sharing.h

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// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the Apache 2.0 License.
#pragma once
 
#include <cstddef>
#include <cstdint>
#include <span>
 
#define FMT_HEADER_ONLY
#include "ccf/crypto/hkdf.h"
#include "ccf/crypto/sha256.h"
#include "ds/serialized.h"
#include "openssl/crypto.h"
 
#include <fmt/format.h>
 
namespace ccf::crypto
{
  namespace sharing
  {
    // We get (almost) 31 bits of entropy per limb, hence to get 256 bits of
    // entropy of derived key material, with 80 bits of safety margin,
    // ((256+80)/31) = 10 limbs.
    static constexpr size_t LIMBS = 10;
    static constexpr const char* key_label = "CCF Wrapping Key v1";
 
    struct Share
    {
      // Index in a re-share, 0 is a full key, and 1+ is a partial share
      uint32_t x = 0;
      uint32_t y[LIMBS];
      constexpr static size_t serialised_size =
        sizeof(uint32_t) + sizeof(uint32_t) * LIMBS;
 
      Share() = default;
      bool operator==(const Share& other) const = default;
 
      ~Share()
      {
        OPENSSL_cleanse(y, sizeof(y));
      };
 
      HashBytes key(size_t key_size) const
      {
        if (x != 0)
        {
          throw std::invalid_argument(
            "Cannot derive a key from a partial share");
        }
        const std::span<const uint8_t> ikm(
          reinterpret_cast<const uint8_t*>(y), sizeof(y));
        const std::span<const uint8_t> label(
          reinterpret_cast<const uint8_t*>(y), sizeof(y));
        auto k = ccf::crypto::hkdf(
          ccf::crypto::MDType::SHA256, key_size, ikm, {}, label);
        return k;
      }
 
      std::vector<uint8_t> serialise() const
      {
        auto size = serialised_size;
        std::vector<uint8_t> serialised(size);
        auto data = serialised.data();
        serialized::write(data, size, x);
        for (size_t i = 0; i < LIMBS; ++i)
        {
          serialized::write(data, size, y[i]);
        }
        return serialised;
      }
 
      Share(const std::span<uint8_t const>& serialised)
      {
        if (serialised.size() != serialised_size)
        {
          throw std::invalid_argument("Invalid serialised share size");
        }
        auto data = serialised.data();
        auto size = serialised.size();
        x = serialized::read<uint32_t>(data, size);
        for (size_t i = 0; i < LIMBS; ++i)
        {
          y[i] = serialized::read<uint32_t>(data, size);
        }
      }
 
      std::string to_str() const
      {
        return fmt::format("x: {} y: {}", x, fmt::join(y, ", "));
      }
    };
 
    // Exposed for testing only
    using element = uint64_t;
    element ct_reduce(element x);
 
    void sample_secret_and_shares(
      Share& raw_secret, const std::span<Share>& shares, size_t threshold);
 
    void recover_unauthenticated_secret(
      Share& raw_secret,
      const std::span<Share const>& shares,
      size_t threshold);
  }
}