Modernise Romulus-M implementation and improve error handling

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+This is romulus-js, a TypeScript implementation of the Romulus-M cryptography specification. It provides authenticated encryption with associated data (AEAD) functionality.
+
+## Development Commands
+
+- `npm run build` - Compile TypeScript to JavaScript (outputs to `dist/`)
+- `npm run test` - Run Jest test suite
+- `npm run lint` - Run ts-standard linter
+- `npm install` - Install dependencies (automatically runs `tsc` via postinstall)
+
+## Architecture
+
+The codebase implements the Romulus-M AEAD cipher with the following structure:
+
+### Core Files
+
+- `src/romulus-m.ts` - Main cryptographic implementation containing `cryptoAeadEncrypt` and `cryptoAeadDecrypt` functions
+- `src/skinny-128-384-plus.ts` - SKINNY-128-384+ block cipher implementation used by Romulus-M
+- `src/constants.ts` - Cryptographic constants
+
+### Public API
+
+- `src/encrypt.ts` - High-level encrypt function that auto-generates nonces using UUID v4
+- `src/decrypt.ts` - High-level decrypt function that handles nonce extraction
+- `src/index.ts` - Main entry point exporting encrypt/decrypt functions
+
+### Key Implementation Details
+
+- Uses 128-bit keys, nonces, and block sizes
+- Implements LFSR-based counter with 56-bit precision
+- Uses domain separation for different encryption phases
+- Nonces are automatically prepended to ciphertext in the high-level API
+- Low-level API (`romulus-m.ts`) works with `number[]` arrays
+- High-level API works with `Uint8Array` for better developer experience
+
+### Testing
+
+Tests are located in `tests/` directory using Jest framework. Test files include:
+
+- `romulus-m.test.ts` - Tests for core cryptographic functions
+- `encrypt.test.ts` and `decrypt.test.ts` - Tests for high-level API
+- `romulus-m-reference.test.ts` - Reference implementation tests
+
+The TypeScript configuration automatically compiles on install and uses ts-jest for testing.