From ea87b30f8907ee11252433811e7a7d0c46758cca Mon Sep 17 00:00:00 2001 From: j-berman Date: Mon, 15 Nov 2021 05:23:53 -0800 Subject: Add view tags to outputs to reduce wallet scanning time Implements view tags as proposed by @UkoeHB in MRL issue https://github.com/monero-project/research-lab/issues/73 At tx construction, the sender adds a 1-byte view tag to each output. The view tag is derived from the sender-receiver shared secret. When scanning for outputs, the receiver can check the view tag for a match, in order to reduce scanning time. When the view tag does not match, the wallet avoids the more expensive EC operations when deriving the output public key using the shared secret. --- src/crypto/crypto.cpp | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) (limited to 'src/crypto/crypto.cpp') diff --git a/src/crypto/crypto.cpp b/src/crypto/crypto.cpp index 1f46164d7..77a36069a 100644 --- a/src/crypto/crypto.cpp +++ b/src/crypto/crypto.cpp @@ -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; + #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(&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)); + } } -- cgit v1.2.3