Modernise Romulus-M implementation and improve error handling

2025-09-02 20:44:07 +01:00
parent b12c05f3b6
commit df9c9a708b
14 changed files with 4823 additions and 8224 deletions
--- a/.prettierignore
+++ b/.prettierignore
@@ -0,0 +1,5 @@
+node_modules/
+dist/
+coverage/
+*.min.js
+package-lock.json
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -0,0 +1,49 @@
+# 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.
--- a/package-lock.json
+++ b/package-lock.json
--- a/package.json
+++ b/package.json
@@ -5,7 +5,8 @@
  "main": "dist/index.js",
  "types": "dist/index.d.ts",
  "scripts": {
-    "lint": "ts-standard",
+    "lint": "prettier --check .",
+    "format": "prettier --write .",
    "test": "jest",
    "build": "tsc",
    "postinstall": "tsc"
@@ -17,12 +18,11 @@
  "author": "Butlersaurus",
  "license": "ISC",
  "devDependencies": {
-    "@types/jest": "^27.4.0",
-    "jest": "^27.4.7",
+    "@types/jest": "^30.0.0",
+    "jest": "^30.1.3",
    "prettier": "^3.6.2",
-    "ts-jest": "^27.1.3",
-    "ts-standard": "^11.0.0",
-    "typescript": "^4.5.5"
+    "ts-jest": "^29.4.1",
+    "typescript": "^5.9.2"
  },
  "jest": {
    "verbose": true,
@@ -31,7 +31,7 @@
    }
  },
  "dependencies": {
-    "@types/uuid": "^8.3.4",
-    "uuid": "^8.3.2"
+    "@types/uuid": "^10.0.0",
+    "uuid": "^11.1.0"
  }
 }
--- a/src/constants.ts
+++ b/src/constants.ts
@@ -69,5 +69,8 @@ export const C = [
 ];

 // Romulus-M cryptography specification constants.
-export const T_LENGTH = 16;
+export const BLOCK_SIZE = 16;
+export const KEY_SIZE = 16;
+export const NONCE_SIZE = 16;
+export const AUTH_TAG_SIZE = 16;
 export const COUNTER_LENGTH = 7;
--- a/src/decrypt.ts
+++ b/src/decrypt.ts
@@ -1,38 +1,60 @@
 import { cryptoAeadDecrypt } from "./romulus-m";
-
-interface DecryptResult {
-  success: boolean;
-  plaintext: Uint8Array;
-}
+import {
+  validateKey,
+  validateAssociatedData,
+  validateCiphertext,
+  DecryptResult,
+  NONCE_SIZE,
+} from "./types";

 /**
- * Decrypt a Romulus-M encrypted message.
- * N.B. Nonces are handled automatically by this function.
- * @param buffer The nonce-prepended data to be decrypted.
- * @param associatedData The associated data.
- * @param key The encryption key.
- * @returns A decrypted DecryptResult object.
+ * Decrypt a Romulus-M encrypted message with automatic nonce handling.
+ * The nonce is automatically extracted from the beginning of the input.
+ * @param buffer The nonce-prepended ciphertext to decrypt (nonce || ciphertext || tag).
+ * @param associatedData The associated data used for authentication.
+ * @param key The 16-byte decryption key.
+ * @returns A DecryptResult object containing success status and plaintext.
+ * @throws {InvalidKeyError} If the key is invalid.
+ * @throws {InvalidInputError} If buffer or associated data is invalid.
+ * @example
+ * ```typescript
+ * const key = new Uint8Array(16); // Same key used for encryption
+ * const associatedData = new TextEncoder().encode("metadata");
+ * const result = decrypt(encryptedBuffer, associatedData, key);
+ * if (result.success) {
+ *   const message = new TextDecoder().decode(result.plaintext);
+ * }
+ * ```
 */
 export function decrypt(
  buffer: Uint8Array,
  associatedData: Uint8Array,
  key: Uint8Array,
 ): DecryptResult {
-  // Split nonce from ciphertext.
-  const nonce = Array.from(buffer.slice(0, 16));
-  const ciphertext = Array.from(buffer.slice(16));
+  // Validate inputs
+  const validatedBuffer = validateCiphertext(buffer);
+  const validatedAssociatedData = validateAssociatedData(associatedData);
+  const validatedKey = validateKey(key);

-  // Decrypt ciphertext using the associated data, nonce and encryption key.
+  // Ensure buffer is long enough to contain a nonce
+  if (validatedBuffer.length < NONCE_SIZE) {
+    return {
+      success: false,
+      plaintext: new Uint8Array(),
+    };
+  }
+
+  // Extract nonce and ciphertext
+  const nonce = validatedBuffer.slice(0, NONCE_SIZE);
+  const ciphertext = validatedBuffer.slice(NONCE_SIZE);
+
+  // Decrypt using the low-level AEAD function
  const result = cryptoAeadDecrypt(
    ciphertext,
-    Array.from(associatedData),
+    validatedAssociatedData,
    nonce,
-    Array.from(key),
+    validatedKey,
  );

-  // Return the ciphertext and decryption status.
-  return {
-    success: result.success,
-    plaintext: Uint8Array.from(result.plaintext),
-  };
+  return result;
 }
--- a/src/encrypt.ts
+++ b/src/encrypt.ts
@@ -1,33 +1,50 @@
 import { cryptoAeadEncrypt } from "./romulus-m";
+import { validateKey, validateAssociatedData, validateMessage } from "./types";
 import { v4 as uuidv4 } from "uuid";

 /**
- * Encrypt a message using the Romulus-M encryption algorithm.
- * N.B. A nonce is automatically prepended to the ciphertext using this function.
+ * Encrypt a message using the Romulus-M AEAD algorithm with auto-generated nonce.
+ * The nonce is automatically prepended to the ciphertext.
 * @param message The plaintext message to encrypt.
- * @param associatedData The associated data.
- * @param key The encryption key.
- * @returns The nonce-prepended ciphertext.
+ * @param associatedData The associated data for authentication.
+ * @param key The 16-byte encryption key.
+ * @returns The nonce-prepended ciphertext (nonce || ciphertext || tag).
+ * @throws {InvalidKeyError} If the key is invalid.
+ * @throws {InvalidInputError} If message or associated data is invalid.
+ * @example
+ * ```typescript
+ * const key = new Uint8Array(16); // 16 zero bytes for example
+ * const message = new TextEncoder().encode("Hello, world!");
+ * const associatedData = new TextEncoder().encode("metadata");
+ * const encrypted = encrypt(message, associatedData, key);
+ * ```
 */
 export function encrypt(
  message: Uint8Array,
  associatedData: Uint8Array,
  key: Uint8Array,
 ): Uint8Array {
-  // Generate a nonce.
+  // Validate inputs
+  const validatedMessage = validateMessage(message);
+  const validatedAssociatedData = validateAssociatedData(associatedData);
+  const validatedKey = validateKey(key);
+
+  // Generate a random nonce using UUID v4
  const nonce = new Uint8Array(16);
  uuidv4({}, nonce);

-  // Encrypt the data using the associated data, newly generated nonce and encryption key.
-  const ciphertext = Uint8Array.from(
-    cryptoAeadEncrypt(
-      Array.from(message),
-      Array.from(associatedData),
-      Array.from(nonce),
-      Array.from(key),
-    ),
+  // Encrypt using the low-level AEAD function
+  const ciphertext = cryptoAeadEncrypt(
+    validatedMessage,
+    validatedAssociatedData,
+    nonce,
+    validatedKey,
  );

