/*
* Copyright (c) 2017 Thomas Pornin <pornin@bolet.org>
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* "Software"), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
* BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
* ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
#include "inner.h"
/*
* Implementation Notes
* ====================
*
* The combined CTR + CBC-MAC functions can only handle full blocks,
* so some buffering is necessary. Moreover, EAX has a special padding
* rule for CBC-MAC, which implies that we cannot compute the MAC over
* the last received full block until we know whether we are at the
* end of the data or not.
*
* - 'ptr' contains a value from 1 to 16, which is the number of bytes
* accumulated in buf[] that still needs to be processed with the
* current OMAC computation. Beware that this can go to 16: a
* complete block cannot be processed until it is known whether it
* is the last block or not. However, it can never be 0, because
* OMAC^t works on an input that is at least one-block long.
*
* - When processing the message itself, CTR encryption/decryption is
* also done at the same time. The first 'ptr' bytes of buf[] then
* contains the encrypted bytes, while the last '16 - ptr' bytes of
* buf[] are the remnants of the stream block, to be used against
* the next input bytes, when available.
*
* - The current counter and running CBC-MAC values are kept in 'ctr'
* and 'cbcmac', respectively.
*
* - The derived keys for padding are kept in L2 and L4 (double and
* quadruple of Enc_K(0^n), in GF(2^128), respectively).
*/
/*
* Start an OMAC computation; the first block is the big-endian
* representation of the provided value ('val' must fit on one byte).
* We make it a delayed block because it may also be the last one,
*/
static void
omac_start(br_eax_context *ctx, unsigned val)
{
memset(ctx->cbcmac, 0, sizeof ctx->cbcmac);
memset(ctx->buf, 0, sizeof ctx->buf);
ctx->buf[15] = val;
ctx->ptr = 16;
}
/*
* Double a value in finite field GF(2^128), defined with modulus
* X^128+X^7+X^2+X+1.
*/
static void
double_gf128(unsigned char *dst, const unsigned char *src)
{
unsigned cc;
int i;
cc = 0x87 & -((unsigned)src[0] >> 7);
for (i = 15; i >= 0; i --) {
unsigned z;
z = (src[i] << 1) ^ cc;
cc = z >> 8;
dst[i] = (unsigned char)z;
}
}
/*
* Apply padding to the last block, currently in ctx->buf (with
* ctx->ptr bytes), and finalize OMAC computation.
*/
static void
do_pad(br_eax_context *ctx)
{
unsigned char *pad;
size_t ptr, u;
ptr = ctx->ptr;
if (ptr == 16) {
pad = ctx->L2;
} else {
ctx->buf[ptr ++] = 0x80;
memset(ctx->buf + ptr, 0x00, 16 - ptr);
pad = ctx->L4;
}
for (u = 0; u < sizeof ctx->buf; u ++) {
ctx->buf[u] ^= pad[u];
}
(*ctx->bctx)->mac(ctx->bctx, ctx->cbcmac, ctx->buf, sizeof ctx->buf);
}
/*
* Apply CBC-MAC on the provided data, with buffering management.
*
* Upon entry, two situations are acceptable:
*
* ctx->ptr == 0: there is no data to process in ctx->buf
* ctx->ptr == 16: there is a full block of unprocessed data in ctx->buf
*
* Upon exit, ctx->ptr may be zero only if it was already zero on entry,
* and len == 0. In all other situations, ctx->ptr will be non-zero on
* exit (and may have value 16).
*/
static void
do_cbcmac_chunk(br_eax_context *ctx, const void *data, size_t len)
{
size_t ptr;
if (len == 0) {
return;
}
ptr = len & (size_t)15;
if (ptr == 0) {
len -= 16;
ptr = 16;
} else {
len -= ptr;
}
if (ctx->ptr == 16) {
(*ctx->bctx)->mac(ctx->bctx, ctx->cbcmac,
ctx->buf, sizeof ctx->buf);
}
(*ctx->bctx)->mac(ctx->bctx, ctx->cbcmac, data, len);
memcpy(ctx->buf, (const unsigned char *)data + len, ptr);
ctx->ptr = ptr;
}
/* see bearssl_aead.h */
void
br_eax_init(br_eax_context *ctx, const br_block_ctrcbc_class **bctx)
{
unsigned char tmp[16], iv[16];
ctx->vtable = &br_eax_vtable;
ctx->bctx = bctx;
/*
* Encrypt a whole-zero block to compute L2 and L4.
*/
memset(tmp, 0, sizeof tmp);
memset(iv, 0, sizeof iv);
(*bctx)->ctr(bctx, iv, tmp, sizeof tmp);
double_gf128(ctx->L2, tmp);
double_gf128(ctx->L4, ctx->L2);
}
/* see bearssl_aead.h */
void
br_eax_capture(const br_eax_context *ctx, br_eax_state *st)
{
/*
* We capture the three OMAC* states _after_ processing the
* initial block (assuming that nonce, message and AAD are
* all non-empty).
