eme_pkcs.cpp

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/*
* PKCS #1 v1.5 Type 2 (encryption) padding
* (C) 1999-2007,2015,2016,2024 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#include <botan/internal/eme_pkcs.h>
 
#include <botan/exceptn.h>
#include <botan/rng.h>
#include <botan/internal/ct_utils.h>
#include <botan/internal/int_utils.h>
#include <botan/internal/stl_util.h>
 
namespace Botan {
 
/*
* PKCS1 Pad Operation
*/
size_t EME_PKCS1v15::pad(std::span<uint8_t> output,
                         std::span<const uint8_t> input,
                         size_t key_length,
                         RandomNumberGenerator& rng) const {
   key_length /= 8;
 
   if(input.size() > maximum_input_size(key_length * 8)) {
      throw Invalid_Argument("PKCS1: Input is too large");
   }
 
   BufferStuffer stuffer(output);
 
   const size_t padding_bytes = [&]() {
      auto d = checked_sub(key_length, input.size() + 2);
      BOTAN_ASSERT_NOMSG(d.has_value());
      return *d;
   }();
 
   stuffer.append(0x02);
   for(size_t i = 0; i != padding_bytes; ++i) {
      stuffer.append(rng.next_nonzero_byte());
   }
   stuffer.append(0x00);
   stuffer.append(input);
 
   return output.size() - stuffer.remaining_capacity();
}
 
/*
* PKCS1 Unpad Operation
*/
CT::Option<size_t> EME_PKCS1v15::unpad(std::span<uint8_t> output, std::span<const uint8_t> input) const {
   BOTAN_ASSERT_NOMSG(output.size() >= input.size());
 
   /*
   * RSA decryption pads the ciphertext up to the modulus size, so this only
   * occurs with very (!) small keys, or when fuzzing.
   *
   * 11 bytes == 00,02 + 8 bytes mandatory padding + 00
   */
   if(input.size() < 11) {
      return {};
   }
 
   auto scope = CT::scoped_poison(input);
 
   CT::Mask<uint8_t> bad_input_m = CT::Mask<uint8_t>::cleared();
   CT::Mask<uint8_t> seen_zero_m = CT::Mask<uint8_t>::cleared();
   size_t delim_idx = 2;  // initial 0002
 
   bad_input_m |= ~CT::Mask<uint8_t>::is_equal(input[0], 0);
   bad_input_m |= ~CT::Mask<uint8_t>::is_equal(input[1], 2);
 
   for(size_t i = 2; i < input.size(); ++i) {
      const auto is_zero_m = CT::Mask<uint8_t>::is_zero(input[i]);
      delim_idx += seen_zero_m.if_not_set_return(1);
      seen_zero_m |= is_zero_m;
   }
 
   // no zero delim -> bad padding
   bad_input_m |= ~seen_zero_m;
   /*
   delim indicates < 8 bytes padding -> bad padding
 
   We require 11 here because we are counting also the 00 delim byte
   */
   bad_input_m |= CT::Mask<uint8_t>(CT::Mask<size_t>::is_lt(delim_idx, 11));
 
   const CT::Choice accept = !(bad_input_m.as_choice());
 
   return CT::copy_output(accept, output, input, delim_idx);
}
 
/*
* Return the max input size for a given key size
*/
size_t EME_PKCS1v15::maximum_input_size(size_t keybits) const {
   if(keybits / 8 > 10) {
      return ((keybits / 8) - 10);
   } else {
      return 0;
   }
}
 
}  // namespace Botan