diff --git a/Xcodes/AppleAPI/Sources/AppleAPI/Client.swift b/Xcodes/AppleAPI/Sources/AppleAPI/Client.swift
index ef43d13..1e8e473 100644
--- a/Xcodes/AppleAPI/Sources/AppleAPI/Client.swift
+++ b/Xcodes/AppleAPI/Sources/AppleAPI/Client.swift
@@ -313,6 +313,7 @@ public class Client {
         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)
         }