*/
int i;
memset(st->st, 0, sizeof st->st);
for (i = 0; i < 3; i ++) {
unsigned char tmp[16];
memset(tmp, 0, sizeof tmp);
tmp[15] = (unsigned char)i;
(*ctx->bctx)->mac(ctx->bctx, st->st[i], tmp, sizeof tmp);
}
}
/* see bearssl_aead.h */
void
br_eax_reset(br_eax_context *ctx, const void *nonce, size_t len)
{
/*
* Process nonce with OMAC^0.
*/
omac_start(ctx, 0);
do_cbcmac_chunk(ctx, nonce, len);
do_pad(ctx);
memcpy(ctx->nonce, ctx->cbcmac, sizeof ctx->cbcmac);
/*
* Start OMAC^1 for the AAD ("header" in the EAX specification).
*/
omac_start(ctx, 1);
/*
* We use ctx->head[0] as temporary flag to mark that we are
* using a "normal" reset().
*/
ctx->head[0] = 0;
}
/* see bearssl_aead.h */
void
br_eax_reset_pre_aad(br_eax_context *ctx, const br_eax_state *st,
const void *nonce, size_t len)
{
if (len == 0) {
omac_start(ctx, 0);
} else {
memcpy(ctx->cbcmac, st->st[0], sizeof ctx->cbcmac);
ctx->ptr = 0;
do_cbcmac_chunk(ctx, nonce, len);
}
do_pad(ctx);
memcpy(ctx->nonce, ctx->cbcmac, sizeof ctx->cbcmac);
memcpy(ctx->cbcmac, st->st[1], sizeof ctx->cbcmac);
ctx->ptr = 0;
memcpy(ctx->ctr, st->st[2], sizeof ctx->ctr);
/*
* We use ctx->head[0] as a flag to indicate that we use a
* a recorded state, with ctx->ctr containing the preprocessed
* first block for OMAC^2.
*/
ctx->head[0] = 1;
}
/* see bearssl_aead.h */
void
br_eax_reset_post_aad(br_eax_context *ctx, const br_eax_state *st,
const void *nonce, size_t len)
{
if (len == 0) {
omac_start(ctx, 0);
} else {
memcpy(ctx->cbcmac, st->st[0], sizeof ctx->cbcmac);
ctx->ptr = 0;
do_cbcmac_chunk(ctx, nonce, len);
}
do_pad(ctx);
memcpy(ctx->nonce, ctx->cbcmac, sizeof ctx->cbcmac);
memcpy(ctx->ctr, ctx->nonce, sizeof ctx->nonce);
memcpy(ctx->head, st->st[1], sizeof ctx->head);
memcpy(ctx->cbcmac, st->st[2], sizeof ctx->cbcmac);
ctx->ptr = 0;
}
/* see bearssl_aead.h */
void
br_eax_aad_inject(br_eax_context *ctx, const void *data, size_t len)
{
size_t ptr;
ptr = ctx->ptr;
/*
* If there is a partial block, first complete it.
*/
if (ptr < 16) {
size_t clen;
clen = 16 - ptr;
if (len <= clen) {
memcpy(ctx->buf + ptr, data, len);
ctx->ptr = ptr + len;
return;
}
memcpy(ctx->buf + ptr, data, clen);
data = (const unsigned char *)data + clen;
len -= clen;
}
/*
* We now have a full block in buf[], and this is not the last
* block.
*/
do_cbcmac_chunk(ctx, data, len);
}
/* see bearssl_aead.h */
void
br_eax_flip(br_eax_context *ctx)
{
int from_capture;
/*
* ctx->head[0] may be non-zero if the context was reset with
* a pre-AAD captured state. In that case, ctx->ctr[] contains
* the state for OMAC^2 _after_ processing the first block.
*/
from_capture = ctx->head[0];
/*
* Complete the OMAC computation on the AAD.
*/
do_pad(ctx);
memcpy(ctx->head, ctx->cbcmac, sizeof ctx->cbcmac);
/*
* Start OMAC^2 for the encrypted data.
* If the context was initialized from a captured state, then
* the OMAC^2 value is in the ctr[] array.
*/
if (from_capture) {
memcpy(ctx->cbcmac, ctx->ctr, sizeof ctx->cbcmac);
ctx->ptr = 0;
} else {
omac_start(ctx, 2);
}
/*
* Initial counter value for CTR is the processed nonce.
*/
memcpy(ctx->ctr, ctx->nonce, sizeof ctx->nonce);
}
/* see bearssl_aead.h */
void
br_eax_run(br_eax_context *ctx, int encrypt, void *data, size_t len)
{
unsigned char *dbuf;
size_t ptr;
/*
* Ensure that there is actual data to process.
