hmac.js
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/**
* Hash-based Message Authentication Code implementation. Requires a message
* digest object that can be obtained, for example, from forge.md.sha1 or
* forge.md.md5.
*
* @author Dave Longley
*
* Copyright (c) 2010-2012 Digital Bazaar, Inc. All rights reserved.
*/
var forge = require('./forge');
require('./md');
require('./util');
/* HMAC API */
var hmac = module.exports = forge.hmac = forge.hmac || {};
/**
* Creates an HMAC object that uses the given message digest object.
*
* @return an HMAC object.
*/
hmac.create = function() {
// the hmac key to use
var _key = null;
// the message digest to use
var _md = null;
// the inner padding
var _ipadding = null;
// the outer padding
var _opadding = null;
// hmac context
var ctx = {};
/**
* Starts or restarts the HMAC with the given key and message digest.
*
* @param md the message digest to use, null to reuse the previous one,
* a string to use builtin 'sha1', 'md5', 'sha256'.
* @param key the key to use as a string, array of bytes, byte buffer,
* or null to reuse the previous key.
*/
ctx.start = function(md, key) {
if(md !== null) {
if(typeof md === 'string') {
// create builtin message digest
md = md.toLowerCase();
if(md in forge.md.algorithms) {
_md = forge.md.algorithms[md].create();
} else {
throw new Error('Unknown hash algorithm "' + md + '"');
}
} else {
// store message digest
_md = md;
}
}
if(key === null) {
// reuse previous key
key = _key;
} else {
if(typeof key === 'string') {
// convert string into byte buffer
key = forge.util.createBuffer(key);
} else if(forge.util.isArray(key)) {
// convert byte array into byte buffer
var tmp = key;
key = forge.util.createBuffer();
for(var i = 0; i < tmp.length; ++i) {
key.putByte(tmp[i]);
}
}
// if key is longer than blocksize, hash it
var keylen = key.length();
if(keylen > _md.blockLength) {
_md.start();
_md.update(key.bytes());
key = _md.digest();
}
// mix key into inner and outer padding
// ipadding = [0x36 * blocksize] ^ key
// opadding = [0x5C * blocksize] ^ key
_ipadding = forge.util.createBuffer();
_opadding = forge.util.createBuffer();
keylen = key.length();
for(var i = 0; i < keylen; ++i) {
var tmp = key.at(i);
_ipadding.putByte(0x36 ^ tmp);
_opadding.putByte(0x5C ^ tmp);
}
// if key is shorter than blocksize, add additional padding
if(keylen < _md.blockLength) {
var tmp = _md.blockLength - keylen;
for(var i = 0; i < tmp; ++i) {
_ipadding.putByte(0x36);
_opadding.putByte(0x5C);
}
}
_key = key;
_ipadding = _ipadding.bytes();
_opadding = _opadding.bytes();
}
// digest is done like so: hash(opadding | hash(ipadding | message))
// prepare to do inner hash
// hash(ipadding | message)
_md.start();
_md.update(_ipadding);
};
/**
* Updates the HMAC with the given message bytes.
*
* @param bytes the bytes to update with.
*/
ctx.update = function(bytes) {
_md.update(bytes);
};
/**
* Produces the Message Authentication Code (MAC).
*
* @return a byte buffer containing the digest value.
*/
ctx.getMac = function() {
// digest is done like so: hash(opadding | hash(ipadding | message))
// here we do the outer hashing
var inner = _md.digest().bytes();
_md.start();
_md.update(_opadding);
_md.update(inner);
return _md.digest();
};
// alias for getMac
ctx.digest = ctx.getMac;
return ctx;
};