dsa_gen.cpp

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/*
* DSA Parameter Generation
* (C) 1999-2007 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#include <botan/internal/primality.h>
 
#include <botan/bigint.h>
#include <botan/hash.h>
#include <botan/numthry.h>
#include <botan/rng.h>
#include <botan/internal/barrett.h>
#include <botan/internal/fmt.h>
 
namespace Botan {
 
namespace {
 
/*
* Check if this size is allowed by FIPS 186-3
*/
bool fips186_3_valid_size(size_t pbits, size_t qbits) {
   if(qbits == 160) {
      return (pbits == 1024);
   }
 
   if(qbits == 224) {
      return (pbits == 2048);
   }
 
   if(qbits == 256) {
      return (pbits == 2048 || pbits == 3072);
   }
 
   return false;
}
 
// qbits assumed to be a valid size for FIPS param gen
std::string hash_function_for(size_t qbits) {
   if(qbits == 160) {
      return "SHA-1";
   }
 
   return "SHA-" + std::to_string(qbits);
}
 
}  // namespace
 
/*
* Attempt DSA prime generation with given seed
*/
bool generate_dsa_primes(RandomNumberGenerator& rng,
                         BigInt& p,
                         BigInt& q,
                         size_t pbits,
                         size_t qbits,
                         const std::vector<uint8_t>& seed_c,
                         size_t offset) {
   if(!fips186_3_valid_size(pbits, qbits)) {
      throw Invalid_Argument(fmt("FIPS 186-3 does not allow DSA domain parameters of {}/{} bits long", pbits, qbits));
   }
 
   if(seed_c.size() * 8 < qbits) {
      throw Invalid_Argument(
         fmt("Generating a DSA parameter set with a {} bit long q requires a seed at least as many bits long", qbits));
   }
 
   const std::string hash_name = hash_function_for(qbits);
   auto hash = HashFunction::create_or_throw(hash_name);
 
   const size_t HASH_SIZE = hash->output_length();
 
   class Seed final {
      public:
         explicit Seed(const std::vector<uint8_t>& s) : m_seed(s) {}
 
         const std::vector<uint8_t>& value() const { return m_seed; }
 
         Seed& operator++() {
            for(size_t j = m_seed.size(); j > 0; --j) {
               if(++m_seed[j - 1]) {
                  break;
               }
            }
            return (*this);
         }
 
      private:
         std::vector<uint8_t> m_seed;
   };
 
   Seed seed(seed_c);
 
   q._assign_from_bytes(hash->process(seed.value()));
   q.set_bit(qbits - 1);
   q.set_bit(0);
 
   if(!is_prime(q, rng, 128, true)) {
      return false;
   }
 
   const size_t n = (pbits - 1) / (HASH_SIZE * 8), b = (pbits - 1) % (HASH_SIZE * 8);
 
   BigInt X;
   std::vector<uint8_t> V(HASH_SIZE * (n + 1));
 
   const BigInt q2 = 2 * q;
 
   for(size_t j = 0; j != 4 * pbits; ++j) {
      for(size_t k = 0; k <= n; ++k) {
         ++seed;
         hash->update(seed.value());
         hash->final(&V[HASH_SIZE * (n - k)]);
      }
 
      if(j >= offset) {
         X._assign_from_bytes(std::span{V}.subspan(HASH_SIZE - 1 - b / 8));
         X.set_bit(pbits - 1);
 
         // Variable time division is OK here since DSA primes are public anyway
         p = X - ((X % q2) - 1);
 
         if(p.bits() == pbits && is_prime(p, rng, 128, true)) {
            return true;
         }
      }
   }
   return false;
}
 
/*
* Generate DSA Primes
*/
std::vector<uint8_t> generate_dsa_primes(RandomNumberGenerator& rng, BigInt& p, BigInt& q, size_t pbits, size_t qbits) {
   while(true) {
      std::vector<uint8_t> seed(qbits / 8);
      rng.randomize(seed.data(), seed.size());
 
      if(generate_dsa_primes(rng, p, q, pbits, qbits, seed)) {
         return seed;
      }
   }
}
 
}  // namespace Botan