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// Written for Graviola by Joe Birr-Pixton, 2024.
// SPDX-License-Identifier: Apache-2.0 OR ISC OR MIT-0
use super::aes::{check_gcm_lengths, expand_key};
use crate::Error;
use crate::low::ghash::{Ghash, GhashTable};
use crate::low::{self, AesKey, Entry, aes_gcm, ct_equal};
/// An AES-GCM key.
///
/// Making one of these is relatively expensive due to key
/// expansion and precomputation.
///
/// This implementation is limited to that which is commonly
/// used:
///
/// - AES-192 is not supported, as it is rarely used.
/// - Only nonces that are 12-bytes/96-bits are supported.
pub(crate) struct AesGcm {
key: AesKey,
gh: GhashTable,
}
impl AesGcm {
/// Expands a 16- or 32-byte key into an AES-128-GCM or AES-256-GCM key.
///
/// Any other length, including AES-192's 24 bytes, is an error.
pub(crate) fn new(key: &[u8]) -> Result<Self, Error> {
let _entry = Entry::new_secret();
let key = expand_key(key)?;
let mut h = [0u8; 16];
key.encrypt_block(&mut h);
let h = u128::from_be_bytes(h);
let gh = GhashTable::new(h);
Ok(Self { key, gh })
}
/// Encrypts the given message.
///
/// On entry, `cipher_inout` contains the plaintext of the message.
/// `nonce` contains the nonce, which must be unique for a given key.
/// `aad` is the additionally-authenticated data. It may be empty.
///
/// On exit, `cipher_inout` contains the ciphertext of the message,
/// and `tag_out` contains the authentication tag.
///
/// Fails with [`Error::WrongLength`], leaving `cipher_inout` unchanged,
/// if the message is longer than GCM's 2^36 - 32 bytes, or `aad` is
/// 2^61 bytes or longer.
pub(crate) fn encrypt(
&self,
nonce: &[u8; 12],
aad: &[u8],
cipher_inout: &mut [u8],
tag_out: &mut [u8; 16],
) -> Result<(), Error> {
let _entry = Entry::new_secret();
check_gcm_lengths(aad.len(), cipher_inout.len())?;
let mut ghash = Ghash::new(&self.gh);
let counter = self.nonce_to_y0(nonce);
let mut e_y0 = counter;
self.key.encrypt_block(&mut e_y0);
// give low-level code opportunity to stitch gf128 and aes
// computations. see low::generic::aes_gcm for model version.
aes_gcm::encrypt(&self.key, &mut ghash, &counter, aad, cipher_inout);
let mut lengths = [0u8; 16];
lengths[..8].copy_from_slice(&((aad.len() * 8) as u64).to_be_bytes());
lengths[8..].copy_from_slice(&((cipher_inout.len() * 8) as u64).to_be_bytes());
ghash.add(&lengths);
let final_xi = ghash.into_bytes();
for ((out, x), e) in tag_out.iter_mut().zip(final_xi.iter()).zip(e_y0.iter()) {
*out = *x ^ *e;
}
low::ct::public_slice(cipher_inout);
low::ct::public_slice(tag_out);
Ok(())
}
/// Decrypts and verifies the given message.
///
/// On entry, `cipher_inout` contains the ciphertext of the message.
/// `nonce` contains the nonce, which must match what was supplied
/// when encrypting this message.
/// `aad` is the additionally-authenticated data. It may be empty.
/// `tag` is the purported authentication tag.
///
/// On success, `cipher_inout` contains the plaintext of the message,
/// and `Ok(())` is returned.
/// A message past [`Self::encrypt`]'s limits fails with
/// `Err(Error::WrongLength)`, leaving `cipher_inout` unchanged.
/// Otherwise, `Err(Error::DecryptFailed)` is returned and `cipher_inout`
/// is cleared.
