tor-browser

cmac.c (9962B)

---

/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifdef FREEBL_NO_DEPEND
#include "stubs.h"
#endif

#include "rijndael.h"
#include "blapi.h"
#include "cmac.h"
#include "secerr.h"
#include "nspr.h"

struct CMACContextStr {
   /* Information about the block cipher to use internally. The cipher should
    * be placed in ECB mode so that we can use it to directly encrypt blocks.
    *
    *
    * To add a new cipher, add an entry to CMACCipher, update CMAC_Init,
    * cmac_Encrypt, and CMAC_Destroy methods to handle the new cipher, and
    * add a new Context pointer to the cipher union with the correct type. */
   CMACCipher cipherType;
   union {
       AESContext *aes;
   } cipher;
   unsigned int blockSize;

   /* Internal keys which are conditionally used by the algorithm. Derived
    * from encrypting the NULL block. We leave the storing of (and the
    * cleanup of) the CMAC key to the underlying block cipher. */
   unsigned char k1[MAX_BLOCK_SIZE];
   unsigned char k2[MAX_BLOCK_SIZE];

   /* When Update is called with data which isn't a multiple of the block
    * size, we need a place to put it. HMAC handles this by passing it to
    * the underlying hash function right away; we can't do that as the
    * contract on the cipher object is different. */
   unsigned int partialIndex;
   unsigned char partialBlock[MAX_BLOCK_SIZE];

   /* Last encrypted block. This gets xor-ed with partialBlock prior to
    * encrypting it. NIST defines this to be the empty string to begin. */
   unsigned char lastBlock[MAX_BLOCK_SIZE];
};

static void
cmac_ShiftLeftOne(unsigned char *out, const unsigned char *in, int length)
{
   int i = 0;
   for (; i < length - 1; i++) {
       out[i] = in[i] << 1;
       out[i] |= in[i + 1] >> 7;
   }
   out[i] = in[i] << 1;
}

static SECStatus
cmac_Encrypt(CMACContext *ctx, unsigned char *output,
            const unsigned char *input,
            unsigned int inputLen)
{
   if (ctx->cipherType == CMAC_AES) {
       unsigned int tmpOutputLen;
       SECStatus rv = AES_Encrypt(ctx->cipher.aes, output, &tmpOutputLen,
                                  ctx->blockSize, input, inputLen);

       /* Assumption: AES_Encrypt (when in ECB mode) always returns an
        * output of length equal to blockSize (what was pass as the value
        * of the maxOutputLen parameter). */
       PORT_Assert(tmpOutputLen == ctx->blockSize);
       return rv;
   }

   return SECFailure;
}

/* NIST SP.800-38B, 6.1 Subkey Generation */
static SECStatus
cmac_GenerateSubkeys(CMACContext *ctx)
{
   unsigned char null_block[MAX_BLOCK_SIZE] = { 0 };
   unsigned char L[MAX_BLOCK_SIZE];
   unsigned char v;
   unsigned char i;

   /* Step 1: L = AES(key, null_block) */
   if (cmac_Encrypt(ctx, L, null_block, ctx->blockSize) != SECSuccess) {
       return SECFailure;
   }

   /* In the following, some effort has been made to be constant time. Rather
    * than conditioning on the value of the MSB (of L or K1), we use the loop
    * to build a mask for the conditional constant. */

   /* Step 2: If MSB(L) = 0, K1 = L << 1. Else, K1 = (L << 1) ^ R_b. */
   cmac_ShiftLeftOne(ctx->k1, L, ctx->blockSize);
   v = L[0] >> 7;
   for (i = 1; i <= 7; i <<= 1) {
       v |= (v << i);
   }
   ctx->k1[ctx->blockSize - 1] ^= (0x87 & v);

   /* Step 3: If MSB(K1) = 0, K2 = K1 << 1. Else, K2 = (K1 <, 1) ^ R_b. */
   cmac_ShiftLeftOne(ctx->k2, ctx->k1, ctx->blockSize);
   v = ctx->k1[0] >> 7;
   for (i = 1; i <= 7; i <<= 1) {
       v |= (v << i);
   }
   ctx->k2[ctx->blockSize - 1] ^= (0x87 & v);

   /* Any intermediate value in the computation of the subkey shall be
    * secret. */
   PORT_Memset(null_block, 0, MAX_BLOCK_SIZE);
   PORT_Memset(L, 0, MAX_BLOCK_SIZE);

   /* Step 4: Return the values. */
   return SECSuccess;
}

/* NIST SP.800-38B, 6.2 MAC Generation step 6 */
static SECStatus
cmac_UpdateState(CMACContext *ctx)
{
   if (ctx == NULL || ctx->partialIndex != ctx->blockSize) {
       PORT_SetError(SEC_ERROR_INVALID_ARGS);
       return SECFailure;
   }

   /* Step 6: C_i = CIPHER(key, C_{i-1} ^ M_i)  for 1 <= i <= n, and
    *         C_0 is defined as the empty string. */

   for (unsigned int index = 0; index < ctx->blockSize; index++) {
       ctx->partialBlock[index] ^= ctx->lastBlock[index];
   }

   return cmac_Encrypt(ctx, ctx->lastBlock, ctx->partialBlock, ctx->blockSize);
}

SECStatus
CMAC_Init(CMACContext *ctx, CMACCipher type,
         const unsigned char *key, unsigned int key_len)
{
   if (ctx == NULL) {
       PORT_SetError(SEC_ERROR_NO_MEMORY);
       return SECFailure;
   }

   /* We only currently support AES-CMAC. */
   if (type != CMAC_AES) {
       PORT_SetError(SEC_ERROR_INVALID_ARGS);
       return SECFailure;
   }