-  // Return the nonce-prepended ciphertext.
-  return new Uint8Array([...nonce, ...ciphertext]);
+  // Return nonce prepended to ciphertext
+  const result = new Uint8Array(nonce.length + ciphertext.length);
+  result.set(nonce, 0);
+  result.set(ciphertext, nonce.length);
+
+  return result;
 }
--- a/src/romulus-m.ts
+++ b/src/romulus-m.ts
@@ -1,458 +1,615 @@
-import { COUNTER_LENGTH } from "./constants";
+import { COUNTER_LENGTH, BLOCK_SIZE, AUTH_TAG_SIZE } from "./constants";
 import { tweakeyEncode, skinnyEncrypt } from "./skinny-128-384-plus";
+import {
+  validateKey,
+  validateNonce,
+  validateAssociatedData,
+  validateMessage,
+  validateCiphertext,
+  createByteArray,
+  xorBytes,
+  copyBytes,
+  DecryptResult,
+  InvalidInputError,
+  RomulusError,
+  Key,
+  Nonce,
+} from "./types";
+
+// Re-export DecryptResult for backward compatibility
+export type { DecryptResult };

 /**
- * Parse message into blocks.
- * @param message The message to parse.
- * @param blockLength The block length.
- * @returns An array of blocks.
+ * Parse message into blocks of specified length.
+ * @param message The message to parse as a Uint8Array.
+ * @param blockLength The block length in bytes (typically 16).
+ * @returns An array of message blocks, with empty block at index 0.
 */
-function parse(message: number[], blockLength: number): number[][] {
-  // Keep track of position in message currently parsed into blocks.
+function parseMessage(message: Uint8Array, blockLength: number): Uint8Array[] {
+  const blocks: Uint8Array[] = [new Uint8Array(0)]; // Empty block at index 0
+
  let cursor = 0;

-  // Slice message into blocks.
-  let ret: number[][] = [];
+  // Create full blocks
  while (message.length - cursor >= blockLength) {
-    ret.push(message.slice(cursor, cursor + blockLength));
-    cursor = cursor + blockLength;
+    blocks.push(message.slice(cursor, cursor + blockLength));
+    cursor += blockLength;
  }

-  // Append any remaining blocks regardless of block length. These will be padded later.
+  // Add final partial block if any bytes remain
  if (message.length - cursor > 0) {
-    ret.push(message.slice(cursor));
+    blocks.push(message.slice(cursor));
  }

-  // If no message, return a single block.
+  // If message is empty, add one empty block (in addition to index 0)
  if (message.length === 0) {
-    ret = [[]];
+    blocks.push(new Uint8Array(0));
  }

-  // Insert empty array at position 0.
-  ret.splice(0, 0, []);
-  return ret;
+  return blocks;
 }

 /**
- * Pads the byte length of message to padLength. The final byte (when padded) contains the original message length.
- * @param message The message to pad.
- * @param padLength The length to pad the message to.
- * @returns A padded block.
+ * Pads a message block to the specified length using Romulus-M padding scheme.
+ * The final byte contains the original message length.
+ * @param message The message block to pad.
+ * @param padLength The target length after padding (typically 16).
+ * @returns A padded block of the specified length.
 */
-function pad(message: number[], padLength: number): number[] {
-  // If there is no message, return a fully padded block.
-  if (message.length === 0) {
-    return Array(16);
+function padBlock(message: Uint8Array, padLength: number): Uint8Array {
+  if (padLength <= 0 || padLength > 255) {
+    throw new InvalidInputError(`Invalid pad length: ${padLength}`);
  }

-  // Return a copy of the message if no padding is required.
+  // If message is empty, return zero-filled block
+  if (message.length === 0) {
+    return createByteArray(padLength, 0);
+  }
+
+  // If message is already the correct length, return a copy
  if (message.length === padLength) {
-    return Array.from(message);
+    return copyBytes(message);
  }

-  // Pad a copy of the message to padLength.
-  const ret = Array.from(message);
-  const requiredPadding = padLength - message.length - 1;
-  ret.push(...Array(requiredPadding));
+  // If message is too long, this is an error
+  if (message.length > padLength) {
+    throw new InvalidInputError(
+      `Message too long for padding: ${message.length} > ${padLength}`,
+    );
+  }

-  // Set the final byte of the padded blocked to the length of the original message.
-  ret[padLength - 1] = message.length;
+  // Create padded block
+  const padded = createByteArray(padLength, 0);
+  padded.set(message, 0);

-  return ret;
+  // Set the final byte to the original message length
+  padded[padLength - 1] = message.length;
+
+  return padded;
 }

 /**
- * Generate the key stream from the internal state by multiplying the state S and the constant matrix G.
- * @param state The state from which the key stream will be generated.
- * @returns The key stream.
+ * Generate the keystream from the internal state using the linear transformation G.
+ * Implements the transformation: G(x) = (x >> 1) ⊕ (x & 0x80) ⊕ ((x & 0x01) << 7)
+ * @param state The 16-byte internal state.
+ * @returns The 16-byte keystream.
 */
-function g(state: number[]): number[] {
-  return state.map((x) => {
-    return (x >> 1) ^ (x & 0x80) ^ ((x & 0x01) << 7);
-  });
+function generateKeystream(state: Uint8Array): Uint8Array {
+  if (state.length !== BLOCK_SIZE) {
+    throw new InvalidInputError(
+      `State must be ${BLOCK_SIZE} bytes, got ${state.length}`,
+    );
+  }
+
+  const keystream = new Uint8Array(BLOCK_SIZE);
+  for (let i = 0; i < BLOCK_SIZE; i++) {
+    const x = state[i];
+    keystream[i] = (x >> 1) ^ (x & 0x80) ^ ((x & 0x01) << 7);
+  }
+  return keystream;
 }

 /**
- * The state update function. Pads an M block.
- * @param state The internal state, S.
- * @param mBlock An M block.
- * @returns [S', C] where S' = M ⊕ S and C = M ⊕ G(S)
+ * State update function for encryption: ρ(S, M) = (S ⊕ M, M ⊕ G(S))
+ * @param state The current 16-byte internal state S.
+ * @param messageBlock The 16-byte message block M.
+ * @returns [nextState, ciphertextBlock] where nextState = S ⊕ M and ciphertextBlock = M ⊕ G(S)
 */
-function rho(state: number[], mBlock: number[]): [number[], number[]] {
-  // G(S)
-  const gOfS = g(state);
+function stateUpdateEncrypt(
+  state: Uint8Array,
+  messageBlock: Uint8Array,
+): [Uint8Array, Uint8Array] {
+  if (state.length !== BLOCK_SIZE) {
+    throw new InvalidInputError(
+      `State must be ${BLOCK_SIZE} bytes, got ${state.length}`,
+    );
+  }
+  if (messageBlock.length !== BLOCK_SIZE) {
+    throw new InvalidInputError(
+      `Message block must be ${BLOCK_SIZE} bytes, got ${messageBlock.length}`,
+    );
+  }
+
+  // Generate keystream G(S)
+  const keystream = generateKeystream(state);

  // C = M ⊕ G(S)
-  const cBlock = Array.from(Array(16).keys()).map((i) => mBlock[i] ^ gOfS[i]);
-
-  // S' = M ⊕ S
-  const nextState = Array.from(Array(16).keys()).map(
-    (i) => state[i] ^ mBlock[i],
-  );
-
-  return [nextState, cBlock];
-}
-
-/**
- * The state update function. Pads a C block.
- * @param state The internal state, S.
- * @param cBlock A C block.
- * @returns [S', M] where M = C ⊕ G(S) and S' = C ⊕ M.
- */
-function inverseRoh(state: number[], cBlock: number[]): [number[], number[]] {
-  // G(S)
-  const gOfS = g(state);
-
-  // M = C ⊕ G(S)
-  const mBlock = Array.from(Array(16).keys()).map((i) => cBlock[i] ^ gOfS[i]);
+  const ciphertextBlock = xorBytes(messageBlock, keystream);

  // S' = S ⊕ M
-  const nextState = Array.from(Array(16).keys()).map(
-    (i) => state[i] ^ mBlock[i],
-  );
-  return [nextState, mBlock];
+  const nextState = xorBytes(state, messageBlock);
+
+  return [nextState, ciphertextBlock];
 }

