diff --git a/package-lock.json b/package-lock.json
index 8bcb070..649f942 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -15,6 +15,7 @@
         "@stablelib/random": "^1.0.2",
         "@stablelib/sha256": "^1.0.1",
         "@stablelib/x25519": "^1.0.1",
+        "bn.js": "^5.2.1",
         "js-base64": "^3.7.3",
         "pkcs7-padding": "^0.1.1",
         "uint8arraylist": "^2.3.2",
@@ -26,6 +27,7 @@
         "@rollup/plugin-node-resolve": "^13.3.0",
         "@size-limit/preset-big-lib": "^8.0.0",
         "@types/app-root-path": "^1.2.4",
+        "@types/bn.js": "^5.1.1",
         "@types/chai": "^4.2.15",
         "@types/debug": "^4.1.7",
         "@types/mocha": "^9.1.0",
@@ -2566,6 +2568,15 @@
       "integrity": "sha512-yhURoXmWN/zfw2MXXcOdUTwe5CWhzRWtb4Rs2+JwGpuhm2hxLCzjASi/aIuyVeJB3Iyks1teaHyOEfNNmltdvQ==",
       "dev": true
     },
+    "node_modules/@types/bn.js": {
+      "version": "5.1.1",
+      "resolved": "https://registry.npmjs.org/@types/bn.js/-/bn.js-5.1.1.tgz",
+      "integrity": "sha512-qNrYbZqMx0uJAfKnKclPh+dTwK33KfLHYqtyODwd5HnXOjnkhc4qgn3BrK6RWyGZm5+sIFE7Q7Vz6QQtJB7w7g==",
+      "dev": true,
+      "dependencies": {
+        "@types/node": "*"
+      }
+    },
     "node_modules/@types/chai": {
       "version": "4.3.4",
       "resolved": "https://registry.npmjs.org/@types/chai/-/chai-4.3.4.tgz",
@@ -3454,6 +3465,11 @@
         "readable-stream": "^3.4.0"
       }
     },
+    "node_modules/bn.js": {
+      "version": "5.2.1",
+      "resolved": "https://registry.npmjs.org/bn.js/-/bn.js-5.2.1.tgz",
+      "integrity": "sha512-eXRvHzWyYPBuB4NBy0cmYQjGitUrtqwbvlzP3G6VFnNRbsZQIxQ10PbKKHt8gZ/HW/D/747aDl+QkDqg3KQLMQ=="
+    },
     "node_modules/body-parser": {
       "version": "1.20.1",
       "resolved": "https://registry.npmjs.org/body-parser/-/body-parser-1.20.1.tgz",
@@ -14021,6 +14037,15 @@
       "integrity": "sha512-yhURoXmWN/zfw2MXXcOdUTwe5CWhzRWtb4Rs2+JwGpuhm2hxLCzjASi/aIuyVeJB3Iyks1teaHyOEfNNmltdvQ==",
       "dev": true
     },
+    "@types/bn.js": {
+      "version": "5.1.1",
+      "resolved": "https://registry.npmjs.org/@types/bn.js/-/bn.js-5.1.1.tgz",
+      "integrity": "sha512-qNrYbZqMx0uJAfKnKclPh+dTwK33KfLHYqtyODwd5HnXOjnkhc4qgn3BrK6RWyGZm5+sIFE7Q7Vz6QQtJB7w7g==",
+      "dev": true,
+      "requires": {
+        "@types/node": "*"
+      }
+    },
     "@types/chai": {
       "version": "4.3.4",
       "resolved": "https://registry.npmjs.org/@types/chai/-/chai-4.3.4.tgz",
@@ -14716,6 +14741,11 @@
         "readable-stream": "^3.4.0"
       }
     },
+    "bn.js": {
+      "version": "5.2.1",
+      "resolved": "https://registry.npmjs.org/bn.js/-/bn.js-5.2.1.tgz",
+      "integrity": "sha512-eXRvHzWyYPBuB4NBy0cmYQjGitUrtqwbvlzP3G6VFnNRbsZQIxQ10PbKKHt8gZ/HW/D/747aDl+QkDqg3KQLMQ=="
+    },
     "body-parser": {
       "version": "1.20.1",
       "resolved": "https://registry.npmjs.org/body-parser/-/body-parser-1.20.1.tgz",
diff --git a/package.json b/package.json
index b018b65..66adecf 100644
--- a/package.json
+++ b/package.json
@@ -49,6 +49,7 @@
     "@rollup/plugin-node-resolve": "^13.3.0",
     "@size-limit/preset-big-lib": "^8.0.0",
     "@types/app-root-path": "^1.2.4",
+    "@types/bn.js": "^5.1.1",
     "@types/chai": "^4.2.15",
     "@types/debug": "^4.1.7",
     "@types/mocha": "^9.1.0",
@@ -119,6 +120,7 @@
     "@stablelib/random": "^1.0.2",
     "@stablelib/sha256": "^1.0.1",
     "@stablelib/x25519": "^1.0.1",
+    "bn.js": "^5.2.1",
     "js-base64": "^3.7.3",
     "pkcs7-padding": "^0.1.1",
     "uint8arraylist": "^2.3.2",
diff --git a/src/crypto.ts b/src/crypto.ts
index 7a02439..e4bd455 100644
--- a/src/crypto.ts
+++ b/src/crypto.ts
@@ -35,6 +35,11 @@ export function getHKDF(ck: bytes32, ikm: Uint8Array): Hkdf {
   return [k1, k2, k3];
 }
 
+export function getHKDFRaw(ck: bytes32, ikm: Uint8Array, numBytes: number): Uint8Array {
+  const hkdf = new HKDF(SHA256, ikm, ck);
+  return hkdf.expand(numBytes);
+}
+
 export function generateX25519KeyPair(): KeyPair {
   const keypair = x25519.generateKeyPair();
 
diff --git a/src/handshake.ts b/src/handshake.ts
index 94998a2..b944e74 100644
--- a/src/handshake.ts
+++ b/src/handshake.ts
@@ -1,9 +1,10 @@
+import { BN } from "bn.js";
 import * as pkcs7 from "pkcs7-padding";
 import { equals as uint8ArrayEquals } from "uint8arrays/equals";
 
 import { bytes32 } from "./@types/basic";
 import { KeyPair } from "./@types/keypair";
-//import { getHKDF } from "./crypto";
+import { getHKDFRaw } from "./crypto";
 import { HandshakeState, NoisePaddingBlockSize } from "./handshake_state";
 import { CipherState } from "./noise";
 import { HandshakePattern, PayloadV2ProtocolIDs } from "./patterns";
@@ -158,11 +159,10 @@ export class Handshake {
 
   // Generates an 8 decimal digits authorization code using HKDF and the handshake state
   genAuthcode(): string {
-    //var output: array[1, array[8, byte]]
-    // const [output0] = getHKDF(this.hs.ss.h, new Uint8Array());
-    // let code = cast[uint64](output[0]) mod 100_000_000
-    // return $code
-    return "TODO: implement";
+    const output0 = getHKDFRaw(this.hs.ss.h, new Uint8Array(), 8);
+    const bn = new BN(output0);
+    const code = bn.mod(new BN(100_000_000));
+    return code.toString();
   }
 
   // Advances 1 step in handshake