AES.js

/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
/*  AES implementation in JavaScript                     (c) Chris Veness 2005-2014 / MIT Licence */
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */

/* jshint node:true *//* global define */
'use strict';


/**
 * AES (Rijndael cipher) encryption routines,
 *
 * Reference implementation of FIPS-197 http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf.
 *
 * @namespace
 */
var Aes = {};


/**
 * AES Cipher function: encrypt 'input' state with Rijndael algorithm [§5.1];
 *   applies Nr rounds (10/12/14) using key schedule w for 'add round key' stage.
 *
 * @param   {number[]}   input - 16-byte (128-bit) input state array.
 * @param   {number[][]} w - Key schedule as 2D byte-array (Nr+1 x Nb bytes).
 * @returns {number[]}   Encrypted output state array.
 */
Aes.cipher = function(input, w) {
    var Nb = 4;               // block size (in words): no of columns in state (fixed at 4 for AES)
    var Nr = w.length/Nb - 1; // no of rounds: 10/12/14 for 128/192/256-bit keys

    var state = [[],[],[],[]];  // initialise 4xNb byte-array 'state' with input [§3.4]
    for (var i=0; i<4*Nb; i++) state[i%4][Math.floor(i/4)] = input[i];

    state = Aes.addRoundKey(state, w, 0, Nb);

    for (var round=1; round<Nr; round++) {
        state = Aes.subBytes(state, Nb);
        state = Aes.shiftRows(state, Nb);
        state = Aes.mixColumns(state, Nb);
        state = Aes.addRoundKey(state, w, round, Nb);
    }

    state = Aes.subBytes(state, Nb);
    state = Aes.shiftRows(state, Nb);
    state = Aes.addRoundKey(state, w, Nr, Nb);

    var output = new Array(4*Nb);  // convert state to 1-d array before returning [§3.4]
    for (var i=0; i<4*Nb; i++) output[i] = state[i%4][Math.floor(i/4)];

    return output;
};


/**
 * Perform key expansion to generate a key schedule from a cipher key [§5.2].
 *
 * @param   {number[]}   key - Cipher key as 16/24/32-byte array.
 * @returns {number[][]} Expanded key schedule as 2D byte-array (Nr+1 x Nb bytes).
 */
Aes.keyExpansion = function(key) {
    var Nb = 4;            // block size (in words): no of columns in state (fixed at 4 for AES)
    var Nk = key.length/4; // key length (in words): 4/6/8 for 128/192/256-bit keys
    var Nr = Nk + 6;       // no of rounds: 10/12/14 for 128/192/256-bit keys

    var w = new Array(Nb*(Nr+1));
    var temp = new Array(4);

    // initialise first Nk words of expanded key with cipher key
    for (var i=0; i<Nk; i++) {
        var r = [key[4*i], key[4*i+1], key[4*i+2], key[4*i+3]];
        w[i] = r;
    }

    // expand the key into the remainder of the schedule
    for (var i=Nk; i<(Nb*(Nr+1)); i++) {
        w[i] = new Array(4);
        for (var t=0; t<4; t++) temp[t] = w[i-1][t];
        // each Nk'th word has extra transformation
        if (i % Nk == 0) {
            temp = Aes.subWord(Aes.rotWord(temp));
            for (var t=0; t<4; t++) temp[t] ^= Aes.rCon[i/Nk][t];
        }
        // 256-bit key has subWord applied every 4th word
        else if (Nk > 6 && i%Nk == 4) {
            temp = Aes.subWord(temp);
        }
        // xor w[i] with w[i-1] and w[i-Nk]
        for (var t=0; t<4; t++) w[i][t] = w[i-Nk][t] ^ temp[t];
    }

    return w;
};


/**
 * Apply SBox to state S [§5.1.1]
 * @private
 */
Aes.subBytes = function(s, Nb) {
    for (var r=0; r<4; r++) {
        for (var c=0; c<Nb; c++) s[r][c] = Aes.sBox[s[r][c]];
    }
    return s;
};


/**
 * Shift row r of state S left by r bytes [§5.1.2]
 * @private
 */
Aes.shiftRows = function(s, Nb) {
    var t = new Array(4);
    for (var r=1; r<4; r++) {
        for (var c=0; c<4; c++) t[c] = s[r][(c+r)%Nb];  // shift into temp copy
        for (var c=0; c<4; c++) s[r][c] = t[c];         // and copy back
    }          // note that this will work for Nb=4,5,6, but not 7,8 (always 4 for AES):
    return s;  // see asmaes.sourceforge.net/rijndael/rijndaelImplementation.pdf
};


