add feistel encryption

apps/shared/feistel.ts

@@ -0,0 +1,45 @@
+const SECRET = process.env.FEISTEL_SECRET || "change-me";
+const ROUNDS = 8;
+const HALF = 20;
+const MASK = (1 << HALF) - 1;
+const CHARS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
+
+function roundFn(right: number, round: number): number {
+  const h = new Bun.CryptoHasher("sha256").update(SECRET + ":" + round + ":" + right).digest();
+  return (h[0] << 12 | h[1] << 4 | h[2] >> 4) & MASK;
+}
+
+function toBase62(n: number): string {
+  let s = "";
+  for (let i = 0; i < 7; i++) { s = CHARS[n % 62] + s; n = Math.floor(n / 62); }
+  return s;
+}
+
+function fromBase62(s: string): number {
+  let n = 0;
+  for (const c of s) n = n * 62 + CHARS.indexOf(c);
+  return n;
+}
+
+export function encodeId(val: number): string {
+  let left = Math.floor(val / (MASK + 1)) & MASK;
+  let right = val & MASK;
+  for (let i = 0; i < ROUNDS; i++) {
+    const tmp = (left ^ roundFn(right, i)) & MASK;
+    left = right;
+    right = tmp;
+  }
+  return toBase62(left * (MASK + 1) + right);
+}
+
+export function decodeId(s: string): number {
+  const val = fromBase62(s);
+  let left = Math.floor(val / (MASK + 1)) & MASK;
+  let right = val & MASK;
+  for (let i = ROUNDS - 1; i >= 0; i--) {
+    const tmp = (right ^ roundFn(left, i)) & MASK;
+    right = left;
+    left = tmp;
+  }
+  return left * (MASK + 1) + right;
+}