Add support for fernet

Sources/CryptoSwift/Fernet.swift

@@ -0,0 +1,203 @@
+//
+//  CryptoSwift
+//
+//  Copyright (C) Marcin Krzyżanowski <marcin@krzyzanowskim.com>
+//  This software is provided 'as-is', without any express or implied warranty.
+//
+//  In no event will the authors be held liable for any damages arising from the use of this software.
+//
+//  Permission is granted to anyone to use this software for any purpose,including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:
+//
+//  - The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation is required.
+//  - Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
+//  - This notice may not be removed or altered from any source or binary distribution.
+//
+
+import Foundation
+
+/// Fernet provides support for the [fernet](https://github.com/fernet/spec) encryption format.
+public struct Fernet {
+  let makeDate: () -> Date
+  let makeIV: (Int) -> [UInt8]
+  let signingKey: Data
+  let encryptionKey: Data
+
+  /// Initialize Fernet with a Base64URL encoded key.
+  public init(
+    encodedKey: Data,
+    makeDate: @escaping () -> Date = Date.init,
+    makeIV: @escaping (Int) -> [UInt8] = AES.randomIV
+  ) throws {
+    guard let fernetKey = Data(base64URLData: encodedKey) else { throw KeyError.invalidFormat }
+    try self.init(key: fernetKey, makeDate: makeDate, makeIV: makeIV)
+  }
+
+  /// Initialize Fernet with raw, unencoded key.
+  public init(
+    key: Data,
+    makeDate: @escaping () -> Date = Date.init,
+    makeIV: @escaping (Int) -> [UInt8] = AES.randomIV
+  ) throws {
+    guard key.count == 32 else { throw KeyError.invalidLength }
+    self.makeDate = makeDate
+    self.makeIV = makeIV
+    signingKey = key.prefix(16)
+    encryptionKey = key.suffix(16)
+  }
+
+  /// Decode fernet data.
+  public func decode(_ encoded: Data) throws -> DecodeOutput {
+    guard let fernetToken = Data(base64URLData: encoded) else { throw DecodingError.tokenDecodingFailed }
+
+    guard fernetToken.count >= 73 && (fernetToken.count - 57) % 16 == 0 else {
+      throw DecodingError.invalidTokenFormat
+    }
+    let version = fernetToken[0]
+    let timestamp = fernetToken[1..<9]
+    let iv = fernetToken[9..<25]
+    let ciphertext = fernetToken[25..<fernetToken.count - 32]
+    let hmac = fernetToken[fernetToken.count - 32..<fernetToken.count]
+
+    guard version == 128 else { throw DecodingError.unknownVersion }
+    let plaintext = try decrypt(ciphertext: ciphertext, key: encryptionKey, iv: iv)
+    let hmacMatches = try verifyHMAC(
+      hmac,
+      authenticating: Data([version]) + timestamp + iv + ciphertext,
+      using: signingKey
+    )
+
+    return DecodeOutput(data: plaintext, hmacSuccess: hmacMatches)
+  }
+
+  /// Encode data in the fernet format.
+  public func encode(_ data: Data) throws -> Data {
+    let timestamp: [UInt8] = {
+      let now = self.makeDate()
+      let timestamp = Int(now.timeIntervalSince1970).bigEndian
+      return withUnsafeBytes(of: timestamp, Array.init)
+    }()
+    guard case let iv = makeIV(16), iv.count == 16 else { throw EncodingError.invalidIV }
+    let ciphertext: [UInt8]
+    do {
+      let aes = try AES(key: encryptionKey.bytes, blockMode: CBC(iv: iv), padding: .pkcs7)
+      ciphertext = try aes.encrypt(data.bytes)
+    } catch {
+      throw EncodingError.aesError(error)
+    }
+    let version: [UInt8] = [0x80]
+    let hmac = try makeVerificationHMAC(data: Data(version + timestamp + iv + ciphertext), key: signingKey)
+    let fernetToken = (version + timestamp + iv + ciphertext + hmac).base64URLEncodedData()
+    return fernetToken
+  }
+}
+
+extension Fernet {
+  /// Errors encountered while processing the fernet key.
+  public enum KeyError: Error {
+    case invalidFormat
+    case invalidLength
+  }
+
+  /// Errors encountered while decoding data.
+  public enum DecodingError: Error {
+    case aesError(any Error)
+    case hmacError(any Error)
+    case invalidTokenFormat
+    case keyDecodingFailed
+    case tokenDecodingFailed
+    case unknownVersion
+  }
+
+  /// Errors encountered while encoding data.
+  public enum EncodingError: Error {
+    case aesError(any Error)
+    case hmacError(any Error)
+    case invalidIV
+  }
+
+  /// Decoding result.
+  public struct DecodeOutput {
+    /// Decoded data.
+    var data: Data
+    /// A boolean indicating if HMAC verification was successful.
+    var hmacSuccess: Bool
+  }
+}
+
+private func computeHMAC(data: Data, key: Data) throws -> Data {
+  Data(try HMAC(key: key.bytes, variant: .sha2(.sha256)).authenticate(data.bytes))
+}
+
+private func decrypt(ciphertext: Data, key: Data, iv: Data) throws -> Data {
+  do {
+    let aes = try AES(key: key.bytes, blockMode: CBC(iv: iv.bytes), padding: .pkcs7)
+    let decryptedData = try aes.decrypt(ciphertext.bytes)
+    return Data(decryptedData)
+  } catch {
+    throw Fernet.DecodingError.aesError(error)
+  }
+}
+
+private func makeVerificationHMAC(data: Data, key: Data) throws -> Data {
+  do {
+    return try computeHMAC(data: data, key: key)
+  } catch {
+    throw Fernet.EncodingError.hmacError(error)
+  }
+}
+
+private func verifyHMAC(_ mac: Data, authenticating data: Data, using key: Data) throws -> Bool {
+  do {
+    let auth = try computeHMAC(data: data, key: key)
+    return constantTimeEquals(auth, mac)
+  } catch {
+    throw Fernet.DecodingError.hmacError(error)
+  }
+}
+
+// Who knows how the compiler will optimize this but at least try to be constant time.
+private func constantTimeEquals<C1, C2>(_ lhs: C1, _ rhs: C2) -> Bool
+  where C1: Collection,
+  C2: Collection,
+  C1.Element == UInt8,
+  C2.Element == UInt8 {
+  guard lhs.count == rhs.count else { return false }
+  return zip(lhs, rhs).reduce(into: 0) { output, pair in output |= pair.0 ^ pair.1 } == 0
+}
+
+private extension Data {
+  init?(base64URLData base64: Data) {
+    var decoded = base64.map { b in
+      switch b {
+        case ASCII.dash.rawValue: ASCII.plus.rawValue
+        case ASCII.underscore.rawValue: ASCII.slash.rawValue
+        default: b
+      }
+    }
+    while decoded.count % 4 != 0 {
+      decoded.append(ASCII.equals.rawValue)
+    }
+    self.init(base64Encoded: Data(decoded))
+  }
+
+  func base64URLEncodedData() -> Data {
+    let bytes = base64EncodedData()
+      .compactMap { b in
+        switch b {
+          case ASCII.plus.rawValue: ASCII.dash.rawValue
+          case ASCII.slash.rawValue: ASCII.underscore.rawValue
+          case ASCII.equals.rawValue: nil
+          default: b
+        }
+      }
+    return Data(bytes)
+  }
+}
+
+private enum ASCII: UInt8 {
+  case plus = 43
+  case dash = 45
+  case slash = 47
+  case equals = 61
+  case underscore = 95
+}