*/
if (len == 0) {
return;
}
dbuf = data;
ptr = ctx->ptr;
/*
* We may have ptr == 0 here if we initialized from a captured
* state. In that case, there is no partially consumed block
* or unprocessed data.
*/
if (ptr != 0 && ptr != 16) {
/*
* We have a partially consumed block.
*/
size_t u, clen;
clen = 16 - ptr;
if (len <= clen) {
clen = len;
}
if (encrypt) {
for (u = 0; u < clen; u ++) {
ctx->buf[ptr + u] ^= dbuf[u];
}
memcpy(dbuf, ctx->buf + ptr, clen);
} else {
for (u = 0; u < clen; u ++) {
unsigned dx, sx;
sx = ctx->buf[ptr + u];
dx = dbuf[u];
ctx->buf[ptr + u] = dx;
dbuf[u] = sx ^ dx;
}
}
if (len <= clen) {
ctx->ptr = ptr + clen;
return;
}
dbuf += clen;
len -= clen;
}
/*
* We now have a complete encrypted block in buf[] that must still
* be processed with OMAC, and this is not the final buf.
* Exception: when ptr == 0, no block has been produced yet.
*/
if (ptr != 0) {
(*ctx->bctx)->mac(ctx->bctx, ctx->cbcmac,
ctx->buf, sizeof ctx->buf);
}
/*
* Do CTR encryption or decryption and CBC-MAC for all full blocks
* except the last.
*/
ptr = len & (size_t)15;
if (ptr == 0) {
len -= 16;
ptr = 16;
} else {
len -= ptr;
}
if (encrypt) {
(*ctx->bctx)->encrypt(ctx->bctx, ctx->ctr, ctx->cbcmac,
dbuf, len);
} else {
(*ctx->bctx)->decrypt(ctx->bctx, ctx->ctr, ctx->cbcmac,
dbuf, len);
}
dbuf += len;
/*
* Compute next block of CTR stream, and use it to finish
* encrypting or decrypting the data.
*/
memset(ctx->buf, 0, sizeof ctx->buf);
(*ctx->bctx)->ctr(ctx->bctx, ctx->ctr, ctx->buf, sizeof ctx->buf);
if (encrypt) {
size_t u;
for (u = 0; u < ptr; u ++) {
ctx->buf[u] ^= dbuf[u];
}
memcpy(dbuf, ctx->buf, ptr);
} else {
size_t u;
for (u = 0; u < ptr; u ++) {
unsigned dx, sx;
sx = ctx->buf[u];
dx = dbuf[u];
ctx->buf[u] = dx;
dbuf[u] = sx ^ dx;
}
}
ctx->ptr = ptr;
}
/*
* Complete tag computation. The final tag is written in ctx->cbcmac.
*/
static void
do_final(br_eax_context *ctx)
{
size_t u;
do_pad(ctx);
/*
* Authentication tag is the XOR of the three OMAC outputs for
* the nonce, AAD and encrypted data.
*/
for (u = 0; u < 16; u ++) {
ctx->cbcmac[u] ^= ctx->nonce[u] ^ ctx->head[u];
}
}
/* see bearssl_aead.h */
void
br_eax_get_tag(br_eax_context *ctx, void *tag)
{
do_final(ctx);
memcpy(tag, ctx->cbcmac, sizeof ctx->cbcmac);
}
/* see bearssl_aead.h */
void
br_eax_get_tag_trunc(br_eax_context *ctx, void *tag, size_t len)
{
do_final(ctx);
memcpy(tag, ctx->cbcmac, len);
}
/* see bearssl_aead.h */
uint32_t
br_eax_check_tag_trunc(br_eax_context *ctx, const void *tag, size_t len)
{
unsigned char tmp[16];
size_t u;
int x;
br_eax_get_tag(ctx, tmp);
x = 0;
for (u = 0; u < len; u ++) {
x |= tmp[u] ^ ((const unsigned char *)tag)[u];
}
return EQ0(x);
}
/* see bearssl_aead.h */
uint32_t
br_eax_check_tag(br_eax_context *ctx, const void *tag)
{
return br_eax_check_tag_trunc(ctx, tag, 16);
}
/* see bearssl_aead.h */
const br_aead_class br_eax_vtable = {
16,
(void (*)(const br_aead_class **, const void *, size_t))
&br_eax_reset,
(void (*)(const br_aead_class **, const void *, size_t))
&br_eax_aad_inject,
(void (*)(const br_aead_class **))
&br_eax_flip,
(void (*)(const br_aead_class **, int, void *, size_t))
&br_eax_run,
(void (*)(const br_aead_class **, void *))
&br_eax_get_tag,
(uint32_t (*)(const br_aead_class **, const void *))
&br_eax_check_tag,
(void (*)(const br_aead_class **, void *, size_t))
&br_eax_get_tag_trunc,
(uint32_t (*)(const br_aead_class **, const void *, size_t))
&br_eax_check_tag_trunc
};