fix: harden hex string parsing (#646)

Improve Data(hexString:) to handle edge cases:
- Reject odd-length strings (return nil)
- Support optional 0x/0X prefix
- Trim leading/trailing whitespace
- Handle empty strings correctly

Add comprehensive unit tests for hex parsing.

bitchat/Protocols/BinaryEncodingUtils.swift

@@ -23,20 +23,42 @@ extension Data {
         return digest.map { String(format: "%02x", $0) }.joined()
     }
     
+    /// Initialize Data from a hex string.
+    /// - Parameter hexString: A hex string, optionally prefixed with "0x" or "0X".
+    ///   Whitespace is trimmed. Must have even length after prefix removal.
+    /// - Returns: nil if the string has odd length or contains invalid hex characters.
     init?(hexString: String) {
-        let len = hexString.count / 2
+        var hex = hexString.trimmingCharacters(in: .whitespaces)
+
+        // Remove optional 0x prefix
+        if hex.hasPrefix("0x") || hex.hasPrefix("0X") {
+            hex = String(hex.dropFirst(2))
+        }
+
+        // Reject odd-length strings
+        guard hex.count % 2 == 0 else {
+            return nil
+        }
+
+        // Reject empty strings
+        guard !hex.isEmpty else {
+            self = Data()
+            return
+        }
+
+        let len = hex.count / 2
         var data = Data(capacity: len)
-        var index = hexString.startIndex
+        var index = hex.startIndex
-        
+
         for _ in 0..<len {
-            let nextIndex = hexString.index(index, offsetBy: 2)
+            let nextIndex = hex.index(index, offsetBy: 2)
-            guard let byte = UInt8(String(hexString[index..<nextIndex]), radix: 16) else {
+            guard let byte = UInt8(String(hex[index..<nextIndex]), radix: 16) else {
                 return nil
             }
             data.append(byte)
             index = nextIndex
         }
-        
+
         self = data
     }
 }

bitchatTests/Utils/HexStringTests.swift

@@ -0,0 +1,108 @@
+//
+// HexStringTests.swift
+// bitchatTests
+//
+// Tests for Data(hexString:) hex parsing
+//
+
+import Testing
+import Foundation
+@testable import bitchat
+
+struct HexStringTests {
+
+    // MARK: - Valid Hex Strings
+
+    @Test func validHexString() {
+        let data = Data(hexString: "0102030405")
+        #expect(data == Data([0x01, 0x02, 0x03, 0x04, 0x05]))
+    }
+
+    @Test func validHexStringUppercase() {
+        let data = Data(hexString: "AABBCCDD")
+        #expect(data == Data([0xAA, 0xBB, 0xCC, 0xDD]))
+    }
+
+    @Test func validHexStringMixedCase() {
+        let data = Data(hexString: "aAbBcCdD")
+        #expect(data == Data([0xAA, 0xBB, 0xCC, 0xDD]))
+    }
+
+    @Test func validHexStringWith0xPrefix() {
+        let data = Data(hexString: "0x0102030405")
+        #expect(data == Data([0x01, 0x02, 0x03, 0x04, 0x05]))
+    }
+
+    @Test func validHexStringWith0XPrefix() {
+        let data = Data(hexString: "0XAABBCCDD")
+        #expect(data == Data([0xAA, 0xBB, 0xCC, 0xDD]))
+    }
+
+    @Test func validHexStringWithWhitespace() {
+        let data = Data(hexString: "  0102030405  ")
+        #expect(data == Data([0x01, 0x02, 0x03, 0x04, 0x05]))
+    }
+
+    @Test func validHexStringWith0xPrefixAndWhitespace() {
+        let data = Data(hexString: "  0x0102030405  ")
+        #expect(data == Data([0x01, 0x02, 0x03, 0x04, 0x05]))
+    }
+
+    @Test func emptyHexString() {
+        let data = Data(hexString: "")
+        #expect(data == Data())
+    }
+
+    @Test func emptyHexStringWithWhitespace() {
+        let data = Data(hexString: "   ")
+        #expect(data == Data())
+    }
+
+    @Test func emptyHexStringWith0xPrefix() {
+        let data = Data(hexString: "0x")
+        #expect(data == Data())
+    }
+
+    // MARK: - Invalid Hex Strings
+
+    @Test func oddLengthHexStringReturnsNil() {
+        let data = Data(hexString: "012")
+        #expect(data == nil)
+    }
+
+    @Test func oddLengthHexStringWith0xPrefixReturnsNil() {
+        let data = Data(hexString: "0x012")
+        #expect(data == nil)
+    }
+
+    @Test func invalidCharactersReturnNil() {
+        let data = Data(hexString: "GHIJ")
+        #expect(data == nil)
+    }
+
+    @Test func mixedValidAndInvalidCharactersReturnNil() {
+        let data = Data(hexString: "01GH")
+        #expect(data == nil)
+    }
+
+    @Test func specialCharactersReturnNil() {
+        let data = Data(hexString: "01-02")
+        #expect(data == nil)
+    }
+
+    // MARK: - Round Trip Tests
+
+    @Test func roundTripConversion() {
+        let original = Data([0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF])
+        let hexString = original.hexEncodedString()
+        let roundTripped = Data(hexString: hexString)
+        #expect(roundTripped == original)
+    }
+
+    @Test func roundTripConversionWith0xPrefix() {
+        let original = Data([0xDE, 0xAD, 0xBE, 0xEF])
+        let hexString = "0x" + original.hexEncodedString()
+        let roundTripped = Data(hexString: hexString)
+        #expect(roundTripped == original)
+    }
+}