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Add view tags to outputs to reduce wallet scanning time #8061

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Apr 20, 2022
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Add view tags to outputs to reduce wallet scanning time #8061

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7 changes: 4 additions & 3 deletions src/blockchain_db/lmdb/db_lmdb.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1045,8 +1045,9 @@ uint64_t BlockchainLMDB::add_output(const crypto::hash& tx_hash,
CURSOR(output_txs)
CURSOR(output_amounts)

if (tx_output.target.type() != typeid(txout_to_key))
throw0(DB_ERROR("Wrong output type: expected txout_to_key"));
crypto::public_key output_public_key;
if (!get_output_public_key(tx_output, output_public_key))
throw0(DB_ERROR("Could not get an output public key from a tx output."));
if (tx_output.amount == 0 && !commitment)
throw0(DB_ERROR("RCT output without commitment"));

Expand Down Expand Up @@ -1074,7 +1075,7 @@ uint64_t BlockchainLMDB::add_output(const crypto::hash& tx_hash,
else
ok.amount_index = 0;
ok.output_id = m_num_outputs;
ok.data.pubkey = boost::get < txout_to_key > (tx_output.target).key;
ok.data.pubkey = output_public_key;
ok.data.unlock_time = unlock_time;
ok.data.height = m_height;
if (tx_output.amount == 0)
Expand Down
24 changes: 24 additions & 0 deletions src/crypto/crypto.cpp
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Expand Up @@ -749,4 +749,28 @@ POP_WARNINGS
sc_sub(&h, &h, &sum);
return sc_isnonzero(&h) == 0;
}

void crypto_ops::derive_view_tag(const key_derivation &derivation, size_t output_index, view_tag &view_tag) {
#pragma pack(push, 1)
struct {
char salt[8]; // view tag domain-separator
key_derivation derivation;
char output_index[(sizeof(size_t) * 8 + 6) / 7];
} buf;
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You may want to disable auto alignment since the whole struct is hashed below, though it should be ok in practice

#pragma pack(pop)

char *end = buf.output_index;
memcpy(buf.salt, "view_tag", 8); // leave off null terminator
buf.derivation = derivation;
tools::write_varint(end, output_index);
assert(end <= buf.output_index + sizeof buf.output_index);

// view_tag_full = H[salt|derivation|output_index]
hash view_tag_full;
cn_fast_hash(&buf, end - reinterpret_cast<char *>(&buf), view_tag_full);

// only need a slice of view_tag_full to realize optimal perf/space efficiency
static_assert(sizeof(crypto::view_tag) <= sizeof(view_tag_full), "view tag should not be larger than hash result");
memcpy(&view_tag, &view_tag_full, sizeof(crypto::view_tag));
}
}
20 changes: 19 additions & 1 deletion src/crypto/crypto.h
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Expand Up @@ -99,6 +99,10 @@ namespace crypto {
ec_scalar c, r;
friend class crypto_ops;
};

POD_CLASS view_tag {
char data;
};
#pragma pack(pop)

void hash_to_scalar(const void *data, size_t length, ec_scalar &res);
Expand All @@ -107,7 +111,7 @@ namespace crypto {
static_assert(sizeof(ec_point) == 32 && sizeof(ec_scalar) == 32 &&
sizeof(public_key) == 32 && sizeof(public_key_memsafe) == 32 && sizeof(secret_key) == 32 &&
sizeof(key_derivation) == 32 && sizeof(key_image) == 32 &&
sizeof(signature) == 64, "Invalid structure size");
sizeof(signature) == 64 && sizeof(view_tag) == 1, "Invalid structure size");

class crypto_ops {
crypto_ops();
Expand Down Expand Up @@ -151,6 +155,8 @@ namespace crypto {
const public_key *const *, std::size_t, const signature *);
friend bool check_ring_signature(const hash &, const key_image &,
const public_key *const *, std::size_t, const signature *);
static void derive_view_tag(const key_derivation &, std::size_t, view_tag &);
friend void derive_view_tag(const key_derivation &, std::size_t, view_tag &);
};

void generate_random_bytes_thread_safe(size_t N, uint8_t *bytes);
Expand Down Expand Up @@ -297,6 +303,14 @@ namespace crypto {
return check_ring_signature(prefix_hash, image, pubs.data(), pubs.size(), sig);
}

/* Derive a 1-byte view tag from the sender-receiver shared secret to reduce scanning time.
* When scanning outputs that were not sent to the user, checking the view tag for a match removes the need to proceed with expensive EC operations
* for an expected 99.6% of outputs (expected false positive rate = 1/2^8 = 1/256 = 0.4% = 100% - 99.6%).
*/
inline void derive_view_tag(const key_derivation &derivation, std::size_t output_index, view_tag &vt) {
crypto_ops::derive_view_tag(derivation, output_index, vt);
}

inline std::ostream &operator <<(std::ostream &o, const crypto::public_key &v) {
epee::to_hex::formatted(o, epee::as_byte_span(v)); return o;
}
Expand All @@ -312,6 +326,9 @@ namespace crypto {
inline std::ostream &operator <<(std::ostream &o, const crypto::signature &v) {
epee::to_hex::formatted(o, epee::as_byte_span(v)); return o;
}
inline std::ostream &operator <<(std::ostream &o, const crypto::view_tag &v) {
epee::to_hex::formatted(o, epee::as_byte_span(v)); return o;
}

const extern crypto::public_key null_pkey;
const extern crypto::secret_key null_skey;
Expand All @@ -325,3 +342,4 @@ CRYPTO_MAKE_HASHABLE_CONSTANT_TIME(secret_key)
CRYPTO_MAKE_HASHABLE_CONSTANT_TIME(public_key_memsafe)
CRYPTO_MAKE_HASHABLE(key_image)
CRYPTO_MAKE_COMPARABLE(signature)
CRYPTO_MAKE_COMPARABLE(view_tag)
19 changes: 18 additions & 1 deletion src/cryptonote_basic/cryptonote_basic.h
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Expand Up @@ -74,13 +74,27 @@ namespace cryptonote
crypto::hash hash;
};

