1 files changed, 82 insertions, 3 deletions
diff --git a/main.go b/main.go
index fdf75d4..ce4fe1c 100644
--- a/main.go
+++ b/main.go
@@ -79,11 +79,89 @@ Encode your selected value in base64 and submit it below:
 </form>
 <br />
 <details>
-<summary>
-Program to solve this
-</summary><pre>` + html.EscapeString(solverProgram) + `</pre>
+<summary>Program to solve this</summary>
+<pre>` + html.EscapeString(solverProgram) + `</pre>
 </details>
+<p id="solver_status"></p>
 </body>
+<script>
+document.addEventListener("DOMContentLoaded", function() {
+	let challenge_b64 = "{{.UnsignedTokenBase64}}";
+	let difficulty = {{.NeedBits}};
+	let form = document.querySelector("form");
+	let field = form.querySelector("input[name='powxy']");
+	let status_el = document.getElementById("solver_status");
+
+	if (!window.crypto || !window.crypto.subtle) {
+		status_el.textContent = "SubtleCrypto not available. You must solve the challenge externally.";
+		return;
+	}
+	status_el.textContent = "SubtleCrypto detected. Attempting to solve in JS...";
+
+	let solver_active = true;
+	form.addEventListener("submit", function() {
+		solver_active = false;
+	});
+
+	async function solve_pow() {
+		let token_bytes = Uint8Array.from(
+			atob(challenge_b64),
+			ch => ch.charCodeAt(0)
+		);
+
+		let nonce = 0n;
+		let buf = new ArrayBuffer(8);
+		let view = new DataView(buf);
+
+		while (solver_active) {
+			view.setBigUint64(0, nonce, true);
+
+			let candidate = new Uint8Array(token_bytes.length + 8);
+			candidate.set(token_bytes, 0);
+			candidate.set(new Uint8Array(buf), token_bytes.length);
+
+			let digest_buffer = await crypto.subtle.digest("SHA-256", candidate);
+			let digest = new Uint8Array(digest_buffer);
+
+			if (has_leading_zero_bits(digest, difficulty)) {
+				let nonce_str = String.fromCharCode(...new Uint8Array(buf));
+				field.value = btoa(nonce_str);
+
+				status_el.textContent = "Solution found. Submitting...";
+				form.submit();
+				return;
+			}
+
+			nonce++;
+
+			// Update status every 65536 tries
+			if ((nonce & 0xFFFFn) === 0n) {
+				status_el.textContent = "Tried " + nonce + " candidates so far...";
+				await new Promise(r => setTimeout(r, 0));
+			}
+		}
+	}
+
+	function has_leading_zero_bits(digest, bits) {
+		let full_bytes = bits >>> 3;
+		for (let i = 0; i < full_bytes; i++) {
+			if (digest[i] !== 0) {
+				return false;
+			}
+		}
+		let remainder = bits & 7;
+		if (remainder !== 0) {
+			let mask = 0xFF << (8 - remainder);
+			if ((digest[full_bytes] & mask) !== 0) {
+				return false;
+			}
+		}
+		return true;
+	}
+
+	solve_pow();
+});
+</script>
 </html>
 `)
 	if err != nil {
@@ -173,6 +251,7 @@ func validateCookie(cookie *http.Cookie, expectedToken []byte) bool {
 
 	return subtle.ConstantTimeCompare(gotToken, expectedToken) == 1
 }
+
 func makeSignedToken(request *http.Request) []byte {
 	buf := make([]byte, 0, 2*sha256.Size)