Add support for existingConfigId (#49)

Sources/PIRService/Controllers/PIRServiceController.swift

@@ -65,12 +65,27 @@ struct PIRServiceController {
             await usecases.get(names: configRequest.usecases)
         }
 
-        let configs = try requestedUsecases.mapValues { usecase in
-            var config = try usecase.config()
-            if let platform = context.platform {
-                config.makeCompatible(with: platform)
+        guard configRequest.existingConfigIds.isEmpty ||
+            configRequest.existingConfigIds.count == requestedUsecases.count
+        else {
+            throw HTTPError(.badRequest, message: """
+                Invalid existingConfigIds count \(configRequest.existingConfigIds.count). \
+                Expected 0 or \(requestedUsecases.count).
+                """)
+        }
+
+        let existingConfigIds = configRequest.existingConfigIds.isEmpty ? Array(
+            repeating: Data(),
+            count: requestedUsecases.count) : configRequest.existingConfigIds
+        var configs = [String: Apple_SwiftHomomorphicEncryption_Api_Pir_V1_Config]()
+        for (usecaseName, configId) in zip(requestedUsecases.keys, existingConfigIds) {
+            if let usecase = requestedUsecases[usecaseName] {
+                var config = try usecase.config(existingConfigId: Array(configId))
+                if let platform = context.platform {
+                    config.makeCompatible(with: platform)
+                }
+                configs[usecaseName] = config
             }
-            return config
         }
 
         let keyConfigs = try requestedUsecases.values.map { try $0.evaluationKeyConfig() }
@@ -81,6 +96,7 @@ struct PIRServiceController {
                 key: key,
                 as: Protobuf<Apple_SwiftHomomorphicEncryption_Api_Shared_V1_EvaluationKey>.self)
             return Apple_SwiftHomomorphicEncryption_Api_Shared_V1_KeyStatus.with { keyStatus in
+                // A timestamp of 0 indicates the evaluation key does not exist on the server
                 keyStatus.timestamp = storedEvaluationKey?.message.metadata.timestamp ?? 0
                 keyStatus.keyConfig = keyConfig
             }

Sources/PIRService/Controllers/Usecase.swift

@@ -29,3 +29,15 @@ protocol Usecase: Sendable {
         evaluationKey: Apple_SwiftHomomorphicEncryption_Api_Shared_V1_EvaluationKey) async throws
         -> Apple_SwiftHomomorphicEncryption_Api_Pir_V1_Response
 }
+
+extension Usecase {
+    func config(existingConfigId: [UInt8]) throws -> Apple_SwiftHomomorphicEncryption_Api_Pir_V1_Config {
+        let config = try config()
+        if Array(config.configID) == existingConfigId {
+            return Apple_SwiftHomomorphicEncryption_Api_Pir_V1_Config.with { apiConfig in
+                apiConfig.reuseExistingConfig = true
+            }
+        }
+        return config
+    }
+}

Sources/PIRService/PIRService.docc/HTTPEndpoints.md

@@ -29,7 +29,10 @@ Header         | `Authorization`    | The value will contain a private access to
 Header         | `User-Agent`       | Identifier for the user's OS type and version.
 Header         | `User-Identifier`  | Pseudorandom identifier tied to a user.
 Request Body   | `ConfigRequest`    | Serialized Protobuf message that list the use-cases that the system is interested in.
+                                      As of iOS 18.2, the client will set the `existing_config_ids` field.
 Response       | `ConfigResponse`   | Serialized Protobuf message. The `ConfigResponse` contains the `configs` and `key_info` response fields.
+                                      As of iOS 18.2, the message may set `reuse_existing_config: true` instead of the `pirConfig` field, reducing the message size.
+                                      This indicates the client should use the config with id specified in `existing_config_ids`.
 Response field | `configs`          | Map from use case names to the corresponding configuration.
 Response field | `key_info`         | List of `KeyStatus` objects.
 

Sources/PIRService/PIRService.docc/TestingInstructions.md

@@ -266,6 +266,8 @@ After introduction in iOS 18.0, `Live Caller ID Lookup` introduced further featu
 
 * `Fixed PIR Shard Config` (iOS 18.2). When all shard configurations are identical, `PIR Fixed Shard Config` allows for a more compact PIR config, saving bandwidth and client-side memory usage. To enable, set the `pirShardConfigs` field in the PIR config. iOS clients prior to iOS 18.2 will still require the `shardConfigs` field to be set. See [Reusing PIR Parameters]( https://swiftpackageindex.com/apple/swift-homomorphic-encryption/main/documentation/privateinformationretrieval/reusingpirparameters) for how to process the database such that all shard configurations are identical.
 
