[BearSSL] / src / aead / gcm.c

/*
 * 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
 * ====================
 *
 * Since CTR and GHASH implementations can handle only full blocks, a
 * 16-byte buffer (buf[]) is maintained in the context:
 *
 *  - When processing AAD, buf[] contains the 0-15 unprocessed bytes.
 *
 *  - When doing CTR encryption / decryption, buf[] contains the AES output
 *    for the last partial block, to be used with the next few bytes of
 *    data, as well as the already encrypted bytes. For instance, if the
 *    processed data length so far is 21 bytes, then buf[0..4] contains
 *    the five last encrypted bytes, and buf[5..15] contains the next 11
 *    AES output bytes to be XORed with the next 11 bytes of input.
 *
 *    The recorded AES output bytes are used to complete the block when
 *    the corresponding bytes are obtained. Note that buf[] always
 *    contains the _encrypted_ bytes, whether we apply encryption or
 *    decryption: these bytes are used as input to GHASH when the block
 *    is complete.
 *
 * In both cases, the low bits of the data length counters (count_aad,
 * count_ctr) are used to work out the current situation.
 */

/* see bearssl_aead.h */
void
br_gcm_init(br_gcm_context *ctx, const br_block_ctr_class **bctx, br_ghash gh)
{
        unsigned char iv[12];

        ctx->vtable = &br_gcm_vtable;
        ctx->bctx = bctx;
        ctx->gh = gh;

        /*
         * The GHASH key h[] is the raw encryption of the all-zero
         * block. Since we only have a CTR implementation, we use it
         * with an all-zero IV and a zero counter, to CTR-encrypt an
         * all-zero block.
         */
        memset(ctx->h, 0, sizeof ctx->h);
        memset(iv, 0, sizeof iv);
        (*bctx)->run(bctx, iv, 0, ctx->h, sizeof ctx->h);
}

/* see bearssl_aead.h */
void
br_gcm_reset(br_gcm_context *ctx, const void *iv, size_t len)
{
        /*
         * If the provided nonce is 12 bytes, then this is the initial
         * IV for CTR mode; it will be used with a counter that starts
         * at 2 (value 1 is for encrypting the GHASH output into the tag).
         *
         * If the provided nonce has any other length, then it is hashed
         * (with GHASH) into a 16-byte value that will be the IV for CTR
         * (both 12-byte IV and 32-bit counter).
         */
        if (len == 12) {
                memcpy(ctx->j0_1, iv, 12);
                ctx->j0_2 = 1;
        } else {
                unsigned char ty[16], tmp[16];

                memset(ty, 0, sizeof ty);
                ctx->gh(ty, ctx->h, iv, len);
                memset(tmp, 0, 8);
                br_enc64be(tmp + 8, (uint64_t)len << 3);
                ctx->gh(ty, ctx->h, tmp, 16);
                memcpy(ctx->j0_1, ty, 12);
                ctx->j0_2 = br_dec32be(ty + 12);
        }
        ctx->jc = ctx->j0_2 + 1;
        memset(ctx->y, 0, sizeof ctx->y);
        ctx->count_aad = 0;
        ctx->count_ctr = 0;
}

/* see bearssl_aead.h */
void
br_gcm_aad_inject(br_gcm_context *ctx, const void *data, size_t len)
{
        size_t ptr, dlen;

        ptr = (size_t)ctx->count_aad & (size_t)15;
        if (ptr != 0) {
                /*
                 * If there is a partial block, then we first try to
                 * complete it.
                 */
                size_t clen;

                clen = 16 - ptr;
                if (len < clen) {
                        memcpy(ctx->buf + ptr, data, len);
                        ctx->count_aad += (uint64_t)len;
                        return;
                }
                memcpy(ctx->buf + ptr, data, clen);
                ctx->gh(ctx->y, ctx->h, ctx->buf, 16);
                data = (const unsigned char *)data + clen;
                len -= clen;
                ctx->count_aad += (uint64_t)clen;
        }

        /*
         * Now AAD is aligned on a 16-byte block (with regards to GHASH).
         * We process all complete blocks, and save the last partial
         * block.
         */
        dlen = len & ~(size_t)15;
        ctx->gh(ctx->y, ctx->h, data, dlen);
        memcpy(ctx->buf, (const unsigned char *)data + dlen, len - dlen);
        ctx->count_aad += (uint64_t)len;
}

/* see bearssl_aead.h */
void
br_gcm_flip(br_gcm_context *ctx)
{
        /*
         * We complete the GHASH computation if there is a partial block.
         * The GHASH implementation automatically applies padding with
         * zeros.
         */
        size_t ptr;

        ptr = (size_t)ctx->count_aad & (size_t)15;
        if (ptr != 0) {
                ctx->gh(ctx->y, ctx->h, ctx->buf, ptr);
        }
}

