feat: Uptake public key hash changes

[sitaram-kalluri]

- What I did

• Uptake "PublicKeyHash" changes into at_client package.
  • In "SharedKeyDecryption.dart", when decrypting the shared key, compare the hash value of the encryption public key with the hash value in atKey.metadata.pubKeyHash. If the value mismatch, throw exception.
  • Add publicKeyHash value in sync_service_impl.dart to sync the value to the remote secondary and fetch the value from the remote secondary.
  • Add the publicKeyHash value in at_notification.metadata.

- How to verify it

• Updated the existing unit test to verify publicKeyHash is populated when encrypting the value.
• Updated the existing unit test to verify the exception is thrown if publicKeyHash mismatch.
• Updated the existing unit test to verify decryption is successful if publicKeyHash value matches.

- Description for the changelog

• feat: Uptake public key hash changes

Note: In the pubspec.yaml, the at_persistence_secondary_server is set to "2121-uptake-public-key-hash-at-persistence_secondary-server" branch to consume the public key hash changes. Once the PR in at_server is merged, will replace with the hosted version.

[gkc]

This seems bad

[sitaram-kalluri]

Sure Gary, will revisit the code and modify the IF condition.

[sitaram-kalluri]

In the MonitorVerbHandler.dart file of at_secondary_server, the pubKeyHash object is converted into a JSON-encoded string and sent to the client. When the pubKeyHash value is null, the jsonEncode method returns a string with the value "null", which causes metadata.pubKeyHash to be set to "null" instead of null.

Code snippet from MonitorVerbHandler.dart:

"pubKeyHash": jsonEncode(atNotification.atMetadata?.pubKeyHash?.toJson())

To ensure that metadata.publicKeyHash contains a null object on the client side, modify the code in MonitorVerbHandler.dart as follows:

notification.metadata?.putIfAbsent(
          "pubKeyHash",
          () => (atNotification.atMetadata?.pubKeyHash != null)
              ? jsonEncode(atNotification.atMetadata?.pubKeyHash?.toJson())
              : null);

@gkc : Let me know if this approach looks good or suggest an alternate approach please.