add check function for BIP39

fixes #393

lib/BIP39.js

@@ -48,6 +48,32 @@ BIP39.entropy2mnemonic = function(wordlist, buf) {
   return mnemonic;
 }
 
+BIP39.check = function(wordlist, mnemonic) {
+    var words = mnemonic.split(' ');
+    var bin = "";
+    for (var i = 0; i < words.length; i++) {
+        var ind = wordlist.indexOf(words[i]);
+        if (ind < 0)
+            return false;
+        bin = bin + ("00000000000" + ind.toString(2)).slice(-11);
+    }
+
+    if (bin.length % 11 != 0) {
+        throw new Error("internal error - entropy not an even multiple of 11 bits - " + bin.length);
+    }
+    var cs = bin.length / 33;
+    var hash_bits = bin.slice(-cs);
+    var nonhash_bits = bin.slice(0, bin.length - cs);
+    var buf = new Buffer(nonhash_bits.length / 8);
+    for (var i = 0; i < nonhash_bits.length / 8; i++) {
+        buf.writeUInt8(parseInt(bin.slice(i * 8, (i + 1) * 8), 2), i);
+    }
+    var hash = coinUtil.sha256(buf);
+    var expected_hash_bits = hash[0].toString(2);
+    expected_hash_bits = ("00000000" + expected_hash_bits).slice(-8).slice(0, cs);
+    return expected_hash_bits == hash_bits;
+}
+
 BIP39.mnemonic2seed = function(mnemonic, passphrase) {
   if (!passphrase)
     passphrase = "";

test/test.BIP39.js

@@ -153,6 +153,7 @@ describe('BIP39', function() {
         var seed = vector[2];
         var mnemonic1 = BIP39.entropy2mnemonic(BIP39WordlistEn, new Buffer(code, 'hex'));
         var seed1 = BIP39.mnemonic2seed(mnemonic, 'TREZOR');
+        BIP39.check(BIP39WordlistEn, mnemonic).should.be.true;
         mnemonic1.should.equal(mnemonic);
         seed1.toString('hex').should.equal(seed)
       }