/*

* HMAC

* (C) 1999-2007,2014,2020 Jack Lloyd

*     2007 Yves Jerschow

*

* Botan is released under the Simplified BSD License (see license.txt)

*/

#include <botan/hmac.h>

#include <botan/internal/ct_utils.h>

namespace Botan {

/*

* Update a HMAC Calculation

*/

void HMAC::add_data(const uint8_t input[], size_t length)

   {

   verify_key_set(m_ikey.empty() == false);

   m_hash->update(input, length);

   }

/*

* Finalize a HMAC Calculation

*/

void HMAC::final_result(uint8_t mac[])

   {

   verify_key_set(m_okey.empty() == false);

   m_hash->final(mac);

   m_hash->update(m_okey);

   m_hash->update(mac, m_hash_output_length);

   m_hash->final(mac);

   m_hash->update(m_ikey);

   }

Key_Length_Specification HMAC::key_spec() const

   {

   // Support very long lengths for things like PBKDF2 and the TLS PRF

   return Key_Length_Specification(0, 4096);

   }

size_t HMAC::output_length() const

   {

   return m_hash_output_length;

   }

/*

* HMAC Key Schedule

*/

void HMAC::key_schedule(const uint8_t key[], size_t length)

   {

   const uint8_t ipad = 0x36;

   const uint8_t opad = 0x5C;

   m_hash->clear();

   m_ikey.resize(m_hash_block_size);

   m_okey.resize(m_hash_block_size);

   clear_mem(m_ikey.data(), m_ikey.size());

   clear_mem(m_okey.data(), m_okey.size());

   /*

   * Sometimes the HMAC key length itself is sensitive, as with PBKDF2 where it

   * reveals the length of the passphrase. Make some attempt to hide this to

   * side channels. Clearly if the secret is longer than the block size then the

   * branch to hash first reveals that. In addition, counting the number of

   * compression functions executed reveals the size at the granularity of the

   * hash function's block size.

   *

   * The greater concern is for smaller keys; being able to detect when a

   * passphrase is say 4 bytes may assist choosing weaker targets. Even though

   * the loop bounds are constant, we can only actually read key[0..length] so

   * it doesn't seem possible to make this computation truly constant time.

   *

   * We don't mind leaking if the length is exactly zero since that's

   * trivial to simply check.

   */

   if(length > m_hash_block_size)

      {

      m_hash->update(key, length);

      m_hash->final(m_ikey.data());

      }

   else if(length > 0)

      {

      for(size_t i = 0, i_mod_length = 0; i != m_hash_block_size; ++i)

         {

         /*

         access key[i % length] but avoiding division due to variable

         time computation on some processors.

         */

         auto needs_reduction = CT::Mask<size_t>::is_lte(length, i_mod_length);

         i_mod_length = needs_reduction.select(0, i_mod_length);

         const uint8_t kb = key[i_mod_length];

         auto in_range = CT::Mask<size_t>::is_lt(i, length);

         m_ikey[i] = static_cast<uint8_t>(in_range.if_set_return(kb));

         i_mod_length += 1;

         }

      }

   for(size_t i = 0; i != m_hash_block_size; ++i)

      {

      m_ikey[i] ^= ipad;

      m_okey[i] = m_ikey[i] ^ ipad ^ opad;

      }

   m_hash->update(m_ikey);

   }

/*

* Clear memory of sensitive data

*/

void HMAC::clear()

   {

   m_hash->clear();

   zap(m_ikey);

   zap(m_okey);

   }

/*

* Return the name of this type

*/

std::string HMAC::name() const

   {

   return "HMAC(" + m_hash->name() + ")";

   }

/*

* Return a clone of this object

*/

MessageAuthenticationCode* HMAC::clone() const

   {

   return new HMAC(m_hash->clone());

   }

/*

* HMAC Constructor

*/

HMAC::HMAC(HashFunction* hash) :

   m_hash(hash),

   m_hash_output_length(m_hash->output_length()),

   m_hash_block_size(m_hash->hash_block_size())

   {

   BOTAN_ARG_CHECK(m_hash_block_size >= m_hash_output_length,

                   "HMAC is not compatible with this hash function");

   }

}