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wownero/src/multisig/multisig_account_kex_impl.cpp

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// Copyright (c) 2021-2022, The Monero Project
//
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without modification, are
// permitted provided that the following conditions are met:
//
// 1. Redistributions of source code must retain the above copyright notice, this list of
// conditions and the following disclaimer.
//
// 2. Redistributions in binary form must reproduce the above copyright notice, this list
// of conditions and the following disclaimer in the documentation and/or other
// materials provided with the distribution.
//
// 3. Neither the name of the copyright holder nor the names of its contributors may be
// used to endorse or promote products derived from this software without specific
// prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#include "multisig_account.h"
#include "crypto/crypto.h"
#include "cryptonote_config.h"
#include "include_base_utils.h"
#include "multisig.h"
#include "multisig_kex_msg.h"
#include "ringct/rctOps.h"
#include <boost/math/special_functions/binomial.hpp>
#include <algorithm>
#include <cmath>
#include <cstdint>
#include <limits>
#include <memory>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#undef MONERO_DEFAULT_LOG_CATEGORY
#define MONERO_DEFAULT_LOG_CATEGORY "multisig"
namespace multisig
{
//----------------------------------------------------------------------------------------------------------------------
/**
* INTERNAL
*
* brief: check_multisig_config - validate multisig configuration details
* param: round - the round of the message that should be produced
* param: threshold - threshold for multisig (M in M-of-N)
* param: num_signers - number of participants in multisig (N)
*/
//----------------------------------------------------------------------------------------------------------------------
static void check_multisig_config(const std::uint32_t round,
const std::uint32_t threshold,
const std::uint32_t num_signers)
{
CHECK_AND_ASSERT_THROW_MES(num_signers > 1, "Must be at least one other multisig signer.");
CHECK_AND_ASSERT_THROW_MES(num_signers <= config::MULTISIG_MAX_SIGNERS,
"Too many multisig signers specified (limit = 16 to prevent dangerous combinatorial explosion during key exchange).");
CHECK_AND_ASSERT_THROW_MES(num_signers >= threshold,
"Multisig threshold may not be larger than number of signers.");
CHECK_AND_ASSERT_THROW_MES(threshold > 0, "Multisig threshold must be > 0.");
CHECK_AND_ASSERT_THROW_MES(round > 0, "Multisig kex round must be > 0.");
CHECK_AND_ASSERT_THROW_MES(round <= multisig_setup_rounds_required(num_signers, threshold),
"Trying to process multisig kex for an invalid round.");
}
//----------------------------------------------------------------------------------------------------------------------
/**
* INTERNAL
*
* brief: calculate_multisig_keypair_from_derivation - wrapper on calculate_multisig_keypair() for an input public key
* Converts an input public key into a crypto private key (type cast, does not change serialization),
* then passes it to get_multisig_blinded_secret_key().
*
* Result:
* - privkey = H(derivation)
* - pubkey = privkey * G
* param: derivation - a curve point
* outparam: derived_pubkey_out - public key of the resulting privkey
* return: multisig private key
*/
//----------------------------------------------------------------------------------------------------------------------
static crypto::secret_key calculate_multisig_keypair_from_derivation(const crypto::public_key_memsafe &derivation,
crypto::public_key &derived_pubkey_out)
{
crypto::secret_key blinded_skey = get_multisig_blinded_secret_key(rct::rct2sk(rct::pk2rct(derivation)));
CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(blinded_skey, derived_pubkey_out), "Failed to derive public key");
return blinded_skey;
}
//----------------------------------------------------------------------------------------------------------------------
/**
* INTERNAL
*
* brief: make_multisig_common_privkey - Create the 'common' multisig privkey, owned by all multisig participants.
* - common privkey = H(sorted base common privkeys)
* param: participant_base_common_privkeys - Base common privkeys contributed by multisig participants.
* outparam: common_privkey_out - result
*/
//----------------------------------------------------------------------------------------------------------------------
static void make_multisig_common_privkey(std::vector<crypto::secret_key> participant_base_common_privkeys,
crypto::secret_key &common_privkey_out)
{
// sort the privkeys for consistency
//TODO: need a constant-time operator< for sorting secret keys
std::sort(participant_base_common_privkeys.begin(), participant_base_common_privkeys.end(),
[](const crypto::secret_key &key1, const crypto::secret_key &key2) -> bool
{
return memcmp(&key1, &key2, sizeof(crypto::secret_key)) < 0;
}
);
// privkey = H(sorted ancillary base privkeys)
crypto::hash_to_scalar(participant_base_common_privkeys.data(),
participant_base_common_privkeys.size()*sizeof(crypto::secret_key),
common_privkey_out);
CHECK_AND_ASSERT_THROW_MES(common_privkey_out != crypto::null_skey, "Unexpected null secret key (danger!).");
}
//----------------------------------------------------------------------------------------------------------------------
/**
* INTERNAL
*
* brief: compute_multisig_aggregation_coefficient - creates aggregation coefficient for a specific public key in a set
* of public keys
*
* WARNING: The coefficient will only be deterministic if...
