fix: assert error when using invalid base64 chars

packages/circuits/helpers/base64.circom

@@ -54,6 +54,22 @@ template Base64Lookup() {
     signal sum_slash <== sum_plus + equal_slash.out * (in + 16);
 
     out <== sum_slash;
+
+    // detect invalid characters
+    // only allow output to be 0 when input is A | =
+    component is_zero = IsZero();
+    is_zero.in <== out;
+
+    component is_A = IsEqual();
+    is_A.in[0] <== in;
+    is_A.in[1] <== 65;
+
+    // handle padding (=)
+    component is_eqsign = IsEqual();
+    is_eqsign.in[0] <== in;
+    is_eqsign.in[1] <== 61;
+
+    0 === (1 - is_A.out - is_eqsign.out) * is_zero.out;
 }
 
 template Base64Decode(N) {

packages/circuits/tests/base64-test.circom

@@ -0,0 +1,5 @@
+pragma circom 2.1.5;
+
+include "../helpers/base64.circom";
+
+component main { public [ in ] } = Base64Lookup();

packages/circuits/tests/base64.test.ts

@@ -0,0 +1,70 @@
+import fs from "fs";
+import { buildMimcSponge } from "circomlibjs";
+import { wasm as wasm_tester } from "circom_tester";
+import { Scalar } from "ffjavascript";
+import path from "path";
+import { DKIMVerificationResult } from "@zk-email/helpers/src/dkim";
+import { generateCircuitInputs } from "@zk-email/helpers/src/input-helpers";
+import { verifyDKIMSignature } from "@zk-email/helpers/src/dkim";
+
+exports.p = Scalar.fromString(
+  "21888242871839275222246405745257275088548364400416034343698204186575808495617"
+);
+
+describe("Base64 Lookup", () => {
+  jest.setTimeout(10 * 60 * 1000); // 10 minutes
+
+  let circuit: any;
+
+  beforeAll(async () => {
+    circuit = await wasm_tester(
+      path.join(__dirname, "./base64-test.circom"),
+      {
+        // @dev During development recompile can be set to false if you are only making changes in the tests.
+        // This will save time by not recompiling the circuit every time.
+        // Compile: circom "./tests/email-verifier-test.circom" --r1cs --wasm --sym --c --wat --output "./tests/compiled-test-circuit"
+        recompile: true,
+        output: path.join(__dirname, "./compiled-test-circuit"),
+        include: path.join(__dirname, "../../../node_modules"),
+      }
+    );
+  });
+
+  it("should decode valid base64 chars", async function () {
+    const inputs = [
+      [65, 0], // A
+      [90, 25], // Z
+      [97, 26], // a
+      [122, 51], // z
+      [48, 52], // 0
+      [57, 61], // 9
+      [43, 62], // +
+      [47, 63], // /
+      [61, 0], // =
+    ]
+
+    for (const [input, output] of inputs) {
+      const witness = await circuit.calculateWitness({
+        in: input
+      });
+      await circuit.checkConstraints(witness);
+      await circuit.assertOut(witness, { out: output })
+    }
+  });
+
+  it("should fail with invalid chars", async function () {
+    const inputs = [34, 64, 91,44];
+
+    expect.assertions(inputs.length);
+    for (const input of inputs) {
+    try {
+      const witness = await circuit.calculateWitness({
+        in: input
+      });
+      await circuit.checkConstraints(witness);
+    } catch (error) {
+      expect((error as Error).message).toMatch("Assert Failed");
+    }
+    }
+  });
+});