View_tag

[Edan3blov]

Hi,

As i saw on IIRC and elsewhere there is a need for view_tag function to create manually tx even after HF.

Thank you.

[j-berman]

Your question is a little unclear. Are you a normal user of Monero, or a developer of software that manually creates Monero txs?

[Edan3blov]

I'm developer that manually creates Monero txs.

Thank you.

[selsta]

If you post more detailed information it will be easier to help you.

[j-berman]

Every output in every tx will need to have a view_tag after the HF. It must be calculated correctly, otherwise other wallets won't be able to recognize received outputs.

A view tag is the first byte of hash["view_tag" | derivation | output index]

• The hash function is the same Keccak used across the code base (see cn_fast_hash)
• derivation is the shared secret

Relevant code

You can see how to derive the view tag here:

monero/src/crypto/crypto.cpp

Lines 753 to 775 in 9750e1f

	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<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));
	}

Here is where the default tx builder derives them:

monero/src/device/device_default.cpp

Lines 341 to 344 in 9f814ed

	if (use_view_tags)
	{
	derive_view_tag(derivation, output_index, view_tag);
	}

Testing

View tags are live on testnet now. You can test by compiling master (commit 9750e1f) and running monerod on testnet. Construct transactions using your tx builder, and then make sure a recipient using monero-wallet-cli is able to see those transactions.

There are also test vectors for derive_view_tag here: https://github.com/monero-project/monero/blob/9750e1fa103539b3e533455500610aae76e253a5/tests/crypto/tests.txt

Original PR

#8061

[Edan3blov]

Hello again,

The best solution is a wallet-cli command line like --create-view-tag-from-spend-key

[selsta]

Again, if you don't explain in detail what you are trying to do we can't help you. And if there is some language barrier maybe you can try to give an example of what you are doing?

[selsta]

Closing this as it's unclear what the issue creator is asking and there was no attempt at clarifying.