+ 5) mod 8 ^ b(i + 6) mod 8 ^
            b(i + 7) mod 8 ^ ci
      for 0 <= i < 8, where bi is the ith bit of the byte, and ci is the
      ith bit of a byte c with the value {63} or {01100011}.

      It is possible to traverse every possible value in a Galois field
      using what is referred to as a 'generator'. There are many
      generators (128 out of 256): 3,5,6,9,11,82 to name a few. To fully
      traverse GF we iterate 255 times, multiplying by our generator
      each time.

      On each iteration we can determine the multiplicative inverse for
      the current element.

      Suppose there is an element in GF 'e'. For a given generator 'g',
      e = g^x. The multiplicative inverse of e is g^(255 - x). It turns
      out that if use the inverse of a generator as another generator
      it will produce all of the corresponding multiplicative inverses
      at the same time. For this reason, we choose 5 as our inverse
      generator because it only requires 2 multiplies and 1 add and its
      inverse, 82, requires relatively few operations as well.

      In order to apply the affine transformation, the multiplicative
      inverse 'ei' of 'e' can be repeatedly XOR'd (4 times) with a
      bit-cycling of 'ei'. To do this 'ei' is first stored in 's' and
      'x'. Then 's' is left shifted and the high bit of 's' is made the
      low bit. The resulting value is stored in 's'. Then 'x' is XOR'd
      with 's' and stored in 'x'. On each subsequent iteration the same
      operation is performed. When 4 iterations are complete, 'x' is
      XOR'd with 'c' (0x63) and the transformed value is stored in 'x'.
      For example:

      s = 01000001
      x = 01000001

      iteration 1: s = 10000010, x ^= s
      iteration 2: s = 00000101, x ^= s
      iteration 3: s = 00001010, x ^= s
      iteration 4: s = 00010100, x ^= s
      x ^= 0x63

      This can be done with a loop where s = (s << 1) | (s >> 7). However,
      it can also be done by using a single 16-bit (in this case 32-bit)
      number 'sx'. Since XOR is an associative operation, we can set 'sx'
      to 'ei' and then XOR it with 'sx' left-shifted 1,2,3, and 4 times.
      The most significant bits will flow into the high 8 bit positions
      and be correctly XOR'd with one another. All that remains will be
      to cycle the high 8 bits by XOR'ing them all with the lower 8 bits
      afterwards.

      At the same time we're populating sbox and isbox we can precompute
      the multiplication we'll need to do to do MixColumns() later.
    */

    // apply affine transformation
    sx = ei ^ (ei << 1) ^ (ei << 2) ^ (ei << 3) ^ (ei << 4);
    sx = (sx >> 8) ^ (sx & 255) ^ 0x63;

    // update tables
    sbox[e] = sx;
    isbox[sx] = e;

    /* Mixing columns is done using matrix multiplication. The columns
      that are to be mixed are each a single word in the current state.
      The state has Nb columns (4 columns). Therefore each column is a
      4 byte word. So to mix the columns in a single column 'c' where
      its rows are r0, r1, r2, and r3, we use the following matrix
      multiplication:

      [2 3 1 1]*[r0,c]=[r'0,c]
      [1 2 3 1] [r1,c] [r'1,c]
      [1 1 2 3] [r2,c] [r'2,c]
      [3 1 1 2] [r3,c] [r'3,c]

      r0, r1, r2, and r3 are each 1 byte of one of the words in the
      state (a column). To do matrix multiplication for each mixed
      column c' we multiply the corresponding row from the left matrix
      with the corresponding column from the right matrix. In total, we
      get 4 equations:

      r0,c' = 2*r0,c + 3*r1,c + 1*r2,c + 1*r3,c
      r1,c' = 1*r0,c + 2*r1,c + 3*r2,c + 1*r3,c
      r2,c' = 1*r0,c + 1*r1,c + 2*r2,c + 3*r3,c
      r3,c' = 3*r0,c + 1*r1,c + 1*r2,c + 2*r3,c

      As usual, the multiplication is as previously defined and the
      addition is XOR. In order to optimize mixing columns we can store
      the multiplication results in tables. If you think of the whole
      column as a word (it might help to visualize by mentally rotating
      the equations above by counterclockwise 90 degrees) then you can
      see that it would be useful to map the multiplications performed on
      each byte (r0, r1, r2, r3) onto a word as well. For instance, we
      could map 2*r0,1*r0,1*r0,3*r0 onto a word by storing 2*r0 in the
      highest 8 bits and 3*r0 in the lowest 8 bits (with the other two
      respectively in the middle). This means that a table can be
      constructed that uses r0 as an index to the word. We can do the
      same with r1, r2, and r3, creating a total of 4 tables.