* 1) input keys are pre-sorted
* - tested here
* 2) input keys are in canonical form (compressed points in the prime-order subgroup of Ed25519)
* - untested here for performance
* param: sorted_keys - set of component public keys that will be merged into a multisig public spend key
* param: aggregation_key - one of the component public keys
* return: aggregation coefficient
*/
//----------------------------------------------------------------------------------------------------------------------
static rct::key compute_multisig_aggregation_coefficient(const std::vector<crypto::public_key> &sorted_keys,
const crypto::public_key &aggregation_key)
{
CHECK_AND_ASSERT_THROW_MES(std::is_sorted(sorted_keys.begin(), sorted_keys.end()),
"Keys for aggregation coefficient aren't sorted.");
// aggregation key must be in sorted_keys
CHECK_AND_ASSERT_THROW_MES(std::find(sorted_keys.begin(), sorted_keys.end(), aggregation_key) != sorted_keys.end(),
"Aggregation key expected to be in input keyset.");
// aggregation coefficient salt
rct::key salt = rct::zero();
static_assert(sizeof(rct::key) >= sizeof(config::HASH_KEY_MULTISIG_KEY_AGGREGATION), "Hash domain separator is too big.");
memcpy(salt.bytes, config::HASH_KEY_MULTISIG_KEY_AGGREGATION, sizeof(config::HASH_KEY_MULTISIG_KEY_AGGREGATION));
// coeff = H(aggregation_key, sorted_keys, domain-sep)
rct::keyV data;
data.reserve(sorted_keys.size() + 2);
data.push_back(rct::pk2rct(aggregation_key));
for (const auto &key : sorted_keys)
data.push_back(rct::pk2rct(key));
data.push_back(salt);
// note: coefficient is considered public knowledge, no need to memwipe data
return rct::hash_to_scalar(data);
}
//----------------------------------------------------------------------------------------------------------------------
/**
* INTERNAL
*
* brief: generate_multisig_aggregate_key - generates a multisig public spend key via key aggregation
* Key aggregation via aggregation coefficients prevents key cancellation attacks.
* See: https://www.getmonero.org/resources/research-lab/pubs/MRL-0009.pdf
* param: final_keys - address components (public keys) obtained from other participants (not shared with local)
* param: privkeys_inout - private keys of address components known by local; each key will be multiplied by an aggregation
* coefficient (return by reference)
* return: final multisig public spend key for the account
*/
//----------------------------------------------------------------------------------------------------------------------
static crypto::public_key generate_multisig_aggregate_key(std::vector<crypto::public_key> final_keys,
std::vector<crypto::secret_key> &privkeys_inout)
{
// collect all public keys that will go into the spend key (these don't need to be memsafe)
final_keys.reserve(final_keys.size() + privkeys_inout.size());
// 1. convert local multisig private keys to pub keys
// 2. insert to final keyset if not there yet
// 3. save the corresponding index of input priv key set for later reference
std::unordered_map<crypto::public_key, std::size_t> own_keys_mapping;
for (std::size_t multisig_keys_index{0}; multisig_keys_index < privkeys_inout.size(); ++multisig_keys_index)
{
crypto::public_key pubkey;
CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(privkeys_inout[multisig_keys_index], pubkey),
"Failed to derive public key");
own_keys_mapping[pubkey] = multisig_keys_index;
final_keys.push_back(pubkey);
}
// sort input final keys for computing aggregation coefficients (lowest to highest)
// note: input should be sanitized (no duplicates)
std::sort(final_keys.begin(), final_keys.end());
CHECK_AND_ASSERT_THROW_MES(std::adjacent_find(final_keys.begin(), final_keys.end()) == final_keys.end(),
"Unexpected duplicate found in input list.");
// key aggregation
rct::key aggregate_key = rct::identity();
for (const crypto::public_key &key : final_keys)
{
// get aggregation coefficient
rct::key coeff = compute_multisig_aggregation_coefficient(final_keys, key);
// convert private key if possible
// note: retain original priv key index in input list, in case order matters upstream
auto found_key = own_keys_mapping.find(key);
if (found_key != own_keys_mapping.end())
{
// k_agg = coeff*k_base
sc_mul((unsigned char*)&(privkeys_inout[found_key->second]),
coeff.bytes,
(const unsigned char*)&(privkeys_inout[found_key->second]));
CHECK_AND_ASSERT_THROW_MES(privkeys_inout[found_key->second] != crypto::null_skey,
"Multisig privkey with aggregation coefficient unexpectedly null.");
}
// convert public key (pre-merge operation)
// K_agg = coeff*K_base
rct::key converted_pubkey = rct::scalarmultKey(rct::pk2rct(key), coeff);
// build aggregate key (merge operation)
rct::addKeys(aggregate_key, aggregate_key, converted_pubkey);
}
return rct::rct2pk(aggregate_key);
}
//----------------------------------------------------------------------------------------------------------------------
/**
* INTERNAL
*
* brief: multisig_kex_make_round_keys - Makes a kex round's keys.
