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<h1 class="page-title">Source: type/openpgp.type.mpi.js</h1>
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<pre class="prettyprint source"><code>// GPG4Browsers - An OpenPGP implementation in javascript
// Copyright (C) 2011 Recurity Labs GmbH
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
// Hint: We hold our MPIs as an array of octets in big endian format preceeding a two
// octet scalar: MPI: [a,b,c,d,e,f]
// - MPI size: (a << 8) | b
// - MPI = c | d << 8 | e << ((MPI.length -2)*8) | f ((MPI.length -2)*8)
/**
* @class
* @classdescImplementation of type MPI (RFC4880 3.2)
* Multiprecision integers (also called MPIs) are unsigned integers used
* to hold large integers such as the ones used in cryptographic
* calculations.
* An MPI consists of two pieces: a two-octet scalar that is the length
* of the MPI in bits followed by a string of octets that contain the
* actual integer.
*/
function openpgp_type_mpi() {
this.MPI = null;
this.mpiBitLength = null;
this.mpiByteLength = null;
this.data = null;
/**
* Parsing function for a mpi (RFC 4880 3.2).
* @param {String} input Payload of mpi data
* @param {Integer} position Position to start reading from the input
* string
* @param {Integer} len Length of the packet or the remaining length of
* input at position
* @return {openpgp_type_mpi} Object representation
*/
function read(input, position, len) {
var mypos = position;
this.mpiBitLength = (input[mypos++].charCodeAt() << 8) | input[mypos++].charCodeAt();
// Additional rules:
//
// The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.
//
// The length field of an MPI describes the length starting from its
// most significant non-zero bit. Thus, the MPI [00 02 01] is not
// formed correctly. It should be [00 01 01].
// TODO: Verification of this size method! This size calculation as
// specified above is not applicable in JavaScript
this.mpiByteLength = (this.mpiBitLength - (this.mpiBitLength % 8)) / 8;
if (this.mpiBitLength % 8 != 0)
this.mpiByteLength++;
this.MPI = input.substring(mypos,mypos+this.mpiByteLength);
this.data = input.substring(position, position+2+this.mpiByteLength);
this.packetLength = this.mpiByteLength +2;
return this;
}
/**
* Generates debug output (pretty print)
* @return {String} String which gives some information about the mpi
*/
function toString() {
var r = " MPI("+this.mpiBitLength+"b/"+this.mpiByteLength+"B) : 0x";
r+=util.hexstrdump(this.MPI);
return r+'\n';
}
/**
* Converts the mpi to an BigInteger object
* @return {BigInteger}
*/
function getBigInteger() {
return new BigInteger(util.hexstrdump(this.MPI),16);
}
function getBits(num) {
for (var i = 0; i < 9; i++)
if (num >> i == 0)
return i;
}
/**
* Gets the length of the mpi in bytes
* @return {Integer} Mpi byte length
*/
function getByteLength() {
return this.mpiByteLength;
}
/**
* Creates an mpi from the specified string
* @param {String} data Data to read the mpi from
* @return {openpgp_type_mpi}
*/
function create(data) {
this.MPI = data;
this.mpiBitLength = (data.length -1) *8 + getBits(data.charCodeAt(0));
this.mpiByteLength = data.length;
return this;
}
/**
* Converts the mpi object to a string as specified in RFC4880 3.2
* @return {String} mpi Byte representation
*/
function toBin() {
var result = String.fromCharCode((this.mpiBitLength >> 8) & 0xFF);
result += String.fromCharCode(this.mpiBitLength & 0xFF);
result += this.MPI;
return result;
}
this.read = read;
this.toBigInteger = getBigInteger;
this.toString = toString;
this.create = create;
this.toBin = toBin;
this.getByteLength = getByteLength;
}
</code></pre>
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