Add "silentpayments" module implementing BIP352 (take 4, limited to full-node scanning)

[theStack]

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 of $n$ secret keys $a_1 ... a_n$, a serialized outpoint, and a list of recipients (each consisting of silent payments scan pubkey and spend pubkey), create the corresponding transaction outputs (x-only public keys) for the sending transaction

Receiver 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 key
• secp256k1_recipient_create_labeled_spend_pubkey: given a spend public key and a label public key, create the corresponding labeled spend public key

Receiver side, scanning [BIP description]:

• secp256k1_recipient_prevouts_summary_create: given a list of $n$ public keys $A_1 ... A_n$ and a serialized outpoint, create a prevouts_summary object needed for scanning
• secp256k1_recipient_scan_outputs: given a prevouts_summary object, 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.

For a higher-level overview on what these functions exactly do, it's suggested to look at a corresponding Python implementation that was created based on the secp256k1lab project (it passes the test vectors, so this "executable pseudo-code" should be correct).

Changes to the previous take

Based on the latest state of the previous PR #1698 (take 3), the following changes have been made:

• rebased on master (adapting to the new unit test framework added in Introduce (mini) unit test framework #1734; thanks to furszy for his mini-guide)
• removed functionality that is only relevant for light clients (_prevout_summary_{parse,serialize}, __recipient_create_output_pubkeys), adapted tests and benchmark accordingly
• adapted examples to have both Bob and Carol do full-node scanning, with a TODO to switch the latter to light-client scanning once available in the future
• addressed various review comments, if still relevant (Add BIP352 module (take 3) #1698 (comment), Add BIP352 module (take 3) #1698 (comment), Add BIP352 module (take 3) #1698 (comment), Add BIP352 module (take 3) #1698 (comment))

The 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

• Recent proposals of reducing the worst-case scanning time (see posts by w0xlt and jonasnick, Add BIP352 module (take 3) #1698 (comment) ff.) are not taken into account yet.
• Not providing 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?

[w0xlt]

Added the optimized version on top of this PR:
w0xlt@8d16914

For more context:
#1698 (comment)

[theStack]

Small supplementary update: I've created a corresponding Python implementation of the provided API functions based on secp256k1lab (https://github.com/theStack/secp256k1lab/blob/add_bip352_module_review_helper/src/secp256k1lab/bip352.py) (also linked in the PR description). The hope is that this makes reviewing this PR a bit easier by having a less noisy, "executable pseudo-code"-like description on what happens under the hood. The code passes the BIP352 test vectors and hence should be correct.

Added the optimized version on top of this PR: w0xlt@8d16914

For more context: #1698 (comment)

Thanks for rebasing on top of this PR, much appreciated! I will take a closer look within the next days.

[w0xlt]

Nit: Not related to optimization, but the diff below removes some redundant public-key serialization code:

diff --git a/src/modules/silentpayments/main_impl.h b/src/modules/silentpayments/main_impl.h
index 106da20..922433d 100644
--- a/src/modules/silentpayments/main_impl.h
+++ b/src/modules/silentpayments/main_impl.h
@@ -21,6 +21,19 @@
 /** magic bytes for ensuring prevouts_summary objects were initialized correctly. */
 static const unsigned char secp256k1_silentpayments_prevouts_summary_magic[4] = { 0xa7, 0x1c, 0xd3, 0x5e };
 
+/* Serialize a ge to compressed 33 bytes. Keeps eckey_pubkey_serialize usage uniform
+ * (expects non-const ge*), and centralizes the VERIFY_CHECK. */
+static SECP256K1_INLINE void secp256k1_sp_ge_serialize33(const secp256k1_ge* in, unsigned char out33[33]) {
+    size_t len = 33;
+    secp256k1_ge tmp = *in;
+    int ok = secp256k1_eckey_pubkey_serialize(&tmp, out33, &len, 1);
+#ifdef VERIFY
+    VERIFY_CHECK(ok && len == 33);
+#else
+    (void)ok;
+#endif
+}
+
 /** Sort an array of silent payment recipients. This is used to group recipients by scan pubkey to
  *  ensure the correct values of k are used when creating multiple outputs for a recipient.
  *
@@ -68,13 +81,11 @@ static int secp256k1_silentpayments_calculate_input_hash_scalar(secp256k1_scalar
     secp256k1_sha256 hash;
     unsigned char pubkey_sum_ser[33];
     unsigned char input_hash[32];
-    size_t len;
     int ret, overflow;
 
