Hi everyone. I’m currently looking into AES256 CBC and how to encrypt/decrypt plaintext using Elixir. The Erlang crypto module provides the crypto_one_time/5 function which seems like the right function to use. crypto — OTP 29.0.2 (crypto 5.9) The code I used: …

Further review turns up the padding option , which mentions this important fact: This option handles padding in the last block. If not set, no padding is done and any bytes in the last unfilled block is silently discarded. You can see this happening with a longer-than-16-byte string: iex(28)> da…

[image] tkuehn: :crypto.crypto_one_time(:aes_256_cbc, key, iv, "Plaintext", true) I am not familiar with how this API works, but is the third parameter Plaintext a valid one?

Thank you for your quick response. It’s the data which should get encrypted. I’ll edit the question to make more clear.

There’s something going on here with the length of the input - making the plaintext longer eventually gives a non-empty result, at exactly 16 bytes :thinking: iex(19)> data_to_encrypt = "the_plaintext1" "the_plaintext1" iex(20)> :crypto.crypto_one_time(:aes_256_cbc, key, iv, data_to_encrypt, true)…

16 bytes (128 bits) is the block size of AES, so perhaps you are expected to pad the input yourself? (I don’t know much about cryptography)

That is quite a gotcha if you use it to encrypt some data and don’t double-check the result! Always read the docs, I guess :slight_smile:

In general that is an API that should not be used if you aren’t aware what you are doing (so unless you are cryptography expert or you are trying to follow well known protocol description). Otherwise you will burn yourself.

I see, if it’s something that’s standardized that makes a lot more sense.

AES256 CBC encryption returning empty string

Questions & Help Questions

D4no0 April 5, 2025, 5:21pm 2

I am not familiar with how this API works, but is the third parameter Plaintext a valid one?