March 14, 2020
If you still use the
createDecipher methods of the
crypto module, you’re likely getting the following deprecation
warnings when running your code:
(node:477082) [DEP0106] DeprecationWarning: crypto.createCipher is deprecated. (node:468005) [DEP0106] DeprecationWarning: crypto.createDecipher is deprecated.
This is because this method didn’t allow for passing an initialization
vector (IV), and instead derived the IV from the key using the OpenSSL
EVP_BytesToKey derivation function, using a
null salt meaning that
the IV would be deterministic for a given key which is an issue for
ciphers with counter mode like CTR, GCM and CCM.
Your code might have looked like:
const cipher = crypto.createCipher('aes256', key)
If you want to make this code backwards compatible, you need to call
EVP_BytesToKey function yourself, typically through
this module which makes
it available in JS userland.
However the reason this function is deprecated in the first place is because you shouldn’t use it, and instead use a random unpredictable IV, which requires you to change your code to something like this:
const iv = crypto.randomBytes(16) const cipher = crypto.createCipher('aes256', key, iv)
Here, for AES-256 in CBC mode (
aes256 being aliased to
OpenSSL), the IV size is expected to be the same as the block size,
which is always 16 bytes.
In order to decrypt the message, you will need the IV as well. Typically you’d store the IV together with the message, as the important part is for the IV to not be predictable ahead of time.