Replace precondition() with throwing errors in XChaCha20Poly1305Compat

precondition() crashes the app in release builds if violated. This is
dangerous for cryptographic code that may receive malformed input from
network data or key derivation bugs.

Changed to guard statements that throw typed errors instead:
- XChaCha20Poly1305Compat.Error.invalidKeyLength
- XChaCha20Poly1305Compat.Error.invalidNonceLength

This allows callers to handle errors gracefully rather than crashing.

bitchat/Nostr/XChaCha20Poly1305Compat.swift

@@ -5,16 +5,27 @@ import CryptoKit
 /// Implements HChaCha20 to derive a subkey and reduces the 24-byte nonce to a 12-byte nonce
 /// as per XChaCha20 construction.
 enum XChaCha20Poly1305Compat {
+
+    /// Errors that can occur during XChaCha20-Poly1305 operations
+    enum Error: Swift.Error {
+        case invalidKeyLength(expected: Int, got: Int)
+        case invalidNonceLength(expected: Int, got: Int)
+    }
+
     struct SealBox {
         let ciphertext: Data
         let tag: Data
     }
 
     static func seal(plaintext: Data, key: Data, nonce24: Data, aad: Data? = nil) throws -> SealBox {
-        precondition(key.count == 32, "XChaCha20 key must be 32 bytes")
+        guard key.count == 32 else {
-        precondition(nonce24.count == 24, "XChaCha20 nonce must be 24 bytes")
+            throw Error.invalidKeyLength(expected: 32, got: key.count)
+        }
+        guard nonce24.count == 24 else {
+            throw Error.invalidNonceLength(expected: 24, got: nonce24.count)
+        }
 
-        let subkey = hchacha20(key: key, nonce16: nonce24.prefix(16))
+        let subkey = try hchacha20(key: key, nonce16: Data(nonce24.prefix(16)))
         let nonce12 = derive12ByteNonce(from24: nonce24)
         let chachaKey = SymmetricKey(data: subkey)
         let nonce = try ChaChaPoly.Nonce(data: nonce12)
@@ -23,10 +34,14 @@ enum XChaCha20Poly1305Compat {
     }
 
     static func open(ciphertext: Data, tag: Data, key: Data, nonce24: Data, aad: Data? = nil) throws -> Data {
-        precondition(key.count == 32, "XChaCha20 key must be 32 bytes")
+        guard key.count == 32 else {
-        precondition(nonce24.count == 24, "XChaCha20 nonce must be 24 bytes")
+            throw Error.invalidKeyLength(expected: 32, got: key.count)
+        }
+        guard nonce24.count == 24 else {
+            throw Error.invalidNonceLength(expected: 24, got: nonce24.count)
+        }
 
-        let subkey = hchacha20(key: key, nonce16: nonce24.prefix(16))
+        let subkey = try hchacha20(key: key, nonce16: Data(nonce24.prefix(16)))
         let nonce12 = derive12ByteNonce(from24: nonce24)
         let chachaKey = SymmetricKey(data: subkey)
         let box = try ChaChaPoly.SealedBox(nonce: ChaChaPoly.Nonce(data: nonce12), ciphertext: ciphertext, tag: tag)
@@ -43,10 +58,14 @@ enum XChaCha20Poly1305Compat {
         return out
     }
 
-    private static func hchacha20(key: Data, nonce16: Data) -> Data {
+    private static func hchacha20(key: Data, nonce16: Data) throws -> Data {
         // HChaCha20 based on the original ChaCha20 core with a 16-byte nonce.
-        precondition(key.count == 32)
+        guard key.count == 32 else {
-        precondition(nonce16.count == 16)
+            throw Error.invalidKeyLength(expected: 32, got: key.count)
+        }
+        guard nonce16.count == 16 else {
+            throw Error.invalidNonceLength(expected: 16, got: nonce16.count)
+        }
 
         // Constants "expand 32-byte k"
         var state: [UInt32] = [