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