   PORT_Memset(ctx, 0, sizeof(*ctx));

   ctx->blockSize = AES_BLOCK_SIZE;
   ctx->cipherType = CMAC_AES;
   ctx->cipher.aes = AES_CreateContext(key, NULL, NSS_AES, 1, key_len,
                                       ctx->blockSize);
   if (ctx->cipher.aes == NULL) {
       return SECFailure;
   }

   return CMAC_Begin(ctx);
}

CMACContext *
CMAC_Create(CMACCipher type, const unsigned char *key,
           unsigned int key_len)
{
   CMACContext *result = PORT_New(CMACContext);

   if (CMAC_Init(result, type, key, key_len) != SECSuccess) {
       CMAC_Destroy(result, PR_TRUE);
       return NULL;
   }

   return result;
}

SECStatus
CMAC_Begin(CMACContext *ctx)
{
   if (ctx == NULL) {
       return SECFailure;
   }

   /* Ensure that our blockSize is less than the maximum. When this fails,
    * a cipher with a larger block size was added and MAX_BLOCK_SIZE needs
    * to be updated accordingly. */
   PORT_Assert(ctx->blockSize <= MAX_BLOCK_SIZE);

   if (cmac_GenerateSubkeys(ctx) != SECSuccess) {
       return SECFailure;
   }

   /* Set the index to write partial blocks at to zero. This saves us from
    * having to clear ctx->partialBlock. */
   ctx->partialIndex = 0;

   /* Step 5: Let C_0 = 0^b. */
   PORT_Memset(ctx->lastBlock, 0, ctx->blockSize);

   return SECSuccess;
}

/* NIST SP.800-38B, 6.2 MAC Generation */
SECStatus
CMAC_Update(CMACContext *ctx, const unsigned char *data,
           unsigned int data_len)
{
   unsigned int data_index = 0;
   if (ctx == NULL) {
       PORT_SetError(SEC_ERROR_INVALID_ARGS);
       return SECFailure;
   }

   if (data == NULL || data_len == 0) {
       return SECSuccess;
   }

   /* Copy as many bytes from data into ctx->partialBlock as we can, up to
    * the maximum of the remaining data and the remaining space in
    * ctx->partialBlock.
    *
    * Note that we swap the order (encrypt *then* copy) because the last
    * block is different from the rest. If we end on an even multiple of
    * the block size, we have to be able to XOR it with K1. But we won't know
    * that it is the last until CMAC_Finish is called (and by then, CMAC_Update
    * has already returned). */
   while (data_index < data_len) {
       if (ctx->partialIndex == ctx->blockSize) {
           if (cmac_UpdateState(ctx) != SECSuccess) {
               return SECFailure;
           }

           ctx->partialIndex = 0;
       }

       unsigned int copy_len = data_len - data_index;
       if (copy_len > (ctx->blockSize - ctx->partialIndex)) {
           copy_len = ctx->blockSize - ctx->partialIndex;
       }

       PORT_Memcpy(ctx->partialBlock + ctx->partialIndex, data + data_index, copy_len);
       data_index += copy_len;
       ctx->partialIndex += copy_len;
   }

   return SECSuccess;
}

/* NIST SP.800-38B, 6.2 MAC Generation */
SECStatus
CMAC_Finish(CMACContext *ctx, unsigned char *result,
           unsigned int *result_len,
           unsigned int max_result_len)
{
   if (ctx == NULL || result == NULL || max_result_len == 0) {
       PORT_SetError(SEC_ERROR_INVALID_ARGS);
       return SECFailure;
   }

   if (max_result_len > ctx->blockSize) {
       /* This is a weird situation. The PKCS #11 soft tokencode passes
        * sizeof(result) here, which is hard-coded as SFTK_MAX_MAC_LENGTH.
        * This later gets truncated to min(SFTK_MAX_MAC_LENGTH, requested). */
       max_result_len = ctx->blockSize;
   }

   /* Step 4: If M_n* is a complete block, M_n = K1 ^ M_n*. Else,
    * M_n = K2 ^ (M_n* || 10^j). */
   if (ctx->partialIndex == ctx->blockSize) {
       /* XOR in K1. */
       for (unsigned int index = 0; index < ctx->blockSize; index++) {
           ctx->partialBlock[index] ^= ctx->k1[index];
       }
   } else {
       /* Use 10* padding on the partial block. */
       ctx->partialBlock[ctx->partialIndex++] = 0x80;
       PORT_Memset(ctx->partialBlock + ctx->partialIndex, 0,
                   ctx->blockSize - ctx->partialIndex);
       ctx->partialIndex = ctx->blockSize;

       /* XOR in K2. */
       for (unsigned int index = 0; index < ctx->blockSize; index++) {
           ctx->partialBlock[index] ^= ctx->k2[index];
       }
   }

   /* Encrypt the block. */
   if (cmac_UpdateState(ctx) != SECSuccess) {
       return SECFailure;
   }

   /* Step 7 & 8: T = MSB_tlen(C_n); return T. */
   PORT_Memcpy(result, ctx->lastBlock, max_result_len);
   if (result_len != NULL) {
       *result_len = max_result_len;
   }
   return SECSuccess;
}

void
CMAC_Destroy(CMACContext *ctx, PRBool free_it)
{
   if (ctx == NULL) {
       return;
   }

   if (ctx->cipherType == CMAC_AES && ctx->cipher.aes != NULL) {
       AES_DestroyContext(ctx->cipher.aes, PR_TRUE);
   }

   /* Destroy everything in the context. This includes sensitive data in
    * K1, K2, and lastBlock. */
   PORT_Memset(ctx, 0, sizeof(*ctx));

   if (free_it == PR_TRUE) {
       PORT_Free(ctx);
   }
}