Implemented the GenerateHashUseCase use case in the library target.

Sources/MarvelService/Internal/Use Cases/GenerateHashUseCase.swift

@@ -0,0 +1,60 @@
+//===----------------------------------------------------------------------===
+//
+// This source file is part of the MarvelService open source project
+//
+// Copyright (c) -2025 Röck+Cöde VoF. and the MarvelService project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of MarvelService project authors
+//
+//===----------------------------------------------------------------------===
+
+import CryptoKit
+
+import struct Foundation.Data
+import struct Foundation.TimeInterval
+
+/// A use case that generates a MD5 hash value, which is required to authenticate any request.
+struct GenerateHashUseCase {
+    
+    // MARK: Properties
+    
+    /// A private key.
+    private let privateKey: String
+    
+    /// A public key.
+    private let publicKey: String
+
+    // MARK: Initializers
+    
+    /// Initializes this use case.
+    /// - Parameters:
+    ///   - privateKey: A private key.
+    ///   - publicKey: A public key.
+    init(
+        privateKey: String,
+        publicKey: String,
+    ) {
+        self.privateKey = privateKey
+        self.publicKey = publicKey
+    }
+    
+    // MARK: Functions
+    
+    /// Generates a MD5 hash value out of a given public key, private key and a timestamp.
+    /// - Parameter timestamp: A timestamp that changes on a request-by-request basis.
+    /// - Returns: A MD5 hash generated out of a private key, an public key, and a timestamp.
+    func callAsFunction(
+        timestamp: TimeInterval
+    ) -> String {
+        let stringToHash = timestamp.asString + self.privateKey + self.publicKey
+        let dataToHash = Data(stringToHash.utf8)
+
+        return Insecure.MD5
+            .hash(data: dataToHash)
+            .map { String(format: "%02x", $0) }
+            .joined()
+    }
+    
+}

Tests/MarvelService/Cases/Internal/Use Cases/GenerateHashUseCaseTests.swift

@@ -0,0 +1,105 @@
+//===----------------------------------------------------------------------===
+//
+// This source file is part of the MarvelService open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the MarvelService project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of MarvelService project authors
+//
+//===----------------------------------------------------------------------===
+
+import Testing
+
+import struct Foundation.TimeInterval
+
+@testable import struct MarvelService.GenerateHashUseCase
+
+@Suite("Generate Hash Use Case", .tags(.useCase))
+struct GenerateHashUseCaseTests {
+    
+    // MARK: Functions
+    
+#if swift(>=6.2)
+    @Test(arguments: zip(
+        Input.timestamps,
+        Output.generatedHashes
+    ))
+    func `hash`(
+        timestamp: TimeInterval,
+        expects hash: String
+    ) async throws {
+        assertHash(
+            timestamp: timestamp,
+            expects: hash
+        )
+    }
+#else
+    @Test("hash", arguments: zip(
+        Input.timestamps,
+        Output.generatedHashes
+    ))
+    func hash(
+        timestamp: TimeInterval,
+        expects hash: String
+    ) async throws {
+        assertHash(
+            timestamp: timestamp,
+            expects: hash
+        )
+    }
+#endif
+    
+}
+
+// MARK: - Assertions
+
+private extension GenerateHashUseCaseTests {
+   
+    // MARK: Functions
+    
+    /// Asserts the MD5 hash generated from the use case.
+    /// - Parameters:
+    ///   - timestamp: A timestamp to use in the hash generation.
+    ///   - hash: An expected MD5 hash string as a result of the use case.
+    func assertHash(
+        timestamp: TimeInterval,
+        expects hash: String
+    ) {
+        // GIVEN
+        let useCase: GenerateHashUseCase = .init(
+            privateKey: .Key.private,
+            publicKey: .Key.public
+        )
+
+        // WHEN
+        let result = useCase(timestamp: timestamp)
+
+        // THEN
+        #expect(result == hash)
+    }
+    
+}
+
+// MARK: - Constants
+
+private extension Output {
+    /// A list of outcomes that are expected from the hash generation.
+    static let generatedHashes: [String] = [
+        "ef9ca6f930e56fb4f8a109a9003580fe",
+        "b500748e9f0aabc67ffc640ae9b87695",
+        "b537f18579112902b7ce046dddad558a",
+        "00fec88a254d42e3a439d49e14cd60d1"
+    ]
+}
+
+private extension String {
+    /// A namespace assigned for Marvel API key samples.
+    enum Key {
+        /// A Marvel API private key sample.
+        static let `private` = "SomePrivateKey"
+        /// A Marvel API public key sample.
+        static let `public` = "SomePublicKey"
+    }
+}

Tests/MarvelService/Types/Extensions/Tag+Customs.swift

@@ -18,4 +18,7 @@ extension Tag {
     
     /// A flag that indicates tests for a type extension.
     @Tag static var `extension`: Self
+    
+    /// A flag that indicates tests for a use case type.
+    @Tag static var useCase: Self
 }