diff --git a/Xcodes/AppleAPI/Sources/AppleAPI/Client.swift b/Xcodes/AppleAPI/Sources/AppleAPI/Client.swift
index 0826472d..1e8e4735 100644
--- a/Xcodes/AppleAPI/Sources/AppleAPI/Client.swift
+++ b/Xcodes/AppleAPI/Sources/AppleAPI/Client.swift
@@ -54,7 +54,7 @@ public class Client {
                 let iterations = srpInit.iteration
                 
                 do {
-                    guard let encryptedPassword = self.pbkdf2(password: password, saltData: decodedSalt, keyByteCount: 32, prf: CCPseudoRandomAlgorithm(kCCPRFHmacAlgSHA256), rounds: iterations) else {
+                    guard let encryptedPassword = self.pbkdf2(password: password, saltData: decodedSalt, keyByteCount: 32, prf: CCPseudoRandomAlgorithm(kCCPRFHmacAlgSHA256), rounds: iterations, protocol: srpInit.protocol) else {
                         return Fail(error: AuthenticationError.srpInvalidPublicKey)
                             .eraseToAnyPublisher()
                     }
@@ -308,9 +308,14 @@ public class Client {
         return Data(hash)
     }
 
-    private func pbkdf2(password: String, saltData: Data, keyByteCount: Int, prf: CCPseudoRandomAlgorithm, rounds: Int) -> Data? {
+    private func pbkdf2(password: String, saltData: Data, keyByteCount: Int, prf: CCPseudoRandomAlgorithm, rounds: Int, protocol srpProtocol: SRPProtocol) -> Data? {
         guard let passwordData = password.data(using: .utf8) else { return nil }
-        let hashedPasswordData = sha256(data: passwordData)
+        let hashedPasswordDataRaw = sha256(data: passwordData)
+        let hashedPasswordData = switch srpProtocol {
+        case .s2k: hashedPasswordDataRaw
+        // the legacy s2k_fo protocol requires hex-encoding the digest before performing PBKDF2.
+        case .s2k_fo: Data(hashedPasswordDataRaw.hexEncodedString().lowercased().utf8)
+        }
 
         var derivedKeyData = Data(repeating: 0, count: keyByteCount)
         let derivedCount = derivedKeyData.count
@@ -584,6 +589,7 @@ public struct ServerSRPInitResponse: Decodable {
     let salt: String
     let b: String
     let c: String
+    let `protocol`: SRPProtocol
 }