/**
 * Combine bytes of each col of state S [§5.1.3]
 * @private
 */
Aes.mixColumns = function(s, Nb) {
    for (var c=0; c<4; c++) {
        var a = new Array(4);  // 'a' is a copy of the current column from 's'
        var b = new Array(4);  // 'b' is a•{02} in GF(2^8)
        for (var i=0; i<4; i++) {
            a[i] = s[i][c];
            b[i] = s[i][c]&0x80 ? s[i][c]<<1 ^ 0x011b : s[i][c]<<1;
        }
        // a[n] ^ b[n] is a•{03} in GF(2^8)
        s[0][c] = b[0] ^ a[1] ^ b[1] ^ a[2] ^ a[3]; // {02}•a0 + {03}•a1 + a2 + a3
        s[1][c] = a[0] ^ b[1] ^ a[2] ^ b[2] ^ a[3]; // a0 • {02}•a1 + {03}•a2 + a3
        s[2][c] = a[0] ^ a[1] ^ b[2] ^ a[3] ^ b[3]; // a0 + a1 + {02}•a2 + {03}•a3
        s[3][c] = a[0] ^ b[0] ^ a[1] ^ a[2] ^ b[3]; // {03}•a0 + a1 + a2 + {02}•a3
    }
    return s;
};


/**
 * Xor Round Key into state S [§5.1.4]
 * @private
 */
Aes.addRoundKey = function(state, w, rnd, Nb) {
    for (var r=0; r<4; r++) {
        for (var c=0; c<Nb; c++) state[r][c] ^= w[rnd*4+c][r];
    }
    return state;
};


/**
 * Apply SBox to 4-byte word w
 * @private
 */
Aes.subWord = function(w) {
    for (var i=0; i<4; i++) w[i] = Aes.sBox[w[i]];
    return w;
};


/**
 * Rotate 4-byte word w left by one byte
 * @private
 */
Aes.rotWord = function(w) {
    var tmp = w[0];
    for (var i=0; i<3; i++) w[i] = w[i+1];
    w[3] = tmp;
    return w;
};


// sBox is pre-computed multiplicative inverse in GF(2^8) used in subBytes and keyExpansion [§5.1.1]
Aes.sBox =  [0x63,0x7c,0x77,0x7b,0xf2,0x6b,0x6f,0xc5,0x30,0x01,0x67,0x2b,0xfe,0xd7,0xab,0x76,
             0xca,0x82,0xc9,0x7d,0xfa,0x59,0x47,0xf0,0xad,0xd4,0xa2,0xaf,0x9c,0xa4,0x72,0xc0,
             0xb7,0xfd,0x93,0x26,0x36,0x3f,0xf7,0xcc,0x34,0xa5,0xe5,0xf1,0x71,0xd8,0x31,0x15,
             0x04,0xc7,0x23,0xc3,0x18,0x96,0x05,0x9a,0x07,0x12,0x80,0xe2,0xeb,0x27,0xb2,0x75,
             0x09,0x83,0x2c,0x1a,0x1b,0x6e,0x5a,0xa0,0x52,0x3b,0xd6,0xb3,0x29,0xe3,0x2f,0x84,
             0x53,0xd1,0x00,0xed,0x20,0xfc,0xb1,0x5b,0x6a,0xcb,0xbe,0x39,0x4a,0x4c,0x58,0xcf,
             0xd0,0xef,0xaa,0xfb,0x43,0x4d,0x33,0x85,0x45,0xf9,0x02,0x7f,0x50,0x3c,0x9f,0xa8,
             0x51,0xa3,0x40,0x8f,0x92,0x9d,0x38,0xf5,0xbc,0xb6,0xda,0x21,0x10,0xff,0xf3,0xd2,
             0xcd,0x0c,0x13,0xec,0x5f,0x97,0x44,0x17,0xc4,0xa7,0x7e,0x3d,0x64,0x5d,0x19,0x73,
             0x60,0x81,0x4f,0xdc,0x22,0x2a,0x90,0x88,0x46,0xee,0xb8,0x14,0xde,0x5e,0x0b,0xdb,
             0xe0,0x32,0x3a,0x0a,0x49,0x06,0x24,0x5c,0xc2,0xd3,0xac,0x62,0x91,0x95,0xe4,0x79,
             0xe7,0xc8,0x37,0x6d,0x8d,0xd5,0x4e,0xa9,0x6c,0x56,0xf4,0xea,0x65,0x7a,0xae,0x08,
             0xba,0x78,0x25,0x2e,0x1c,0xa6,0xb4,0xc6,0xe8,0xdd,0x74,0x1f,0x4b,0xbd,0x8b,0x8a,
             0x70,0x3e,0xb5,0x66,0x48,0x03,0xf6,0x0e,0x61,0x35,0x57,0xb9,0x86,0xc1,0x1d,0x9e,
             0xe1,0xf8,0x98,0x11,0x69,0xd9,0x8e,0x94,0x9b,0x1e,0x87,0xe9,0xce,0x55,0x28,0xdf,
             0x8c,0xa1,0x89,0x0d,0xbf,0xe6,0x42,0x68,0x41,0x99,0x2d,0x0f,0xb0,0x54,0xbb,0x16];


