Systemsicherheit/Assignment 7 - SGX Hands-on/lib/rsa.c

#include "rsa.h"
#include <gmp.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static int random_prime(mpz_t prime, const size_t size) {
    u8 tmp[size];
    FILE *urandom = fopen("/dev/urandom", "rb");
    
    if((urandom == NULL) || (prime == NULL))
        return 0;

    fread(tmp, 1, size, urandom);
    mpz_import(prime, size, 1, 1, 1, 0, tmp);
    mpz_nextprime(prime, prime);

    fclose(urandom);
    return 1;
}

static int rsa_keygen(rsa_key *key) {
    // null pointer handling
    if(key == NULL)
        return 0;

    // init bignums
    mpz_init_set_ui(key->e, 65537);
    mpz_inits(key->p, key->q, key->n, key->d, NULL);
    
    // prime gen
    if ((!random_prime(key->p, MODULUS_SIZE/2)) || (!random_prime(key->q, MODULUS_SIZE/2)))
        return 0;

    // compute n
    mpz_mul(key->n, key->p, key->q);

    // compute phi(n)
    mpz_t phi_n; mpz_init(phi_n);
    mpz_sub_ui(key->p, key->p, 1);
    mpz_sub_ui(key->q, key->q, 1);
    mpz_mul(phi_n, key->p, key->q);
    mpz_add_ui(key->p, key->p, 1);
    mpz_add_ui(key->q, key->q, 1);
    
    // compute d
    if(mpz_invert(key->d, key->e, phi_n) == 0) {
        return 0;
    }
    
    // free temporary phi_n and return true
    mpz_clear(phi_n);
    return 1;
}

static int rsa_export(rsa_key *key) {
    
}

static int rsa_import(rsa_key *key) {
    return 0;
}

int rsa_init(rsa_key *key) {
    if(rsa_import(key)) {
        return 1;
    } else {
        return rsa_keygen(key);
    }
    return 0;
}

int rsa_public_init(rsa_public_key *key) {
    // null pointer handling
    if(key == NULL)
        return 0;

    mpz_init_set_ui(key->e, 65537);
    mpz_init_set_str(key->n, "", 0);
}

void rsa_free(rsa_key *key) {
    // free bignums
    mpz_clears(key->p, key->q, key->n, key->e, key->d, NULL);
}

void rsa_public_free(rsa_public_key *key) {
    // free bignums
    mpz_clears(key->e, key->n, NULL);
}

static int pkcs1(mpz_t message, const u8 *data, const size_t length) {
    // temporary buffer
    u8 padded_bytes[MODULUS_SIZE];

    // calculate padding size (how many 0xff bytes)
    size_t padding_length = MODULUS_SIZE - length - 3;

    if ((padding_length < 8) || (message == NULL) || (data == NULL)) {
        // message to big
        // or null pointer
        return 0;
    }

    // set padding bytes
    padded_bytes[0] = 0x00;
    padded_bytes[1] = 0x01;
    padded_bytes[2 + padding_length] = 0x00;

    for (size_t i = 2; i < padding_length + 2; i++) {
        padded_bytes[i] = 0xff;
    }

    // copy message bytes
    memcpy(padded_bytes + padding_length + 3, data, length);

    // convert padded message to mpz_t
    mpz_import(message, MODULUS_SIZE, 1, 1, 0, 0, padded_bytes);
    return 1;
}

size_t rsa_sign(u8 *sig, const u8 *sha256, const rsa_key *key) {
    // null pointer handling
    if((sig == NULL) || (sha256 == NULL) || (key == NULL))
        return 0;
    
    // init bignum message
    mpz_t message; mpz_init(message);
    mpz_t blinder; mpz_init(blinder);

    // get random blinder
    random_prime(blinder, MODULUS_SIZE - 10);

    // add padding
    if(!pkcs1(message, sha256, 32)) {
        return 0;
    }

    // blind
    mpz_mul(message, message, blinder);
    mpz_mod(message, message, key->n);
    mpz_invert(blinder, blinder, key->n);
    mpz_powm(blinder, blinder, key->d, key->n);

    // compute signature
    mpz_powm(message, message, key->d, key->n);

    // unblind
    mpz_mul(message, message, blinder);
    mpz_mod(message, message, key->n);

    // export signature
    size_t size = (mpz_sizeinbase(message, 2) + 7) / 8;
    mpz_export(sig, &size, 1, 1, 0, 0, message);

    // free bignum and return true
    mpz_clears(message, blinder, NULL);
    return size;
}

int rsa_verify(const u8 *sig, const size_t sig_length, const u8 *sha256, const rsa_public_key *pk) {
    // null pointer handling
    if((sig == NULL) || (sha256 == NULL) || (pk == NULL))
        return 0;

    // initialize bignums
    mpz_t signature, message; 
    mpz_inits(signature, message, NULL);

    // import signature
    mpz_import(signature, (sig_length < MODULUS_SIZE) ? sig_length : MODULUS_SIZE, 1, 1, 0, 0, sig);

    // revert rsa signing process
    mpz_powm(signature, signature, pk->e, pk->n);

    // rebuild signed message
    if(!pkcs1(message, sha256, 32))
        return 0;
    
    // compare signature with expected value
    if(mpz_cmp(signature, message) != 0)
        return 0;

    // free bignums and return valid signature
    mpz_clears(signature, message, NULL);
    return 1;
}

void rsa_print(const rsa_key *key) {
    gmp_printf("%Zx\n", key->p);
    gmp_printf("%Zx\n", key->q);
    gmp_printf("%Zx\n", key->n);
    gmp_printf("%Zx\n", key->e);
    gmp_printf("%Zx\n", key->d);
}

void rsa_public_print(const rsa_public_key *pk) {
    gmp_printf("%Zx\n", pk->e);
    gmp_printf("%Zx\n", pk->n);
}