mceliece(3) mceliece(3)

mceliece - C API for the libmceliece implementation of the Classic McEliece cryptosystem

Using libmceliece:

#include <mceliece.h>

Link with -lmceliece.

Key generation (for, e.g., mceliece6960119):

unsigned char pk[mceliece6960119_PUBLICKEYBYTES];
unsigned char sk[mceliece6960119_SECRETKEYBYTES];
mceliece6960119_keypair(pk,sk);

Encapsulation (for, e.g., mceliece6960119):

unsigned char ct[mceliece6960119_CIPHERTEXTBYTES];
unsigned char k[mceliece6960119_BYTES];
const unsigned char pk[mceliece6960119_PUBLICKEYBYTES];
int ret;
ret = mceliece6960119_enc(ct,k,pk);

Decapsulation (for, e.g., mceliece6960119):

unsigned char k[mceliece6960119_BYTES];
const unsigned char ct[mceliece6960119_CIPHERTEXTBYTES];
const unsigned char sk[mceliece6960119_SECRETKEYBYTES];
int ret;
ret = mceliece6960119_dec(k,ct,sk);

libmceliece is an implementation of the Classic McEliece (https://classic.mceliece.org) cryptosystem. The C API for libmceliece provides the following functions:

mceliece{6960119,6688128,8192128,460896,348864}_keypair
mceliece{6960119,6688128,8192128,460896,348864}_enc
mceliece{6960119,6688128,8192128,460896,348864}_dec
mceliece{6960119,6688128,8192128,460896,348864}f_keypair
mceliece{6960119,6688128,8192128,460896,348864}f_enc
mceliece{6960119,6688128,8192128,460896,348864}f_dec

All of these functions follow the SUPERCOP API for KEMs (https://bench.cr.yp.to/call-kem.html) except that

•
the function names are libmceliece-specific instead of crypto_kem_*,
•
message lengths are long long instead of unsigned long long, and
•
the keypair functions return void instead of int.

The details below use mceliece6960119 as an example.

The mceliece6960119_keypair function randomly generates Alice’s secret key sk[0], sk[1], ..., sk[mceliece6960119_SECRETKEYBYTES-1] and Alice’s corresponding public key pk[0], pk[1], ..., pk[mceliece6960119_PUBLICKEYBYTES-1].

The mceliece6960119_enc function randomly generates a ciphertext ct[0], ct[1], ..., ct[mceliece6960119_CIPHERTEXTBYTES-1] and the corresponding session key k[0], k[1], ..., k[mceliece6960119_BYTES-1] given Alice’s public key pk[0], pk[1], ..., pk[mceliece6960119_PUBLICKEYBYTES-1]. This function then returns 0.

Exception: If the input public key is not “narrowly decodable” (i.e., if bits at particular positions in pk are set), this function returns -1. Currently the function also handles such public keys by clearing ct and k, but callers should not rely on this.

For {6688128,8192128,460896,348864}{,f}, all byte strings of the correct length are narrowly decodable, and the return value is always 0. For 6960119{,f}, the return value can be -1.

The mceliece6960119_dec function, given Alice’s secret key sk[0], sk[1], ..., sk[mceliece6960119_SECRETKEYBYTES-1], computes the session key k[0], k[1], ..., k[mceliece6960119_BYTES-1] corresponding to a ciphertext ct[0], ct[1], ..., ct[mceliece6960119_CIPHERTEXTBYTES-1] that was encapsulated to Alice. This function then returns 0.

Exception: If the input ciphertext is not “narrowly decodable” (i.e., if bits at particular positions in ct are set), this function returns -1. Currently this function also handles such ciphertexts by setting all bytes of k to 255, but callers should not rely on this.

For {6688128,8192128,460896,348864}{,f}, all byte strings of the correct length are narrowly decodable, and the return value is always 0. For 6960119{,f}, the return value can be -1.

The f variants are internally more complicated than the non-f variants but provide faster key generation. The f variants are interoperable with the non-f variants: for example, a key generated with mceliece6960119f_keypair can decapsulate ciphertexts that were encapsulated with mceliece6960119_enc. The secret-key sizes (and formats) are the same, the enc functions are the same, and the dec functions are the same.

mceliece(1), randombytes(3)