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/*
* Helper functions to implement Keccak-derived functions from NIST SP.800-185
* (C) 2023 Jack Lloyd
* (C) 2023 René Meusel - Rohde & Schwarz Cybersecurity
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
#include <botan/internal/keccak_helpers.h>
#include <botan/internal/bit_ops.h>
#include <botan/internal/loadstor.h>
#include <limits>
namespace Botan {
namespace {
size_t int_encoding_size(uint64_t x) {
BOTAN_ASSERT_NOMSG(x < std::numeric_limits<uint64_t>::max());
return ceil_tobytes(std::max(uint8_t(1), ceil_log2(x + 1)));
}
uint8_t encode(std::span<uint8_t> out, uint64_t x) {
const auto bytes_needed = int_encoding_size(x);
BOTAN_ASSERT_NOMSG(out.size() >= bytes_needed);
std::array<uint8_t, sizeof(x)> bigendian_x;
store_be(x, bigendian_x.data());
auto begin = bigendian_x.begin();
std::advance(begin, sizeof(x) - bytes_needed);
std::copy(begin, bigendian_x.end(), out.begin());
return static_cast<uint8_t>(bytes_needed);
}
} // namespace
std::span<const uint8_t> keccak_int_left_encode(std::span<uint8_t> out, size_t x) {
BOTAN_ASSERT_NOMSG(!out.empty());
out[0] = encode(out.last(out.size() - 1), x);
return out.first(out[0] + 1 /* the length tag */);
}
std::span<const uint8_t> keccak_int_right_encode(std::span<uint8_t> out, size_t x) {
const auto bytes_needed = encode(out, x);
BOTAN_ASSERT_NOMSG(out.size() >= bytes_needed + size_t(1));
out[bytes_needed] = bytes_needed;
return out.first(bytes_needed + 1 /* the length tag */);
}
size_t keccak_int_encoding_size(size_t x) {
return int_encoding_size(x) + 1 /* the length tag */;
}
} // namespace Botan