iso9796.cpp

Go to the documentation of this file.

/*
 * ISO-9796-2 - Digital signature schemes giving message recovery schemes 2 and 3
 * (C) 2016 Tobias Niemann, Hackmanit GmbH
 *     2025 Jack Lloyd
 *
 * Botan is released under the Simplified BSD License (see license.txt)
 */
 
#include <botan/internal/iso9796.h>
 
#include <botan/exceptn.h>
#include <botan/hash.h>
#include <botan/rng.h>
#include <botan/internal/buffer_stuffer.h>
#include <botan/internal/ct_utils.h>
#include <botan/internal/fmt.h>
#include <botan/internal/hash_id.h>
#include <botan/internal/mgf1.h>
 
namespace Botan {
 
namespace {
 
std::vector<uint8_t> iso9796_hash(HashFunction& hash,
                                  std::span<const uint8_t> msg1,
                                  std::span<const uint8_t> hmsg2,
                                  std::span<const uint8_t> salt) {
   // Compute H(C || msg1 || H(msg2) || S) as described in the ISO text
   hash.update_be(static_cast<uint64_t>(msg1.size()) * 8);
   hash.update(msg1);
   hash.update(hmsg2);
   hash.update(salt);
   return hash.final_stdvec();
}
 
std::vector<uint8_t> iso9796_encoding(std::span<const uint8_t> msg,
                                      size_t output_bits,
                                      std::unique_ptr<HashFunction>& hash,
                                      size_t salt_len,
                                      bool implicit,
                                      RandomNumberGenerator& rng) {
   const size_t output_length = (output_bits + 7) / 8;
 
   //set trailer length
   const size_t trailer_len = (implicit) ? 1 : 2;
 
   const size_t hash_len = hash->output_length();
 
   if(output_length <= hash_len + salt_len + trailer_len) {
      throw Encoding_Error("ISO9796-2::encoding_of: Output length is too small");
   }
 
   //calculate message capacity
   const size_t capacity = output_length - hash_len - salt_len - trailer_len - 1;
 
   // msg1 is the recoverable part and hmsg2 is the hash of the unrecoverable message part.
   const size_t msg1_len = std::min(capacity, msg.size());
   const auto msg1 = msg.first(msg1_len);    // the first capacity bytes
   const auto msg2 = msg.subspan(msg1_len);  // the rest; possibly empty
 
   const auto hmsg2 = hash->process<std::vector<uint8_t>>(msg2);
   const auto salt = rng.random_vec<std::vector<uint8_t>>(salt_len);
 
   const auto H = iso9796_hash(*hash, msg1, hmsg2, salt);
 
   std::vector<uint8_t> EM(output_length);
 
   BufferStuffer stuffer(EM);
   stuffer.append(0x00, stuffer.remaining_capacity() - (hash_len + salt_len + trailer_len + msg1_len + 1));
   stuffer.append(0x01);
   stuffer.append(msg1);
   stuffer.append(salt);
 
   //apply mask
   const size_t mgf1_bytes = EM.size() - hash_len - trailer_len;
   mgf1_mask(*hash, H, std::span{EM}.first(mgf1_bytes));
 
   //clear the unused leftmost bits so the representative is < the modulus
   EM[0] &= 0xFF >> (8 * output_length - output_bits);
 
   stuffer.append(H);
 
   // set implicit/ISO trailer
 
   if(implicit) {
      stuffer.append(0xBC);
   } else {
      const uint8_t hash_id = ieee1363_hash_id(hash->name());
      if(hash_id == 0) {
         throw Encoding_Error("ISO-9796: no hash identifier for " + hash->name());
      }
      stuffer.append(hash_id);
      stuffer.append(0xCC);
   }
 
