Ensure P256 derived field length

NearbyShare/Connection/NearbyConnection.swift

@@ -511,17 +511,12 @@ class NearbyConnection {
         guard peerKey.hasEcP256PublicKey else { throw NearbyError.requiredFieldMissing("peerKey.ecP256PublicKey") }
         
         let domain = Domain.instance(curve: .EC256r1)
-        var clientX = peerKey.ecP256PublicKey.x
-        var clientY = peerKey.ecP256PublicKey.y
-        if clientX.count > 32 {
-            clientX = clientX.suffix(32)
-        }
-        if clientY.count > 32 {
-            clientY = clientY.suffix(32)
-        }
+        let clientX = peerKey.ecP256PublicKey.x.ensureLength(length: 32)
+        let clientY = peerKey.ecP256PublicKey.y.ensureLength(length: 32)
         let key = try ECPublicKey(domain: domain, w: Point(BInt(magnitude: [UInt8](clientX)), BInt(magnitude: [UInt8](clientY))))
         
-        let dhs = try (privateKey?.domain.multiplyPoint(key.w, privateKey!.s).x.asMagnitudeBytes())!
+        let dhsRaw = try (privateKey?.domain.multiplyPoint(key.w, privateKey!.s).x.asMagnitudeBytes())!
+        let dhs = Data(dhsRaw).ensureLength(length: 32)
         var sha = SHA256()
         sha.update(data: dhs)
         let derivedSecretKey = Data(sha.finalize())

NearbyShare/Connection/NearbyConnectionManager.swift

@@ -628,7 +628,7 @@ public class NearbyConnectionManager: NSObject, NetServiceDelegate, InboundNearb
         
         var keyData = Data()
         keyData.append(contentsOf: [0, 0, 2])
-        keyData.append(Data(pubKey.w.x.asSignedBytes()).suffixOfAtMost(numBytes: 32))
+        keyData.append(Data(pubKey.w.x.asSignedBytes()).ensureLength(length: 32))
         
         let ikm = SymmetricKey(data: keyData)
         

NearbyShare/Util/Helpers.swift

@@ -31,17 +31,6 @@ extension Data {
         return data
     }
     
-    
-    func suffixOfAtMost(numBytes: Int) -> Data {
-        
-        if count <= numBytes {
-            return self
-        }
-        
-        return subdata(in: count-numBytes..<count)
-    }
-    
-    
     static func dataFromUrlSafeBase64(_ str: String) -> Data? {
         var regularB64 = String(str.map {
             if $0 == "_" {