classic-mceliece-rust

This is a pure-rust safe-rust implementation of the Classic McEliece post-quantum scheme.

• Classic McEliece is a code-based key encapsulation mechanism (KEM)
• The implementation is based on the Classic McEliece reference implementation of NIST round 3
• The implementation does not utilize any concurrency techniques (SIMD/threading/…, except maybe auto-vectorization on your CPU)
• It depends on sha3 as SHA-3 implementation and aes as AES block cipher (used as RNG) implementation
• It passes the 100 testcases of the C reference implementation
• It implements all 10 variants of the Classic McEliece KEM
• The implementation takes between 100 milliseconds (mceliece348864) and 500 milliseconds (mceliece8192128f) to run on a modern computer
• The implementation is constant-time on software instruction level
• The random number generator is based on AES256 in counter mode
• First described in 1978, the cryptographic scheme has a rich history in security analysis. Its large public key size, however, often limits adoption.

The 10 variants have the following designated identifiers:

• mceliece348864
• mceliece348864f
• mceliece460896
• mceliece460896f
• mceliece6688128
• mceliece6688128f
• mceliece6960119
• mceliece6960119f
• mceliece8192128
• mceliece8192128f

Who should use it?

Anyone, how wants to use Classic McEliece to negotiate a key between two parties.

How does one use it?

Add this to your Cargo.toml: toml [dependencies] classic-mceliece-rust = "1.0"

To use a specific Classic McEliece variant, you need to import it with the corresponding feature flag:

toml [dependencies] classic-mceliece-rust = { version = "1.0", features = ["mceliece6960119"] }

If you have access to feature alloc (you are not on no_std), then the simplest and most ergonomic way of using the library is with heap allocated keys and the *_boxed helper methods: ```rust

[cfg(feature = "alloc")] {

use classic_mceliece_rust::{keypair_boxed, encapsulate_boxed, decapsulate_boxed};

fn run_kem() {
    let mut rng = rand::thread_rng();

    // Alice computes the keypair
    let (public_key, secret_key) = keypair_boxed(&mut rng);

    // Send `secret_key` over to Bob.
    // Bob computes the shared secret and a ciphertext
    let (ciphertext, shared_secret_bob) = encapsulate_boxed(&public_key, &mut rng);

    // Send `ciphertext` back to Alice.
    // Alice decapsulates the ciphertext...
    let shared_secret_alice = decapsulate_boxed(&ciphertext, &secret_key);

    // ... and ends up with the same key material as Bob.
    assert_eq!(shared_secret_bob.as_array(), shared_secret_alice.as_array());
}

std::thread::Builder::new()
    // This library needs quite a lot of stack space to work
    .stack_size(2 * 1024 * 1024)
    .spawn(run_kem)
    .unwrap()
    .join()
    .unwrap();

} ```

You can also use this crate in a no_std environment. Then you don't have access to boxed keys and you need to provide the kem functions with the storage they should use. You can store the key material directly on the stack. However, see the stack usage section further down for known issues with this.

This test does not run on Windows due to the default stack size being too small. ```rust

[cfg(not(windows))] {

use classic_mceliece_rust::{keypair, encapsulate, decapsulate};
use classic_mceliece_rust::{CRYPTO_BYTES, CRYPTO_PUBLICKEYBYTES, CRYPTO_SECRETKEYBYTES};

let mut rng = rand::thread_rng();

// Please mind that `public_key_buf` is very large.
let mut public_key_buf = [0u8; CRYPTO_PUBLICKEYBYTES];
let mut secret_key_buf = [0u8; CRYPTO_SECRETKEYBYTES];
let (public_key, secret_key) = keypair(&mut public_key_buf, &mut secret_key_buf, &mut rng);

let mut shared_secret_bob_buf = [0u8; CRYPTO_BYTES];
let (ciphertext, shared_secret_bob) = encapsulate(&public_key, &mut shared_secret_bob_buf, &mut rng);

let mut shared_secret_alice_buf = [0u8; CRYPTO_BYTES];
let shared_secret_alice = decapsulate(&ciphertext, &secret_key, &mut shared_secret_alice_buf);

assert_eq!(shared_secret_bob.as_array(), shared_secret_alice.as_array());