// rCon is Round Constant used for the Key Expansion [1st col is 2^(r-1) in GF(2^8)] [§5.2]
Aes.rCon = [ [0x00, 0x00, 0x00, 0x00],
             [0x01, 0x00, 0x00, 0x00],
             [0x02, 0x00, 0x00, 0x00],
             [0x04, 0x00, 0x00, 0x00],
             [0x08, 0x00, 0x00, 0x00],
             [0x10, 0x00, 0x00, 0x00],
             [0x20, 0x00, 0x00, 0x00],
             [0x40, 0x00, 0x00, 0x00],
             [0x80, 0x00, 0x00, 0x00],
             [0x1b, 0x00, 0x00, 0x00],
             [0x36, 0x00, 0x00, 0x00] ]; 


/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
if (typeof module != 'undefined' && module.exports) module.exports = Aes; // CommonJs export
if (typeof define == 'function' && define.amd) define([], function() { return Aes; }); // AMD


/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
/*  AES Counter-mode implementation in JavaScript       (c) Chris Veness 2005-2014 / MIT Licence  */
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */

/* jshint node:true *//* global define, escape, unescape, btoa, atob */
'use strict';
if (typeof module!='undefined' && module.exports) var Aes = require('./aes'); // CommonJS (Node.js)


/**
 * Aes.Ctr: Counter-mode (CTR) wrapper for AES.
 *
 * This encrypts a Unicode string to produces a base64 ciphertext using 128/192/256-bit AES,
 * and the converse to decrypt an encrypted ciphertext.
 *
 * See http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
 *
 * @augments Aes
 */
Aes.Ctr = {};


/**
 * Encrypt a text using AES encryption in Counter mode of operation.
 *
 * Unicode multi-byte character safe
 *
 * @param   {string} plaintext - Source text to be encrypted.
 * @param   {string} password - The password to use to generate a key.
 * @param   {number} nBits - Number of bits to be used in the key; 128 / 192 / 256.
 * @returns {string} Encrypted text.
 *
 * @example
 *   var encr = Aes.Ctr.encrypt('big secret', 'pāşšŵōřđ', 256); // encr: 'lwGl66VVwVObKIr6of8HVqJr'
 */
Aes.Ctr.encrypt = function(plaintext, password, nBits) {
    var blockSize = 16;  // block size fixed at 16 bytes / 128 bits (Nb=4) for AES
    if (!(nBits==128 || nBits==192 || nBits==256)) return ''; // standard allows 128/192/256 bit keys
    plaintext = String(plaintext).utf8Encode();
    password = String(password).utf8Encode();

    // use AES itself to encrypt password to get cipher key (using plain password as source for key
    // expansion) - gives us well encrypted key (though hashed key might be preferred for prod'n use)
    var nBytes = nBits/8;  // no bytes in key (16/24/32)
    var pwBytes = new Array(nBytes);
    for (var i=0; i<nBytes; i++) {  // use 1st 16/24/32 chars of password for key
        pwBytes[i] = isNaN(password.charCodeAt(i)) ? 0 : password.charCodeAt(i);
    }
    var key = Aes.cipher(pwBytes, Aes.keyExpansion(pwBytes)); // gives us 16-byte key
    key = key.concat(key.slice(0, nBytes-16));  // expand key to 16/24/32 bytes long

