hotp.cpp

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/*
* HOTP
* (C) 2017 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#include <botan/otp.h>
#include <botan/internal/loadstor.h>
#include <botan/exceptn.h>
 
namespace Botan {
 
HOTP::HOTP(const uint8_t key[], size_t key_len,
           std::string_view hash_algo, size_t digits)
   {
   BOTAN_ARG_CHECK(digits == 6 || digits == 7 || digits == 8, "Invalid HOTP digits");
 
   if(digits == 6)
      m_digit_mod = 1000000;
   else if(digits == 7)
      m_digit_mod = 10000000;
   else if(digits == 8)
      m_digit_mod = 100000000;
 
   /*
   RFC 4228 only supports SHA-1 but TOTP allows SHA-256 and SHA-512
   and some HOTP libs support one or both as extensions
   */
   if(hash_algo == "SHA-1")
      m_mac = MessageAuthenticationCode::create_or_throw("HMAC(SHA-1)");
   else if(hash_algo == "SHA-256")
      m_mac = MessageAuthenticationCode::create_or_throw("HMAC(SHA-256)");
   else if(hash_algo == "SHA-512")
      m_mac = MessageAuthenticationCode::create_or_throw("HMAC(SHA-512)");
   else
      throw Invalid_Argument("Unsupported HOTP hash function");
 
   m_mac->set_key(key, key_len);
   }
 
uint32_t HOTP::generate_hotp(uint64_t counter)
   {
   m_mac->update_be(counter);
   const secure_vector<uint8_t> mac = m_mac->final();
 
   const size_t offset = mac[mac.size()-1] & 0x0F;
   const uint32_t code = load_be<uint32_t>(mac.data() + offset, 0) & 0x7FFFFFFF;
   return code % m_digit_mod;
   }
 
std::pair<bool,uint64_t> HOTP::verify_hotp(uint32_t otp, uint64_t starting_counter, size_t resync_range)
   {
   for(size_t i = 0; i <= resync_range; ++i)
      {
      if(generate_hotp(starting_counter + i) == otp)
         return std::make_pair(true, starting_counter + i + 1);
      }
   return std::make_pair(false, starting_counter);
   }
 
}