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/*
* Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License").
* You may not use this file except in compliance with the License.
* A copy of the License is located at
*
* http://aws.amazon.com/apache2.0
*
* or in the "license" file accompanying this file. This file 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.
*/
#include <sys/param.h>
#include <openssl/evp.h>
#include "crypto/s2n_drbg.h"
#include "utils/s2n_safety.h"
#include "utils/s2n_random.h"
#include "utils/s2n_blob.h"
#define s2n_drbg_key_size(drgb) EVP_CIPHER_CTX_key_length((drbg)->ctx)
#define s2n_drbg_seed_size(drgb) (S2N_DRBG_BLOCK_SIZE + s2n_drbg_key_size(drgb))
/* This function is the same as s2n_increment_sequence_number
but it does not check for overflow, since overflow is
acceptable in DRBG */
int s2n_increment_drbg_counter(struct s2n_blob *counter)
{
for (int i = counter->size - 1; i >= 0; i--) {
counter->data[i] += 1;
if (counter->data[i]) {
break;
}
/* seq[i] wrapped, so let it carry */
}
return 0;
}
static int s2n_drbg_block_encrypt(EVP_CIPHER_CTX * ctx, uint8_t in[S2N_DRBG_BLOCK_SIZE], uint8_t out[S2N_DRBG_BLOCK_SIZE])
{
notnull_check(ctx);
int len = S2N_DRBG_BLOCK_SIZE;
GUARD_OSSL(EVP_EncryptUpdate(ctx, out, &len, in, S2N_DRBG_BLOCK_SIZE), S2N_ERR_DRBG);
eq_check(len, S2N_DRBG_BLOCK_SIZE);
return 0;
}
static int s2n_drbg_bits(struct s2n_drbg *drbg, struct s2n_blob *out)
{
notnull_check(drbg);
notnull_check(drbg->ctx);
notnull_check(out);
struct s2n_blob value = {0};
GUARD(s2n_blob_init(&value, drbg->v, sizeof(drbg->v)));
int block_aligned_size = out->size - (out->size % S2N_DRBG_BLOCK_SIZE);
/* Per NIST SP800-90A 10.2.1.2: */
for (int i = 0; i < block_aligned_size; i += S2N_DRBG_BLOCK_SIZE) {
GUARD(s2n_increment_drbg_counter(&value));
GUARD(s2n_drbg_block_encrypt(drbg->ctx, drbg->v, out->data + i));
drbg->bytes_used += S2N_DRBG_BLOCK_SIZE;
}
if (out->size <= block_aligned_size) {
return 0;
}
uint8_t spare_block[S2N_DRBG_BLOCK_SIZE];
GUARD(s2n_increment_drbg_counter(&value));
GUARD(s2n_drbg_block_encrypt(drbg->ctx, drbg->v, spare_block));
drbg->bytes_used += S2N_DRBG_BLOCK_SIZE;
memcpy_check(out->data + block_aligned_size, spare_block, out->size - block_aligned_size);
return 0;
}
static int s2n_drbg_update(struct s2n_drbg *drbg, struct s2n_blob *provided_data)
{
notnull_check(drbg);
notnull_check(drbg->ctx);
s2n_stack_blob(temp_blob, s2n_drbg_seed_size(drgb), S2N_DRBG_MAX_SEED_SIZE);
eq_check(provided_data->size, s2n_drbg_seed_size(drbg));
GUARD(s2n_drbg_bits(drbg, &temp_blob));
/* XOR in the provided data */
for (int i = 0; i < provided_data->size; i++) {
temp_blob.data[i] ^= provided_data->data[i];
}
/* Update the key and value */
GUARD_OSSL(EVP_EncryptInit_ex(drbg->ctx, NULL, NULL, temp_blob.data, NULL), S2N_ERR_DRBG);
memcpy_check(drbg->v, temp_blob.data + s2n_drbg_key_size(drbg), S2N_DRBG_BLOCK_SIZE);
return 0;
}