    // initialise 1st 8 bytes of counter block with nonce (NIST SP800-38A §B.2): [0-1] = millisec,
    // [2-3] = random, [4-7] = seconds, together giving full sub-millisec uniqueness up to Feb 2106
    var counterBlock = new Array(blockSize);

    var nonce = (new Date()).getTime();  // timestamp: milliseconds since 1-Jan-1970
    var nonceMs = nonce%1000;
    var nonceSec = Math.floor(nonce/1000);
    var nonceRnd = Math.floor(Math.random()*0xffff);
    // for debugging: nonce = nonceMs = nonceSec = nonceRnd = 0;

    for (var i=0; i<2; i++) counterBlock[i]   = (nonceMs  >>> i*8) & 0xff;
    for (var i=0; i<2; i++) counterBlock[i+2] = (nonceRnd >>> i*8) & 0xff;
    for (var i=0; i<4; i++) counterBlock[i+4] = (nonceSec >>> i*8) & 0xff;

    // and convert it to a string to go on the front of the ciphertext
    var ctrTxt = '';
    for (var i=0; i<8; i++) ctrTxt += String.fromCharCode(counterBlock[i]);

    // generate key schedule - an expansion of the key into distinct Key Rounds for each round
    var keySchedule = Aes.keyExpansion(key);

    var blockCount = Math.ceil(plaintext.length/blockSize);
    var ciphertxt = new Array(blockCount);  // ciphertext as array of strings

    for (var b=0; b<blockCount; b++) {
        // set counter (block #) in last 8 bytes of counter block (leaving nonce in 1st 8 bytes)
        // done in two stages for 32-bit ops: using two words allows us to go past 2^32 blocks (68GB)
        for (var c=0; c<4; c++) counterBlock[15-c] = (b >>> c*8) & 0xff;
        for (var c=0; c<4; c++) counterBlock[15-c-4] = (b/0x100000000 >>> c*8);

        var cipherCntr = Aes.cipher(counterBlock, keySchedule);  // -- encrypt counter block --

        // block size is reduced on final block
        var blockLength = b<blockCount-1 ? blockSize : (plaintext.length-1)%blockSize+1;
        var cipherChar = new Array(blockLength);

        for (var i=0; i<blockLength; i++) {  // -- xor plaintext with ciphered counter char-by-char --
            cipherChar[i] = cipherCntr[i] ^ plaintext.charCodeAt(b*blockSize+i);
            cipherChar[i] = String.fromCharCode(cipherChar[i]);
        }
        ciphertxt[b] = cipherChar.join('');
    }

    // use Array.join() for better performance than repeated string appends
    var ciphertext = ctrTxt + ciphertxt.join('');
    ciphertext = ciphertext.base64Encode();

    return ciphertext;
};


/**
 * Decrypt a text encrypted by AES in counter mode of operation
 *
 * @param   {string} ciphertext - Source text to be encrypted.
 * @param   {string} password - Password to use to generate a key.
 * @param   {number} nBits - Number of bits to be used in the key; 128 / 192 / 256.
 * @returns {string} Decrypted text
 *
 * @example
 *   var decr = Aes.Ctr.encrypt('lwGl66VVwVObKIr6of8HVqJr', 'pāşšŵōřđ', 256); // decr: 'big secret'
 */
Aes.Ctr.decrypt = function(ciphertext, password, nBits) {
    var blockSize = 16;  // block size fixed at 16 bytes / 128 bits (Nb=4) for AES
    if (!(nBits==128 || nBits==192 || nBits==256)) return ''; // standard allows 128/192/256 bit keys
    ciphertext = String(ciphertext).base64Decode();
    password = String(password).utf8Encode();

    // use AES to encrypt password (mirroring encrypt routine)
    var nBytes = nBits/8;  // no bytes in key
    var pwBytes = new Array(nBytes);
    for (var i=0; i<nBytes; i++) {
        pwBytes[i] = isNaN(password.charCodeAt(i)) ? 0 : password.charCodeAt(i);
    }
    var key = Aes.cipher(pwBytes, Aes.keyExpansion(pwBytes));
    key = key.concat(key.slice(0, nBytes-16));  // expand key to 16/24/32 bytes long