   BOTAN_ASSERT_NOMSG(stuffer.full());
 
   return EM;
}
 
bool iso9796_verification(std::span<const uint8_t> repr,
                          std::span<const uint8_t> raw,
                          size_t key_bits,
                          std::unique_ptr<HashFunction>& hash,
                          size_t salt_len,
                          bool implicit) {
   const size_t key_bytes = ceil_tobytes(key_bits);
 
   if(repr.size() > key_bytes) {
      return false;
   }
 
   // The recovered representative is minimally encoded, so left-pad a value
   // with leading zero bytes back to the full width before parsing (cf PSS).
   std::vector<uint8_t> coded(key_bytes);
   {
      BufferStuffer stuffer(coded);
      stuffer.append(0x00, key_bytes - repr.size());
      stuffer.append(repr);
   }
 
   if(coded.empty()) {
      return false;
   }
 
   //the trailer length is fixed by the configured mode, and the verifier
   //rejects a signature whose trailer does not match that mode
 
   const size_t trailer_len = implicit ? 1 : 2;
 
   const size_t hash_len = hash->output_length();
 
   // The encoder requires output_length > hash_len + salt_len + trailer_len so
   // that the message capacity is at least zero; match that here (the <= rejects
   // the boundary case where the later capacity computation would underflow).
   if(coded.size() <= hash_len + trailer_len + salt_len) {
      return false;
   }
 
   if(implicit) {
      if(coded[coded.size() - 1] != 0xBC) {
         return false;
      }
   } else {
      const uint8_t hash_id = ieee1363_hash_id(hash->name());
      if(hash_id == 0) {
         throw Decoding_Error("ISO-9796: no hash identifier for " + hash->name());
      }
 
      const uint8_t trailer_0 = coded[coded.size() - 2];
      const uint8_t trailer_1 = coded[coded.size() - 1];
 
      if(trailer_0 != hash_id || trailer_1 != 0xCC) {
         return false;
      }
   }
 
   // The encoder clears the unused leftmost bits of the representative, so a
   // valid signature always has them zero; reject otherwise (cf PSS).
   const size_t top_bits = 8 * key_bytes - key_bits;
   if(top_bits > 8 - high_bit(coded[0])) {
      return false;
   }
 
   CT::poison(coded.data(), coded.size());
   //remove mask
   uint8_t* DB = coded.data();
   const size_t DB_size = coded.size() - hash_len - trailer_len;
 
   const uint8_t* H = &coded[DB_size];
 
   mgf1_mask(*hash, {H, hash_len}, {DB, DB_size});
   //clear the unused leftmost bits (matching the encoding mask)
   DB[0] &= 0xFF >> top_bits;
 
   //recover msg1 and salt
   size_t msg1_offset = 1;
 
   auto waiting_for_delim = CT::Mask<uint8_t>::set();
   auto bad_input = CT::Mask<uint8_t>::cleared();
 
   for(size_t j = 0; j < DB_size; ++j) {
      const auto is_zero = CT::Mask<uint8_t>::is_zero(DB[j]);
      const auto is_one = CT::Mask<uint8_t>::is_equal(DB[j], 0x01);
 
      const auto add_m = waiting_for_delim & is_zero;
 
      bad_input |= waiting_for_delim & ~(is_zero | is_one);
      msg1_offset += add_m.if_set_return(1);
 
      waiting_for_delim &= is_zero;
   }
 
   //invalid, if delimiter 0x01 was not found or msg1_offset is too big
   bad_input |= waiting_for_delim;
 
   const auto bad_offset = CT::Mask<size_t>::is_lt(coded.size(), trailer_len + hash_len + msg1_offset + salt_len);
   bad_input |= CT::Mask<uint8_t>(bad_offset);
 
   //in case that msg1_offset is too big, just continue with offset = 0.
   msg1_offset = CT::Mask<size_t>::expand(bad_input.value()).if_not_set_return(msg1_offset);
 
   CT::unpoison(coded.data(), coded.size());
   CT::unpoison(msg1_offset);
 
   const size_t msg1_len = coded.size() - (trailer_len + hash_len + msg1_offset + salt_len);
 
   const auto msg1 = std::span(coded).subspan(msg1_offset, msg1_len);
   const auto salt = std::span(coded).subspan(msg1_offset + msg1.size(), salt_len);
 
   //compute H2(C||msg1||H(msg2)||S*). * indicates a recovered value
   // key_bytes == ceil_tobytes(key_bits) == the encoder's output_length, so the
   // verifier splits the message at exactly the capacity the encoder used.
   const size_t capacity = key_bytes - hash_len - salt_len - trailer_len - 1;
 
   std::span<const uint8_t> msg1raw = raw;
   if(msg1raw.size() > capacity) {
      hash->update(msg1raw.subspan(capacity));
      msg1raw = msg1raw.first(capacity);
   }
 
   const auto hmsg2 = hash->final_stdvec();
 