* - Involves DH exchanges with pubkeys provided by other participants.
* - Conserves mapping [pubkey -> DH derivation] : [origin keys of participants that share this secret with you].
* param: base_privkey - account's base private key, for performing DH exchanges and signing messages
* param: pubkey_origins_map - map between pubkeys to produce DH derivations with and identity keys of
* participants who will share each derivation with you
* outparam: derivation_origins_map_out - map between DH derivations (shared secrets) and identity keys
*/
//----------------------------------------------------------------------------------------------------------------------
static void multisig_kex_make_round_keys(const crypto::secret_key &base_privkey,
multisig_keyset_map_memsafe_t pubkey_origins_map,
multisig_keyset_map_memsafe_t &derivation_origins_map_out)
{
// make shared secrets with input pubkeys
derivation_origins_map_out.clear();
for (auto &pubkey_and_origins : pubkey_origins_map)
{
// D = 8 * k_base * K_pubkey
// note: must be mul8 (cofactor), otherwise it is possible to leak to a malicious participant if the local
// base_privkey is a multiple of 8 or not
// note2: avoid making temporaries that won't be memwiped
rct::key derivation_rct;
auto a_wiper = epee::misc_utils::create_scope_leave_handler([&]{
memwipe(&derivation_rct, sizeof(rct::key));
});
rct::scalarmultKey(derivation_rct, rct::pk2rct(pubkey_and_origins.first), rct::sk2rct(base_privkey));
rct::scalarmultKey(derivation_rct, derivation_rct, rct::EIGHT);
// retain mapping between pubkey's origins and the DH derivation
// note: if working on last kex round, then caller must know how to handle these derivations properly
derivation_origins_map_out[rct::rct2pk(derivation_rct)] = std::move(pubkey_and_origins.second);
}
}
//----------------------------------------------------------------------------------------------------------------------
/**
* INTERNAL
*
* brief: check_messages_round - Check that a set of messages have an expected round number.
* param: expanded_msgs - set of multisig kex messages to process
* param: expected_round - round number the kex messages should have
*/
//----------------------------------------------------------------------------------------------------------------------
static void check_messages_round(const std::vector<multisig_kex_msg> &expanded_msgs,
const std::uint32_t expected_round)
{
CHECK_AND_ASSERT_THROW_MES(expanded_msgs.size() > 0, "At least one input message expected.");
const std::uint32_t round{expanded_msgs[0].get_round()};
CHECK_AND_ASSERT_THROW_MES(round == expected_round, "Messages don't have the expected kex round number.");
for (const auto &expanded_msg : expanded_msgs)
CHECK_AND_ASSERT_THROW_MES(expanded_msg.get_round() == round, "All messages must have the same kex round number.");
}
//----------------------------------------------------------------------------------------------------------------------
/**
* INTERNAL
*
* brief: multisig_kex_msgs_sanitize_pubkeys - Sanitize multisig kex messages.
* - Removes duplicates from msg pubkeys, ignores keys found in input 'exclusion set',
* constructs map of pubkey:origins.
* - Requires that all input msgs have the same round number.
*
* origins = all the signing pubkeys that recommended a given pubkey found in input msgs
*
* - If the messages' round numbers are all '1', then only the message signing pubkey is considered
* 'recommended'. Furthermore, the 'exclusion set' is ignored.
* param: expanded_msgs - set of multisig kex messages to process
* param: exclude_pubkeys - pubkeys to exclude from output set
* outparam: sanitized_pubkeys_out - processed pubkeys obtained from msgs, mapped to their origins
* return: round number shared by all input msgs
*/
//----------------------------------------------------------------------------------------------------------------------
static std::uint32_t multisig_kex_msgs_sanitize_pubkeys(const std::vector<multisig_kex_msg> &expanded_msgs,
const std::vector<crypto::public_key> &exclude_pubkeys,
multisig_keyset_map_memsafe_t &sanitized_pubkeys_out)
{
// all messages should have the same round (redundant sanity check)
CHECK_AND_ASSERT_THROW_MES(expanded_msgs.size() > 0, "At least one input message expected.");
const std::uint32_t round{expanded_msgs[0].get_round()};
check_messages_round(expanded_msgs, round);
sanitized_pubkeys_out.clear();
// get all pubkeys from input messages, add them to pubkey:origins map
// - origins = all the signing pubkeys that recommended a given msg pubkey
for (const auto &expanded_msg : expanded_msgs)
{
// in round 1, only the signing pubkey is treated as a msg pubkey
if (round == 1)
{
// note: ignores duplicates
sanitized_pubkeys_out[expanded_msg.get_signing_pubkey()].insert(expanded_msg.get_signing_pubkey());
}
// in other rounds, only the msg pubkeys are treated as msg pubkeys
else
{
// copy all pubkeys from message into list
for (const auto &pubkey : expanded_msg.get_msg_pubkeys())
{
// ignore pubkeys in 'ignore' set
if (std::find(exclude_pubkeys.begin(), exclude_pubkeys.end(), pubkey) != exclude_pubkeys.end())
continue;
// note: ignores duplicates
sanitized_pubkeys_out[pubkey].insert(expanded_msg.get_signing_pubkey());
}
}
}
return round;
}
//----------------------------------------------------------------------------------------------------------------------
/**
* INTERNAL
*
* brief: remove_key_from_mapped_sets - Remove a specified key from the mapped sets in a multisig keyset map.