    // recover nonce from 1st 8 bytes of ciphertext
    var counterBlock = new Array(8);
    var ctrTxt = ciphertext.slice(0, 8);
    for (var i=0; i<8; i++) counterBlock[i] = ctrTxt.charCodeAt(i);

    // generate key schedule
    var keySchedule = Aes.keyExpansion(key);

    // separate ciphertext into blocks (skipping past initial 8 bytes)
    var nBlocks = Math.ceil((ciphertext.length-8) / blockSize);
    var ct = new Array(nBlocks);
    for (var b=0; b<nBlocks; b++) ct[b] = ciphertext.slice(8+b*blockSize, 8+b*blockSize+blockSize);
    ciphertext = ct;  // ciphertext is now array of block-length strings

    // plaintext will get generated block-by-block into array of block-length strings
    var plaintxt = new Array(ciphertext.length);

    for (var b=0; b<nBlocks; b++) {
        // set counter (block #) in last 8 bytes of counter block (leaving nonce in 1st 8 bytes)
        for (var c=0; c<4; c++) counterBlock[15-c] = ((b) >>> c*8) & 0xff;
        for (var c=0; c<4; c++) counterBlock[15-c-4] = (((b+1)/0x100000000-1) >>> c*8) & 0xff;

        var cipherCntr = Aes.cipher(counterBlock, keySchedule);  // encrypt counter block

        var plaintxtByte = new Array(ciphertext[b].length);
        for (var i=0; i<ciphertext[b].length; i++) {
            // -- xor plaintxt with ciphered counter byte-by-byte --
            plaintxtByte[i] = cipherCntr[i] ^ ciphertext[b].charCodeAt(i);
            plaintxtByte[i] = String.fromCharCode(plaintxtByte[i]);
        }
        plaintxt[b] = plaintxtByte.join('');
    }

    // join array of blocks into single plaintext string
    var plaintext = plaintxt.join('');
    plaintext = plaintext.utf8Decode();  // decode from UTF8 back to Unicode multi-byte chars

    return plaintext;
};


/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */


/** Extend String object with method to encode multi-byte string to utf8
 *  - monsur.hossa.in/2012/07/20/utf-8-in-javascript.html */
if (typeof String.prototype.utf8Encode == 'undefined') {
    String.prototype.utf8Encode = function() {
        return unescape( encodeURIComponent( this ) );
    };
}

/** Extend String object with method to decode utf8 string to multi-byte */
if (typeof String.prototype.utf8Decode == 'undefined') {
    String.prototype.utf8Decode = function() {
        try {
            return decodeURIComponent( escape( this ) );
        } catch (e) {
            return this; // invalid UTF-8? return as-is
        }
    };
}


/** Extend String object with method to encode base64
 *  - developer.mozilla.org/en-US/docs/Web/API/window.btoa, nodejs.org/api/buffer.html
 *  note: if btoa()/atob() are not available (eg IE9-), try github.com/davidchambers/Base64.js */
if (typeof String.prototype.base64Encode == 'undefined') {
    String.prototype.base64Encode = function() {
        if (typeof btoa != 'undefined') return btoa(this); // browser
        if (typeof Buffer != 'undefined') return new Buffer(this, 'utf8').toString('base64'); // Node.js
        throw new Error('No Base64 Encode');
    };
}

/** Extend String object with method to decode base64 */
if (typeof String.prototype.base64Decode == 'undefined') {
    String.prototype.base64Decode = function() {
        if (typeof atob != 'undefined') return atob(this); // browser
        if (typeof Buffer != 'undefined') return new Buffer(this, 'base64').toString('utf8'); // Node.js
        throw new Error('No Base64 Decode');
    };
}

/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
if (typeof module != 'undefined' && module.exports) module.exports = Aes.Ctr; // CommonJs export
if (typeof define == 'function' && define.amd) define(['Aes'], function() { return Aes.Ctr; }); // AMD


/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
/*  Encrypt/decrypt files                                                                         */
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */

function encryptFile(file) {
    // use FileReader.readAsArrayBuffer to handle binary files
    var reader = new FileReader();
    reader.readAsArrayBuffer(file);
    reader.onload = function(evt) {
        $('body').css({'cursor':'wait'});