 /**
- * Increments the 56 bit LFSR-based counter.
- * @param counter The old counter.
- * @returns An incremented counter.
+ * State update function for decryption: ρ⁻¹(S, C) = (S ⊕ M, C ⊕ G(S)) where M = C ⊕ G(S)
+ * @param state The current 16-byte internal state S.
+ * @param ciphertextBlock The 16-byte ciphertext block C.
+ * @returns [nextState, messageBlock] where M = C ⊕ G(S) and nextState = S ⊕ M
 */
-function increaseCounter(counter: number[]): number[] {
-  const fb0 = counter[6] >> 7;
-
-  counter[6] = (counter[6] << 1) | (counter[5] >> 7);
-  counter[5] = (counter[5] << 1) | (counter[4] >> 7);
-  counter[4] = (counter[4] << 1) | (counter[3] >> 7);
-  counter[3] = (counter[3] << 1) | (counter[2] >> 7);
-  counter[2] = (counter[2] << 1) | (counter[1] >> 7);
-  counter[1] = (counter[1] << 1) | (counter[0] >> 7);
-
-  if (fb0 === 1) {
-    counter[0] = (counter[0] << 1) ^ 0x95;
-  } else {
-    counter[0] = counter[0] << 1;
+function stateUpdateDecrypt(
+  state: Uint8Array,
+  ciphertextBlock: Uint8Array,
+): [Uint8Array, Uint8Array] {
+  if (state.length !== BLOCK_SIZE) {
+    throw new InvalidInputError(
+      `State must be ${BLOCK_SIZE} bytes, got ${state.length}`,
+    );
+  }
+  if (ciphertextBlock.length !== BLOCK_SIZE) {
+    throw new InvalidInputError(
+      `Ciphertext block must be ${BLOCK_SIZE} bytes, got ${ciphertextBlock.length}`,
+    );
  }

-  return counter;
+  // Generate keystream G(S)
+  const keystream = generateKeystream(state);
+
+  // M = C ⊕ G(S)
+  const messageBlock = xorBytes(ciphertextBlock, keystream);
+
+  // S' = S ⊕ M
+  const nextState = xorBytes(state, messageBlock);
+
+  return [nextState, messageBlock];
 }

 /**
- * Returns a reset counter.
- * @returns A reset counter.
+ * Increments the 56-bit LFSR-based counter (pure function).
+ * @param counter The current 7-byte counter state.
+ * @returns A new incremented counter (does not modify input).
 */
-function resetCounter(): number[] {
-  const counter = Array(COUNTER_LENGTH);
+function incrementCounter(counter: Uint8Array): Uint8Array {
+  if (counter.length !== COUNTER_LENGTH) {
+    throw new InvalidInputError(
+      `Counter must be ${COUNTER_LENGTH} bytes, got ${counter.length}`,
+    );
+  }
+
+  // Create a copy to avoid mutation
+  const newCounter = copyBytes(counter);
+
+  // Extract feedback bit from MSB of byte 6
+  const fb0 = newCounter[6] >> 7;
+
+  // Shift left by 1 bit across all bytes (big-endian)
+  newCounter[6] = (newCounter[6] << 1) | (newCounter[5] >> 7);
+  newCounter[5] = (newCounter[5] << 1) | (newCounter[4] >> 7);
+  newCounter[4] = (newCounter[4] << 1) | (newCounter[3] >> 7);
+  newCounter[3] = (newCounter[3] << 1) | (newCounter[2] >> 7);
+  newCounter[2] = (newCounter[2] << 1) | (newCounter[1] >> 7);
+  newCounter[1] = (newCounter[1] << 1) | (newCounter[0] >> 7);
+
+  // Apply LFSR feedback polynomial if feedback bit is set
+  if (fb0 === 1) {
+    newCounter[0] = (newCounter[0] << 1) ^ 0x95;
+  } else {
+    newCounter[0] = newCounter[0] << 1;
+  }
+
+  // Ensure all bytes remain in valid range
+  for (let i = 0; i < COUNTER_LENGTH; i++) {
+    newCounter[i] &= 0xff;
+  }
+
+  return newCounter;
+}
+
+/**
+ * Creates a reset counter initialized to the starting state.
+ * @returns A new 7-byte counter initialized to [1, 0, 0, 0, 0, 0, 0].
+ */
+function createResetCounter(): Uint8Array {
+  const counter = createByteArray(COUNTER_LENGTH, 0);
  counter[0] = 1;
  return counter;
 }

 /**
- * Calculate the domain separation.
- * @param combinedData The parsed and concatenated message and associated data,
- * @param parsedMessageLength The length of the parsed message.
- * @param parsedAssociatedDataLength The length of the parsed associated data.
+ * Calculate the domain separation value based on message and associated data characteristics.
+ * @param combinedBlocks The parsed and concatenated message and associated data blocks.
+ * @param messageBlockCount The number of message blocks.
+ * @param associatedDataBlockCount The number of associated data blocks.
+ * @returns The domain separation value for the final encryption step.
 */
 function calculateDomainSeparation(
-  combinedData: number[][],
-  parsedMessageLength: number,
-  parsedAssociatedDataLength: number,
+  combinedBlocks: Uint8Array[],
+  messageBlockCount: number,
+  associatedDataBlockCount: number,
 ): number {
  let domainSeparation = 16;

-  if (combinedData[parsedAssociatedDataLength].length < 16) {
-    domainSeparation = domainSeparation ^ 2;
+  // Check if the final associated data block is incomplete
+  if (combinedBlocks[associatedDataBlockCount].length < BLOCK_SIZE) {
+    domainSeparation ^= 2;
  }

+  // Check if the final message block is incomplete
  if (
-    combinedData[parsedAssociatedDataLength + parsedMessageLength].length < 16
+    combinedBlocks[associatedDataBlockCount + messageBlockCount].length <
+    BLOCK_SIZE
  ) {
-    domainSeparation = domainSeparation ^ 1;
+    domainSeparation ^= 1;
  }

-  if (parsedAssociatedDataLength % 2 === 0) {
-    domainSeparation = domainSeparation ^ 8;
+  // Check if associated data block count is even
+  if (associatedDataBlockCount % 2 === 0) {
+    domainSeparation ^= 8;
  }

-  if (parsedMessageLength % 2 === 0) {
-    domainSeparation = domainSeparation ^ 4;
+  // Check if message block count is even
+  if (messageBlockCount % 2 === 0) {
+    domainSeparation ^= 4;
  }

  return domainSeparation;
 }

 /**
- * Encrypt a message using the Romulus-M cryptography specification.
+ * Encrypt a message using the Romulus-M AEAD cryptography specification.
 * See https://romulusae.github.io/romulus/docs/Romulusv1.3.pdf for more information.
- * @param message The message to encrypt.
- * @param associatedData The associated data to encrypt.
- * @param nonce A 128 bit nonce.
- * @param key A 128 bit encryption key.
- * @returns The encrypted ciphertext.
+ * @param message The message to encrypt as a byte array.
+ * @param associatedData The associated data as a byte array.
+ * @param nonce A 16-byte nonce.
+ * @param key A 16-byte encryption key.
+ * @returns The encrypted ciphertext with authentication tag appended.
+ * @throws {InvalidInputError} If inputs have incorrect lengths or invalid values.
 */
 export function cryptoAeadEncrypt(
-  message: number[],
-  associatedData: number[],
-  nonce: number[],
-  key: number[],
-): number[] {
-  // Buffer for ciphertext.
-  const ciphertext = [];
+  message: Uint8Array,
+  associatedData: Uint8Array,
+  nonce: Uint8Array,
+  key: Uint8Array,
+): Uint8Array {
+  // Validate all inputs
+  const validatedMessage = validateMessage(message);
+  const validatedAssociatedData = validateAssociatedData(associatedData);
+  const validatedNonce = validateNonce(nonce);
+  const validatedKey = validateKey(key);

-  // Reset state and counter.
-  let state = Array(16);
-  let counter = resetCounter();
+  try {
+    // Initialize state and counter
+    let state = createByteArray(BLOCK_SIZE, 0);
+    let counter = createResetCounter();