* param: key_to_remove - specified key to remove
* inoutparam: keyset_inout - keyset to update
*/
//----------------------------------------------------------------------------------------------------------------------
static void remove_key_from_mapped_sets(const crypto::public_key &key_to_remove,
multisig_keyset_map_memsafe_t &keyset_inout)
{
// remove specified key from each mapped set
for (auto keyset_it = keyset_inout.begin(); keyset_it != keyset_inout.end();)
{
// remove specified key from this set
keyset_it->second.erase(key_to_remove);
// remove empty keyset positions or increment iterator
if (keyset_it->second.size() == 0)
keyset_it = keyset_inout.erase(keyset_it);
else
++keyset_it;
}
}
//----------------------------------------------------------------------------------------------------------------------
/**
* INTERNAL
*
* brief: evaluate_multisig_kex_round_msgs - Evaluate pubkeys from a kex round in order to prepare for the next round.
* - Sanitizes input msgs.
* - Require uniqueness in: 'exclude_pubkeys'.
* - Requires each input pubkey be recommended by 'num_recommendations = expected_round' msg signers.
* - For a final multisig key to be truly 'M-of-N', each of the the private key's components must be
* shared by (N - M + 1) signers.
* - Requires that msgs are signed by only keys in 'signers'.
* - Requires that each key in 'signers' recommends [num_signers - 2 CHOOSE (expected_round - 1)] pubkeys.
* - These should be derivations each signer recommends for round 'expected_round', excluding derivations shared
* with the local account.
* - Requires that 'exclude_pubkeys' has [num_signers - 1 CHOOSE (expected_round - 1)] pubkeys.
* - These should be derivations the local account has corresponding to round 'expected_round'.
* param: base_pubkey - multisig account's base public key
* param: expected_round - expected kex round of input messages
* param: signers - expected participants in multisig kex
* param: expanded_msgs - set of multisig kex messages to process
* param: exclude_pubkeys - derivations held by the local account corresponding to round 'expected_round'
* param: incomplete_signer_set - only require the minimum number of signers to complete this round
* minimum = num_signers - (round num - 1) (including local signer)
* return: fully sanitized and validated pubkey:origins map for building the account's next kex round message
*/
//----------------------------------------------------------------------------------------------------------------------
static multisig_keyset_map_memsafe_t evaluate_multisig_kex_round_msgs(
const crypto::public_key &base_pubkey,
const std::uint32_t expected_round,
const std::vector<crypto::public_key> &signers,
const std::vector<multisig_kex_msg> &expanded_msgs,
const std::vector<crypto::public_key> &exclude_pubkeys,
const bool incomplete_signer_set)
{
// exclude_pubkeys should all be unique
for (auto it = exclude_pubkeys.begin(); it != exclude_pubkeys.end(); ++it)
{
CHECK_AND_ASSERT_THROW_MES(std::find(exclude_pubkeys.begin(), it, *it) == it,
"Found duplicate pubkeys for exclusion unexpectedly.");
}
// sanitize input messages
multisig_keyset_map_memsafe_t pubkey_origins_map; //map: [pubkey : [origins]]
const std::uint32_t round = multisig_kex_msgs_sanitize_pubkeys(expanded_msgs, exclude_pubkeys, pubkey_origins_map);
CHECK_AND_ASSERT_THROW_MES(round == expected_round,
"Kex messages were for round [" << round << "], but expected round is [" << expected_round << "]");
// remove the local signer from each origins set in the sanitized pubkey map
// note: intermediate kex rounds only need keys from other signers to make progress (keys from self are useless)
remove_key_from_mapped_sets(base_pubkey, pubkey_origins_map);
// evaluate pubkeys collected
std::unordered_map<crypto::public_key, std::unordered_set<crypto::public_key>> origin_pubkeys_map; //map: [origin: [pubkeys]]
// 1. each pubkey should be recommended by a precise number of signers
const std::size_t num_recommendations_per_pubkey_required{
incomplete_signer_set
? 1
: round
};
for (const auto &pubkey_and_origins : pubkey_origins_map)
{
// expected amount = round_num
// With each successive round, pubkeys are shared by incrementally larger groups,
// starting at 1 in round 1 (i.e. the local multisig key to start kex with).
