CODEWITHSUNDEEP
pythonnode.jscryptographyaesaes-gcm
QuinnF
QuinnF

Reputation: 2209

correct nonce/iv size for AES-GCM mode

EDIT: The issue can be simplified to this: The following Node.js code give an "Invalid IV length" Error. Why? What should the IV be?

const crypto = require('crypto')
const decipher = crypto.createDecipheriv('aes-128-gcm', crypto.randomBytes(16), crypto.randomBytes(16))

I'm using AES in GCM mode to encrypt some data, but I'm using two different languages and libraries for encryption and decryption and they seem to have different vocabularies about what I need.

I'm encrypting with a Python library (Crypto). The encrypt_and_digest method takes a 128 bit key and a message and returns a 128 bit nonce, 128 bit tag, and a ciphertext.

(Encryption code taken from this example)

I'm decrypting with the default Node.js crypto library. That library expects a session key, a tag, and an IV. When I pass the nonce from the Python library as the IV, it gives me an “invalid iv size” error. Examples of the Node library seem to use a 12-character string as an IV.

My decryption code looks like this (taken from here):

var decipher = crypto.createDecipheriv(algorithm, password, nonce)
decipher.setAuthTag(encrypted.tag);
var dec = decipher.update(encrypted.content, 'hex', 'utf8')

What is the difference between IV and nonce for this scheme? How should I resolve this? Thanks!

Upvotes: 3

Views: 10792

Answers (1)

QuinnF
QuinnF

Reputation: 2209

It turns out the nonce for GCM should be 12 bytes long. I'm not sure why the python library defaults to auto-generating a 16-byte nonce, but you can generate your own and specify it manually in the AES constructor, so thats what I did. The whole system works perfectly now

Upvotes: 10

Related Questions