hkdf.cpp

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/*
* HKDF
* (C) 2013,2015,2017 Jack Lloyd
* (C) 2016 René Korthaus, Rohde & Schwarz Cybersecurity
*
* Botan is released under the Simplified BSD License (see license.txt)
*/

#include <botan/hkdf.h>
#include <botan/loadstor.h>

namespace Botan {

size_t HKDF::kdf(uint8_t key[], size_t key_len,
                 const uint8_t secret[], size_t secret_len,
                 const uint8_t salt[], size_t salt_len,
                 const uint8_t label[], size_t label_len) const
   {
   HKDF_Extract extract(m_prf->clone());
   HKDF_Expand expand(m_prf->clone());
   secure_vector<uint8_t> prk(m_prf->output_length());

   extract.kdf(prk.data(), prk.size(), secret, secret_len, salt, salt_len, nullptr, 0);
   return expand.kdf(key, key_len, prk.data(), prk.size(), nullptr, 0, label, label_len);
   }

size_t HKDF_Extract::kdf(uint8_t key[], size_t key_len,
                         const uint8_t secret[], size_t secret_len,
                         const uint8_t salt[], size_t salt_len,
                         const uint8_t[], size_t) const
   {
   secure_vector<uint8_t> prk;
   if(salt_len == 0)
      {
      m_prf->set_key(std::vector<uint8_t>(m_prf->output_length()));
      }
   else
      {
      m_prf->set_key(salt, salt_len);
      }

   m_prf->update(secret, secret_len);
   m_prf->final(prk);

   const size_t written = std::min(prk.size(), key_len);
   copy_mem(&key[0], prk.data(), written);
   return written;
   }

size_t HKDF_Expand::kdf(uint8_t key[], size_t key_len,
                        const uint8_t secret[], size_t secret_len,
                        const uint8_t salt[], size_t salt_len,
                        const uint8_t label[], size_t label_len) const
   {
   m_prf->set_key(secret, secret_len);

   uint8_t counter = 1;
   secure_vector<uint8_t> h;
   size_t offset = 0;

   while(offset != key_len && counter != 0)
      {
      m_prf->update(h);
      m_prf->update(label, label_len);
      m_prf->update(salt, salt_len);
      m_prf->update(counter++);
      m_prf->final(h);

      const size_t written = std::min(h.size(), key_len - offset);
      copy_mem(&key[offset], h.data(), written);
      offset += written;
      }

   return offset;
   }

secure_vector<uint8_t>
hkdf_expand_label(const std::string& hash_fn,
                  const uint8_t secret[], size_t secret_len,
                  const std::string& label,
                  const uint8_t hash_val[], size_t hash_val_len,
                  size_t length)
   {
   BOTAN_ARG_CHECK(length <= 0xFFFF, "HKDF-Expand-Label requested output too large");
   BOTAN_ARG_CHECK(label.size() <= 0xFF, "HKDF-Expand-Label label too long");
   BOTAN_ARG_CHECK(hash_val_len <= 0xFF, "HKDF-Expand-Label hash too long");

   const uint16_t length16 = static_cast<uint16_t>(length);

   auto mac = MessageAuthenticationCode::create_or_throw("HMAC(" + hash_fn + ")");

   HKDF_Expand hkdf(mac.release());

   secure_vector<uint8_t> output(length16);
   std::vector<uint8_t> prefix(3 + label.size() + 1);

   prefix[0] = get_byte(0, length16);
   prefix[1] = get_byte(1, length16);
   prefix[2] = static_cast<uint8_t>(label.size());

   copy_mem(prefix.data() + 3,
            cast_char_ptr_to_uint8(label.data()),
            label.size());

   prefix[3 + label.size()] = static_cast<uint8_t>(hash_val_len);

   /*
   * We do something a little dirty here to avoid copying the hash_val,
   * making use of the fact that Botan's KDF interface supports label+salt,
   * and knowing that our HKDF hashes first param label then param salt.
   */
   hkdf.kdf(output.data(), output.size(),
            secret, secret_len,
            hash_val, hash_val_len,
            prefix.data(), prefix.size());

   return output;
   }

}