CHECK_AND_ASSERT_THROW_MES(pubkey_and_origins.second.size() >= num_recommendations_per_pubkey_required,
"A pubkey recommended by multisig kex messages had an unexpected number of recommendations.");
// map (sanitized) pubkeys back to origins
for (const auto &origin : pubkey_and_origins.second)
origin_pubkeys_map[origin].insert(pubkey_and_origins.first);
}
// 2. the number of unique signers recommending pubkeys should equal the number of signers passed in (minus the local signer)
// - if an incomplete set is allowed, then we need at least one signer to represent each subgroup in this round that
// doesn't include the local signer
const std::size_t num_signers_required{
incomplete_signer_set
? signers.size() - 1 - (round - 1)
: signers.size() - 1
};
CHECK_AND_ASSERT_THROW_MES(origin_pubkeys_map.size() >= num_signers_required,
"Number of unique other signers recommending pubkeys does not equal number of required other signers "
"(kex round: " << round << ", num signers found: " << origin_pubkeys_map.size() << ", num signers required: " <<
num_signers_required << ").");
// 3. each origin should recommend a precise number of pubkeys
// TODO: move to a 'math' library, with unit tests
auto n_choose_k_f =
[](const std::uint32_t n, const std::uint32_t k) -> std::uint32_t
{
static_assert(std::numeric_limits<std::int32_t>::digits <= std::numeric_limits<double>::digits,
"n_choose_k requires no rounding issues when converting between int32 <-> double.");
if (n < k)
return 0;
double fp_result = boost::math::binomial_coefficient<double>(n, k);
if (fp_result < 0)
return 0;
if (fp_result > std::numeric_limits<std::int32_t>::max()) // note: std::round() returns std::int32_t
return 0;
return static_cast<std::uint32_t>(std::round(fp_result));
};
// other signers: (N - 2) choose (msg_round_num - 1)
// - Each signer recommends keys they share with other signers.
// - In each round, every group of size 'round num' will have a key. From a single signer's perspective,
// they will share a key with every group of size 'round num - 1' of other signers.
// - Since 'origins pubkey map' excludes keys shared with the local account, only keys shared with participants
// 'other than local and self' will be in the map (e.g. N - 2 signers).
// - Other signers will recommend (N - 2) choose (msg_round_num - 1) pubkeys (after removing keys shared with local).
// Note: Keys shared with local are filtered out to facilitate kex round boosting, where one or more signers may
// have boosted the local signer (implying they didn't have access to the local signer's previous round msg).
const std::uint32_t expected_recommendations_others = n_choose_k_f(signers.size() - 2, round - 1);
// local: (N - 1) choose (msg_round_num - 1)
const std::uint32_t expected_recommendations_self = n_choose_k_f(signers.size() - 1, round - 1);
// note: expected_recommendations_others would be 0 in the last round of 1-of-N, but we don't call this function for
// that case
CHECK_AND_ASSERT_THROW_MES(expected_recommendations_self > 0 && expected_recommendations_others > 0,
"Bad num signers or round num (possibly numerical limits exceeded).");
// check that local account recommends expected number of keys
CHECK_AND_ASSERT_THROW_MES(exclude_pubkeys.size() == expected_recommendations_self,
"Local account did not recommend expected number of multisig keys.");
// check that other signers recommend expected number of keys
for (const auto &origin_and_pubkeys : origin_pubkeys_map)
{
CHECK_AND_ASSERT_THROW_MES(origin_and_pubkeys.second.size() == expected_recommendations_others,
"A multisig signer recommended an unexpected number of pubkeys.");
// 2 (continued). only expected signers should be recommending keys
CHECK_AND_ASSERT_THROW_MES(std::find(signers.begin(), signers.end(), origin_and_pubkeys.first) != signers.end(),
"Multisig kex message with unexpected signer encountered.");
}
// note: above tests implicitly detect if the total number of recommended keys is correct or not
return pubkey_origins_map;
}
//----------------------------------------------------------------------------------------------------------------------
/**
* INTERNAL
*
* brief: evaluate_multisig_post_kex_round_msgs - Evaluate messages for the post-kex verification round.
* - Sanitizes input msgs.
* - Requires that only one pubkey is recommended.
* - Requires that all signers (other than self) recommend that one pubkey.