static int s2n_drbg_mix_in_entropy(struct s2n_drbg *drbg, struct s2n_blob *entropy, struct s2n_blob *ps)
{
notnull_check(drbg);
notnull_check(drbg->ctx);
notnull_check(entropy);
gte_check(entropy->size, ps->size);
for (int i = 0; i < ps->size; i++) {
entropy->data[i] ^= ps->data[i];
}
GUARD(s2n_drbg_update(drbg, entropy));
return 0;
}
static int s2n_drbg_seed(struct s2n_drbg *drbg, struct s2n_blob *ps)
{
s2n_stack_blob(blob, s2n_drbg_seed_size(drbg), S2N_DRBG_MAX_SEED_SIZE);
GUARD_AS_POSIX(s2n_get_seed_entropy(&blob));
GUARD(s2n_drbg_mix_in_entropy(drbg, &blob, ps));
drbg->bytes_used = 0;
return 0;
}
static int s2n_drbg_mix(struct s2n_drbg *drbg, struct s2n_blob *ps)
{
s2n_stack_blob(blob, s2n_drbg_seed_size(drbg), S2N_DRBG_MAX_SEED_SIZE);
GUARD_AS_POSIX(s2n_get_mix_entropy(&blob));
GUARD(s2n_drbg_mix_in_entropy(drbg, &blob, ps));
drbg->mixes += 1;
return 0;
}
int s2n_drbg_instantiate(struct s2n_drbg *drbg, struct s2n_blob *personalization_string, const s2n_drbg_mode mode)
{
notnull_check(drbg);
drbg->ctx = EVP_CIPHER_CTX_new();
S2N_ERROR_IF(!drbg->ctx, S2N_ERR_DRBG);
s2n_evp_ctx_init(drbg->ctx);
switch(mode) {
case S2N_AES_128_CTR_NO_DF_PR:
GUARD_OSSL(EVP_EncryptInit_ex(drbg->ctx, EVP_aes_128_ecb(), NULL, NULL, NULL), S2N_ERR_DRBG);
break;
case S2N_AES_256_CTR_NO_DF_PR:
GUARD_OSSL(EVP_EncryptInit_ex(drbg->ctx, EVP_aes_256_ecb(), NULL, NULL, NULL), S2N_ERR_DRBG);
break;
default:
S2N_ERROR(S2N_ERR_DRBG);
}
lte_check(s2n_drbg_key_size(drbg), S2N_DRBG_MAX_KEY_SIZE);
lte_check(s2n_drbg_seed_size(drbg), S2N_DRBG_MAX_SEED_SIZE);
static const uint8_t zero_key[S2N_DRBG_MAX_KEY_SIZE] = {0};
/* Start off with zeroed data, per 10.2.1.3.1 item 4 and 5 */
memset(drbg->v, 0, sizeof(drbg->v));
GUARD_OSSL(EVP_EncryptInit_ex(drbg->ctx, NULL, NULL, zero_key, NULL), S2N_ERR_DRBG);
/* Copy the personalization string */
s2n_stack_blob(ps, s2n_drbg_seed_size(drbg), S2N_DRBG_MAX_SEED_SIZE);
GUARD(s2n_blob_zero(&ps));
memcpy_check(ps.data, personalization_string->data, MIN(ps.size, personalization_string->size));
/* Seed the DRBG */
GUARD(s2n_drbg_seed(drbg, &ps));
return 0;
}
int s2n_drbg_generate(struct s2n_drbg *drbg, struct s2n_blob *blob)
{
notnull_check(drbg);
notnull_check(drbg->ctx);
s2n_stack_blob(zeros, s2n_drbg_seed_size(drbg), S2N_DRBG_MAX_SEED_SIZE);
S2N_ERROR_IF(blob->size > S2N_DRBG_GENERATE_LIMIT, S2N_ERR_DRBG_REQUEST_SIZE);
/* Always mix in additional entropy, for prediction resistance */
GUARD(s2n_drbg_mix(drbg, &zeros));
GUARD(s2n_drbg_bits(drbg, blob));
GUARD(s2n_drbg_update(drbg, &zeros));
return 0;
}
int s2n_drbg_wipe(struct s2n_drbg *drbg)
{
notnull_check(drbg);
if (drbg->ctx) {
GUARD_OSSL(EVP_CIPHER_CTX_cleanup(drbg->ctx), S2N_ERR_DRBG);
EVP_CIPHER_CTX_free(drbg->ctx);
drbg->ctx = NULL;
}
*drbg = (struct s2n_drbg) {0};
return 0;
}
int s2n_drbg_bytes_used(struct s2n_drbg *drbg, uint64_t *bytes_used)
{
notnull_check(drbg);
notnull_check(bytes_used);
*bytes_used = drbg->bytes_used;
return 0;
}
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