-  // Carve message and associated data into blocks.
-  const messageBlocks = parse(message, 16);
+    // Parse message and associated data into blocks
+    const messageBlocks = parseMessage(validatedMessage, BLOCK_SIZE);
    const messageBlockCount = messageBlocks.length - 1;

-  const associatedDataBlocks = parse(associatedData, 16);
+    const associatedDataBlocks = parseMessage(
+      validatedAssociatedData,
+      BLOCK_SIZE,
+    );
    const associatedDataBlockCount = associatedDataBlocks.length - 1;

-  // Concatenate the message and associated data blocks, excluding each array's first element.
-  const combinedDataBlocks = associatedDataBlocks
-    .slice(1)
-    .concat(messageBlocks.slice(1));
+    // Concatenate blocks (excluding empty block at index 0)
+    const combinedBlocks = [
+      createByteArray(0), // Empty block at index 0
+      ...associatedDataBlocks.slice(1),
+      ...messageBlocks.slice(1),
+    ];

-  // Insert empty array at position 0.
-  combinedDataBlocks.splice(0, 0, []);
-
-  // Calculate domain separation for final encryption stage.
+    // Calculate domain separation for final encryption stage
    const domainSeparation = calculateDomainSeparation(
-    combinedDataBlocks,
+      combinedBlocks,
      messageBlockCount,
      associatedDataBlockCount,
    );

-  // Pad combined data.
-  combinedDataBlocks[associatedDataBlockCount] = pad(
-    combinedDataBlocks[associatedDataBlockCount],
-    16,
+    // Pad the final blocks
+    if (associatedDataBlockCount > 0) {
+      combinedBlocks[associatedDataBlockCount] = padBlock(
+        combinedBlocks[associatedDataBlockCount],
+        BLOCK_SIZE,
      );
-  combinedDataBlocks[associatedDataBlockCount + messageBlockCount] = pad(
-    combinedDataBlocks[associatedDataBlockCount + messageBlockCount],
-    16,
-  );
-
-  // Process the associated data.
-  let x = 8;
-  for (
-    let i = 1;
-    i < Math.floor((associatedDataBlockCount + messageBlockCount) / 2) + 1;
-    i++
-  ) {
-    [state] = rho(state, combinedDataBlocks[2 * i - 1]);
-    counter = increaseCounter(counter);
-    if (i === Math.floor(associatedDataBlockCount / 2) + 1) {
-      x = x ^ 4;
    }
+    if (messageBlockCount > 0) {
+      combinedBlocks[associatedDataBlockCount + messageBlockCount] = padBlock(
+        combinedBlocks[associatedDataBlockCount + messageBlockCount],
+        BLOCK_SIZE,
+      );
+    }
+
+    // Process associated data and message blocks
+    let x = 8;
+    const totalRounds =
+      Math.floor((associatedDataBlockCount + messageBlockCount) / 2) + 1;
+
+    for (let i = 1; i < totalRounds; i++) {
+      // State update with odd-indexed block
+      [state] = stateUpdateEncrypt(state, combinedBlocks[2 * i - 1]);
+      counter = incrementCounter(counter);
+
+      // Switch domain constant at transition point
+      if (i === Math.floor(associatedDataBlockCount / 2) + 1) {
+        x ^= 4;
+      }
+
+      // SKINNY encryption with even-indexed block
      state = skinnyEncrypt(
        state,
-      tweakeyEncode(counter, x, combinedDataBlocks[2 * i], key),
+        tweakeyEncode(counter, x, combinedBlocks[2 * i], validatedKey),
      );
-    counter = increaseCounter(counter);
+      counter = incrementCounter(counter);
    }

+    // Handle final block based on parity
+    const emptyBlock = createByteArray(BLOCK_SIZE, 0);
    if (associatedDataBlockCount % 2 === messageBlockCount % 2) {
-    [state] = rho(state, Array(16));
+      [state] = stateUpdateEncrypt(state, emptyBlock);
    } else {
-    [state] = rho(
+      [state] = stateUpdateEncrypt(
        state,
-      combinedDataBlocks[associatedDataBlockCount + messageBlockCount],
+        combinedBlocks[associatedDataBlockCount + messageBlockCount],
      );
-    counter = increaseCounter(counter);
+      counter = incrementCounter(counter);
    }

-  // Generate authentication tag.
-  const [, authenticationTag] = rho(
-    skinnyEncrypt(state, tweakeyEncode(counter, domainSeparation, nonce, key)),
-    Array(16),
+    // Generate authentication tag
+    const encryptedState = skinnyEncrypt(
+      state,
+      tweakeyEncode(counter, domainSeparation, validatedNonce, validatedKey),
+    );
+    const [, authenticationTag] = stateUpdateEncrypt(
+      encryptedState,
+      emptyBlock,
    );

-  if (message.length === 0) {
+    // If message is empty, return only the authentication tag
+    if (validatedMessage.length === 0) {
      return authenticationTag;
    }

-  state = Array.from(authenticationTag);
-  counter = resetCounter();
+    // Encrypt the message blocks
+    state = copyBytes(authenticationTag);
+    counter = createResetCounter();

-  // Encrypt the message.
-  const originalFinalMessageBlockLength =
-    messageBlocks[messageBlockCount].length;
-  messageBlocks[messageBlockCount] = pad(messageBlocks[messageBlockCount], 16);
-
-  for (let i = 1; i < messageBlockCount + 1; i++) {
-    state = skinnyEncrypt(state, tweakeyEncode(counter, 4, nonce, key));
-
-    let cBlock;
-    [state, cBlock] = rho(state, messageBlocks[i]);
-    counter = increaseCounter(counter);
-
-    if (i < messageBlockCount) {
-      ciphertext.push(...cBlock);
-    } else {
-      ciphertext.push(...cBlock.slice(0, originalFinalMessageBlockLength));
-    }
-  }
-
-  // Store the authentication tag in the final 16 bytes of the ciphertext.
-  ciphertext.push(...authenticationTag);
-
-  return ciphertext;
-}
-
-/**
- * Return interface for decrypting a message.
- */
-export interface DecryptResult {
-  success: boolean;
-  plaintext: number[];
-}
-
-/**
- * Decrypt a message using the Romulus-M cryptography specification.
- * See https://romulusae.github.io/romulus/docs/Romulusv1.3.pdf for more information.
- * @param ciphertext The ciphertext to decrypt.
- * @param associatedData The associated data.
- * @param nonce The nonce.
- * @param key The key.
- * @returns The decrypted plaintext.
- */
-export function cryptoAeadDecrypt(
-  ciphertext: number[],
-  associatedData: number[],
-  nonce: number[],
-  key: number[],
-): DecryptResult {
-  // Buffer for decrypted message.
-  const cleartext = [];
-
-  // The authentication tag is represented by the final 16 bytes of the ciphertext.
-  const authenticationTag = ciphertext.slice(-16);
-  ciphertext.length -= 16;
-
-  // Reset state and counter.
-  let state = Array(16);
-  let counter = resetCounter();
-
-  if (ciphertext.length !== 0) {
-    // Combine the ciphertext.
-    state = Array.from(authenticationTag);
-    const ciphertextBlocks = parse(ciphertext, 16);
-    const ciphertextBlockCount = ciphertextBlocks.length - 1;
-    const finalCiphertextBlockLength =
-      ciphertextBlocks[ciphertextBlockCount].length;
-    ciphertextBlocks[ciphertextBlockCount] = pad(
-      ciphertextBlocks[ciphertextBlockCount],
-      16,
+    const ciphertext: number[] = [];
+    const originalFinalBlockLength = messageBlocks[messageBlockCount].length;
+    messageBlocks[messageBlockCount] = padBlock(
+      messageBlocks[messageBlockCount],
+      BLOCK_SIZE,
    );