pub(crate) fn decrypt(
&self,
nonce: &[u8; 12],
aad: &[u8],
cipher_inout: &mut [u8],
tag: &[u8; 16],
) -> Result<(), Error> {
let _entry = Entry::new_secret();
check_gcm_lengths(aad.len(), cipher_inout.len())?;
let mut ghash = Ghash::new(&self.gh);
let counter = self.nonce_to_y0(nonce);
let mut e_y0 = counter;
self.key.encrypt_block(&mut e_y0);
aes_gcm::decrypt(&self.key, &mut ghash, &counter, aad, cipher_inout);
let mut lengths = [0u8; 16];
lengths[..8].copy_from_slice(&((aad.len() * 8) as u64).to_be_bytes());
lengths[8..].copy_from_slice(&((cipher_inout.len() * 8) as u64).to_be_bytes());
ghash.add(&lengths);
let mut actual_tag = ghash.into_bytes();
for (out, e) in actual_tag.iter_mut().zip(e_y0.iter()) {
*out ^= *e;
}
if low::ct::into_public(ct_equal(&actual_tag, tag)) {
low::ct::public_slice(cipher_inout);
Ok(())
} else {
// avoid unauthenticated plaintext leak
cipher_inout.fill(0x00);
Err(Error::DecryptFailed)
}
}
fn nonce_to_y0(&self, nonce: &[u8; 12]) -> [u8; 16] {
let mut y0 = [0u8; 16];
y0[..12].copy_from_slice(nonce);
y0[15] = 0x01;
y0
}
}
#[cfg(test)]
#[cfg_attr(coverage_nightly, coverage(off))]
mod tests {
use super::*;
use crate::test::*;
#[test]
fn smoketest() {
let t = AesGcm::new(&[0; 16]).unwrap();
let mut tag = [0u8; 16];
t.encrypt(&[0u8; 12], &[], &mut [], &mut tag).unwrap();
assert_eq!(
&tag,
b"\x58\xe2\xfc\xce\xfa\x7e\x30\x61\x36\x7f\x1d\x57\xa4\xe7\x45\x5a"
);
}
#[test]
fn long_encrypt_test() {
let t = AesGcm::new(&[b'k'; 16]).unwrap();
let mut tag = [0u8; 16];
// not divisible by 128, 64, 16 to cover by-8, by-4, and trailing cases
let mut cipher = [b'p'; 4164];
t.encrypt(b"noncenonceno", b"aad", &mut cipher, &mut tag)
.unwrap();
let expected = include_bytes!("../testdata/aes-gcm-ciphertext.bin");
let (expected_cipher, expected_tag) = expected.split_at(expected.len() - 16);
assert_eq!(expected_cipher, cipher);
assert_eq!(expected_tag, tag);
}
#[test]
fn long_decrypt_test() {
let t = AesGcm::new(&[b'k'; 16]).unwrap();
let expected = include_bytes!("../testdata/aes-gcm-ciphertext.bin");
let (cipher, tag) = expected.split_at(expected.len() - 16);
let mut plain = cipher.to_vec();
t.decrypt(b"noncenonceno", b"aad", &mut plain, tag.try_into().unwrap())
.unwrap();
assert_eq!(plain, &[b'p'; 4164]);
}
#[test]
fn cavp() {
#[derive(Default)]
struct State {
encrypt: bool,
key: Option<AesGcm>,
nonce: Vec<u8>,
ct: Vec<u8>,
aad: Vec<u8>,
pt: Vec<u8>,
tag: Vec<u8>,
}
impl CavpSink for State {
fn on_meta(&mut self, _meta: &str) {}
fn on_value(&mut self, name: &str, value: Value<'_>) {
match name {
"Count" => println!(" test {}", value.int()),
"Key" => self.key = Some(AesGcm::new(&value.bytes()).unwrap()),
"IV" => self.nonce = value.bytes(),
"CT" => self.ct = value.bytes(),
"AAD" => self.aad = value.bytes(),
"Tag" if !self.encrypt => self.tag = value.bytes(),
"Tag" if self.encrypt => {
if self.nonce.len() != 12 {
println!("skip unhandled nonce len");
return;
}
let mut got_tag = [0u8; 16];
self.key
.as_ref()
.unwrap()
.encrypt(
&self.nonce[..].try_into().unwrap(),
&self.aad,
&mut self.pt,
&mut got_tag,
)
.unwrap();
assert_eq!(self.pt, self.ct);
let tag = value.bytes();
assert_eq!(&got_tag[..tag.len()], &tag[..]);
}
"FAIL" => {
assert!(!self.encrypt);
// A truncated tag never reaches `decrypt`: `aead::Tag` refuses it.
if self.nonce.len() != 12 || self.tag.len() != 16 {
println!("skip unhandled nonce/tag len");
return;
}
assert_eq!(
self.key
.as_ref()
.unwrap()
.decrypt(
&self.nonce[..].try_into().unwrap(),
&self.aad,
&mut self.ct,
&self.tag[..].try_into().unwrap(),
)
.unwrap_err(),
Error::DecryptFailed,
);
}
"PT" if !self.encrypt => {
if self.nonce.len() != 12 || self.tag.len() != 16 {
println!("skip unhandled nonce/tag len");
return;
}
self.key
.as_ref()
.unwrap()
.decrypt(
&self.nonce[..].try_into().expect("only 96-bit nonce"),
&self.aad,
&mut self.ct,
&self.tag[..].try_into().unwrap(),
)
.unwrap();
assert_eq!(self.ct, value.bytes());
}
"PT" if self.encrypt => self.pt = value.bytes(),
_ => {
todo!("unhandled value {name} = {value:?}");
}
}
}
}
process_cavp(
"../thirdparty/cavp/gcm/gcmDecrypt128.rsp",
&mut State::default(),
);
process_cavp(
"../thirdparty/cavp/gcm/gcmDecrypt256.rsp",
&mut State::default(),
);
let mut state = State {
encrypt: true,
..Default::default()
};
process_cavp("../thirdparty/cavp/gcm/gcmEncryptExtIV128.rsp", &mut state);
process_cavp("../thirdparty/cavp/gcm/gcmEncryptExtIV256.rsp", &mut state);
}
}