      To construct a full c', we can just look up each byte of c in
      their respective tables and XOR the results together.

      Also, to build each table we only have to calculate the word
      for 2,1,1,3 for every byte ... which we can do on each iteration
      of this loop since we will iterate over every byte. After we have
      calculated 2,1,1,3 we can get the results for the other tables
      by cycling the byte at the end to the beginning. For instance
      we can take the result of table 2,1,1,3 and produce table 3,2,1,1
      by moving the right most byte to the left most position just like
      how you can imagine the 3 moved out of 2,1,1,3 and to the front
      to produce 3,2,1,1.

      There is another optimization in that the same multiples of
      the current element we need in order to advance our generator
      to the next iteration can be reused in performing the 2,1,1,3
      calculation. We also calculate the inverse mix column tables,
      with e,9,d,b being the inverse of 2,1,1,3.

      When we're done, and we need to actually mix columns, the first
      byte of each state word should be put through mix[0] (2,1,1,3),
      the second through mix[1] (3,2,1,1) and so forth. Then they should
      be XOR'd together to produce the fully mixed column.
    */

    // calculate mix and imix table values
    sx2 = xtime[sx];
    e2 = xtime[e];
    e4 = xtime[e2];
    e8 = xtime[e4];
    me =
      (sx2 << 24) ^  // 2
      (sx << 16) ^   // 1
      (sx << 8) ^    // 1
      (sx ^ sx2);    // 3
    ime =
      (e2 ^ e4 ^ e8) << 24 ^  // E (14)
      (e ^ e8) << 16 ^        // 9
      (e ^ e4 ^ e8) << 8 ^    // D (13)
      (e ^ e2 ^ e8);          // B (11)
    // produce each of the mix tables by rotating the 2,1,1,3 value
    for(var n = 0; n < 4; ++n) {
      mix[n][e] = me;
      imix[n][sx] = ime;
      // cycle the right most byte to the left most position
      // ie: 2,1,1,3 becomes 3,2,1,1
      me = me << 24 | me >>> 8;
      ime = ime << 24 | ime >>> 8;
    }

    // get next element and inverse
    if(e === 0) {
      // 1 is the inverse of 1
      e = ei = 1;
    } else {
      // e = 2e + 2*2*2*(10e)) = multiply e by 82 (chosen generator)
      // ei = ei + 2*2*ei = multiply ei by 5 (inverse generator)
      e = e2 ^ xtime[xtime[xtime[e2 ^ e8]]];
      ei ^= xtime[xtime[ei]];
    }
  }
}

/**
 * Generates a key schedule using the AES key expansion algorithm.
 *
 * The AES algorithm takes the Cipher Key, K, and performs a Key Expansion
 * routine to generate a key schedule. The Key Expansion generates a total
 * of Nb*(Nr + 1) words: the algorithm requires an initial set of Nb words,
 * and each of the Nr rounds requires Nb words of key data. The resulting
 * key schedule consists of a linear array of 4-byte words, denoted [wi ],
 * with i in the range 0 <= i < Nb(Nr + 1).
 *
 * KeyExpansion(byte key[4*Nk], word w[Nb*(Nr+1)], Nk)
 * AES-128 (Nb=4, Nk=4, Nr=10)
 * AES-192 (Nb=4, Nk=6, Nr=12)
 * AES-256 (Nb=4, Nk=8, Nr=14)
 * Note: Nr=Nk+6.
 *
 * Nb is the number of columns (32-bit words) comprising the State (or
 * number of bytes in a block). For AES, Nb=4.
 *
 * @param key the key to schedule (as an array of 32-bit words).
 * @param decrypt true to modify the key schedule to decrypt, false not to.
 *
 * @return the generated key schedule.
 */
function _expandKey(key, decrypt) {
  // copy the key's words to initialize the key schedule
  var w = key.slice(0);

  /* RotWord() will rotate a word, moving the first byte to the last
    byte's position (shifting the other bytes left).