-    for (let i = 1; i < ciphertextBlockCount + 1; i++) {
-      state = skinnyEncrypt(state, tweakeyEncode(counter, 4, nonce, key));
+    for (let i = 1; i <= messageBlockCount; i++) {
+      state = skinnyEncrypt(
+        state,
+        tweakeyEncode(counter, 4, validatedNonce, validatedKey),
+      );

-      let mBlock;
-      [state, mBlock] = inverseRoh(state, ciphertextBlocks[i]);
-      counter = increaseCounter(counter);
+      const [nextState, ciphertextBlock] = stateUpdateEncrypt(
+        state,
+        messageBlocks[i],
+      );
+      state = nextState;
+      counter = incrementCounter(counter);

+      // Add ciphertext to output (truncate final block to original length)
+      if (i < messageBlockCount) {
+        ciphertext.push(...Array.from(ciphertextBlock));
+      } else {
+        ciphertext.push(
+          ...Array.from(ciphertextBlock.slice(0, originalFinalBlockLength)),
+        );
+      }
+    }
+
+    // Append authentication tag to ciphertext
+    ciphertext.push(...Array.from(authenticationTag));
+
+    return new Uint8Array(ciphertext);
+  } catch (error) {
+    if (error instanceof InvalidInputError) {
+      throw error;
+    }
+    throw new RomulusError(
+      `Encryption failed: ${error instanceof Error ? error.message : "Unknown error"}`,
+    );
+  }
+}
+
+/**
+ * Decrypt a message using the Romulus-M AEAD cryptography specification.
+ * See https://romulusae.github.io/romulus/docs/Romulusv1.3.pdf for more information.
+ * @param ciphertext The ciphertext to decrypt (includes authentication tag).
+ * @param associatedData The associated data as a byte array.
+ * @param nonce The 16-byte nonce.
+ * @param key The 16-byte decryption key.
+ * @returns The decryption result with success status and plaintext.
+ * @throws {InvalidInputError} If inputs have incorrect lengths or invalid values.
+ */
+export function cryptoAeadDecrypt(
+  ciphertext: Uint8Array,
+  associatedData: Uint8Array,
+  nonce: Uint8Array,
+  key: Uint8Array,
+): DecryptResult {
+  // Validate all inputs
+  const validatedCiphertext = validateCiphertext(ciphertext);
+  const validatedAssociatedData = validateAssociatedData(associatedData);
+  const validatedNonce = validateNonce(nonce);
+  const validatedKey = validateKey(key);
+
+  try {
+    // Extract authentication tag from the end of ciphertext
+    const authenticationTag = validatedCiphertext.slice(-AUTH_TAG_SIZE);
+    const actualCiphertext = validatedCiphertext.slice(0, -AUTH_TAG_SIZE);
+
+    let plaintextBytes: number[] = [];
+
+    // Decrypt message blocks if ciphertext is not empty
+    if (actualCiphertext.length > 0) {
+      let state = copyBytes(authenticationTag);
+      let counter = createResetCounter();
+
+      // Parse ciphertext into blocks
+      const ciphertextBlocks = parseMessage(actualCiphertext, BLOCK_SIZE);
+      const ciphertextBlockCount = ciphertextBlocks.length - 1;
+      const finalBlockLength = ciphertextBlocks[ciphertextBlockCount].length;
+
+      // Pad final block
+      ciphertextBlocks[ciphertextBlockCount] = padBlock(
+        ciphertextBlocks[ciphertextBlockCount],
+        BLOCK_SIZE,
+      );
+
+      // Decrypt each block
+      for (let i = 1; i <= ciphertextBlockCount; i++) {
+        state = skinnyEncrypt(
+          state,
+          tweakeyEncode(counter, 4, validatedNonce, validatedKey),
+        );
+
+        const [nextState, messageBlock] = stateUpdateDecrypt(
+          state,
+          ciphertextBlocks[i],
+        );
+        state = nextState;
+        counter = incrementCounter(counter);
+
+        // Add plaintext to output (truncate final block to original length)
        if (i < ciphertextBlockCount) {
-        cleartext.push(...mBlock);
+          plaintextBytes.push(...Array.from(messageBlock));
        } else {
-        cleartext.push(...mBlock.slice(0, finalCiphertextBlockLength));
+          plaintextBytes.push(
+            ...Array.from(messageBlock.slice(0, finalBlockLength)),
+          );
        }
      }
-  } else {
-    state = [];
    }

-  // Reset state and counter.
-  state = Array(16);
-  counter = resetCounter();
+    // Verify authentication by recomputing the tag
+    const recoveredPlaintext = new Uint8Array(plaintextBytes);

-  // Carve the message and associated data into blocks.
-  const messageBlocks = parse(cleartext, 16);
-  const messageBlockLength = messageBlocks.length - 1;
+    // Initialize for verification
+    let state = createByteArray(BLOCK_SIZE, 0);
+    let counter = createResetCounter();

-  const associatedDataBlocks = parse(associatedData, 16);
+    // Parse recovered message and associated data
+    const messageBlocks = parseMessage(recoveredPlaintext, BLOCK_SIZE);
+    const messageBlockCount = messageBlocks.length - 1;
+
+    const associatedDataBlocks = parseMessage(
+      validatedAssociatedData,
+      BLOCK_SIZE,
+    );
    const associatedDataBlockCount = associatedDataBlocks.length - 1;

-  // Concatenate the message and associated data blocks, excluding each array's first element.
-  const combinedData = associatedDataBlocks
-    .slice(1)
-    .concat(messageBlocks.slice(1));
+    // Combine blocks for verification
+    const combinedBlocks = [
+      createByteArray(0),
+      ...associatedDataBlocks.slice(1),
+      ...messageBlocks.slice(1),
+    ];

-  // Insert empty array at position 0.
-  combinedData.splice(0, 0, []);
-
-  // Calculate domain separation for final decryption stage.
+    // Calculate domain separation
    const domainSeparation = calculateDomainSeparation(
-    combinedData,
-    messageBlockLength,
+      combinedBlocks,
+      messageBlockCount,
      associatedDataBlockCount,
    );

-  // Pad combined data.
-  combinedData[associatedDataBlockCount] = pad(
-    combinedData[associatedDataBlockCount],
-    16,
+    // Pad final blocks
+    if (associatedDataBlockCount > 0) {
+      combinedBlocks[associatedDataBlockCount] = padBlock(
+        combinedBlocks[associatedDataBlockCount],
+        BLOCK_SIZE,
      );
-  combinedData[associatedDataBlockCount + messageBlockLength] = pad(
-    combinedData[associatedDataBlockCount + messageBlockLength],
-    16,
-  );
-
-  // Verifiy associated data.
-  let x = 8;
-  for (
-    let i = 1;
-    i < Math.floor((associatedDataBlockCount + messageBlockLength) / 2) + 1;
-    i++
-  ) {
-    [state] = rho(state, combinedData[2 * i - 1]);
-    counter = increaseCounter(counter);
-    if (i === Math.floor(associatedDataBlockCount / 2) + 1) {
-      x = x ^ 4;
    }
+    if (messageBlockCount > 0) {
+      combinedBlocks[associatedDataBlockCount + messageBlockCount] = padBlock(
+        combinedBlocks[associatedDataBlockCount + messageBlockCount],
+        BLOCK_SIZE,
+      );
+    }
+
+    // Process blocks for verification
+    let x = 8;
+    const totalRounds =
+      Math.floor((associatedDataBlockCount + messageBlockCount) / 2) + 1;
+
+    for (let i = 1; i < totalRounds; i++) {
+      [state] = stateUpdateEncrypt(state, combinedBlocks[2 * i - 1]);
+      counter = incrementCounter(counter);
+
+      if (i === Math.floor(associatedDataBlockCount / 2) + 1) {
+        x ^= 4;
+      }
+
      state = skinnyEncrypt(
        state,
-      tweakeyEncode(counter, x, combinedData[2 * i], key),
+        tweakeyEncode(counter, x, combinedBlocks[2 * i], validatedKey),
      );
-    counter = increaseCounter(counter);
+      counter = incrementCounter(counter);
    }