     secp256k1_silentpayments_sha256_init_inputs(&hash);
     secp256k1_sha256_write(&hash, outpoint_smallest36, 36);
-    ret = secp256k1_eckey_pubkey_serialize(pubkey_sum, pubkey_sum_ser, &len, 1);
-    VERIFY_CHECK(ret && len == sizeof(pubkey_sum_ser));
+    secp256k1_sp_ge_serialize33(pubkey_sum, pubkey_sum_ser);
     secp256k1_sha256_write(&hash, pubkey_sum_ser, sizeof(pubkey_sum_ser));
     secp256k1_sha256_finalize(&hash, input_hash);
     /* Convert input_hash to a scalar.
@@ -85,15 +96,13 @@ static int secp256k1_silentpayments_calculate_input_hash_scalar(secp256k1_scalar
      * an error to ensure strict compliance with BIP0352.
      */
     secp256k1_scalar_set_b32(input_hash_scalar, input_hash, &overflow);
-    ret &= !secp256k1_scalar_is_zero(input_hash_scalar);
+    ret = !secp256k1_scalar_is_zero(input_hash_scalar);
     return ret & !overflow;
 }
 
 static void secp256k1_silentpayments_create_shared_secret(const secp256k1_context *ctx, unsigned char *shared_secret33, const secp256k1_ge *public_component, const secp256k1_scalar *secret_component) {
     secp256k1_gej ss_j;
     secp256k1_ge ss;
-    size_t len;
-    int ret;
 
     secp256k1_ecmult_const(&ss_j, public_component, secret_component);
     secp256k1_ge_set_gej(&ss, &ss_j);
@@ -103,12 +112,7 @@ static void secp256k1_silentpayments_create_shared_secret(const secp256k1_contex
      * impossible at this point considering we have already validated the public key and
      * the secret key.
      */
-    ret = secp256k1_eckey_pubkey_serialize(&ss, shared_secret33, &len, 1);
-#ifdef VERIFY
-    VERIFY_CHECK(ret && len == 33);
-#else
-    (void)ret;
-#endif
+    secp256k1_sp_ge_serialize33(&ss, shared_secret33);
 
     /* Leaking these values would break indistinguishability of the transaction, so clear them. */
     secp256k1_ge_clear(&ss);
@@ -585,7 +589,6 @@ int secp256k1_silentpayments_recipient_scan_outputs(
                 secp256k1_ge output_negated_ge, tx_output_ge;
                 secp256k1_gej tx_output_gej, label_gej;
                 unsigned char label33[33];
-                size_t len;
 
