKeyAndMac() {

        try {
            KeyGenerator kg = KeyGenerator.getInstance(KEY_ALGORITHM);
            kg.init(KEY_LENGTH);   // 256 if you're using the Unlimited Policy Files
            sk = kg.generateKey(); 

        } catch (Exception e) {
            throw new FacesException(e);
        }
    }

    /**
     * This method concatenates two byte arrays
     * @return a byte array of array1||array2
     * @param array1 first byte array to be concatenated
     * @param array2 second byte array to be concatenated
     */
    private static byte[] concatBytes(byte[] array1, byte[] array2) {
        byte[] cBytes = new byte[array1.length + array2.length];
        try {
            System.arraycopy(array1, 0, cBytes, 0, array1.length);
            System.arraycopy(array2, 0, cBytes, array1.length, array2.length);
        } catch(Exception e) {
            throw new FacesException(e);
        }
        return cBytes;
    }    
}
And reload this jar. JSF_REFERENCE