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Filename : decrypter.js
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var AuthCipher = require('./authCipher')
var Buffer = require('safe-buffer').Buffer
var MODES = require('./modes')
var StreamCipher = require('./streamCipher')
var Transform = require('cipher-base')
var aes = require('./aes')
var ebtk = require('evp_bytestokey')
var inherits = require('inherits')

function Decipher (mode, key, iv) {
  Transform.call(this)

  this._cache = new Splitter()
  this._last = void 0
  this._cipher = new aes.AES(key)
  this._prev = Buffer.from(iv)
  this._mode = mode
  this._autopadding = true
}

inherits(Decipher, Transform)

Decipher.prototype._update = function (data) {
  this._cache.add(data)
  var chunk
  var thing
  var out = []
  while ((chunk = this._cache.get(this._autopadding))) {
    thing = this._mode.decrypt(this, chunk)
    out.push(thing)
  }
  return Buffer.concat(out)
}

Decipher.prototype._final = function () {
  var chunk = this._cache.flush()
  if (this._autopadding) {
    return unpad(this._mode.decrypt(this, chunk))
  } else if (chunk) {
    throw new Error('data not multiple of block length')
  }
}

Decipher.prototype.setAutoPadding = function (setTo) {
  this._autopadding = !!setTo
  return this
}

function Splitter () {
  this.cache = Buffer.allocUnsafe(0)
}

Splitter.prototype.add = function (data) {
  this.cache = Buffer.concat([this.cache, data])
}

Splitter.prototype.get = function (autoPadding) {
  var out
  if (autoPadding) {
    if (this.cache.length > 16) {
      out = this.cache.slice(0, 16)
      this.cache = this.cache.slice(16)
      return out
    }
  } else {
    if (this.cache.length >= 16) {
      out = this.cache.slice(0, 16)
      this.cache = this.cache.slice(16)
      return out
    }
  }

  return null
}

Splitter.prototype.flush = function () {
  if (this.cache.length) return this.cache
}

function unpad (last) {
  var padded = last[15]
  if (padded < 1 || padded > 16) {
    throw new Error('unable to decrypt data')
  }
  var i = -1
  while (++i < padded) {
    if (last[(i + (16 - padded))] !== padded) {
      throw new Error('unable to decrypt data')
    }
  }
  if (padded === 16) return

  return last.slice(0, 16 - padded)
}

function createDecipheriv (suite, password, iv) {
  var config = MODES[suite.toLowerCase()]
  if (!config) throw new TypeError('invalid suite type')

  if (typeof iv === 'string') iv = Buffer.from(iv)
  if (config.mode !== 'GCM' && iv.length !== config.iv) throw new TypeError('invalid iv length ' + iv.length)

  if (typeof password === 'string') password = Buffer.from(password)
  if (password.length !== config.key / 8) throw new TypeError('invalid key length ' + password.length)

  if (config.type === 'stream') {
    return new StreamCipher(config.module, password, iv, true)
  } else if (config.type === 'auth') {
    return new AuthCipher(config.module, password, iv, true)
  }

  return new Decipher(config.module, password, iv)
}

function createDecipher (suite, password) {
  var config = MODES[suite.toLowerCase()]
  if (!config) throw new TypeError('invalid suite type')

  var keys = ebtk(password, false, config.key, config.iv)
  return createDecipheriv(suite, keys.key, keys.iv)
}

exports.createDecipher = createDecipher
exports.createDecipheriv = createDecipheriv