                 secp256k1_xonly_pubkey_load(ctx, &tx_output_ge, tx_outputs[j]);
                 secp256k1_gej_set_ge(&tx_output_gej, &tx_output_ge);
@@ -595,7 +598,6 @@ int secp256k1_silentpayments_recipient_scan_outputs(
                 secp256k1_ge_neg(&output_negated_ge, &output_ge);
                 secp256k1_gej_add_ge_var(&label_gej, &tx_output_gej, &output_negated_ge, NULL);
                 secp256k1_ge_set_gej_var(&label_ge, &label_gej);
-                ret = secp256k1_eckey_pubkey_serialize(&label_ge, label33, &len, 1);
                 /* Serialize must succeed because the point was just loaded.
                  *
                  * Note: serialize will also fail if label_ge is the point at infinity, but we know
@@ -603,7 +605,7 @@ int secp256k1_silentpayments_recipient_scan_outputs(
                  * Thus, we know that label_ge = tx_output_gej + output_negated_ge cannot be the
                  * point at infinity.
                  */
-                VERIFY_CHECK(ret && len == 33);
+                secp256k1_sp_ge_serialize33(&label_ge, label33);
                 label_tweak = label_lookup(label33, label_context);
                 if (label_tweak != NULL) {
                     found = 1;
@@ -617,7 +619,6 @@ int secp256k1_silentpayments_recipient_scan_outputs(
                 secp256k1_gej_neg(&label_gej, &tx_output_gej);
                 secp256k1_gej_add_ge_var(&label_gej, &label_gej, &output_negated_ge, NULL);
                 secp256k1_ge_set_gej_var(&label_ge, &label_gej);
-                ret = secp256k1_eckey_pubkey_serialize(&label_ge, label33, &len, 1);
                 /* Serialize must succeed because the point was just loaded.
                  *
                  * Note: serialize will also fail if label_ge is the point at infinity, but we know
@@ -625,7 +626,7 @@ int secp256k1_silentpayments_recipient_scan_outputs(
                  * Thus, we know that label_ge = tx_output_gej + output_negated_ge cannot be the
                  * point at infinity.
                  */
-                VERIFY_CHECK(ret && len == 33);
+                secp256k1_sp_ge_serialize33(&label_ge, label33);
                 label_tweak = label_lookup(label33, label_context);
                 if (label_tweak != NULL) {
                     found = 1;

[w0xlt]

nit: The following diff removes the implicit cast and clarifies that k is 4 bytes

diff --git a/src/modules/silentpayments/main_impl.h b/src/modules/silentpayments/main_impl.h
index 922433d..d94aed6 100644
--- a/src/modules/silentpayments/main_impl.h
+++ b/src/modules/silentpayments/main_impl.h
@@ -512,7 +512,8 @@ int secp256k1_silentpayments_recipient_scan_outputs(
     secp256k1_xonly_pubkey output_xonly;
     unsigned char shared_secret[33];
     const unsigned char *label_tweak = NULL;
-    size_t j, k, found_idx;
+    size_t j, found_idx;
+    uint32_t k;
     int found, combined, valid_scan_key, ret;
 
     /* Sanity check inputs */

[jonasnick]

Thanks @theStack for the new PR. I can confirm that this PR is a rebased version of #1698, with the light client functionality removed and comments addressed, except for:

• #1698 (comment)
• #1698 (comment)
• #1698 (review) (only the last one, "elemement")

[jonasnick]

Not providing 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?

Is there a reason for serializing prevouts_summary without light client functionality? If not, I think the don't do this comment is sufficient. Right now, in contrast to the docs of all other opaque objects, this is missing, however:

The exact representation of data inside the opaque data structures is implementation defined and not guaranteed to be portable between different platforms or versions.

[theStack]

@w0xlt, @jonasnick: Thanks for the reviews! I've addressed the suggested changes:

• in _recpient_scan_outputs: changed the type of k to uint32_t (comment above)
• in _recipient_create_label: added a scan key validity check (+added a test for that) (#1698 - comment)
• unified all mentions of "Silent Payments" to title case in the header API and example (#1698 - comment)
• fixed typo s/elemement/element/ (#1698 - review)
• in _recipient_scan_outputs: fixed comment in second label candidate (review above)
• extended the API header comment for the _prevouts_summary opaque data structure, to point out that the data structure is implementation defined (like docs of all other opaque structs) (comment above)

Nit: Not related to optimization, but the diff below removes some redundant public-key serialization code:

Given that this compressed-pubkey-serialization pattern shows up repeatedly also in other modules (ellswift, musig), I think it would make the most sense to add a general helper (e.g. in eckey{,_impl}.h), which could be done in an independent PR. I've opened issue #1773 to see if there is conceptual support for doing this.

Not providing 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?

Is there a reason for serializing prevouts_summary without light client functionality? If not, I think the don't do this comment is sufficient.

Good point, I can't think of a good reason for full nodes wanting to serialize prevouts_summary.