fix: prevent oracle failure on tag computation for invalid recipient

noir-projects/aztec-nr/aztec/src/messages/logs/utils.nr

@@ -21,7 +21,9 @@ pub(crate) fn compute_discovery_tag(recipient: AztecAddress) -> Field {
 }
 
 mod test {
+    use crate::oracle::notes::set_sender_for_tags;
     use crate::protocol::{address::AztecAddress, traits::FromField};
+    use crate::test::helpers::test_environment::TestEnvironment;
     use super::compute_discovery_tag;
     use std::test::OracleMock;
 
@@ -42,4 +44,21 @@ mod test {
         let _ = OracleMock::mock("aztec_prv_getNextAppTagAsSender").returns(expected_tag);
         assert_eq(compute_discovery_tag(recipient), expected_tag);
     }
+
+    #[test]
+    unconstrained fn does_not_fail_on_an_invalid_recipient() {
+        let mut env = TestEnvironment::new();
+
+        let sender = env.create_light_account();
+
+        env.private_context(|_context| {
+            set_sender_for_tags(sender);
+
+            // The recipient is an invalid address
+            let recipient = AztecAddress { inner: 3 };
+            assert(!recipient.is_valid());
+
+            let _ = compute_discovery_tag(recipient);
+        });
+    }
 }

yarn-project/foundation/src/curves/grumpkin/point.ts

@@ -65,7 +65,7 @@ export class Point {
   }
 
   /**
-   * Generate a random Point instance.
+   * Generate a random Point instance that is on the curve.
    *
    * @returns A randomly generated Point instance.
    */

yarn-project/pxe/src/contract_function_simulator/oracle/private_execution_oracle.ts

@@ -209,6 +209,16 @@ export class PrivateExecutionOracle extends UtilityExecutionOracle implements IP
       recipient,
     );
 
+    if (!extendedSecret) {
+      // We'd only fail to compute an extended secret if the recipient is an invalid address. To prevent
+      // king-of-the-hill attacks, instead of failing we use a random tag. By including a correct-looking tag in the
+      // log, the transaction shape is preserved and no privacy is leaked, even if the tag is bogus.
+      this.logger.warn(`Computing a tag for invalid recipient ${recipient} - returning a random tag instead`, {
+        contractAddress: this.contractAddress,
+      });
+      return new Tag(Fr.random());
+    }
+
     const index = await this.#getIndexToUseForSecret(extendedSecret);
     this.logger.debug(
       `Incrementing tagging index for sender: ${sender}, recipient: ${recipient}, contract: ${this.contractAddress} to ${index}`,

yarn-project/pxe/src/logs/log_service.ts

@@ -176,14 +176,24 @@ export class LogService {
     );
 
     return Promise.all(
-      deduplicatedSenders.map(sender => {
-        return ExtendedDirectionalAppTaggingSecret.compute(
+      deduplicatedSenders.map(async sender => {
+        const secret = await ExtendedDirectionalAppTaggingSecret.compute(
           recipientCompleteAddress,
           recipientIvsk,
           sender,
           contractAddress,
           recipient,
         );
+
+        if (!secret) {
+          // Note that all senders originate from either the SenderAddressBookStore or the KeyStore.
+          // TODO(F-512): make sure we actually prevent registering invalid senders.
+          throw new Error(
+            `Failed to compute a tagging secret for sender ${sender} - this implies this is an invalid address, which should not happen as they have been previously registered in PXE.`,
+          );
+        }
+
+        return secret;
       }),
     );
   }

yarn-project/stdlib/src/logs/extended_directional_app_tagging_secret.ts

@@ -23,14 +23,14 @@ import { computeAddressSecret, computePreaddress } from '../keys/derivation.js';
  * doesn't seem to be a good way around this.
  */
 export class ExtendedDirectionalAppTaggingSecret {
-  private constructor(
+  constructor(
     public readonly secret: Fr,
     public readonly app: AztecAddress,
   ) {}
 
   /**
    * Derives shared tagging secret and from that, the app address and recipient derives the directional app tagging
-   * secret.
+   * secret. Returns undefined if `externalAddress` is an invalid address.
    *
    * @param localAddress - The complete address of entity A in the shared tagging secret derivation scheme
    * @param localIvsk - The incoming viewing secret key of entity A
@@ -45,8 +45,12 @@ export class ExtendedDirectionalAppTaggingSecret {
     externalAddress: AztecAddress,
     app: AztecAddress,
     recipient: AztecAddress,
-  ): Promise<ExtendedDirectionalAppTaggingSecret> {
+  ): Promise<ExtendedDirectionalAppTaggingSecret | undefined> {
     const taggingSecretPoint = await computeSharedTaggingSecret(localAddress, localIvsk, externalAddress);
+    if (!taggingSecretPoint) {
+      return undefined;
+    }
+
     const appTaggingSecret = await poseidon2Hash([taggingSecretPoint.x, taggingSecretPoint.y, app]);
     const directionalAppTaggingSecret = await poseidon2Hash([appTaggingSecret, recipient]);
 
@@ -63,18 +67,25 @@ export class ExtendedDirectionalAppTaggingSecret {
   }
 }
 
-// Returns shared tagging secret computed with Diffie-Hellman key exchange.
+// Returns shared tagging secret computed with Diffie-Hellman key exchange, or undefined if `externalAddress` is an
+// invalid address.
 async function computeSharedTaggingSecret(
   localAddress: CompleteAddress,
   localIvsk: Fq,
   externalAddress: AztecAddress,
-): Promise<Point> {
-  const knownPreaddress = await computePreaddress(await localAddress.publicKeys.hash(), localAddress.partialAddress);
-  // TODO: #8970 - Computation of address point from x coordinate might fail
-  const externalAddressPoint = await externalAddress.toAddressPoint();
+): Promise<Point | undefined> {
   // Given A (local complete address) -> B (external address) and h == preaddress
   // Compute shared secret as S = (h_A + local_ivsk_A) * Addr_Point_B
 
+  const knownPreaddress = await computePreaddress(await localAddress.publicKeys.hash(), localAddress.partialAddress);
+
+  // An invalid address has no corresponding address point
+  if (!(await externalAddress.isValid())) {
+    return undefined;
+  }
+
+  const externalAddressPoint = await externalAddress.toAddressPoint();
+
   // Beware! h_a + local_ivsk_a (also known as the address secret) can lead to an address point with a negative
   // y-coordinate, since there's two possible candidates computeAddressSecret takes care of selecting the one that
   // leads to a positive y-coordinate, which is the only valid address point