    We will be getting the value of Rcon at i / Nk. 'i' will iterate
    from Nk to (Nb * Nr+1). Nk = 4 (4 byte key), Nb = 4 (4 words in
    a block), Nr = Nk + 6 (10). Therefore 'i' will iterate from
    4 to 44 (exclusive). Each time we iterate 4 times, i / Nk will
    increase by 1. We use a counter iNk to keep track of this.
   */

  // go through the rounds expanding the key
  var temp, iNk = 1;
  var Nk = w.length;
  var Nr1 = Nk + 6 + 1;
  var end = Nb * Nr1;
  for(var i = Nk; i < end; ++i) {
    temp = w[i - 1];
    if(i % Nk === 0) {
      // temp = SubWord(RotWord(temp)) ^ Rcon[i / Nk]
      temp =
        sbox[temp >>> 16 & 255] << 24 ^
        sbox[temp >>> 8 & 255] << 16 ^
        sbox[temp & 255] << 8 ^
        sbox[temp >>> 24] ^ (rcon[iNk] << 24);
      iNk++;
    } else if(Nk > 6 && (i % Nk === 4)) {
      // temp = SubWord(temp)
      temp =
        sbox[temp >>> 24] << 24 ^
        sbox[temp >>> 16 & 255] << 16 ^
        sbox[temp >>> 8 & 255] << 8 ^
        sbox[temp & 255];
    }
    w[i] = w[i - Nk] ^ temp;
  }

  /* When we are updating a cipher block we always use the code path for
     encryption whether we are decrypting or not (to shorten code and
     simplify the generation of look up tables). However, because there
     are differences in the decryption algorithm, other than just swapping
     in different look up tables, we must transform our key schedule to
     account for these changes:

     1. The decryption algorithm gets its key rounds in reverse order.
     2. The decryption algorithm adds the round key before mixing columns
       instead of afterwards.

     We don't need to modify our key schedule to handle the first case,
     we can just traverse the key schedule in reverse order when decrypting.

     The second case requires a little work.

     The tables we built for performing rounds will take an input and then
     perform SubBytes() and MixColumns() or, for the decrypt version,
     InvSubBytes() and InvMixColumns(). But the decrypt algorithm requires
     us to AddRoundKey() before InvMixColumns(). This means we'll need to
     apply some transformations to the round key to inverse-mix its columns
     so they'll be correct for moving AddRoundKey() to after the state has
     had its columns inverse-mixed.

     To inverse-mix the columns of the state when we're decrypting we use a
     lookup table that will apply InvSubBytes() and InvMixColumns() at the
     same time. However, the round key's bytes are not inverse-substituted
     in the decryption algorithm. To get around this problem, we can first
     substitute the bytes in the round key so that when we apply the
     transformation via the InvSubBytes()+InvMixColumns() table, it will
     undo our substitution leaving us with the original value that we
     want -- and then inverse-mix that value.

     This change will correctly alter our key schedule so that we can XOR
     each round key with our already transformed decryption state. This
     allows us to use the same code path as the encryption algorithm.

     We make one more change to the decryption key. Since the decryption
     algorithm runs in reverse from the encryption algorithm, we reverse
     the order of the round keys to avoid having to iterate over the key
     schedule backwards when running the encryption algorithm later in
     decryption mode. In addition to reversing the order of the round keys,
     we also swap each round key's 2nd and 4th rows. See the comments
     section where rounds are performed for more details about why this is
     done. These changes are done inline with the other substitution
     described above.
  */
  if(decrypt) {
    var tmp;
    var m0 = imix[0];
    var m1 = imix[1];
    var m2 = imix[2];
    var m3 = imix[3];
    var wnew = w.slice(0);
    end = w.length;
    for(var i = 0, wi = end - Nb; i < end; i += Nb, wi -= Nb) {
      // do not sub the first or last round key (round keys are Nb
      // words) as no column mixing is performed before they are added,
      // but do change the key order
      if(i === 0 || i === (end - Nb)) {
        wnew[i] = w[wi];
        wnew[i + 1] = w[wi + 3];
        wnew[i + 2] = w[wi + 2];
        wnew[i + 3] = w[wi + 1];
      } else {
        // substitute each round key byte because the inverse-mix
        // table will inverse-substitute it (effectively cancel the
        // substitution because round key bytes aren't sub'd in
        // decryption mode) and swap indexes 3 and 1
        for(var n = 0; n < Nb; ++n) {
          tmp = w[wi + n];
          wnew[i + (3&-n)] =
            m0[sbox[tmp >>> 24]] ^
            m1[sbox[tmp >>> 16 & 255]] ^
            m2[sbox[tmp >>> 8 & 255]] ^
            m3[sbox[tmp & 255]];
        }
      }
    }
    w = wnew;
  }

