[#1019] Memo has trailing garbled text (#1023)

Changes the way unpadded bytes are turned into a UTF-8 Swift String
without using cString assuming APIs that would overflow memory and
add garbled trailing bytes.

Closes #1019

add changelog entry

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Unreleased
 
+### [#1019] Memo has trailing garbled text
+    
+Changes the way unpadded bytes are turned into a UTF-8 Swift String
+without using cString assuming APIs that would overflow memory and
+add garbled trailing bytes.
+
 ### Changed
 - `WalletTransactionEncoder` now uses a `LightWalletService` to submit the
 encoded transactions.

Sources/ZcashLightClientKit/Model/Memo.swift

@@ -165,8 +165,8 @@ public extension MemoBytes {
     func intoMemo() throws -> Memo {
         switch self.bytes[0] {
         case 0x00 ... 0xF4:
-            let unpadded = self.unpaddedRawBytes()
-            guard let validatedUTF8String = String(validatingUTF8: unpadded) else {
+            let unpaddedBytes = self.unpaddedRawBytes()
+            guard let validatedUTF8String = String(bytes: unpaddedBytes, encoding: .utf8) else {
                 throw ZcashError.memoBytesInvalidUTF8
             }
 
@@ -218,19 +218,6 @@ extension Array where Element == UInt8 {
     }
 }
 
-extension String {
-    public init?(validatingUTF8 cString: UnsafePointer<UInt8>) {
-        guard let (str, _) = String.decodeCString(
-            cString,
-            as: UTF8.self,
-            repairingInvalidCodeUnits: false
-        ) else {
-            return nil
-        }
-        self = str
-    }
-}
-
 extension Optional where WrappedType == String {
     func intoMemo() throws -> Memo {
         switch self {