} ```

Stack usage

The public keys in Classic McEliece are huge. As a result, running the algorithm requires a lot of stack space. This can be somewhat mitigated by storing the public key on the heap, like shown in an example above. However, even when doing this the call to keypair uses more stack than available by default on some platforms (Windows). So if you want your program to be portable and not unexpectedly crash, you should probably run the entire key exchange in a dedicated thread with a large enough stack size. Something like this:

rust,no_run std::thread::Builder::new() .stack_size(4 * 1024 * 1024) .spawn(|| {/* Run the KEM here */}) .unwrap();

Feature kem: RustCrypto APIs

If the kem feature is enabled, key encapsulation and decapsulation can also be done via the standard traits in the kem crate.

Feature zeroize: Clear out secrets from memory

If the zeroize feature is enabled (it is by default), all key types that contain anything secret implements Zeroize and ZeroizeOnDrop. This makes them clear their memory when they go out of scope, and lowers the risk of secret key material leaking in one way or another.

Please mind that this of course makes any buffers you pass into the library useless for reading out the key from. Instead of trying to fetch the key material from the buffers you pass in, get it from the as_array method.

```rust

[cfg(not(windows))] {

use classic_mceliece_rust::keypair;
use classic_mceliece_rust::{CRYPTO_PUBLICKEYBYTES, CRYPTO_SECRETKEYBYTES};

let mut rng = rand::thread_rng();

let mut pk_buf = [0u8; CRYPTO_PUBLICKEYBYTES];
// Initialize to non-zero to show that it has been set to zero by the drop later
let mut sk_buf = [255u8; CRYPTO_SECRETKEYBYTES];

// This is the WRONG way of accessing your keys. The buffer will
// be cleared once the `PrivateKey` returned from `keypair` goes out of scope.
// You should not rely on that array for anything except providing a temporary storage
// location to this library.
#[cfg(feature = "zeroize")]
{
    let (_, secret_key) = keypair(&mut pk_buf, &mut sk_buf, &mut rng);
    drop(secret_key);
    // Ouch! The array only has zeroes now.
    assert_eq!(sk_buf, [0; CRYPTO_SECRETKEYBYTES]);
}

// Correct way of getting the secret key bytes if you do need them. However,
// if you want the secrets to stay secret, you should try to not read them out of their
// storage at all
{
    let (_, secret_key) = keypair(&mut pk_buf, &mut sk_buf, &mut rng);
    assert_ne!(secret_key.as_array(), &[0; CRYPTO_SECRETKEYBYTES]);
}

} ```

How does one run it?

This library comes with two examples:

bash $ cargo run --example basic

The output annotates messages with Alice/Bob to illustrate which data is processed by which party. The katkem example implements the classic request/response file structure which is part of the NIST PQC framework.

bash $ cargo run --example katkem PQCkemKAT_935.req PQCkemKAT_935.rsp $ cargo run --example katkem PQCkemKAT_935.rsp

The different variants can be enabled through feature flags:

bash $ cargo run --example katkem --features mceliece6960119 -- PQCkemKAT_1450.req PQCkemKAT_1450.rsp

mceliece348864 is the default variant. You cannot enable two variants simultaneously.

How fast is it?

All data uses clock cycles as unit (the smaller the better). The rust implementation yielded the following runtime results:

The C reference implementation yielded the following runtime results:

The tests were done on a Lenovo Thinkpad x260 (Intel Core i5-6200U CPU @ 2.30GHz). In the case of rust, criterion 0.3.5 has been used as given in benches/ and in case of C, Google's benchmark with PFM support and disabled CPU frequency scaling. You can run the benchmark suite yourself with the bench subcommand and optionally some variant feature flag:

bash $ cargo bench --features mceliece348864

Is it correct?

Yes, besides passing unittests (derived from the C implementation), the generated KAT KEM test files have equivalent MD5 hashes. Namely …

Where is the source code?

On github.

What is the content's license?

MIT License

Changelog

• 2022-09-06 version 2.0.0: refined API with heap-allocated keys and RustCrypto integration
• 2022-09-06 version 1.1.0: add CI, clippy, infallible SHAKE impl, forbid unsafe code
• 2022-04-12 version 1.0.1: fix C&P mistakes in documentation
• 2022-04-01 version 1.0.0: public release (no April fools though)

Where can I ask you to fix a bug?

On github.