---

utils/preprocessKey.ts

@@ -0,0 +1,34 @@
+export const preprocessKey = (key: string): number[][] => {
+  /**
+   * @param {string} key - A string of 128, 192, or 256 bits.
+   * @returns {number[][]} A 4x4/6/8 matrix representing the key.
+  */
+
+  const encoder = new TextEncoder()
+  const byteArray = encoder.encode(key)
+  
+  const validKeyLengths = [128, 192, 256]
+  
+  if (!validKeyLengths.includes(byteArray.length * 8)) {
+    throw new Error('Invalid Key!')
+  }
+
+  const N_k = (byteArray.length * 8) / 32
+
+  const matrix: number[][] = [
+    Array(N_k).fill(0),
+    Array(N_k).fill(0),
+    Array(N_k).fill(0),
+    Array(N_k).fill(0)
+  ]
+
+  // Fill the matrix in column-major order, i.e., top-to-bottom + left-to-right
+  // byteArray.length is either 16, 24, or 32.
+  for (let i = 0;i < byteArray.length;i++) {
+    const row = i % 4
+    const col = Math.floor(i / 4)
+    matrix[row][col] = byteArray[i]
+  }
+
+  return matrix
+}