diff --git a/acs-visualiser/public/oidc.js b/acs-visualiser/public/oidc.js
index 10ec32360..ebc463d10 100644
--- a/acs-visualiser/public/oidc.js
+++ b/acs-visualiser/public/oidc.js
@@ -15,6 +15,80 @@ function b64url (buf) {
         .replace(/\+/g, "-").replace(/\//g, "_").replace(/=+$/, "");
 }
 
+/* Pure-JS SHA-256 (FIPS 180-4). crypto.subtle only exists in secure
+ * contexts and several ACS deployments (kiosks especially) are served
+ * over plain http, so we need a fallback for the PKCE challenge. */
+function sha256_js (bytes) {
+    const K = [
+        0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5,
+        0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
+        0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3,
+        0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
+        0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc,
+        0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
+        0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7,
+        0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
+        0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13,
+        0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
+        0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3,
+        0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
+        0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5,
+        0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
+        0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208,
+        0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2,
+    ];
+    const H = [
+        0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a,
+        0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19,
+    ];
+    const rr = (x, n) => (x >>> n) | (x << (32 - n));
+
+    const len = bytes.length;
+    const padded = new Uint8Array((len + 9 + 63) & ~63);
+    padded.set(bytes);
+    padded[len] = 0x80;
+    new DataView(padded.buffer).setUint32(padded.length - 4, len * 8);
+
+    const w = new Int32Array(64);
+    const view = new DataView(padded.buffer);
+    for (let off = 0; off < padded.length; off += 64) {
+        for (let i = 0; i < 16; i++)
+            w[i] = view.getUint32(off + i * 4);
+        for (let i = 16; i < 64; i++) {
+            const s0 = rr(w[i-15], 7) ^ rr(w[i-15], 18) ^ (w[i-15] >>> 3);
+            const s1 = rr(w[i-2], 17) ^ rr(w[i-2], 19) ^ (w[i-2] >>> 10);
+            w[i] = (w[i-16] + s0 + w[i-7] + s1) | 0;
+        }
+
+        let [a, b, c, d, e, f, g, h] = H;
+        for (let i = 0; i < 64; i++) {
+            const S1 = rr(e, 6) ^ rr(e, 11) ^ rr(e, 25);
+            const ch = (e & f) ^ (~e & g);
+            const t1 = (h + S1 + ch + K[i] + w[i]) | 0;
+            const S0 = rr(a, 2) ^ rr(a, 13) ^ rr(a, 22);
+            const maj = (a & b) ^ (a & c) ^ (b & c);
+            const t2 = (S0 + maj) | 0;
+            h = g; g = f; f = e; e = (d + t1) | 0;
+            d = c; c = b; b = a; a = (t1 + t2) | 0;
+        }
+        H[0] = (H[0] + a) | 0; H[1] = (H[1] + b) | 0;
+        H[2] = (H[2] + c) | 0; H[3] = (H[3] + d) | 0;
+        H[4] = (H[4] + e) | 0; H[5] = (H[5] + f) | 0;
+        H[6] = (H[6] + g) | 0; H[7] = (H[7] + h) | 0;
+    }
+
+    const out = new Uint8Array(32);
+    const ov = new DataView(out.buffer);
+    H.forEach((v, i) => ov.setUint32(i * 4, v >>> 0));
+    return out;
+}
+
+async function sha256 (bytes) {
+    if (crypto.subtle)
+        return new Uint8Array(await crypto.subtle.digest("SHA-256", bytes));
+    return sha256_js(bytes);
+}
+
 function jwt_claims (token) {
     try {
         const b64 = token.split(".")[1]
@@ -102,7 +176,7 @@ export class OidcClient {
 
     async start_code_flow () {
         const verifier = b64url(crypto.getRandomValues(new Uint8Array(32)));
-        const challenge = b64url(await crypto.subtle.digest("SHA-256",
+        const challenge = b64url(await sha256(
             new TextEncoder().encode(verifier)));
         const state = b64url(crypto.getRandomValues(new Uint8Array(16)));