crypto/kyber/internal.h

// Copyright 2023 The BoringSSL Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef OPENSSL_HEADER_CRYPTO_KYBER_INTERNAL_H
#define OPENSSL_HEADER_CRYPTO_KYBER_INTERNAL_H

#include <openssl/base.h>


BSSL_NAMESPACE_BEGIN

// Kyber is the pre-standard version of ML-KEM. This was once exported as public
// API, but is now internal and only used by libssl. It will be removed entirely
// in the future.
//
// This implements the round-3 specification of Kyber, defined at
// https://pq-crystals.org/kyber/data/kyber-specification-round3-20210804.pdf

// KYBER_public_key contains a Kyber768 public key. The contents of this
// object should never leave the address space since the format is unstable.
struct KYBER_public_key {
  union {
    uint8_t bytes[512 * (3 + 9) + 32 + 32];
    uint16_t alignment;
  } opaque;
};

// KYBER_private_key contains a Kyber768 private key. The contents of this
// object should never leave the address space since the format is unstable.
struct KYBER_private_key {
  union {
    uint8_t bytes[512 * (3 + 3 + 9) + 32 + 32 + 32];
    uint16_t alignment;
  } opaque;
};

// KYBER_PUBLIC_KEY_BYTES is the number of bytes in an encoded Kyber768 public
// key.
#define KYBER_PUBLIC_KEY_BYTES 1184

// KYBER_SHARED_SECRET_BYTES is the number of bytes in the Kyber768 shared
// secret. Although the round-3 specification has a variable-length output, the
// final ML-KEM construction is expected to use a fixed 32-byte output. To
// simplify the future transition, we apply the same restriction.
#define KYBER_SHARED_SECRET_BYTES 32

// KYBER_generate_key generates a random public/private key pair, writes the
// encoded public key to |out_encoded_public_key| and sets |out_private_key| to
// the private key.
OPENSSL_EXPORT void KYBER_generate_key(
    uint8_t out_encoded_public_key[KYBER_PUBLIC_KEY_BYTES],
    struct KYBER_private_key *out_private_key);

// KYBER_public_from_private sets |*out_public_key| to the public key that
// corresponds to |private_key|. (This is faster than parsing the output of
// |KYBER_generate_key| if, for some reason, you need to encapsulate to a key
// that was just generated.)
OPENSSL_EXPORT void KYBER_public_from_private(
    struct KYBER_public_key *out_public_key,
    const struct KYBER_private_key *private_key);

// KYBER_CIPHERTEXT_BYTES is number of bytes in the Kyber768 ciphertext.
#define KYBER_CIPHERTEXT_BYTES 1088

// KYBER_encap encrypts a random shared secret for |public_key|, writes the
// ciphertext to |out_ciphertext|, and writes the random shared secret to
// |out_shared_secret|.
OPENSSL_EXPORT void KYBER_encap(
    uint8_t out_ciphertext[KYBER_CIPHERTEXT_BYTES],
    uint8_t out_shared_secret[KYBER_SHARED_SECRET_BYTES],
    const struct KYBER_public_key *public_key);

// KYBER_decap decrypts a shared secret from |ciphertext| using |private_key|
// and writes it to |out_shared_secret|. If |ciphertext| is invalid,
// |out_shared_secret| is filled with a key that will always be the same for the
// same |ciphertext| and |private_key|, but which appears to be random unless
// one has access to |private_key|. These alternatives occur in constant time.
// Any subsequent symmetric encryption using |out_shared_secret| must use an
// authenticated encryption scheme in order to discover the decapsulation
// failure.
OPENSSL_EXPORT void KYBER_decap(
    uint8_t out_shared_secret[KYBER_SHARED_SECRET_BYTES],
    const uint8_t ciphertext[KYBER_CIPHERTEXT_BYTES],
    const struct KYBER_private_key *private_key);


// Serialisation of keys.

// KYBER_marshal_public_key serializes |public_key| to |out| in the standard
// format for Kyber public keys. It returns one on success or zero on allocation
// error.
OPENSSL_EXPORT int KYBER_marshal_public_key(
    CBB *out, const struct KYBER_public_key *public_key);

// KYBER_parse_public_key parses a public key, in the format generated by
// |KYBER_marshal_public_key|, from |in| and writes the result to
// |out_public_key|. It returns one on success or zero on parse error or if
// there are trailing bytes in |in|.
OPENSSL_EXPORT int KYBER_parse_public_key(
    struct KYBER_public_key *out_public_key, CBS *in);

// KYBER_marshal_private_key serializes |private_key| to |out| in the standard
// format for Kyber private keys. It returns one on success or zero on
// allocation error.
OPENSSL_EXPORT int KYBER_marshal_private_key(
    CBB *out, const struct KYBER_private_key *private_key);

// KYBER_PRIVATE_KEY_BYTES is the length of the data produced by
// |KYBER_marshal_private_key|.
#define KYBER_PRIVATE_KEY_BYTES 2400

// KYBER_parse_private_key parses a private key, in the format generated by
// |KYBER_marshal_private_key|, from |in| and writes the result to
// |out_private_key|. It returns one on success or zero on parse error or if
// there are trailing bytes in |in|.
OPENSSL_EXPORT int KYBER_parse_private_key(
    struct KYBER_private_key *out_private_key, CBS *in);


// Internal symbols.

// KYBER_ENCAP_ENTROPY is the number of bytes of uniformly random entropy
// necessary to encapsulate a secret. The entropy will be leaked to the
// decapsulating party.
#define KYBER_ENCAP_ENTROPY 32

// KYBER_GENERATE_KEY_ENTROPY is the number of bytes of uniformly random entropy
// necessary to generate a key.
#define KYBER_GENERATE_KEY_ENTROPY 64

// KYBER_generate_key_external_entropy is a deterministic function to create a
// pair of Kyber768 keys, using the supplied entropy. The entropy needs to be
// uniformly random generated. This function is should only be used for tests,
// regular callers should use the non-deterministic |KYBER_generate_key|
// directly.
OPENSSL_EXPORT void KYBER_generate_key_external_entropy(
    uint8_t out_encoded_public_key[KYBER_PUBLIC_KEY_BYTES],
    struct KYBER_private_key *out_private_key,
    const uint8_t entropy[KYBER_GENERATE_KEY_ENTROPY]);

// KYBER_encap_external_entropy behaves like |KYBER_encap|, but uses
// |KYBER_ENCAP_ENTROPY| bytes of |entropy| for randomization. The decapsulating
// side will be able to recover |entropy| in full. This function should only be
// used for tests, regular callers should use the non-deterministic
// |KYBER_encap| directly.
OPENSSL_EXPORT void KYBER_encap_external_entropy(
    uint8_t out_ciphertext[KYBER_CIPHERTEXT_BYTES],
    uint8_t out_shared_secret[KYBER_SHARED_SECRET_BYTES],
    const struct KYBER_public_key *public_key,
    const uint8_t entropy[KYBER_ENCAP_ENTROPY]);

BSSL_NAMESPACE_END

#endif  // OPENSSL_HEADER_CRYPTO_KYBER_INTERNAL_H