+* `Reusing existing config id` (iOS 18.2). During the `config` request, if a client has a cached configuration, it will send the config id of that cached configuration. Then, if the configuration is unchanged, the server may respond with a config setting `reuseExistingConfig = true` and omit any other fields. This helps reduce the response size for the config fetch.
+
 ## Writing the application extension
 
 > Important: Please see [Getting up-to-date calling and blocking information for your

Tests/PIRServiceTests/PIRServiceControllerTests.swift

@@ -19,6 +19,7 @@ import HummingbirdTesting
 @testable import PIRService
 import PrivateInformationRetrieval
 import PrivateInformationRetrievalProtobuf
+import Util
 import XCTest
 
 class PIRServiceControllerTests: XCTestCase {
@@ -94,10 +95,65 @@ class PIRServiceControllerTests: XCTestCase {
         }
     }
 
+    func testCachedConfigFetch() async throws {
+        let usecaseStore = UsecaseStore()
+        let exampleUsecase = ExampleUsecase.repeatedShardConfig
+        try await usecaseStore.set(name: "test", usecase: exampleUsecase)
+        let app = try await buildApplication(usecaseStore: usecaseStore)
+        let user = UserIdentifier()
+
+        try await app.test(.live) { client in // swiftlint:disable:this closure_body_length
+            for platform: Platform in [.macOS15, .macOS15_2, .iOS18, .iOS18_2] {
+                // No or wrong existing configId
+                for existingConfigId in [Data(), Data([UInt8(1), 2])] {
+                    let configRequest = Apple_SwiftHomomorphicEncryption_Api_Pir_V1_ConfigRequest.with { configReq in
+                        configReq.usecases = ["test"]
+                        configReq.existingConfigIds = [existingConfigId]
+                    }
+                    try await client.execute(
+                        uri: "/config",
+                        userIdentifier: user,
+                        message: configRequest,
+                        platform: platform)
+                    { response in
+                        XCTAssertEqual(response.status, .ok)
+                        var expectedConfig = try exampleUsecase.config()
+                        expectedConfig.makeCompatible(with: platform)
+                        let configResponse = try response
+                            .message(as: Apple_SwiftHomomorphicEncryption_Api_Pir_V1_ConfigResponse.self)
+                        XCTAssertEqual(configResponse.configs["test"], expectedConfig)
+                        try XCTAssertEqual(configResponse.keyInfo[0].keyConfig, exampleUsecase.evaluationKeyConfig())
+                    }
+                }
+                // Existing configId
+                let configRequestWithConfigId = try Apple_SwiftHomomorphicEncryption_Api_Pir_V1_ConfigRequest
+                    .with { configReq in
+                        configReq.usecases = ["test"]
+                        configReq.existingConfigIds = try [exampleUsecase.config().configID]
+                    }
+                try await client.execute(
+                    uri: "/config",
+                    userIdentifier: user,
+                    message: configRequestWithConfigId,
+                    platform: platform)
+                { response in
+                    XCTAssertEqual(response.status, .ok)
+                    let configResponse = try response
+                        .message(as: Apple_SwiftHomomorphicEncryption_Api_Pir_V1_ConfigResponse.self)
+                    XCTAssertEqual(configResponse.configs["test"]?.reuseExistingConfig, true)
+                    XCTAssertEqual(
+                        configResponse.configs["test"]?.pirConfig,
+                        Apple_SwiftHomomorphicEncryption_Api_Pir_V1_PIRConfig())
+                    try XCTAssertEqual(configResponse.keyInfo[0].keyConfig, exampleUsecase.evaluationKeyConfig())
+                }
+            }
+        }
+    }
+
     func testCompressedConfigFetch() async throws {
         // Mock usecase that has a large config with 10K randomized shardConfigs.
         struct TestUseCaseWithLargeConfig: Usecase {
-            init() {
+            init() throws {
                 let shardConfigs = (0..<10000).map { _ in
                     Apple_SwiftHomomorphicEncryption_Api_Pir_V1_PIRShardConfig.with { shardConfig in
                         shardConfig.numEntries = UInt64.random(in: 0..<1000)
@@ -106,10 +162,12 @@ class PIRServiceControllerTests: XCTestCase {
                     }
                 }
 
-                self.randomConfig = Apple_SwiftHomomorphicEncryption_Api_Pir_V1_Config.with { config in
-                    config.pirConfig = .with { pirConfig in
-                        pirConfig.shardConfigs = shardConfigs
-                    }
+                let pirConfg = Apple_SwiftHomomorphicEncryption_Api_Pir_V1_PIRConfig.with { pirConfig in
+                    pirConfig.shardConfigs = shardConfigs
+                }
+                self.randomConfig = try Apple_SwiftHomomorphicEncryption_Api_Pir_V1_Config.with { config in
+                    config.pirConfig = pirConfg
+                    config.configID = try pirConfg.sha256()
                 }
             }
 
@@ -133,7 +191,7 @@ class PIRServiceControllerTests: XCTestCase {
         }
 
         let usecaseStore = UsecaseStore()
-        let exampleUsecase = TestUseCaseWithLargeConfig()
+        let exampleUsecase = try TestUseCaseWithLargeConfig()
         try await usecaseStore.set(name: "test", usecase: exampleUsecase)
 
         let app = try await buildApplication(usecaseStore: usecaseStore)