diff --git a/Sources/MarvelService/Internal/Extensions/TimeInterval+Computed.swift b/Sources/MarvelService/Internal/Extensions/TimeInterval+Computed.swift
new file mode 100644
index 00000000..467ff0c2
--- /dev/null
+++ b/Sources/MarvelService/Internal/Extensions/TimeInterval+Computed.swift
@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===
+//
+// 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 struct Foundation.TimeInterval
+
+extension TimeInterval {
+    
+    // MARK: Functions
+    
+    /// Converts a time interval to a string value.
+    /// - Returns: A time interval as a string.
+    var asString: String {
+        .init(format: "%f", self)
+    }
+    
+}
diff --git a/Sources/MarvelService/Internal/Use Cases/GenerateHashUseCase.swift b/Sources/MarvelService/Internal/Use Cases/GenerateHashUseCase.swift
new file mode 100644
index 00000000..f0803e08
--- /dev/null
+++ b/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()
+    }
+    
+}
diff --git a/Sources/MarvelService/Public/Middlewares/AuthMiddleware.swift b/Sources/MarvelService/Public/Middlewares/AuthMiddleware.swift
new file mode 100644
index 00000000..1070841c
--- /dev/null
+++ b/Sources/MarvelService/Public/Middlewares/AuthMiddleware.swift
@@ -0,0 +1,104 @@
+//===----------------------------------------------------------------------===
+//
+// 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 class OpenAPIRuntime.HTTPBody
+
+import protocol OpenAPIRuntime.ClientMiddleware
+
+import struct Foundation.Date
+import struct Foundation.TimeInterval
+import struct Foundation.URL
+import struct Foundation.URLComponents
+import struct HTTPTypes.HTTPRequest
+import struct HTTPTypes.HTTPResponse
+
+/// A middleware that attaches the necessary authentication parameters to the path of the request.
+public struct AuthMiddleware {
+    
+    // MARK: Properties
+    
+    /// A use case that generates a MD5 hash value to use as an authentication parameter.
+    private let hash: GenerateHashUseCase
+    
+    /// A Marvel API public key.
+    private let publicKey: String
+    
+    // MARK: Initializers
+
+    /// Initializes this middleware with private and public keys.
+    ///
+    /// The middleware attaches the required `apikey`, `ts`, and `hash` parameters to the URI path of the intercepted request.
+    /// This initializer should be used for server-side applications, as indicated in the [Marvel API documentation](https://developer.marvel.com/documentation/authorization)
+    ///
+    /// - Parameters:
+    ///   - privateKey: A Marvel API private key.
+    ///   - publicKey: A Marvel API public key.
+    public init(
+        privateKey: String,
+        publicKey: String
+    ) {
+        self.hash = .init(
+            privateKey: privateKey,
+            publicKey: publicKey
+        )
+        self.publicKey = publicKey
+    }
+    
+}
+
+// MARK: - ClientMiddleware
+
+extension AuthMiddleware: ClientMiddleware {
+    
+    // MARK: Functions
+    
+    public func intercept(
+        _ request: HTTPRequest,
+        body: HTTPBody?,
+        baseURL: URL,
+        operationID: String,
+        next: @Sendable (HTTPRequest, HTTPBody?, URL) async throws -> (HTTPResponse, HTTPBody?)
