add enc size compute

src/encrypt.ts

@@ -165,3 +165,23 @@ export const decryptBase64urlToString = async (
     )
   );
 };
+
+export const getSizeFromOrigToEnc = (x: number) => {
+  if (x < 0 || !Number.isInteger(x)) {
+    throw Error(`x=${x} is not a valid size`);
+  }
+  return (Math.floor(x / 16) + 1) * 16 + 16;
+};
+
+export const getSizeFromEncToOrig = (x: number) => {
+  if (x < 32 || !Number.isInteger(x)) {
+    throw Error(`${x} is not a valid size`);
+  }
+  if (x % 16 !== 0) {
+    throw Error(`${x} is not a valid encrypted file size`);
+  }
+  return {
+    minSize: ((x - 16) / 16 - 1) * 16,
+    maxSize: ((x - 16) / 16 - 1) * 16 + 15,
+  };
+};