  return w;
}

/**
 * Updates a single block (16 bytes) using AES. The update will either
 * encrypt or decrypt the block.
 *
 * @param w the key schedule.
 * @param input the input block (an array of 32-bit words).
 * @param output the updated output block.
 * @param decrypt true to decrypt the block, false to encrypt it.
 */
function _updateBlock(w, input, output, decrypt) {
  /*
  Cipher(byte in[4*Nb], byte out[4*Nb], word w[Nb*(Nr+1)])
  begin
    byte state[4,Nb]
    state = in
    AddRoundKey(state, w[0, Nb-1])
    for round = 1 step 1 to Nr-1
      SubBytes(state)
      ShiftRows(state)
      MixColumns(state)
      AddRoundKey(state, w[round*Nb, (round+1)*Nb-1])
    end for
    SubBytes(state)
    ShiftRows(state)
    AddRoundKey(state, w[Nr*Nb, (Nr+1)*Nb-1])
    out = state
  end

  InvCipher(byte in[4*Nb], byte out[4*Nb], word w[Nb*(Nr+1)])
  begin
    byte state[4,Nb]
    state = in
    AddRoundKey(state, w[Nr*Nb, (Nr+1)*Nb-1])
    for round = Nr-1 step -1 downto 1
      InvShiftRows(state)
      InvSubBytes(state)
      AddRoundKey(state, w[round*Nb, (round+1)*Nb-1])
      InvMixColumns(state)
    end for
    InvShiftRows(state)
    InvSubBytes(state)
    AddRoundKey(state, w[0, Nb-1])
    out = state
  end
  */

  // Encrypt: AddRoundKey(state, w[0, Nb-1])
  // Decrypt: AddRoundKey(state, w[Nr*Nb, (Nr+1)*Nb-1])
  var Nr = w.length / 4 - 1;
  var m0, m1, m2, m3, sub;
  if(decrypt) {
    m0 = imix[0];
    m1 = imix[1];
    m2 = imix[2];
    m3 = imix[3];
    sub = isbox;
  } else {
    m0 = mix[0];
    m1 = mix[1];
    m2 = mix[2];
    m3 = mix[3];
    sub = sbox;
  }
  var a, b, c, d, a2, b2, c2;
  a = input[0] ^ w[0];
  b = input[decrypt ? 3 : 1] ^ w[1];
  c = input[2] ^ w[2];
  d = input[decrypt ? 1 : 3] ^ w[3];
  var i = 3;

  /* In order to share code we follow the encryption algorithm when both
    encrypting and decrypting. To account for the changes required in the
    decryption algorithm, we use different lookup tables when decrypting
    and use a modified key schedule to account for the difference in the
    order of transformations applied when performing rounds. We also get
    key rounds in reverse order (relative to encryption). */
  for(var round = 1; round < Nr; ++round) {
    /* As described above, we'll be using table lookups to perform the
      column mixing. Each column is stored as a word in the state (the
      array 'input' has one column as a word at each index). In order to
      mix a column, we perform these transformations on each row in c,
      which is 1 byte in each word. The new column for c0 is c'0:

               m0      m1      m2      m3
      r0,c'0 = 2*r0,c0 + 3*r1,c0 + 1*r2,c0 + 1*r3,c0
      r1,c'0 = 1*r0,c0 + 2*r1,c0 + 3*r2,c0 + 1*r3,c0
      r2,c'0 = 1*r0,c0 + 1*r1,c0 + 2*r2,c0 + 3*r3,c0
      r3,c'0 = 3*r0,c0 + 1*r1,c0 + 1*r2,c0 + 2*r3,c0

      So using mix tables where c0 is a word with r0 being its upper
      8 bits and r3 being its lower 8 bits:

      m0[c0 >> 24] will yield this word: [2*r0,1*r0,1*r0,3*r0]
      ...
      m3[c0 & 255] will yield this word: [1*r3,1*r3,3*r3,2*r3]