* param: base_pubkey - multisig account's base public key
* param: expected_round - expected kex round of input messages
* param: signers - expected participants in multisig kex
* param: expanded_msgs - set of multisig kex messages to process
* param: incomplete_signer_set - only require the minimum amount of messages to complete this round (1 message)
* return: sanitized and validated pubkey:origins map
*/
//----------------------------------------------------------------------------------------------------------------------
static multisig_keyset_map_memsafe_t evaluate_multisig_post_kex_round_msgs(
const crypto::public_key &base_pubkey,
const std::uint32_t expected_round,
const std::vector<crypto::public_key> &signers,
const std::vector<multisig_kex_msg> &expanded_msgs,
const bool incomplete_signer_set)
{
// sanitize input messages
const std::vector<crypto::public_key> dummy;
multisig_keyset_map_memsafe_t pubkey_origins_map; //map: [pubkey : [origins]]
const std::uint32_t round = multisig_kex_msgs_sanitize_pubkeys(expanded_msgs, dummy, pubkey_origins_map);
CHECK_AND_ASSERT_THROW_MES(round == expected_round,
"Kex messages were for round [" << round << "], but expected round is [" << expected_round << "]");
// note: do NOT remove the local signer from the pubkey origins map, since the post-kex round can be force-updated with
// just the local signer's post-kex message (if the local signer were removed, then the post-kex message's pubkeys
// would be completely lost)
// evaluate pubkeys collected
// 1) there should only be two pubkeys
CHECK_AND_ASSERT_THROW_MES(pubkey_origins_map.size() == 2,
"Multisig post-kex round messages from other signers did not all contain two pubkeys.");
// 2) both keys should be recommended by the same set of signers
CHECK_AND_ASSERT_THROW_MES(pubkey_origins_map.begin()->second == (++(pubkey_origins_map.begin()))->second,
"Multisig post-kex round messages from other signers did not all recommend the same pubkey pair.");
// 3) all signers should be present in the recommendation list (unless an incomplete list is permitted)
auto origins = pubkey_origins_map.begin()->second;
origins.insert(base_pubkey); //add self if missing
const std::size_t num_signers_required{
incomplete_signer_set
? 1
: signers.size()
};
CHECK_AND_ASSERT_THROW_MES(origins.size() >= num_signers_required,
"Multisig post-kex round message origins don't line up with multisig signer set "
"(num signers found: " << origins.size() << ", num signers required: " << num_signers_required << ").");
for (const crypto::public_key &origin : origins)
{
// note: if num_signers_required == signers.size(), then this test will ensure all signers are present in 'origins',
// which contains only unique pubkeys
CHECK_AND_ASSERT_THROW_MES(std::find(signers.begin(), signers.end(), origin) != signers.end(),
"An unknown origin recommended a multisig post-kex verification messsage.");
}
return pubkey_origins_map;
}
//----------------------------------------------------------------------------------------------------------------------
/**
* INTERNAL
*
* brief: multisig_kex_process_round_msgs - Process kex messages for the active kex round.
* - A wrapper around evaluate_multisig_kex_round_msgs() -> multisig_kex_make_next_msg().
* - In other words, evaluate the input messages and try to make a message for the next round.
* - Note: Must be called on the final round's msgs to evaluate the final key components
* recommended by other participants.
* param: base_privkey - multisig account's base private key
* param: current_round - round of kex the input messages should be designed for
* param: threshold - threshold for multisig (M in M-of-N)
* param: signers - expected participants in multisig kex
* param: expanded_msgs - set of multisig kex messages to process
* param: exclude_pubkeys - keys held by the local account corresponding to round 'current_round'
* - If 'current_round' is the final round, these are the local account's shares of the final aggregate key.
* param: incomplete_signer_set - allow messages from an incomplete signer set
* outparam: keys_to_origins_map_out - map between round keys and identity keys
* - If in the final round, these are key shares recommended by other signers for the final aggregate key.
* - Otherwise, these are the local account's DH derivations for the next round.
* - See multisig_kex_make_next_msg() for an explanation.