// outputs <= HF_VERSION_VIEW_TAGS
struct txout_to_key
{
txout_to_key() { }
txout_to_key(const crypto::public_key &_key) : key(_key) { }
crypto::public_key key;
};

// outputs >= HF_VERSION_VIEW_TAGS
struct txout_to_tagged_key
{
txout_to_tagged_key() { }
txout_to_tagged_key(const crypto::public_key &_key, const crypto::view_tag &_view_tag) : key(_key), view_tag(_view_tag) { }
crypto::public_key key;
crypto::view_tag view_tag; // optimization to reduce scanning time

BEGIN_SERIALIZE_OBJECT()
FIELD(key)
FIELD(view_tag)
END_SERIALIZE()
};

/* inputs */

Expand Down Expand Up @@ -137,7 +151,7 @@ namespace cryptonote

typedef boost::variant<txin_gen, txin_to_script, txin_to_scripthash, txin_to_key> txin_v;

typedef boost::variant<txout_to_script, txout_to_scripthash, txout_to_key> txout_target_v;
typedef boost::variant<txout_to_script, txout_to_scripthash, txout_to_key, txout_to_tagged_key> txout_target_v;

//typedef std::pair<uint64_t, txout> out_t;
struct tx_out
Expand Down Expand Up @@ -562,6 +576,7 @@ VARIANT_TAG(binary_archive, cryptonote::txin_to_key, 0x2);
VARIANT_TAG(binary_archive, cryptonote::txout_to_script, 0x0);
VARIANT_TAG(binary_archive, cryptonote::txout_to_scripthash, 0x1);
VARIANT_TAG(binary_archive, cryptonote::txout_to_key, 0x2);
VARIANT_TAG(binary_archive, cryptonote::txout_to_tagged_key, 0x3);
VARIANT_TAG(binary_archive, cryptonote::transaction, 0xcc);
VARIANT_TAG(binary_archive, cryptonote::block, 0xbb);

Expand All @@ -572,6 +587,7 @@ VARIANT_TAG(json_archive, cryptonote::txin_to_key, "key");
VARIANT_TAG(json_archive, cryptonote::txout_to_script, "script");
VARIANT_TAG(json_archive, cryptonote::txout_to_scripthash, "scripthash");
VARIANT_TAG(json_archive, cryptonote::txout_to_key, "key");
VARIANT_TAG(json_archive, cryptonote::txout_to_tagged_key, "tagged_key");
VARIANT_TAG(json_archive, cryptonote::transaction, "tx");
VARIANT_TAG(json_archive, cryptonote::block, "block");

Expand All @@ -582,5 +598,6 @@ VARIANT_TAG(debug_archive, cryptonote::txin_to_key, "key");
VARIANT_TAG(debug_archive, cryptonote::txout_to_script, "script");
VARIANT_TAG(debug_archive, cryptonote::txout_to_scripthash, "scripthash");
VARIANT_TAG(debug_archive, cryptonote::txout_to_key, "key");
VARIANT_TAG(debug_archive, cryptonote::txout_to_tagged_key, "tagged_key");
VARIANT_TAG(debug_archive, cryptonote::transaction, "tx");
VARIANT_TAG(debug_archive, cryptonote::block, "block");
13 changes: 12 additions & 1 deletion src/cryptonote_basic/cryptonote_boost_serialization.h
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Expand Up @@ -71,7 +71,11 @@ namespace boost
{
a & reinterpret_cast<char (&)[sizeof(crypto::key_image)]>(x);
}

template <class Archive>
inline void serialize(Archive &a, crypto::view_tag &x, const boost::serialization::version_type ver)
{
a & reinterpret_cast<char (&)[sizeof(crypto::view_tag)]>(x);
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With this complicated construct instead of a & x.data; do you want to be prepared for view tags that are possibly more than 1 character in the future?

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Was following the pattern for the other crypto classes right above, I figure deviating from the pattern is more complicated than not

I'd guess the intention with the pattern is to avoid referencing data to maintain a higher level abstraction when working with the crypto classes across the code base (there seems to be a conscious effort not to reference data with the others in other places in the code), but I don't know for sure

}
template <class Archive>
inline void serialize(Archive &a, crypto::signature &x, const boost::serialization::version_type ver)
{
Expand Down Expand Up @@ -102,6 +106,13 @@ namespace boost
a & x.key;
}

template <class Archive>
inline void serialize(Archive &a, cryptonote::txout_to_tagged_key &x, const boost::serialization::version_type ver)
{
a & x.key;
a & x.view_tag;
}

template <class Archive>
inline void serialize(Archive &a, cryptonote::txout_to_scripthash &x, const boost::serialization::version_type ver)
{
Expand Down
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