Man page - mceliece460896_dec(3)

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libmceliece-dev

Manual

mceliece

NAME
SYNOPSIS
DESCRIPTION
KEY GENERATION
ENCAPSULATION
DECAPSULATION
THE f VARIANTS
SEE ALSO

NAME

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

SYNOPSIS

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);

DESCRIPTION

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.

KEY GENERATION

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] .

ENCAPSULATION

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 .

DECAPSULATION

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

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.