-  if (associatedDataBlockCount % 2 === messageBlockLength % 2) {
-    [state] = rho(state, Array(16));
+    // Handle final block
+    const emptyBlock = createByteArray(BLOCK_SIZE, 0);
+    if (associatedDataBlockCount % 2 === messageBlockCount % 2) {
+      [state] = stateUpdateEncrypt(state, emptyBlock);
    } else {
-    [state] = rho(
+      [state] = stateUpdateEncrypt(
        state,
-      combinedData[associatedDataBlockCount + messageBlockLength],
+        combinedBlocks[associatedDataBlockCount + messageBlockCount],
      );
-    counter = increaseCounter(counter);
+      counter = incrementCounter(counter);
    }

-  // Calculate authentication tag.
-  const [, computedTag] = rho(
-    skinnyEncrypt(state, tweakeyEncode(counter, domainSeparation, nonce, key)),
-    Array(16),
+    // Compute authentication tag
+    const encryptedState = skinnyEncrypt(
+      state,
+      tweakeyEncode(counter, domainSeparation, validatedNonce, validatedKey),
    );
+    const [, computedTag] = stateUpdateEncrypt(encryptedState, emptyBlock);

-  // Validate authentication tag.
+    // Constant-time comparison of authentication tags
    let compare = 0;
-  for (let i = 0; i < 16; i++) {
+    for (let i = 0; i < AUTH_TAG_SIZE; i++) {
      compare |= authenticationTag[i] ^ computedTag[i];
    }

    if (compare !== 0) {
-    // Authentication failed.
+      // Authentication failed
      return {
        success: false,
-      plaintext: [],
+        plaintext: new Uint8Array(),
      };
    } else {
-    // Decrypted successfully.
+      // Decryption successful
      return {
        success: true,
-      plaintext: cleartext,
+        plaintext: recoveredPlaintext,
+      };
+    }
+  } catch (error) {
+    if (error instanceof InvalidInputError) {
+      throw error;
+    }
+    // Authentication failure or other error - return failure without throwing
+    return {
+      success: false,
+      plaintext: new Uint8Array(),
    };
  }
 }
--- a/src/skinny-128-384-plus.ts
+++ b/src/skinny-128-384-plus.ts
@@ -7,93 +7,170 @@ import {
  LFSR_8_TK3,
  S8,
  C,
+  BLOCK_SIZE,
 } from "./constants";
+import {
+  validateKey,
+  validateNonce,
+  InvalidInputError,
+  copyBytes,
+} from "./types";

 /**
 * Create a tweakey based on the specified domain separation, nonce, key and current counter state.
- * @param counter The counter.
- * @param domainSeparation The domain separation.
- * @param nonce The nonce.
- * @param key The encryption key.
- * @returns The tweakey.
+ * @param counter The counter state as a byte array.
+ * @param domainSeparation The domain separation value.
+ * @param nonce The 16-byte nonce.
+ * @param key The 16-byte encryption key.
+ * @returns The tweakey as a byte array.
+ * @throws {InvalidInputError} If inputs have incorrect lengths or invalid values.
 */
 export function tweakeyEncode(
-  counter: number[],
+  counter: Uint8Array,
  domainSeparation: number,
-  nonce: number[],
-  key: number[],
-): number[] {
-  return counter.concat([domainSeparation ^ MEMBER_MASK], Array(8), nonce, key);
+  nonce: Uint8Array,
+  key: Uint8Array,
+): Uint8Array {
+  if (counter.length !== 7) {
+    throw new InvalidInputError(
+      `Counter must be 7 bytes, got ${counter.length}`,
+    );
+  }
+  if (
+    !Number.isInteger(domainSeparation) ||
+    domainSeparation < 0 ||
+    domainSeparation > 255
+  ) {
+    throw new InvalidInputError(
+      `Domain separation must be a valid byte (0-255), got ${domainSeparation}`,
+    );
+  }
+
+  const validatedNonce = validateNonce(nonce);
+  const validatedKey = validateKey(key);
+
+  // Create tweakey: counter (7) + domain_sep^MEMBER_MASK (1) + padding (8) + nonce (16) + key (16) = 48 bytes
+  const tweakey = new Uint8Array(TWEAK_LENGTH);
+  let offset = 0;
+
+  // Copy counter
+  tweakey.set(counter, offset);
+  offset += counter.length;
+
+  // Add domain separation XORed with member mask
+  tweakey[offset] = domainSeparation ^ MEMBER_MASK;
+  offset += 1;
+
+  // Skip 8 bytes of padding (already zero-filled)
+  offset += 8;
+
+  // Copy nonce
+  tweakey.set(validatedNonce, offset);
+  offset += BLOCK_SIZE;
+
+  // Copy key
+  tweakey.set(validatedKey, offset);
+
+  return tweakey;
 }

 /**
- * Perform a round of SKINNY-188/384+ encryption.
- * @param plaintext The plaintext to encrypt.
- * @param tweakey The tweakey to use for encryption.
- * @returns The ciphertext.
+ * Perform SKINNY-128/384+ block cipher encryption.
+ * @param plaintext The 16-byte plaintext block to encrypt.
+ * @param tweakey The 48-byte tweakey for encryption.
+ * @returns The 16-byte ciphertext block.
+ * @throws {InvalidInputError} If inputs have incorrect lengths.
 */
 export function skinnyEncrypt(
-  plaintext: number[],
-  tweakey: number[],
-): number[] {
-  const tk = Array(NB_ROUNDS + 1).fill(Array(TWEAK_LENGTH).fill(0));
+  plaintext: Uint8Array,
+  tweakey: Uint8Array,
+): Uint8Array {
+  if (plaintext.length !== BLOCK_SIZE) {
+    throw new InvalidInputError(
+      `Plaintext must be ${BLOCK_SIZE} bytes, got ${plaintext.length}`,
+    );
+  }
+  if (tweakey.length !== TWEAK_LENGTH) {
+    throw new InvalidInputError(
+      `Tweakey must be ${TWEAK_LENGTH} bytes, got ${tweakey.length}`,
+    );
+  }

-  tk[0] = Array.from(Array(TWEAK_LENGTH).keys()).map((i) => tweakey[i]);
+  // Pre-compute all round tweakeys (fix the array sharing bug)
+  const tk: Uint8Array[] = new Array(NB_ROUNDS + 1);

+  // Initialize first round tweakey
+  tk[0] = copyBytes(tweakey);
+
+  // Generate remaining round tweakeys
  for (let i = 0; i < NB_ROUNDS - 1; i++) {
-    tk[i + 1] = Array.from(tk[i]);
+    tk[i + 1] = copyBytes(tk[i]);

+    // Apply permutation table
    for (let j = 0; j < TWEAK_LENGTH; j++) {
      tk[i + 1][j] = tk[i][j - (j % 16) + PT[j % 16]];
    }

+    // Apply LFSR to TK2 and TK3 parts
    for (let j = 0; j < 8; j++) {
      tk[i + 1][j + 16] = LFSR_8_TK2[tk[i + 1][j + 16]];
      tk[i + 1][j + 32] = LFSR_8_TK3[tk[i + 1][j + 32]];
    }
  }

-  let s = Array.from(Array(16).keys()).map((i) => plaintext[i]);
-  for (let i = 0; i < NB_ROUNDS; i++) {
-    for (let j = 0; j < 16; j++) {
+  // Initialize state with plaintext (fix inefficient array creation)
+  const s = copyBytes(plaintext);
+
+  // Perform encryption rounds
+  for (let round = 0; round < NB_ROUNDS; round++) {
+    // SubCells: apply S-box to all bytes
+    for (let j = 0; j < BLOCK_SIZE; j++) {
      s[j] = S8[s[j]];
    }

-    s[0] ^= C[i] & 0xf;
-    s[4] ^= (C[i] >> 4) & 0xf;
+    // AddConstants
+    s[0] ^= C[round] & 0xf;
+    s[4] ^= (C[round] >> 4) & 0xf;
    s[8] ^= 0x2;