+    ) async throws -> (HTTPResponse, HTTPBody?) {
+        guard
+            let uriPath = request.path,
+            var urlComponents = URLComponents(string: uriPath)
+        else {
+            return try await next(request, body, baseURL)
+        }
+
+        let queryItems = urlComponents.queryItems ?? []
+        let timestamp = Date().timeIntervalSince1970
+
+        urlComponents.queryItems = queryItems + [
+            .init(name: "ts", value: timestamp.asString),
+            .init(name: "apikey", value: publicKey),
+            .init(name: "hash", value: hash(timestamp: timestamp))
+        ]
+        
+        let newPath = if let urlQuery = urlComponents.query {
+            urlComponents.path + "?" + urlQuery
+        } else {
+            urlComponents.path
+        }
+
+        let newRequest = HTTPRequest(
+            method: request.method,
+            scheme: request.scheme,
+            authority: request.authority,
+            path: newPath,
+            headerFields: request.headerFields
+        )
+
+        return try await next(newRequest, body, baseURL)
+    }
+    
+}
diff --git a/Tests/MarvelService/Cases/Internal/Extensions/TimeInterval+ComputedTests.swift b/Tests/MarvelService/Cases/Internal/Extensions/TimeInterval+ComputedTests.swift
new file mode 100644
index 00000000..3ccdb0fc
--- /dev/null
+++ b/Tests/MarvelService/Cases/Internal/Extensions/TimeInterval+ComputedTests.swift
@@ -0,0 +1,90 @@
+//===----------------------------------------------------------------------===
+//
+// 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 MarvelService
+
+@Suite("Time Interval computed properties", .tags(.extension))
+struct TimeIntervalComputedTests {
+    
+    // MARK: Properties tests
+    
+#if swift(>=6.2)
+    @Test(arguments: zip(
+        Input.timestamps,
+        Output.timestampAsString
+    ))
+    func `asString`(
+        timestamp: TimeInterval,
+        expects string: String
+    ) {
+        assertAsString(
+            timestamp: timestamp,
+            expects: string
+        )
+    }
+#else
+    @Test("asString", arguments: zip(
+        Input.timestamps,
+        Output.timestampAsString
+    ))
+    func asString(
+        timestamp: TimeInterval,
+        expects string: String
+    ) {
+        assertAsString(
+            timestamp: timestamp,
+            expects: string
+        )
+    }
+#endif
+    
+}
+
+// MARK: - Assertions
+
+private extension TimeIntervalComputedTests {
+    
+    // MARK: Functions
+    
+    /// Asserts the timestamp to string conversion.
+    /// - Parameters:
+    ///   - timestamp: A timestamp to convert.
+    ///   - string: An expected timestamp converted to string.
+    func assertAsString(
+        timestamp: TimeInterval,
+        expects string: String
+    ) {
+        // GIVEN
+        // WHEN
+        let result = timestamp.asString
+        
+        // THEN
+        #expect(result == string)
+    }
+
+}
+
+// MARK: - Constants
+
+private extension Output {
+    /// A list of outcomes that are expected from converting timestamps to string.
+    static let timestampAsString: [String] = [
+        "0.000000",
+        "1000.000000",
+        "1000000.000000",
+        "1000000000.000000"
+    ]
+}
diff --git a/Tests/MarvelService/Cases/Internal/Use Cases/GenerateHashUseCaseTests.swift b/Tests/MarvelService/Cases/Internal/Use Cases/GenerateHashUseCaseTests.swift
new file mode 100644
index 00000000..18f35c7f
--- /dev/null
+++ b/Tests/MarvelService/Cases/Internal/Use Cases/GenerateHashUseCaseTests.swift	
@@ -0,0 +1,95 @@
+//===----------------------------------------------------------------------===
+//
+// 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"
+    ]
+}
diff --git a/Tests/MarvelService/Cases/Public/Middlewares/AuthMiddlewareTests.swift b/Tests/MarvelService/Cases/Public/Middlewares/AuthMiddlewareTests.swift
new file mode 100644
index 00000000..5b848957
--- /dev/null
+++ b/Tests/MarvelService/Cases/Public/Middlewares/AuthMiddlewareTests.swift
@@ -0,0 +1,135 @@
+//===----------------------------------------------------------------------===
+//
+// 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.URL
+import struct Foundation.URLComponents
+import struct HTTPTypes.HTTPRequest
+import struct HTTPTypes.HTTPResponse
+import struct MarvelService.AuthMiddleware
+
+@Suite("Auth Middleware", .tags(.middleware))
+struct AuthMiddlewareTest {
+    
+    // MARK: Functions
+    
+#if swift(>=6.2)
+    @Test(arguments: Input.pathRequests)
+    func `intercept`(path: String?) async throws {
+        try await assertIntercept(path: path)
+    }
+#else
+    @Test("intercept", arguments: Input.pathRequests)
+    func intercept(path: String?) async throws {
+        try await assertIntercept(path: path)
+    }
+#endif
+    
+}
+
+// MARK: - Assertions
+
+private extension AuthMiddlewareTest {
+    
+    // MARK: Functions
+    
+    /// Asserts the interception of a request to add authentication parameters in it.
+    /// - Parameter path: A URI path for a request.