      Therefore to mix the columns in each word in the state we
      do the following (& 255 omitted for brevity):
      c'0,r0 = m0[c0 >> 24] ^ m1[c1 >> 16] ^ m2[c2 >> 8] ^ m3[c3]
      c'0,r1 = m0[c0 >> 24] ^ m1[c1 >> 16] ^ m2[c2 >> 8] ^ m3[c3]
      c'0,r2 = m0[c0 >> 24] ^ m1[c1 >> 16] ^ m2[c2 >> 8] ^ m3[c3]
      c'0,r3 = m0[c0 >> 24] ^ m1[c1 >> 16] ^ m2[c2 >> 8] ^ m3[c3]

      However, before mixing, the algorithm requires us to perform
      ShiftRows(). The ShiftRows() transformation cyclically shifts the
      last 3 rows of the state over different offsets. The first row
      (r = 0) is not shifted.

      s'_r,c = s_r,(c + shift(r, Nb) mod Nb
      for 0 < r < 4 and 0 <= c < Nb and
      shift(1, 4) = 1
      shift(2, 4) = 2
      shift(3, 4) = 3.

      This causes the first byte in r = 1 to be moved to the end of
      the row, the first 2 bytes in r = 2 to be moved to the end of
      the row, the first 3 bytes in r = 3 to be moved to the end of
      the row:

      r1: [c0 c1 c2 c3] => [c1 c2 c3 c0]
      r2: [c0 c1 c2 c3]    [c2 c3 c0 c1]
      r3: [c0 c1 c2 c3]    [c3 c0 c1 c2]

      We can make these substitutions inline with our column mixing to
      generate an updated set of equations to produce each word in the
      state (note the columns have changed positions):

      c0 c1 c2 c3 => c0 c1 c2 c3
      c0 c1 c2 c3    c1 c2 c3 c0  (cycled 1 byte)
      c0 c1 c2 c3    c2 c3 c0 c1  (cycled 2 bytes)
      c0 c1 c2 c3    c3 c0 c1 c2  (cycled 3 bytes)

      Therefore:

      c'0 = 2*r0,c0 + 3*r1,c1 + 1*r2,c2 + 1*r3,c3
      c'0 = 1*r0,c0 + 2*r1,c1 + 3*r2,c2 + 1*r3,c3
      c'0 = 1*r0,c0 + 1*r1,c1 + 2*r2,c2 + 3*r3,c3
      c'0 = 3*r0,c0 + 1*r1,c1 + 1*r2,c2 + 2*r3,c3

      c'1 = 2*r0,c1 + 3*r1,c2 + 1*r2,c3 + 1*r3,c0
      c'1 = 1*r0,c1 + 2*r1,c2 + 3*r2,c3 + 1*r3,c0
      c'1 = 1*r0,c1 + 1*r1,c2 + 2*r2,c3 + 3*r3,c0
      c'1 = 3*r0,c1 + 1*r1,c2 + 1*r2,c3 + 2*r3,c0

      ... and so forth for c'2 and c'3. The important distinction is
      that the columns are cycling, with c0 being used with the m0
      map when calculating c0, but c1 being used with the m0 map when
      calculating c1 ... and so forth.

      When performing the inverse we transform the mirror image and
      skip the bottom row, instead of the top one, and move upwards:

      c3 c2 c1 c0 => c0 c3 c2 c1  (cycled 3 bytes) *same as encryption
      c3 c2 c1 c0    c1 c0 c3 c2  (cycled 2 bytes)
      c3 c2 c1 c0    c2 c1 c0 c3  (cycled 1 byte)  *same as encryption
      c3 c2 c1 c0    c3 c2 c1 c0

      If you compare the resulting matrices for ShiftRows()+MixColumns()
      and for InvShiftRows()+InvMixColumns() the 2nd and 4th columns are
      different (in encrypt mode vs. decrypt mode). So in order to use
      the same code to handle both encryption and decryption, we will
      need to do some mapping.