* return: multisig kex message for next round, or empty message if 'current_round' is the final round
*/
//----------------------------------------------------------------------------------------------------------------------
static void multisig_kex_process_round_msgs(const crypto::secret_key &base_privkey,
const crypto::public_key &base_pubkey,
const std::uint32_t current_round,
const std::uint32_t threshold,
const std::vector<crypto::public_key> &signers,
const std::vector<multisig_kex_msg> &expanded_msgs,
const std::vector<crypto::public_key> &exclude_pubkeys,
const bool incomplete_signer_set,
multisig_keyset_map_memsafe_t &keys_to_origins_map_out)
{
check_multisig_config(current_round, threshold, signers.size());
const std::uint32_t kex_rounds_required{multisig_kex_rounds_required(signers.size(), threshold)};
// process messages into a [pubkey : {origins}] map
multisig_keyset_map_memsafe_t evaluated_pubkeys;
if (threshold == 1 && current_round == kex_rounds_required)
{
// in the last main kex round of 1-of-N, all signers share a key so the local signer doesn't care about evaluating
// recommendations from other signers
}
else if (current_round <= kex_rounds_required)
{
// for normal kex rounds, fully evaluate kex round messages
evaluated_pubkeys = evaluate_multisig_kex_round_msgs(base_pubkey,
current_round,
signers,
expanded_msgs,
exclude_pubkeys,
incomplete_signer_set);
}
else //(current_round == kex_rounds_required + 1)
{
// for the post-kex verification round, validate the last kex round's messages
evaluated_pubkeys = evaluate_multisig_post_kex_round_msgs(base_pubkey,
current_round,
signers,
expanded_msgs,
incomplete_signer_set);
}
// prepare keys-to-origins map for updating the multisig account
if (current_round < kex_rounds_required)
{
// normal kex round: make new keys
multisig_kex_make_round_keys(base_privkey, std::move(evaluated_pubkeys), keys_to_origins_map_out);
}
else if (current_round >= kex_rounds_required)
{
// last kex round: collect the key shares recommended by other signers for the final aggregate key
// post-kex verification round: save the keys found in input messages
keys_to_origins_map_out = std::move(evaluated_pubkeys);
}
}
//----------------------------------------------------------------------------------------------------------------------
// multisig_account: INTERNAL
//----------------------------------------------------------------------------------------------------------------------
void multisig_account::initialize_kex_update(const std::vector<multisig_kex_msg> &expanded_msgs,
const std::uint32_t kex_rounds_required,
std::vector<crypto::public_key> &exclude_pubkeys_out)
{
if (m_kex_rounds_complete == 0)
{
// the first round of kex msgs will contain each participant's base pubkeys and ancillary privkeys
// collect participants' base common privkey shares
// note: duplicate privkeys are acceptable, and duplicates due to duplicate signers
// will be blocked by duplicate-signer errors after this function is called
std::vector<crypto::secret_key> participant_base_common_privkeys;
participant_base_common_privkeys.reserve(expanded_msgs.size() + 1);
// add local ancillary base privkey
participant_base_common_privkeys.emplace_back(m_base_common_privkey);
// add other signers' base common privkeys
for (const auto &expanded_msg : expanded_msgs)
{
if (expanded_msg.get_signing_pubkey() != m_base_pubkey)
{
participant_base_common_privkeys.emplace_back(expanded_msg.get_msg_privkey());
}
}
// make common privkey
make_multisig_common_privkey(std::move(participant_base_common_privkeys), m_common_privkey);
// set common pubkey
CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(m_common_privkey, m_common_pubkey),
"Failed to derive public key");
// if N-of-N, then the base privkey will be used directly to make the account's share of the final key
if (kex_rounds_required == 1)
{
m_multisig_privkeys.clear();
m_multisig_privkeys.emplace_back(m_base_privkey);
}
// exclude all keys the local account recommends
// - in the first round, only the local pubkey is recommended by the local signer
exclude_pubkeys_out.emplace_back(m_base_pubkey);
}
else
{
// in other rounds, kex msgs will contain participants' shared keys
// ignore shared keys the account helped create for this round
for (const auto &shared_key_with_origins : m_kex_keys_to_origins_map)
{
exclude_pubkeys_out.emplace_back(shared_key_with_origins.first);
}
}
}
//----------------------------------------------------------------------------------------------------------------------
// multisig_account: INTERNAL
//----------------------------------------------------------------------------------------------------------------------
void multisig_account::finalize_kex_update(const std::uint32_t kex_rounds_required,
multisig_keyset_map_memsafe_t result_keys_to_origins_map)
{
std::vector<crypto::public_key> next_msg_keys;
// prepare for next round (or complete the multisig account fully)
if (m_kex_rounds_complete == kex_rounds_required)
{
// post-kex verification round: check that the multisig pubkey and common pubkey were recommended by other signers
CHECK_AND_ASSERT_THROW_MES(result_keys_to_origins_map.count(m_multisig_pubkey) > 0,
"Multisig post-kex round: expected multisig pubkey wasn't found in input messages.");
CHECK_AND_ASSERT_THROW_MES(result_keys_to_origins_map.count(m_common_pubkey) > 0,
"Multisig post-kex round: expected common pubkey wasn't found in input messages.");
// save keys that should be recommended to other signers
// - for convenience, re-recommend the post-kex verification message once an account is complete
next_msg_keys.reserve(2);
next_msg_keys.push_back(m_multisig_pubkey);
next_msg_keys.push_back(m_common_pubkey);
}
else if (m_kex_rounds_complete + 1 == kex_rounds_required)
{
// finished with main kex rounds (have set of msgs to complete address)
// when 'completing the final round', result keys are other signers' shares of the final key
std::vector<crypto::public_key> result_keys;
result_keys.reserve(result_keys_to_origins_map.size());
for (const auto &result_key_and_origins : result_keys_to_origins_map)
{
result_keys.emplace_back(result_key_and_origins.first);
}
// compute final aggregate key, update local multisig privkeys with aggregation coefficients applied
m_multisig_pubkey = generate_multisig_aggregate_key(std::move(result_keys), m_multisig_privkeys);
// no longer need the account's pubkeys saved for this round (they were only used to build exclude_pubkeys)
// TODO: record [pre-aggregation pubkeys : origins] map for aggregation-style signing
m_kex_keys_to_origins_map.clear();
// save keys that should be recommended to other signers
// - for post-kex verification, recommend the multisig pubkeys to notify other signers that the local signer is done
next_msg_keys.reserve(2);
next_msg_keys.push_back(m_multisig_pubkey);
next_msg_keys.push_back(m_common_pubkey);
}
else if (m_kex_rounds_complete + 2 == kex_rounds_required)
{
// one more round (must send/receive one more set of kex msgs)
// - at this point, have local signer's pre-aggregation private key shares of the final address
// result keys are the local signer's DH derivations for the next round
// derivations are shared secrets between each group of N - M + 1 signers of which the local account is a member
// - convert them to private keys: multisig_key = H(derivation)
// - note: shared key = multisig_key[i]*G is recorded in the kex msg for sending to other participants
// instead of the original 'derivation' value (which MUST be kept secret!)
