Add "silentpayments" module implementing BIP352 (take 4, limited to full-node scanning) #1765
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Add a routine for the entire sending flow which takes a set of private keys,
the smallest outpoint, and list of recipients and returns a list of
x-only public keys by performing the following steps:
1. Sum up the private keys
2. Calculate the input_hash
3. For each recipient group:
3a. Calculate a shared secret
3b. Create the requested number of outputs
This function assumes a single sender context in that it requires the
sender to have access to all of the private keys. In the future, this
API may be expanded to allow for a multiple senders or for a single
sender who does not have access to all private keys at any given time,
but for now these modes are considered out of scope / unsafe.
Internal to the library, add:
1. A function for creating shared secrets (i.e., a*B or b*A)
2. A function for generating the "SharedSecret" tagged hash
3. A function for creating a single output public key
Add function for creating a label tweak. This requires a tagged hash function for labels. This function is used by the receiver for creating labels to be used for a) creating labeled addresses and b) to populate a labels cache when scanning. Add function for creating a labeled spend pubkey. This involves taking a label tweak, turning it into a public key and adding it to the spend public key. This function is used by the receiver to create a labeled silent payment address. Add tests for the label API.
Add routine for scanning a transaction and returning the necessary spending data for any found outputs. This function works with labels via a lookup callback and requires access to the transaction outputs. Requiring access to the transaction outputs is not suitable for light clients, but light client support is enabled in the next commit. Add an opaque data type for passing around the prevout public key sum and the input hash tweak (input_hash). This data is passed to the scanner before the ECDH step as two separate elements so that the scanner can multiply the scan_key * input_hash before doing ECDH. Finally, add test coverage for the receiving API.
Demonstrate sending and scanning on full nodes.
Add a benchmark for a full transaction scan. Only benchmarks for scanning are added as this is the most performance critical portion of the protocol. Co-authored-by: Sebastian Falbesoner <[email protected]>
Add the BIP-352 test vectors. The vectors are generated with a Python script that converts the .json file from the BIP to C code: $ ./tools/tests_silentpayments_generate.py test_vectors.json > ./src/modules/silentpayments/vectors.h Co-authored-by: Ron <[email protected]> Co-authored-by: Sebastian Falbesoner <[email protected]> Co-authored-by: Tim Ruffing <[email protected]>
Co-authored-by: Jonas Nick <[email protected]> Co-authored-by: Sebastian Falbesoner <[email protected]>
Test midstate tags used in silent payments.
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Description
This PR implements BIP352 with scanning limited to full-nodes. Light-client scanning is planned to be added in a separate PR in the future. The following 5 API functions are currently introduced:
Sender side [BIP description]:
secp256k1_silentpayments_sender_create_outputs: given a list ofReceiver side, label creation [BIP description]:
secp256k1_recipient_create_label: given a scan secret key and label integer, calculate the corresponding label_tweak and label public keysecp256k1_recipient_create_labeled_spend_pubkey: given a spend public key and a label public key, create the corresponding labeled spend public keyReceiver side, scanning [BIP description]:
secp256k1_recipient_prevouts_summary_create: given a list ofprevouts_summaryobject needed for scanningsecp256k1_recipient_scan_outputs: given aprevouts_summaryobject, a recipients scan secret key and spend public key, and the relevant transaction outputs (x-only public keys), scan for outputs belonging to the recipients and and return the tweak(s) needed for spending the output(s). Optionally, a label_lookup callback function can be provided to also scan for labels.Changes to the previous take
Based on the latest state of the previous PR #1698 (take 3), the following changes have been made:
_prevout_summary_{parse,serialize},__recipient_create_output_pubkeys), adapted tests and benchmark accordinglyThe scope reduction isn't immediately visible in commit count (only one commit was only introducing light-client relevant functionality and could be completely removed), but the review burden compared #1698 is still significantly lower in terms of LOC, especially in the receiving commit.
Open questions / TODOs
prevouts_summary(de)serialization functionality yet in the API poses the risk that users try to do it anyway by treating the opaque object as "serialized". How to cope with that? Is adding a "don't do this" comment in API header sufficient?