/* see bearssl_aead.h */
void
br_gcm_run(br_gcm_context *ctx, int encrypt, void *data, size_t len)
{
        unsigned char *buf;
        size_t ptr, dlen;

        buf = data;
        ptr = (size_t)ctx->count_ctr & (size_t)15;
        if (ptr != 0) {
                /*
                 * If we have a partial block, then we try to complete it.
                 */
                size_t u, clen;

                clen = 16 - ptr;
                if (len < clen) {
                        clen = len;
                }
                for (u = 0; u < clen; u ++) {
                        unsigned x, y;

                        x = buf[u];
                        y = x ^ ctx->buf[ptr + u];
                        ctx->buf[ptr + u] = encrypt ? y : x;
                        buf[u] = y;
                }
                ctx->count_ctr += (uint64_t)clen;
                buf += clen;
                len -= clen;
                if (ptr + clen < 16) {
                        return;
                }
                ctx->gh(ctx->y, ctx->h, ctx->buf, 16);
        }

        /*
         * Process full blocks.
         */
        dlen = len & ~(size_t)15;
        if (!encrypt) {
                ctx->gh(ctx->y, ctx->h, buf, dlen);
        }
        ctx->jc = (*ctx->bctx)->run(ctx->bctx, ctx->j0_1, ctx->jc, buf, dlen);
        if (encrypt) {
                ctx->gh(ctx->y, ctx->h, buf, dlen);
        }
        buf += dlen;
        len -= dlen;
        ctx->count_ctr += (uint64_t)dlen;

        if (len > 0) {
                /*
                 * There is a partial block.
                 */
                size_t u;

                memset(ctx->buf, 0, sizeof ctx->buf);
                ctx->jc = (*ctx->bctx)->run(ctx->bctx, ctx->j0_1,
                        ctx->jc, ctx->buf, 16);
                for (u = 0; u < len; u ++) {
                        unsigned x, y;

                        x = buf[u];
                        y = x ^ ctx->buf[u];
                        ctx->buf[u] = encrypt ? y : x;
                        buf[u] = y;
                }
                ctx->count_ctr += (uint64_t)len;
        }
}

/* see bearssl_aead.h */
void
br_gcm_get_tag(br_gcm_context *ctx, void *tag)
{
        size_t ptr;
        unsigned char tmp[16];

        ptr = (size_t)ctx->count_ctr & (size_t)15;
        if (ptr > 0) {
                /*
                 * There is a partial block: encrypted/decrypted data has
                 * been produced, but the encrypted bytes must still be
                 * processed by GHASH.
                 */
                ctx->gh(ctx->y, ctx->h, ctx->buf, ptr);
        }

        /*
         * Final block for GHASH: the AAD and plaintext lengths (in bits).
         */
        br_enc64be(tmp, ctx->count_aad << 3);
        br_enc64be(tmp + 8, ctx->count_ctr << 3);
        ctx->gh(ctx->y, ctx->h, tmp, 16);

        /*
         * Tag is the GHASH output XORed with the encryption of the
         * nonce with the initial counter value.
         */
        memcpy(tag, ctx->y, 16);
        (*ctx->bctx)->run(ctx->bctx, ctx->j0_1, ctx->j0_2, tag, 16);
}

/* see bearssl_aead.h */
void
br_gcm_get_tag_trunc(br_gcm_context *ctx, void *tag, size_t len)
{
        unsigned char tmp[16];

        br_gcm_get_tag(ctx, tmp);
        memcpy(tag, tmp, len);
}

/* see bearssl_aead.h */
uint32_t
br_gcm_check_tag_trunc(br_gcm_context *ctx, const void *tag, size_t len)
{
        unsigned char tmp[16];
        size_t u;
        int x;

        br_gcm_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_gcm_check_tag(br_gcm_context *ctx, const void *tag)
{
        return br_gcm_check_tag_trunc(ctx, tag, 16);
}

/* see bearssl_aead.h */
const br_aead_class br_gcm_vtable = {
        16,
        (void (*)(const br_aead_class **, const void *, size_t))
                &br_gcm_reset,
        (void (*)(const br_aead_class **, const void *, size_t))
                &br_gcm_aad_inject,
        (void (*)(const br_aead_class **))
                &br_gcm_flip,
        (void (*)(const br_aead_class **, int, void *, size_t))
                &br_gcm_run,
        (void (*)(const br_aead_class **, void *))
                &br_gcm_get_tag,
        (uint32_t (*)(const br_aead_class **, const void *))
                &br_gcm_check_tag,
        (void (*)(const br_aead_class **, void *, size_t))
                &br_gcm_get_tag_trunc,
        (uint32_t (*)(const br_aead_class **, const void *, size_t))
                &br_gcm_check_tag_trunc
};