   // Compute H(C*||msg1*||H(msg2)||S*) where '*' indicates a recovered value
   const auto H2 = iso9796_hash(*hash, msg1, hmsg2, salt);
 
   // Check if H == H2
   bad_input |= CT::is_not_equal(H, H2.data(), hash_len);
 
   // Check that msg after MGF1 matches msg in the original
   bad_input |= ~CT::Mask<uint8_t>(CT::Mask<size_t>::is_equal(msg1.size(), msg1raw.size()));
   bad_input |= ~CT::is_equal(msg1.data(), msg1raw.data(), std::min(msg1.size(), msg1raw.size()));
 
   CT::unpoison(bad_input);
   return (bad_input.as_bool() == false);
}
 
}  // namespace
 
/*
 *  ISO-9796-2 signature scheme 2
 *  DS 2 is probabilistic
 */
void ISO_9796_DS2::update(const uint8_t input[], size_t length) {
   //need to buffer message completely, before digest
   if(length > 0) {
      m_msg_buffer.insert(m_msg_buffer.end(), input, input + length);
   }
}
 
/*
 * Return the raw (unencoded) data
 */
std::vector<uint8_t> ISO_9796_DS2::raw_data() {
   std::vector<uint8_t> retbuffer = m_msg_buffer;
   m_msg_buffer.clear();
   return retbuffer;
}
 
/*
 *  ISO-9796-2 scheme 2 encode operation
 */
std::vector<uint8_t> ISO_9796_DS2::encoding_of(std::span<const uint8_t> msg,
                                               size_t output_bits,
                                               RandomNumberGenerator& rng) {
   return iso9796_encoding(msg, output_bits, m_hash, m_salt_len, m_implicit, rng);
}
 
/*
 * ISO-9796-2 scheme 2 verify operation
 */
bool ISO_9796_DS2::verify(std::span<const uint8_t> repr, std::span<const uint8_t> raw, size_t key_bits) {
   return iso9796_verification(repr, raw, key_bits, m_hash, m_salt_len, m_implicit);
}
 
std::string ISO_9796_DS2::hash_function() const {
   return m_hash->name();
}
 
/*
 * Return the SCAN name
 */
std::string ISO_9796_DS2::name() const {
   return fmt("ISO_9796_DS2({},{},{})", m_hash->name(), (m_implicit ? "imp" : "exp"), m_salt_len);
}
 
/*
 *  ISO-9796-2 signature scheme 3
 *  DS 3 is deterministic and equals DS2 without salt
 */
void ISO_9796_DS3::update(const uint8_t input[], size_t length) {
   //need to buffer message completely, before digest
   if(length > 0) {
      m_msg_buffer.insert(m_msg_buffer.end(), input, input + length);
   }
}
 
/*
 * Return the raw (unencoded) data
 */
std::vector<uint8_t> ISO_9796_DS3::raw_data() {
   std::vector<uint8_t> retbuffer = m_msg_buffer;
   m_msg_buffer.clear();
   return retbuffer;
}
 
/*
 *  ISO-9796-2 scheme 3 encode operation
 */
std::vector<uint8_t> ISO_9796_DS3::encoding_of(std::span<const uint8_t> msg,
                                               size_t output_bits,
                                               RandomNumberGenerator& rng) {
   return iso9796_encoding(msg, output_bits, m_hash, 0, m_implicit, rng);
}
 
/*
 * ISO-9796-2 scheme 3 verify operation
 */
bool ISO_9796_DS3::verify(std::span<const uint8_t> repr, std::span<const uint8_t> raw, size_t key_bits) {
   return iso9796_verification(repr, raw, key_bits, m_hash, 0, m_implicit);
}
 
std::string ISO_9796_DS3::hash_function() const {
   return m_hash->name();
}
 
/*
 * Return the SCAN name
 */
std::string ISO_9796_DS3::name() const {
   return fmt("ISO_9796_DS3({},{})", m_hash->name(), (m_implicit ? "imp" : "exp"));
}
 
}  // namespace Botan