+    /// - Throws: An error in case
+    func assertIntercept(path: String?) async throws {
+        // GIVEN
+        let baseURL: URL = .baseURL
+        let request: HTTPRequest = .init(path: path)
+        
+        let middleware: AuthMiddleware = .init(
+            privateKey: .Key.private,
+            publicKey: .Key.public
+        )
+        
+        // WHEN
+        _ = try await confirmation { confirmation in
+            try await middleware.intercept(
+                request,
+                body: nil,
+                baseURL: baseURL,
+                operationID: .operationId
+            ) { request, _, _ in
+                // THEN
+                if path != nil {
+                    let pathRequest = try #require(request.path)
+                    let urlComponents = try #require(URLComponents(string: pathRequest))
+                    let queryItems = try #require(urlComponents.queryItems)
+                    
+                    #expect(queryItems.count >= 3)
+                    #expect(queryItems.contains(where: { $0.name == "ts" }))
+                    #expect(queryItems.contains(where: { $0.name == "apikey" }))
+                    #expect(queryItems.contains(where: { $0.name == "hash" }))
+                } else {
+                    #expect(request.path == nil)
+                }
+
+                confirmation()
+                
+                return (.init(status: .ok) , nil)
+            }
+        }
+    }
+    
+}
+
+// MARK: - Helpers
+
+private extension HTTPRequest {
+    
+    // MARK: Initializers
+    
+    /// Initializes a HTTP request with a method and a path.
+    /// - Parameters:
+    ///   - method: The request method.
+    ///   - path: The value of the “:path” pseudo header field.
+    init(
+        method: HTTPRequest.Method = .get,
+        path: String?
+    ) {
+        self.init(
+            method: method,
+            scheme: nil,
+            authority: nil,
+            path: path
+        )
+    }
+
+}
+
+// MARK: - Constants
+
+private extension Input {
+    /// A list of URI path to a resource for a request.
+    static let pathRequests: [String?] = [
+        nil,
+        "/path/to/resource",
+        "/path/to/resource?boolean",
+        "/path/to/resource?query=value",
+        "/path/to/resource?query=value&anotherQuery=anotherValue"
+    ]
+}
+
+private extension String {
+    /// An operation ID sample.
+    static let operationId = "SomeOperationId"
+}
+
+private extension URL {
+    /// A base URL sample.
+    static let baseURL = URL(string: "https://sample.domain.com")!
+}
diff --git a/Tests/MarvelService/Types/Extensions/String+Samples.swift b/Tests/MarvelService/Types/Extensions/String+Samples.swift
new file mode 100644
index 00000000..3b2c79bc
--- /dev/null
+++ b/Tests/MarvelService/Types/Extensions/String+Samples.swift
@@ -0,0 +1,21 @@
+//===----------------------------------------------------------------------===
+//
+// 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
+//
+//===----------------------------------------------------------------------===
+
+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"
+    }
+}
diff --git a/Tests/MarvelService/Types/Extensions/Tag+Customs.swift b/Tests/MarvelService/Types/Extensions/Tag+Customs.swift
new file mode 100644
index 00000000..1565b12e
--- /dev/null
+++ b/Tests/MarvelService/Types/Extensions/Tag+Customs.swift
@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===
+//
+// 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
+
+extension Tag {
+    
+    // MARK: Constants
+    
+    /// A flag that indicates tests for a type extension.
+    @Tag static var `extension`: Self
+    
+    /// A flag that indicates tests for a middleware type.
+    @Tag static var middleware: Self
+    
+    /// A flag that indicates tests for a use case type.
+    @Tag static var useCase: Self
+}
diff --git a/Tests/MarvelService/Types/Namespaces/Input.swift b/Tests/MarvelService/Types/Namespaces/Input.swift
new file mode 100644
index 00000000..42ed60c9
--- /dev/null
+++ b/Tests/MarvelService/Types/Namespaces/Input.swift
@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===
+//
+// 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 struct Foundation.Date
+import struct Foundation.TimeInterval
+
+/// A namespace assigned for input arguments on test cases.
+enum Input {
+    
+    // MARK: Constants
+
+    /// A list of timestamps samples.
+    static let timestamps: [TimeInterval] = [
+        Date(timeIntervalSince1970: 0).timeIntervalSince1970,
+        Date(timeIntervalSince1970: 1_000).timeIntervalSince1970,
+        Date(timeIntervalSince1970: 1_000_000).timeIntervalSince1970,
+        Date(timeIntervalSince1970: 1_000_000_000).timeIntervalSince1970,
+    ]
+}
diff --git a/Tests/MarvelService/Types/Namespaces/Output.swift b/Tests/MarvelService/Types/Namespaces/Output.swift
new file mode 100644
index 00000000..0bc713eb
--- /dev/null
+++ b/Tests/MarvelService/Types/Namespaces/Output.swift
@@ -0,0 +1,14 @@
+//===----------------------------------------------------------------------===
+//
+// 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
+//
+//===----------------------------------------------------------------------===
+
+/// A namespace assigned for output arguments on test cases, that are expected results.
+enum Output {}