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AES-CFB

Algorithm​

Cipher feedback (CFB) turns a block cipher into a stream-like primitive: a 16-byte shift register is encrypted with AES; up to 16 bytes of keystream are taken from the front of that block and XORed with the next segment of input. After each segment, the register is shifted left and the last segment’s bytes are appended (CFB-128 style with segment size matching the AES block size in this implementation).

For encryption, the bytes fed back into the register are the ciphertext segments. For decryption, feedback uses ciphertext segments as well (so decryption still calls the block cipher in the “encrypt” direction on the register). Output length always equals input length; there is no authentication tag—CFB provides confidentiality only when used correctly, not integrity.

Purpose​

Expose AES-CFB-128 on a shared AesCipher key schedule for legacy or interoperability scenarios (for example protocols that specify CFB). Prefer an AEAD such as AES-GCM or AES-CCM for new designs that need integrity.

Rust API​

  • Crate: noxtls-crypto
  • Module path (conceptual): noxtls_crypto::sym (re-exported at crate root)
  • Primary symbols:
    • AesCipher
    • noxtls_aes_cfb_encrypt
    • noxtls_aes_cfb_decrypt
    • noxtls_aes_cfb_apply

Functions and types:

  • noxtls_aes_cfb_encrypt(cipher, iv, plaintext) -> Vec<u8> - Parameters: cipher is an initialized AesCipher; iv is a 16-byte initial register (often called the IV); plaintext is arbitrary length. Behavior: runs AES-CFB-128 encryption segment by segment. Returns: ciphertext bytes, same length as plaintext.
  • noxtls_aes_cfb_decrypt(cipher, iv, ciphertext) -> Vec<u8> - Parameters: same cipher and 16-byte iv as used at the start of that ciphertext stream; ciphertext is the CFB ciphertext. Behavior: inverts CFB-128 to recover plaintext. Returns: plaintext bytes, same length as ciphertext.
  • noxtls_aes_cfb_apply(cipher, iv, input) -> Vec<u8> - Same behavior as noxtls_aes_cfb_encrypt (convenience entry point for the CFB “forward” transform).

Feature flags and policy​

Standard noxtls-crypto build.

Examples​

use noxtls_crypto::{AesCipher, noxtls_aes_cfb_decrypt, noxtls_aes_cfb_encrypt};

let key = [0x11u8; 24];
let cipher = AesCipher::new(&key)?;
let iv = [0xAAu8; 16];
let plaintext = b"stream-style payload";
let ciphertext = noxtls_aes_cfb_encrypt(&cipher, &iv, plaintext);
let recovered = noxtls_aes_cfb_decrypt(&cipher, &iv, &ciphertext);
assert_eq!(recovered.as_slice(), plaintext.as_slice());
# Ok::<(), noxtls_core::Error>(())

Security and compatibility​

Use an unpredictable unique IV (register seed) per key and message (or follow the profile’s IV construction rules). Ciphertext is malleable: an attacker flipping bits in the ciphertext causes predictable plaintext changes unless you add a separate MAC or use AEAD. Do not confuse CFB with authenticated encryption.