Use PoW difficulty returned by snodes

SignalServiceKit/src/Loki/API/LokiAPI.swift

@@ -12,6 +12,7 @@ public final class LokiAPI : NSObject {
     private static let defaultTimeout: TimeInterval = 20
     private static let longPollingTimeout: TimeInterval = 40
     public static let defaultMessageTTL: UInt64 = 24 * 60 * 60 * 1000
+    internal static var powDifficulty: UInt = 100
     
     // MARK: Types
     public typealias RawResponse = Any
@@ -81,7 +82,16 @@ public final class LokiAPI : NSObject {
             let powPromise = lokiMessage.calculatePoW()
             let swarmPromise = getTargetSnodes(for: destination)
             return when(fulfilled: powPromise, swarmPromise).map { lokiMessageWithPoW, swarm in
-                return Set(swarm.map { sendLokiMessage(lokiMessageWithPoW, to: $0) })
+                return Set(swarm.map {
+                    sendLokiMessage(lokiMessageWithPoW, to: $0).map { rawResponse in
+                        if let json = rawResponse as? JSON, let powDifficulty = json["difficulty"] as? Int {
+                            LokiAPI.powDifficulty = UInt(powDifficulty)
+                        } else {
+                            print("[Loki] Failed to update PoW difficulty from: \(rawResponse).")
+                        }
+                        return rawResponse
+                    }
+                })
             }.retryingIfNeeded(maxRetryCount: maxRetryCount)
         }
         if let peer = LokiP2PAPI.getInfo(for: destination), (lokiMessage.isPing || peer.isOnline) {

SignalServiceKit/src/Loki/Crypto/ProofOfWork.swift

@@ -48,13 +48,6 @@ public enum ProofOfWork {
     // If this changes then we also have to use something other than UInt64 to support the new length
     private static let nonceLength = 8
     
-    private static let nonceTrialCount: Int = {
-        switch BuildConfiguration.current {
-        case .debug: return 10
-        case .production: return 100
-        }
-    }()
-    
     /// Calculate a proof of work with the given configuration
     ///
     /// Ref: https://bitmessage.org/wiki/Proof_of_work
@@ -67,7 +60,7 @@ public enum ProofOfWork {
     /// - Returns: A nonce string or `nil` if it failed
     public static func calculate(data: String, pubKey: String, timestamp: UInt64, ttl: UInt64) -> String? {
         let payload = createPayload(pubKey: pubKey, data: data, timestamp: timestamp, ttl: ttl)
-        let target = calcTarget(ttl: ttl, payloadLength: payload.count, nonceTrials: nonceTrialCount)
+        let target = calcTarget(ttl: ttl, payloadLength: payload.count, nonceTrials: Int(LokiAPI.powDifficulty))
         
         // Start with the max value
         var trialValue = UInt64.max