+    // AddRoundTweakey: XOR first 8 bytes of state with tweakey
    for (let j = 0; j < 8; j++) {
-      s[j] ^= tk[i][j] ^ tk[i][j + 16] ^ tk[i][j + 32];
+      s[j] ^= tk[round][j] ^ tk[round][j + 16] ^ tk[round][j + 32];
    }

-    s = [
-      s[0],
-      s[1],
-      s[2],
-      s[3],
-      s[7],
-      s[4],
-      s[5],
-      s[6],
-      s[10],
-      s[11],
-      s[8],
-      s[9],
-      s[13],
-      s[14],
-      s[15],
-      s[12],
-    ];
+    // ShiftRows: apply shift row transformation
+    const temp = new Uint8Array(BLOCK_SIZE);
+    temp[0] = s[0];
+    temp[1] = s[1];
+    temp[2] = s[2];
+    temp[3] = s[3];
+    temp[4] = s[7];
+    temp[5] = s[4];
+    temp[6] = s[5];
+    temp[7] = s[6];
+    temp[8] = s[10];
+    temp[9] = s[11];
+    temp[10] = s[8];
+    temp[11] = s[9];
+    temp[12] = s[13];
+    temp[13] = s[14];
+    temp[14] = s[15];
+    temp[15] = s[12];
+    s.set(temp);

-    for (let j = 0; j < 4; j++) {
-      const tmp = Array.from(s);
-      s[j] = tmp[j] ^ tmp[8 + j] ^ tmp[12 + j];
-      s[4 + j] = tmp[j];
-      s[8 + j] = tmp[4 + j] ^ tmp[8 + j];
-      s[12 + j] = tmp[0 + j] ^ tmp[8 + j];
+    // MixColumns: apply linear transformation to columns
+    for (let col = 0; col < 4; col++) {
+      const c0 = s[col];
+      const c1 = s[4 + col];
+      const c2 = s[8 + col];
+      const c3 = s[12 + col];
+
+      s[col] = c0 ^ c2 ^ c3;
+      s[4 + col] = c0;
+      s[8 + col] = c1 ^ c2;
+      s[12 + col] = c0 ^ c2;
    }
  }

-  return Array.from(Array(16).keys()).map((i) => s[i]);
+  return s;
 }
--- a/src/types.ts
+++ b/src/types.ts
@@ -0,0 +1,161 @@
+/**
+ * Type definitions and validation utilities for the Romulus-M cryptographic library.
+ */
+
+// Branded types for type safety
+declare const KeyBrand: unique symbol;
+declare const NonceBrand: unique symbol;
+declare const BlockBrand: unique symbol;
+declare const CounterBrand: unique symbol;
+
+export type Key = Uint8Array & { readonly [KeyBrand]: true };
+export type Nonce = Uint8Array & { readonly [NonceBrand]: true };
+export type Block = Uint8Array & { readonly [BlockBrand]: true };
+export type Counter = Uint8Array & { readonly [CounterBrand]: true };
+
+// Constants
+export const BLOCK_SIZE = 16;
+export const KEY_SIZE = 16;
+export const NONCE_SIZE = 16;
+export const COUNTER_SIZE = 7;
+export const AUTH_TAG_SIZE = 16;
+
+// Custom error classes
+export class RomulusError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "RomulusError";
+  }
+}
+
+export class InvalidInputError extends RomulusError {
+  constructor(message: string) {
+    super(`Invalid input: ${message}`);
+    this.name = "InvalidInputError";
+  }
+}
+
+export class InvalidKeyError extends InvalidInputError {
+  constructor(message: string = "Key must be exactly 16 bytes") {
+    super(`Key error: ${message}`);
+    this.name = "InvalidKeyError";
+  }
+}
+
+export class InvalidNonceError extends InvalidInputError {
+  constructor(message: string = "Nonce must be exactly 16 bytes") {
+    super(`Nonce error: ${message}`);
+    this.name = "InvalidNonceError";
+  }
+}
+
+// Validation functions
+export function isValidByteArray(arr: Uint8Array): boolean {
+  return arr.every(
+    (byte) => byte >= 0 && byte <= 255 && Number.isInteger(byte),
+  );
+}
+
+export function validateKey(key: Uint8Array): Key {
+  if (!key || !(key instanceof Uint8Array)) {
+    throw new InvalidKeyError("Key must be a Uint8Array");
+  }
+  if (key.length !== KEY_SIZE) {
+    throw new InvalidKeyError(
+      `Key must be exactly ${KEY_SIZE} bytes, got ${key.length}`,
+    );
+  }
+  if (!isValidByteArray(key)) {
+    throw new InvalidKeyError("Key must contain only valid bytes (0-255)");
+  }
+  return key as Key;
+}
+
+export function validateNonce(nonce: Uint8Array): Nonce {
+  if (!nonce || !(nonce instanceof Uint8Array)) {
+    throw new InvalidNonceError("Nonce must be a Uint8Array");
+  }
+  if (nonce.length !== NONCE_SIZE) {
+    throw new InvalidNonceError(
+      `Nonce must be exactly ${NONCE_SIZE} bytes, got ${nonce.length}`,
+    );
+  }
+  if (!isValidByteArray(nonce)) {
+    throw new InvalidNonceError("Nonce must contain only valid bytes (0-255)");
+  }
+  return nonce as Nonce;
+}
+
+export function validateAssociatedData(data: Uint8Array): Uint8Array {
+  if (!data || !(data instanceof Uint8Array)) {
+    throw new InvalidInputError("Associated data must be a Uint8Array");
+  }
+  if (!isValidByteArray(data)) {
+    throw new InvalidInputError(
+      "Associated data must contain only valid bytes (0-255)",
+    );
+  }
+  return data;
+}
+
+export function validateMessage(message: Uint8Array): Uint8Array {
+  if (!message || !(message instanceof Uint8Array)) {
+    throw new InvalidInputError("Message must be a Uint8Array");
+  }
+  if (!isValidByteArray(message)) {
+    throw new InvalidInputError(
+      "Message must contain only valid bytes (0-255)",
+    );
+  }
+  return message;
+}
+
+export function validateCiphertext(ciphertext: Uint8Array): Uint8Array {
+  if (!ciphertext || !(ciphertext instanceof Uint8Array)) {
+    throw new InvalidInputError("Ciphertext must be a Uint8Array");
+  }
+  if (ciphertext.length < AUTH_TAG_SIZE) {
+    throw new InvalidInputError(
+      `Ciphertext must be at least ${AUTH_TAG_SIZE} bytes (authentication tag size)`,
+    );
+  }
+  if (!isValidByteArray(ciphertext)) {
+    throw new InvalidInputError(
+      "Ciphertext must contain only valid bytes (0-255)",
+    );
+  }
+  return ciphertext;
+}
+
+// Utility functions for efficient array operations
+export function createByteArray(size: number, fill: number = 0): Uint8Array {
+  if (fill < 0 || fill > 255 || !Number.isInteger(fill)) {
+    throw new InvalidInputError(
+      `Fill value must be a valid byte (0-255), got ${fill}`,
+    );
+  }
+  return new Uint8Array(size).fill(fill);
+}
+
+export function xorBytes(a: Uint8Array, b: Uint8Array): Uint8Array {
+  if (a.length !== b.length) {
+    throw new InvalidInputError(
+      `Array lengths must match: ${a.length} !== ${b.length}`,
+    );
+  }
+  const result = new Uint8Array(a.length);
+  for (let i = 0; i < a.length; i++) {
+    result[i] = a[i] ^ b[i];
+  }
+  return result;
+}
+
+export function copyBytes(source: Uint8Array): Uint8Array {
+  return new Uint8Array(source);
+}
+
+// Result interface
+export interface DecryptResult {
+  success: boolean;
+  plaintext: Uint8Array;
+}
--- a/tests/romulus-m-reference.test.ts
+++ b/tests/romulus-m-reference.test.ts
@@ -5,13 +5,12 @@ import {
  DecryptResult,
 } from "../src/romulus-m";