Tests/CryptoSwiftTests/FernetTests.swift

@@ -0,0 +1,48 @@
+//
+//  CryptoSwift
+//
+//  Copyright (C) Marcin Krzyżanowski <marcin@krzyzanowskim.com>
+//  This software is provided 'as-is', without any express or implied warranty.
+//
+//  In no event will the authors be held liable for any damages arising from the use of this software.
+//
+//  Permission is granted to anyone to use this software for any purpose,including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:
+//
+//  - The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation is required.
+//  - Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
+//  - This notice may not be removed or altered from any source or binary distribution.
+//
+
+import XCTest
+@testable import CryptoSwift
+
+final class FernetTests: XCTestCase {
+  func testEncode() throws {
+    let key = "3b-Nqg6ry-jrAuDyVjSwEe8wrdyEPQfPuOQNH1q5olE="
+    let plaintext = "my deep dark secret"
+
+    let now = Date(timeIntervalSince1970: 1_627_721_798)
+    let iv: [UInt8] = [41, 44, 26, 236, 9, 110, 52, 150, 33, 193, 102, 135, 173, 1, 176, 0]
+
+    let fernet = try Fernet(
+      encodedKey: Data(key.utf8),
+      makeDate: { now },
+      makeIV: { _ in iv }
+    )
+    let encoded = try fernet.encode(Data(plaintext.utf8))
+
+    XCTAssertEqual(
+      String(data: encoded, encoding: .utf8),
+      "gAAAAABhBRBGKSwa7AluNJYhwWaHrQGwAA8UpMH8Wtw3tEoTD2E_-nbeoAvxbtBpFiC0ZjbVne_ZetFinKSyMjxwWaPRnXVSVqz5QqpUXp6h-34_TL7BaDs"
+    )
+  }
+
+  func testDecode() throws {
+    let key = "3b-Nqg6ry-jrAuDyVjSwEe8wrdyEPQfPuOQNH1q5olE"
+    let encrypted = "gAAAAABhBRBGKSwa7AluNJYhwWaHrQGwAA8UpMH8Wtw3tEoTD2E_-nbeoAvxbtBpFiC0ZjbVne_ZetFinKSyMjxwWaPRnXVSVqz5QqpUXp6h-34_TL7BaDs"
+    let fernet = try Fernet(encodedKey: Data(key.utf8))
+    let decoded = try fernet.decode(Data(encrypted.utf8))
+    XCTAssertEqual(String(data: decoded.data, encoding: .utf8), "my deep dark secret")
+    XCTAssertTrue(decoded.hmacSuccess)
+  }
+}