        // Aes.Ctr.encrypt expects a string, but converting binary file directly to string could
        // give invalid Unicode sequences, so convert bytestream ArrayBuffer to single-byte chars
        var contentBytes = new Uint8Array(reader.result); // ≡ evt.target.result
        var contentStr = '';
        for (var i=0; i<contentBytes.length; i++) {
            contentStr += String.fromCharCode(contentBytes[i]);
        }

        var password = $('#password-file').val();

        var t1 = new Date();
        var ciphertext = Aes.Ctr.encrypt(contentStr, password, 256);
        var t2 = new Date();

        // use Blob to save encrypted file
        var blob = new Blob([ciphertext], { type: 'text/plain' });
        var filename = file.name+'.encrypted';
        saveAs(blob, filename);

        $('#encrypt-file-time').html(((t2 - t1)/1000)+'s'); // display time taken
        $('body').css({'cursor':'default'});
    }
}

function decryptFile(file) {
    // use FileReader.ReadAsText to read (base64-encoded) ciphertext file
    var reader = new FileReader();
    reader.readAsText(file);
    reader.onload = function(evt) {
        $('body').css({'cursor':'wait'});

        var content = reader.result; // ≡ evt.target.result
        var password = $('#password-file').val();

        var t1 = new Date();
        var plaintext = Aes.Ctr.decrypt(content, password, 256);
        var t2 = new Date();

        // convert single-byte character stream to ArrayBuffer bytestream
        var contentBytes = new Uint8Array(plaintext.length);
        for (var i=0; i<plaintext.length; i++) {
            contentBytes[i] = plaintext.charCodeAt(i);
        }

        // use Blob to save decrypted file
        var blob = new Blob([contentBytes], { type: 'application/octet-stream' });
        var filename = file.name.replace(/\.encrypted$/,'')+'.decrypted';
        saveAs(blob, filename);

        $('#decrypt-file-time').html(((t2 - t1)/1000)+'s'); // display time taken
        $('body').css({'cursor':'default'});
    }
}




/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
/*  Encrypt/decrypt files                                                                         */
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */

function encryptFile(file) {
    // use FileReader.readAsArrayBuffer to handle binary files
    var reader = new FileReader();
    reader.readAsArrayBuffer(file);
    reader.onload = function(evt) {
        $('body').css({'cursor':'wait'});

        // Aes.Ctr.encrypt expects a string, but converting binary file directly to string could
        // give invalid Unicode sequences, so convert bytestream ArrayBuffer to single-byte chars
        var contentBytes = new Uint8Array(reader.result); // ≡ evt.target.result
        var contentStr = '';
        for (var i=0; i<contentBytes.length; i++) {
            contentStr += String.fromCharCode(contentBytes[i]);
        }

        var password = $('#password-file').val();

        var t1 = new Date();
        var ciphertext = Aes.Ctr.encrypt(contentStr, password, 256);
        var t2 = new Date();

        // use Blob to save encrypted file
        var blob = new Blob([ciphertext], { type: 'text/plain' });
        var filename = file.name+'.encrypted';
        saveAs(blob, filename);

        $('#encrypt-file-time').html(((t2 - t1)/1000)+'s'); // display time taken
        $('body').css({'cursor':'default'});
    }
}

function decryptFile(file) {
    // use FileReader.ReadAsText to read (base64-encoded) ciphertext file
    var reader = new FileReader();
    reader.readAsText(file);
    reader.onload = function(evt) {
        $('body').css({'cursor':'wait'});

        var content = reader.result; // ≡ evt.target.result
        var password = $('#password-file').val();

        var t1 = new Date();
        var plaintext = Aes.Ctr.decrypt(content, password, 256);
        var t2 = new Date();

        // convert single-byte character stream to ArrayBuffer bytestream
        var contentBytes = new Uint8Array(plaintext.length);
        for (var i=0; i<plaintext.length; i++) {
            contentBytes[i] = plaintext.charCodeAt(i);
        }

        // use Blob to save decrypted file
        var blob = new Blob([contentBytes], { type: 'application/octet-stream' });
        var filename = file.name.replace(/\.encrypted$/,'')+'.decrypted';
        saveAs(blob, filename);

        $('#decrypt-file-time').html(((t2 - t1)/1000)+'s'); // display time taken
        $('body').css({'cursor':'default'});
    }
}

Anything about how to go on about decrypting it?

RTFM

Anything about how to go on about decrypting it?