m_multisig_privkeys.clear();
m_multisig_privkeys.reserve(result_keys_to_origins_map.size());
m_kex_keys_to_origins_map.clear();
next_msg_keys.reserve(result_keys_to_origins_map.size());
for (const auto &derivation_and_origins : result_keys_to_origins_map)
{
// multisig_privkey = H(derivation)
// derived pubkey = multisig_key * G
crypto::public_key_memsafe derived_pubkey;
m_multisig_privkeys.push_back(
calculate_multisig_keypair_from_derivation(derivation_and_origins.first, derived_pubkey));
// save the account's kex key mappings for this round [derived pubkey : other signers who will have the same key]
m_kex_keys_to_origins_map[derived_pubkey] = std::move(derivation_and_origins.second);
// save keys that should be recommended to other signers
// - The keys multisig_key*G are sent to other participants in the message, so they can be used to produce the final
// multisig key via generate_multisig_spend_public_key().
next_msg_keys.push_back(derived_pubkey);
}
}
else //(m_kex_rounds_complete + 3 <= kex_rounds_required)
{
// next round is an 'intermediate' key exchange round, so there is nothing special to do here
// save keys that should be recommended to other signers
// - Send this round's DH derivations to other participants, who will make more DH derivations for the following round.
next_msg_keys.reserve(result_keys_to_origins_map.size());
for (const auto &derivation_and_origins : result_keys_to_origins_map)
next_msg_keys.push_back(derivation_and_origins.first);
// save the account's kex keys for this round [DH derivation : other signers who should have the same derivation]
m_kex_keys_to_origins_map = std::move(result_keys_to_origins_map);
}
// a full set of msgs has been collected and processed, so the 'round is complete'
++m_kex_rounds_complete;
// make next round's message (or reproduce the post-kex verification round if kex is complete)
m_next_round_kex_message = multisig_kex_msg{
(m_kex_rounds_complete > kex_rounds_required ? kex_rounds_required : m_kex_rounds_complete) + 1,
m_base_privkey,
std::move(next_msg_keys)}.get_msg();
}
//----------------------------------------------------------------------------------------------------------------------
// multisig_account: INTERNAL
//----------------------------------------------------------------------------------------------------------------------
void multisig_account::kex_update_impl(const std::vector<multisig_kex_msg> &expanded_msgs,
const bool incomplete_signer_set)
{
// check messages are for the expected kex round
check_messages_round(expanded_msgs, m_kex_rounds_complete + 1);
// check kex round count
const std::uint32_t kex_rounds_required{multisig_kex_rounds_required(m_signers.size(), m_threshold)};
CHECK_AND_ASSERT_THROW_MES(kex_rounds_required > 0, "Multisig kex rounds required unexpectedly 0.");
CHECK_AND_ASSERT_THROW_MES(m_kex_rounds_complete < kex_rounds_required + 1,
"Multisig kex has already completed all required rounds (including post-kex verification).");
// initialize account update
std::vector<crypto::public_key> exclude_pubkeys;
initialize_kex_update(expanded_msgs, kex_rounds_required, exclude_pubkeys);
// process messages into a [pubkey : {origins}] map
multisig_keyset_map_memsafe_t result_keys_to_origins_map;
multisig_kex_process_round_msgs(
m_base_privkey,
m_base_pubkey,
m_kex_rounds_complete + 1,
m_threshold,
m_signers,
expanded_msgs,
exclude_pubkeys,
incomplete_signer_set,
result_keys_to_origins_map);
// finish account update
finalize_kex_update(kex_rounds_required, std::move(result_keys_to_origins_map));
}
//----------------------------------------------------------------------------------------------------------------------
} //namespace multisig
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