      If in encryption mode we let a=c0, b=c1, c=c2, d=c3, and r<N> be
      a row number in the state, then the resulting matrix in encryption
      mode for applying the above transformations would be:

      r1: a b c d
      r2: b c d a
      r3: c d a b
      r4: d a b c

      If we did the same in decryption mode we would get:

      r1: a d c b
      r2: b a d c
      r3: c b a d
      r4: d c b a

      If instead we swap d and b (set b=c3 and d=c1), then we get:

      r1: a b c d
      r2: d a b c
      r3: c d a b
      r4: b c d a

      Now the 1st and 3rd rows are the same as the encryption matrix. All
      we need to do then to make the mapping exactly the same is to swap
      the 2nd and 4th rows when in decryption mode. To do this without
      having to do it on each iteration, we swapped the 2nd and 4th rows
      in the decryption key schedule. We also have to do the swap above
      when we first pull in the input and when we set the final output. */
    a2 =
      m0[a >>> 24] ^
      m1[b >>> 16 & 255] ^
      m2[c >>> 8 & 255] ^
      m3[d & 255] ^ w[++i];
    b2 =
      m0[b >>> 24] ^
      m1[c >>> 16 & 255] ^
      m2[d >>> 8 & 255] ^
      m3[a & 255] ^ w[++i];
    c2 =
      m0[c >>> 24] ^
      m1[d >>> 16 & 255] ^
      m2[a >>> 8 & 255] ^
      m3[b & 255] ^ w[++i];
    d =
      m0[d >>> 24] ^
      m1[a >>> 16 & 255] ^
      m2[b >>> 8 & 255] ^
      m3[c & 255] ^ w[++i];
    a = a2;
    b = b2;
    c = c2;
  }

  /*
    Encrypt:
    SubBytes(state)
    ShiftRows(state)
    AddRoundKey(state, w[Nr*Nb, (Nr+1)*Nb-1])

    Decrypt:
    InvShiftRows(state)
    InvSubBytes(state)
    AddRoundKey(state, w[0, Nb-1])
   */
  // Note: rows are shifted inline
  output[0] =
    (sub[a >>> 24] << 24) ^
    (sub[b >>> 16 & 255] << 16) ^
    (sub[c >>> 8 & 255] << 8) ^
    (sub[d & 255]) ^ w[++i];
  output[decrypt ? 3 : 1] =
    (sub[b >>> 24] << 24) ^
    (sub[c >>> 16 & 255] << 16) ^
    (sub[d >>> 8 & 255] << 8) ^
    (sub[a & 255]) ^ w[++i];
  output[2] =
    (sub[c >>> 24] << 24) ^
    (sub[d >>> 16 & 255] << 16) ^
    (sub[a >>> 8 & 255] << 8) ^
    (sub[b & 255]) ^ w[++i];
  output[decrypt ? 1 : 3] =
    (sub[d >>> 24] << 24) ^
    (sub[a >>> 16 & 255] << 16) ^
    (sub[b >>> 8 & 255] << 8) ^
    (sub[c & 255]) ^ w[++i];
}

/**
 * Deprecated. Instead, use:
 *
 * forge.cipher.createCipher('AES-<mode>', key);
 * forge.cipher.createDecipher('AES-<mode>', key);
 *
 * Creates a deprecated AES cipher object. This object's mode will default to
 * CBC (cipher-block-chaining).
 *
 * The key and iv may be given as a string of bytes, an array of bytes, a
 * byte buffer, or an array of 32-bit words.
 *
 * @param options the options to use.
 *          key the symmetric key to use.
 *          output the buffer to write to.
 *          decrypt true for decryption, false for encryption.
 *          mode the cipher mode to use (default: 'CBC').
 *
 * @return the cipher.
 */
function _createCipher(options) {
  options = options || {};
  var mode = (options.mode || 'CBC').toUpperCase();
  var algorithm = 'AES-' + mode;

  var cipher;
  if(options.decrypt) {
    cipher = forge.cipher.createDecipher(algorithm, options.key);
  } else {
    cipher = forge.cipher.createCipher(algorithm, options.key);
  }

  // backwards compatible start API
  var start = cipher.start;
  cipher.start = function(iv, options) {
    // backwards compatibility: support second arg as output buffer
    var output = null;
    if(options instanceof forge.util.ByteBuffer) {
      output = options;
      options = {};
    }
    options = options || {};
    options.output = output;
    options.iv = iv;
    start.call(cipher, options);
  };

  return cipher;
}
