Tech Tips: Node.js Crypto Module Examples

I recently had the need to use the node crypto module in a project. To get this to work properly, paying attention to the last two sentences on the cipher.update function documentation is critical.

cipher.update(data, [input_encoding], [output_encoding])#

Updates the cipher with data, the encoding of which is given in input_encoding and can be 'utf8','ascii' or 'binary'. If no encoding is provided, then a buffer is expected. If data is a Buffer theninput_encoding is ignored.

The output_encoding specifies the output format of the enciphered data, and can be 'binary', 'base64' or'hex'. If no encoding is provided, then a buffer is returned.

To illustrate, here are 3 simple examples:

1. Concat cipher text strings with in/out encoding specified.

var plainText = '1234567812345678'; 
var cipher1 = crypto.createCipher('aes256', 'password'); 
var cipherText1 = cipher1.update(plainText, 'ascii','binary'); 
cipherText1 += cipher1.final('binary'); 
console.log('Method 1 - plainText:' + plainText ); 
console.log('Method 1 - cipherText length:' + cipherText1.length); 
var decipher1 = crypto.createDecipher('aes256', 'password'); 
var result1 = decipher1.update(cipherText1); 
result1 += decipher1.final(); 
console.log('Method 1 - result:' + result1);

This one works as advertised. Output below:

Method 1 - plainText:1234567812345678
Method 1 - cipherText length:32
Method 1 - result:1234567812345678

2. Concat buffers with no in/out encoding specified.

var plainText = '1234567812345678'; 
var cipher2 = crypto.createCipher('aes256', 'password'); 
var cipherText2 = Buffer.concat([cipher2.update(new Buffer(plainText)), cipher2.final()]); 
console.log('Method 2 - plainText:' + plainText ); 
console.log('Method 2 - cipherText length:' + cipherText2.length); 
var decipher2 = crypto.createDecipher('aes256', 'password'); 
var result2 = decipher2.update(cipherText2); 
result2 += decipher2.final(); 
console.log('Method 2 - result:' + result2);

Also works. Output below


Method 2 - plainText:1234567812345678
Method 2 - cipherText length:32
Method 2 - result:1234567812345678








3.  String concat with NO encoding specification.





var cipher3 = crypto.createCipher('aes256', 'password'); 
var cipherText3 = cipher3.update(plainText); 
cipherText3 += cipher3.final(); 
console.log('Method 3 - plainText:' + plainText ); 
console.log('Method 3 - cipherText length:' + cipherText3.length); 
var decipher3 = crypto.createDecipher('aes256', 'password'); 
var result3 = decipher3.update(cipherText3); 
result3 += decipher3.final(); //Boom 
console.log('Method 3 - result:' + result3);


Method 3 - plainText:1234567812345678 

Method 3 - cipherText length:29
error:0606506D:digital envelope routines:EVP_DecryptFinal_ex:wrong final block length



The tip off as to what is happening is that cipherText length.  It's only 29 in this example,but was 32 in the others.  cipher.update() is returning a Buffer object, just as advertised. Concat'ing those buffers leads to an implicit toString() call on those buffer objects. That  call leads to a default UTF-8 conversion in Buffer (also as advertised in the documentation).Cipher text is mangled in that conversion and the result can't be decrypted.

Source code used in this discussion here.

Saturday, February 1, 2014

Node.js Crypto Module Examples

cipher.update(data, [input_encoding], [output_encoding])#