precompute every time is faster use that

@@ -49,59 +49,6 @@ const ntoh = function(word) {
     (word >>> 24);
 };
 
-/**
- * Expand the S-box tables.
- *
- * @private
- */
-const precompute = function() {
-  let _tables = [[[], [], [], [], []], [[], [], [], [], []]];
-  let encTable = _tables[0];
-  let decTable = _tables[1];
-  let sbox = encTable[4];
-  let sboxInv = decTable[4];
-  let i;
-  let x;
-  let xInv;
-  let d = [];
-  let th = [];
-  let x2;
-  let x4;
-  let x8;
-  let s;
-  let tEnc;
-  let tDec;
-
-  // Compute double and third tables
-  for (i = 0; i < 256; i++) {
-    th[(d[i] = i << 1 ^ (i >> 7) * 283) ^ i] = i;
-  }
-
-  for (x = xInv = 0; !sbox[x]; x ^= x2 || 1, xInv = th[xInv] || 1) {
-    // Compute sbox
-    s = xInv ^ xInv << 1 ^ xInv << 2 ^ xInv << 3 ^ xInv << 4;
-    s = s >> 8 ^ s & 255 ^ 99;
-    sbox[x] = s;
-    sboxInv[s] = x;
-
-    // Compute MixColumns
-    x8 = d[x4 = d[x2 = d[x]]];
-    tDec = x8 * 0x1010101 ^ x4 * 0x10001 ^ x2 * 0x101 ^ x * 0x1010100;
-    tEnc = d[s] * 0x101 ^ s * 0x1010100;
-
-    for (i = 0; i < 4; i++) {
-      encTable[i][x] = tEnc = tEnc << 24 ^ tEnc >>> 8;
-      decTable[i][s] = tDec = tDec << 24 ^ tDec >>> 8;
-    }
-  }
-
-  // Compactify. Considerable speedup on Firefox.
-  for (i = 0; i < 5; i++) {
-    encTable[i] = encTable[i].slice(0);
-    decTable[i] = decTable[i].slice(0);
-  }
-  return _tables;
-};
 
 let aesTables;
 
@@ -126,10 +73,7 @@ class AES {
     *
     * @private
     */
-    if (!aesTables) {
-      aesTables = precompute();
-    }
-    this._tables = JSON.parse(JSON.stringify(aesTables));
+    this._tables = this._precompute();
     let i;
     let j;
     let tmp;
@@ -183,6 +127,61 @@ class AES {
     }
   }
 
+
+  /**
+   * Expand the S-box tables.
+   *
+   * @private
+   */
+  _precompute() {
+    let _tables = [[[], [], [], [], []], [[], [], [], [], []]];
+    let encTable = _tables[0];
+    let decTable = _tables[1];
+    let sbox = encTable[4];
+    let sboxInv = decTable[4];
+    let i;
+    let x;
+    let xInv;
+    let d = [];
+    let th = [];
+    let x2;
+    let x4;
+    let x8;
+    let s;
+    let tEnc;
+    let tDec;
+
+    // Compute double and third tables
+    for (i = 0; i < 256; i++) {
+      th[(d[i] = i << 1 ^ (i >> 7) * 283) ^ i] = i;
+    }
+
+    for (x = xInv = 0; !sbox[x]; x ^= x2 || 1, xInv = th[xInv] || 1) {
+      // Compute sbox
+      s = xInv ^ xInv << 1 ^ xInv << 2 ^ xInv << 3 ^ xInv << 4;
+      s = s >> 8 ^ s & 255 ^ 99;
+      sbox[x] = s;
+      sboxInv[s] = x;
+
+      // Compute MixColumns
+      x8 = d[x4 = d[x2 = d[x]]];
+      tDec = x8 * 0x1010101 ^ x4 * 0x10001 ^ x2 * 0x101 ^ x * 0x1010100;
+      tEnc = d[s] * 0x101 ^ s * 0x1010100;
+
+      for (i = 0; i < 4; i++) {
+        encTable[i][x] = tEnc = tEnc << 24 ^ tEnc >>> 8;
+        decTable[i][s] = tDec = tDec << 24 ^ tDec >>> 8;
+      }
+    }
+
+    // Compactify. Considerable speedup on Firefox.
+    for (i = 0; i < 5; i++) {
+      encTable[i] = encTable[i].slice(0);
+      decTable[i] = decTable[i].slice(0);
+    }
+    return _tables;
+  }
+
   /**
    * Decrypt 16 bytes, specified as four 32-bit words.
    * @param encrypted0 {number} the first word to decrypt