Add a ChaCha20 utility for encrypting in place

Similar to ChaCha20::encrypt_single_block only encrypts in-place.

lightning/src/util/chacha20.rs

@@ -173,6 +173,16 @@ mod real_chacha {
 			}
 		}
 
+		/// Same as `encrypt_single_block` only operates on a fixed-size input in-place.
+		pub fn encrypt_single_block_in_place(
+			key: &[u8; 32], nonce: &[u8; 16], bytes: &mut [u8; 32]
+		) {
+			let block = ChaCha20::get_single_block(key, nonce);
+			for i in 0..bytes.len() {
+				bytes[i] = block[i] ^ bytes[i];
+			}
+		}
+
 		fn expand(key: &[u8], nonce: &[u8]) -> ChaChaState {
 			let constant = match key.len() {
 				16 => b"expand 16-byte k",
@@ -311,6 +321,10 @@ mod fuzzy_chacha {
 			debug_assert!(dest.len() <= 32);
 		}
 
+		pub fn encrypt_single_block_in_place(
+			_key: &[u8; 32], _nonce: &[u8; 16], _bytes: &mut [u8; 32]
+		) {}
+
 		pub fn process(&mut self, input: &[u8], output: &mut [u8]) {
 			output.copy_from_slice(input);
 		}
@@ -662,4 +676,26 @@ mod test {
 
 		assert_eq!(bytes, decrypted_bytes);
 	}
+
+	#[test]
+	fn encrypt_single_block_in_place() {
+		let key = [
+			0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+			0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
+			0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
+			0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
+		];
+		let nonce = [
+			0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,
+			0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
+		];
+		let unencrypted_bytes = [1; 32];
+		let mut bytes = unencrypted_bytes;
+
+		ChaCha20::encrypt_single_block_in_place(&key, &nonce, &mut bytes);
+		assert_ne!(bytes, unencrypted_bytes);
+
+		ChaCha20::encrypt_single_block_in_place(&key, &nonce, &mut bytes);
+		assert_eq!(bytes, unencrypted_bytes);
+	}
 }