-function parseHexString(string: string): number[] {
-  const ret = [];
+function parseHexString(string: string): Uint8Array {
+  const bytes: number[] = [];
  for (let i = 0; i < string.length; i += 2) {
-    ret.push(parseInt(string.slice(i, i + 2), 16));
+    bytes.push(parseInt(string.slice(i, i + 2), 16));
  }
-
-  return ret;
+  return new Uint8Array(bytes);
 }

 test.each(referenceTests)(
@@ -27,7 +26,7 @@ test.each(referenceTests)(

    // Then
    const expectedResult = parseHexString(ciphertext);
-    expect(result).toMatchObject(expectedResult);
+    expect(result).toEqual(expectedResult);
  },
 );

@@ -47,6 +46,6 @@ test.each(referenceTests)(
      success: true,
      plaintext: parseHexString(plaintext),
    };
-    expect(result).toMatchObject(expectedResult);
+    expect(result).toEqual(expectedResult);
  },
 );
--- a/tests/romulus-m.test.ts
+++ b/tests/romulus-m.test.ts
@@ -1,18 +1,18 @@
 import { cryptoAeadDecrypt, cryptoAeadEncrypt } from "../src/romulus-m";

-function stringToArray(string: string): number[] {
+function stringToUint8Array(string: string): Uint8Array {
  const encoder = new TextEncoder();
-  return Array.from(encoder.encode(string));
+  return encoder.encode(string);
 }

 test("Encrypt a message with no associated data.", () => {
  // Given
-  const message = stringToArray("Hello, World! This is a test message.");
-  const associatedData = stringToArray("");
-  const nonce = stringToArray(
+  const message = stringToUint8Array("Hello, World! This is a test message.");
+  const associatedData = stringToUint8Array("");
+  const nonce = stringToUint8Array(
    "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
  );
-  const key = stringToArray(
+  const key = stringToUint8Array(
    "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
  );

@@ -20,23 +20,23 @@ test("Encrypt a message with no associated data.", () => {
  const result = cryptoAeadEncrypt(message, associatedData, nonce, key);

  // Then
-  const expectedResult = [
+  const expectedResult = new Uint8Array([
    85, 125, 23, 244, 73, 241, 140, 72, 166, 113, 114, 78, 239, 211, 84, 113,
    222, 153, 207, 183, 69, 142, 174, 15, 38, 46, 112, 162, 229, 27, 136, 184,
    163, 78, 132, 42, 107, 160, 74, 115, 28, 251, 209, 37, 48, 57, 184, 204,
    199, 247, 93, 5, 208,
-  ];
-  expect(result).toMatchObject(expectedResult);
+  ]);
+  expect(result).toEqual(expectedResult);
 });

 test("Encrypt a message with associated data.", () => {
  // Given
-  const message = stringToArray("Hello, World! This is a test message.");
-  const associatedData = stringToArray("Some associated data.");
-  const nonce = stringToArray(
+  const message = stringToUint8Array("Hello, World! This is a test message.");
+  const associatedData = stringToUint8Array("Some associated data.");
+  const nonce = stringToUint8Array(
    "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
  );
-  const key = stringToArray(
+  const key = stringToUint8Array(
    "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
  );

@@ -44,28 +44,28 @@ test("Encrypt a message with associated data.", () => {
  const result = cryptoAeadEncrypt(message, associatedData, nonce, key);

  // Then
-  const expectedResult = [
+  const expectedResult = new Uint8Array([
    225, 53, 3, 212, 22, 112, 246, 194, 61, 171, 230, 187, 157, 102, 32, 76, 62,
    65, 25, 202, 255, 201, 206, 49, 60, 58, 82, 216, 72, 116, 106, 129, 162,
    142, 69, 40, 167, 88, 94, 195, 174, 217, 242, 149, 224, 125, 196, 237, 172,
    165, 116, 119, 128,
-  ];
-  expect(result).toMatchObject(expectedResult);
+  ]);
+  expect(result).toEqual(expectedResult);
 });

 test("Decrypt a message with no associated data.", () => {
  // Given
-  const ciphertext = [
+  const ciphertext = new Uint8Array([
    85, 125, 23, 244, 73, 241, 140, 72, 166, 113, 114, 78, 239, 211, 84, 113,
    222, 153, 207, 183, 69, 142, 174, 15, 38, 46, 112, 162, 229, 27, 136, 184,
    163, 78, 132, 42, 107, 160, 74, 115, 28, 251, 209, 37, 48, 57, 184, 204,
    199, 247, 93, 5, 208,
-  ];
-  const associatedData = stringToArray("");
-  const nonce = stringToArray(
+  ]);
+  const associatedData = stringToUint8Array("");
+  const nonce = stringToUint8Array(
    "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
  );
-  const key = stringToArray(
+  const key = stringToUint8Array(
    "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
  );

@@ -73,24 +73,26 @@ test("Decrypt a message with no associated data.", () => {
  const result = cryptoAeadDecrypt(ciphertext, associatedData, nonce, key);

  // Then
-  const expectedResult = stringToArray("Hello, World! This is a test message.");
+  const expectedResult = stringToUint8Array(
+    "Hello, World! This is a test message.",
+  );
  expect(result.success).toBe(true);
-  expect(result.plaintext).toMatchObject(expectedResult);
+  expect(result.plaintext).toEqual(expectedResult);
 });

 test("Decrypt a message with associated data.", () => {
  // Given
-  const ciphertext = [
+  const ciphertext = new Uint8Array([
    225, 53, 3, 212, 22, 112, 246, 194, 61, 171, 230, 187, 157, 102, 32, 76, 62,
    65, 25, 202, 255, 201, 206, 49, 60, 58, 82, 216, 72, 116, 106, 129, 162,
    142, 69, 40, 167, 88, 94, 195, 174, 217, 242, 149, 224, 125, 196, 237, 172,
    165, 116, 119, 128,
-  ];
-  const associatedData = stringToArray("Some associated data.");
-  const nonce = stringToArray(
+  ]);
+  const associatedData = stringToUint8Array("Some associated data.");
+  const nonce = stringToUint8Array(
    "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
  );
-  const key = stringToArray(
+  const key = stringToUint8Array(
    "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
  );

@@ -98,7 +100,9 @@ test("Decrypt a message with associated data.", () => {
  const result = cryptoAeadDecrypt(ciphertext, associatedData, nonce, key);

  // Then
-  const expectedResult = stringToArray("Hello, World! This is a test message.");
+  const expectedResult = stringToUint8Array(
+    "Hello, World! This is a test message.",
+  );
  expect(result.success).toBe(true);
-  expect(result.plaintext).toMatchObject(expectedResult);
+  expect(result.plaintext).toEqual(expectedResult);
 });
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,8 +1,9 @@
 {
  "compilerOptions": {
    /* Visit https://aka.ms/tsconfig.json to read more about this file */
-    "target": "es2016" /* Set the JavaScript language version for emitted JavaScript and include compatible library declarations. */,
-    "module": "commonjs" /* Specify what module code is generated. */,
+    "target": "es2022" /* Set the JavaScript language version for emitted JavaScript and include compatible library declarations. */,
+    "module": "node16" /* Specify what module code is generated. */,
+    "moduleResolution": "node16" /* Specify how TypeScript looks up a file from a given module specifier. */,
    "rootDir": "src" /* Specify the root folder within your source files. */,
    "sourceMap": true /* Create source map files for emitted JavaScript files. */,
    "outDir": "dist" /* Specify an output folder for all emitted files. */,
@@ -10,6 +11,7 @@
    "forceConsistentCasingInFileNames": true /* Ensure that casing is correct in imports. */,
    "strict": true /* Enable all strict type-checking options. */,
    "skipLibCheck": true /* Skip type checking all .d.ts files. */,
+    "isolatedModules": true /* Ensure that each file can be safely transpiled without relying on other imports. */,
    "declaration": true
  },
  "exclude": ["tests", "dist"]