/*

* 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/mem_ops.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