Browse WireGuard specs (17 of 38)

Proofs / WireGuard / Encryption and the primitives

ae_split_blake2s

BLAKE2s in one call gives the same hash as init, update over the message in any pieces, and final, for any key of 0 to 32 bytes and any output length of 1 to 32 bytes.

spec
37 lines of C
with its headers
77 lines
status
Proved
17 of 38
On this page

The spec

The spec is the C program harness/ae_split_blake2s.c, 37 lines, shown whole as it is in the repository at 3c1fc13. It includes harness/wg_laws.h (40 lines). Its first comment states the property in full, with every bound.

C
// Law ae_split_blake2s: For any output length 1..32, any key of 0..32 bytes and any message (up to PTRDIFF_MAX
// bytes), the one-call blake2s() gives the same hash as blake2s_init (no key) or blake2s_init_key, blake2s_update
// over the message in any pieces of any lengths (empty pieces too), and blake2s_final.
#include "wg_laws.h"

int main(void) {
	// Choose the inputs: any output length and key length in the documented ranges, any key, any message
	size_t out_len = 1 + upto(BLAKE2S_HASH_SIZE - 1);
	size_t key_len = upto(BLAKE2S_KEY_SIZE);
	u8 key[BLAKE2S_KEY_SIZE];
	nondet_bytes(key, sizeof key);
	size_t len = upto(PTRDIFF_MAX);
	u8 *message = any_bytes(len);
	if (message == NULL)
		return 0;
	// In one call. include/crypto/blake2s.h: "@outlen: length of output hash value in bytes, at most
	// BLAKE2S_HASH_SIZE"; "@keylen: the key length in bytes (at most BLAKE2S_KEY_SIZE), or 0 for an unkeyed hash"
	u8 whole[BLAKE2S_HASH_SIZE], pieces[BLAKE2S_HASH_SIZE];
	blake2s(key_len == 0 ? NULL : key, key_len, message, len, whole, out_len);
	// In any pieces. include/crypto/blake2s.h, blake2s_update: "This can be called any number of times."; RFC 7693,
	// 3.3: "Key and data input are split and padded into "dd" message blocks".
	struct blake2s_ctx ctx;
	if (key_len == 0)
		blake2s_init(&ctx, out_len);
	else
		blake2s_init_key(&ctx, out_len, key, key_len);
	size_t done = 0;
	while (done < len || nondet_u8() != 0) {
		size_t take = upto(len - done);
		blake2s_update(&ctx, message + done, take);
		done += take;
	}
	blake2s_final(&ctx, pieces);
	// Check
	gp_assert(bytes_eq(whole, pieces, out_len));
	return 0;
}
harness/wg_laws.h, 40 lines
C
// wg_laws.h: trivial helpers shared by the WireGuard laws (each law includes it once). No models, no algorithms.
#include "quickproof.h"
#include "noise.h"
#include <crypto/blake2s.h>
#include <crypto/chacha.h>
#include <crypto/poly1305.h>
#include <crypto/chacha20poly1305.h>
bool wg_counter_validate(struct noise_replay_counter *counter, u64 nonce);   // receive.c's counter_validate (S19)
#define REJECT_AFTER (U64_MAX - COUNTER_WINDOW_SIZE - 1)   // messages.h: REJECT_AFTER_MESSAGES, 2^64 - 8130
static size_t upto(size_t max) {                      // any value from 0 to max
	size_t value = nondet_size();
	return value < max ? value : max;
}
static u64 any_u64(void) {                            // any 64-bit value
	u64 high = nondet_u32();
	return high << 32 | nondet_u32();
}
static u8 *any_bytes(size_t n) {                      // a new object of n arbitrary bytes (NULL if malloc fails)
	u8 *bytes = malloc(n);
	if (bytes != NULL && n != 0)
		nondet_bytes(bytes, n);
	return bytes;
}
static u8 *copy_of(const u8 *source, size_t n) {      // a new object holding source[0..n-1]
	u8 *copy = malloc(n);
	for (size_t i = 0; copy != NULL && i < n; i++)
		copy[i] = source[i];
	return copy;
}
static int bytes_eq(const u8 *a, const u8 *b, size_t n) {   // a[0..n-1] == b[0..n-1]
	for (size_t i = 0; i < n; i++)
		if (a[i] != b[i])
			return 0;
	return 1;
}
static void new_counter(struct noise_replay_counter *c) {    // a new key's replay counter: zero (keypair_create)
	c->counter = 0;
	for (size_t i = 0; i < COUNTER_BITS_TOTAL / BITS_PER_LONG; i++)
		c->backtrack[i] = 0;
}

The theorem

QuickProof generated this statement from the spec, in src/contracts/ae_split_blake2s.v:

Rocq
Theorem ae_split_blake2s : harness_safe whole_prog.

src/contracts/laws.v states it on the spec's own program, whole_prog (the spec linked with the code under test), with its proof:

Rocq
Theorem ae_split_blake2s : harness_safe WgNoise.contracts.ae_split_blake2s.whole_prog.
Proof. exact (proj1 (proj2 (proj2 WgNoise.contracts.suite.suite))). Qed.

Read as a statement about runs, it says that no run of the program has undefined behaviour or a failing check, for every value the spec can draw. That reading rests on VST's adequacy theorem, which is not yet instantiated in Rocq (formerly Coq) for QuickProof's environment.

The C files (units) its program links
  • lib/crypto/blake2s.c
  • quickproof/libc/libc.c

What it leaves out

  • What the first comment of the spec excludes, and its bounds.
  • Like every spec of this proof: runs of more than one thread (the locks and RCU are those of one thread), termination (a run that never returns is not a counterexample), and binaries built by a compiler (the theorem is about the code as CompCert defines C for 64-bit x86).
  • The kernel around the code is a shim of plain C for one thread, and the code carries rewrites of idioms that VST cannot handle. Both are tested against the kernel's code, not proved.

The summary page says what the proof as a whole covers.

How it was proved

AI agents wrote the proof, and Rocq checks it. qp verify passes on this repository: this statement is the one generated from the spec at the contract commit, and its proof uses only Rocq's standard axioms and VST's abstract malloc library.