bintzwing/core/ccm.js

/** @fileOverview CCM mode implementation.
 *
 * Special thanks to Roy Nicholson for pointing out a bug in our
 * implementation.
 *
 * @author Emily Stark
 * @author Mike Hamburg
 * @author Dan Boneh
 */

/** @namespace CTR mode with CBC MAC. */
sjcl.mode.ccm = {
  /** The name of the mode.
   * @constant
   */
  name: "ccm",
  
  /** Encrypt in CCM mode.
   * @static
   * @param {Object} prf The pseudorandom function.  It must have a block size of 16 bytes.
   * @param {bitArray} plaintext The plaintext data.
   * @param {bitArray} iv The initialization value.
   * @param {bitArray} [adata=[]] The authenticated data.
   * @param {Number} [tlen=64] the desired tag length, in bits.
   * @return {bitArray} The encrypted data, an array of bytes.
   */
  encrypt: function(prf, plaintext, iv, adata, tlen) {
    var L, i, out = plaintext.slice(0), tag, w=sjcl.bitArray, ivl = w.bitLength(iv) / 8, ol = w.bitLength(out) / 8;
    tlen = tlen || 64;
    adata = adata || [];
    
    if (ivl < 7) {
      throw new sjcl.exception.invalid("ccm: iv must be at least 7 bytes");
    }
    
    // compute the length of the length
    for (L=2; L<4 && ol >>> 8*L; L++) {}
    if (L < 15 - ivl) { L = 15-ivl; }
    iv = w.clamp(iv,8*(15-L));
    
    // compute the tag
    tag = sjcl.mode.ccm._computeTag(prf, plaintext, iv, adata, tlen, L);
    
    // encrypt
    out = sjcl.mode.ccm._ctrMode(prf, out, iv, tag, tlen, L);
    
    return w.concat(out.data, out.tag);
  },
  
  /** Decrypt in CCM mode.
   * @static
   * @param {Object} prf The pseudorandom function.  It must have a block size of 16 bytes.
   * @param {bitArray} ciphertext The ciphertext data.
   * @param {bitArray} iv The initialization value.
   * @param {bitArray} [[]] adata The authenticated data.
   * @param {Number} [64] tlen the desired tag length, in bits.
   * @return {bitArray} The decrypted data.
   */
  decrypt: function(prf, ciphertext, iv, adata, tlen) {
    tlen = tlen || 64;
    adata = adata || [];
    var L, i, 
        w=sjcl.bitArray,
        ivl = w.bitLength(iv) / 8,
        ol = w.bitLength(ciphertext), 
        out = w.clamp(ciphertext, ol - tlen),
        tag = w.bitSlice(ciphertext, ol - tlen), tag2;
    

    ol = (ol - tlen) / 8;
        
    if (ivl < 7) {
      throw new sjcl.exception.invalid("ccm: iv must be at least 7 bytes");
    }
    
    // compute the length of the length
    for (L=2; L<4 && ol >>> 8*L; L++) {}
    if (L < 15 - ivl) { L = 15-ivl; }
    iv = w.clamp(iv,8*(15-L));
    
    // decrypt
    out = sjcl.mode.ccm._ctrMode(prf, out, iv, tag, tlen, L);
    
    // check the tag
    tag2 = sjcl.mode.ccm._computeTag(prf, out.data, iv, adata, tlen, L);
    if (!w.equal(out.tag, tag2)) {
      throw new sjcl.exception.corrupt("ccm: tag doesn't match");
    }
    
    return out.data;
  },

  /* Compute the (unencrypted) authentication tag, according to the CCM specification
   * @param {Object} prf The pseudorandom function.
   * @param {bitArray} plaintext The plaintext data.
   * @param {bitArray} iv The initialization value.
   * @param {bitArray} adata The authenticated data.
   * @param {Number} tlen the desired tag length, in bits.
   * @return {bitArray} The tag, but not yet encrypted.
   * @private
   */
  _computeTag: function(prf, plaintext, iv, adata, tlen, L) {
    // compute B[0]
    var q, mac, field = 0, offset = 24, tmp, i, macData = [], w=sjcl.bitArray, xor = w._xor4;

    tlen /= 8;
  
    // check tag length and message length
    if (tlen % 2 || tlen < 4 || tlen > 16) {
      throw new sjcl.exception.invalid("ccm: invalid tag length");
    }
  
    if (adata.length > 0xFFFFFFFF || plaintext.length > 0xFFFFFFFF) {
      // I don't want to deal with extracting high words from doubles.
      throw new sjcl.exception.bug("ccm: can't deal with 4GiB or more data");
    }

    // mac the flags
    mac = [w.partial(8, (adata.length ? 1<<6 : 0) | (tlen-2) << 2 | L-1)];

    // mac the iv and length
    mac = w.concat(mac, iv);
    mac[3] |= w.bitLength(plaintext)/8;
    mac = prf.encrypt(mac);
    
  
    if (adata.length) {
      // mac the associated data.  start with its length...
      tmp = w.bitLength(adata)/8;
      if (tmp <= 0xFEFF) {
        macData = [w.partial(16, tmp)];
      } else if (tmp <= 0xFFFFFFFF) {
        macData = w.concat([w.partial(16,0xFFFE)], [tmp]);
      } // else ...
    
      // mac the data itself
      macData = w.concat(macData, adata);
      for (i=0; i<macData.length; i += 4) {
        mac = prf.encrypt(xor(mac, macData.slice(i,i+4)));
      }
    }
  
    // mac the plaintext
    for (i=0; i<plaintext.length; i+=4) {
      mac = prf.encrypt(xor(mac, plaintext.slice(i,i+4)));
    }

    return w.clamp(mac, tlen * 8);
  },

  /** CCM CTR mode.
   * Encrypt or decrypt data and tag with the prf in CCM-style CTR mode.
   * May mutate its arguments.
   * @param {Object} prf The PRF.
   * @param {bitArray} data The data to be encrypted or decrypted.
   * @param {bitArray} iv The initialization vector.
   * @param {bitArray} tag The authentication tag.
   * @param {Number} tlen The length of th etag, in bits.
   * @param {Number} L The CCM L value.
   * @return {Object} An object with data and tag, the en/decryption of data and tag values.
   * @private
   */
  _ctrMode: function(prf, data, iv, tag, tlen, L) {
    var enc, i, w=sjcl.bitArray, xor = w._xor4, ctr, b, l = data.length, bl=w.bitLength(data);

    // start the ctr
    ctr = w.concat([w.partial(8,L-1)],iv).concat([0,0,0]).slice(0,4);
    
    // en/decrypt the tag
    tag = w.bitSlice(xor(tag,prf.encrypt(ctr)), 0, tlen);
  
    // en/decrypt the data
    if (!l) { return {tag:tag, data:[]}; }
    
    for (i=0; i<l; i+=4) {
      ctr[3]++;
      enc = prf.encrypt(ctr);
      data[i]   ^= enc[0];
      data[i+1] ^= enc[1];
      data[i+2] ^= enc[2];
      data[i+3] ^= enc[3];
    }
    return { tag:tag, data:w.clamp(data,bl) };
  }
};