Feat: Kalshi RSA auth signing for API requests

lib/kalshi/auth.js

@@ -0,0 +1,34 @@
+import crypto from 'crypto';
+
+const KALSHI_API_BASE = 'https://api.elections.kalshi.com';
+
+/**
+ * Signs a Kalshi API request using RSA-PSS with SHA-256.
+ * Returns headers needed for authenticated requests.
+ */
+export function signRequest(method, path, timestampMs = Date.now()) {
+  const keyId = process.env.KALSHI_API_KEY_ID;
+  const privateKeyPem = process.env.KALSHI_RSA_PRIVATE_KEY?.replace(/\\n/g, '\n');
+
+  if (!keyId || !privateKeyPem) {
+    throw new Error('Missing KALSHI_API_KEY_ID or KALSHI_RSA_PRIVATE_KEY');
+  }
+
+  const ts = String(timestampMs);
+  const message = `${ts}${method.toUpperCase()}${path}`;
+
+  const signature = crypto.sign('sha256', Buffer.from(message), {
+    key: privateKeyPem,
+    padding: crypto.constants.RSA_PKCS1_PSS_PADDING,
+    saltLength: crypto.constants.RSA_PSS_SALTLEN_DIGEST
+  });
+
+  return {
+    'KALSHI-ACCESS-KEY': keyId,
+    'KALSHI-ACCESS-SIGNATURE': signature.toString('base64'),
+    'KALSHI-ACCESS-TIMESTAMP': ts,
+    'Content-Type': 'application/json'
+  };
+}
+
+export { KALSHI_API_BASE };