Add parseAssertionAuthData

2 files changed, 36 insertions, 0 deletions

diff --git a/src/assertion/parseAssertionAuthData.ts b/src/assertion/parseAssertionAuthData.ts
new file mode 100644
index 0000000..5b11059
--- /dev/null
+++ b/src/assertion/parseAssertionAuthData.ts
@@ -0,0 +1,28 @@
+import { ParsedAssertionAuthData } from "@types";
+
+/**
+ * Make sense of the authData buffer contained in an Assertion
+ */
+export default function parseAssertionAuthData(authData: Buffer): ParsedAssertionAuthData {
+  let intBuffer = authData;
+
+  const rpIdHash = intBuffer.slice(0, 32);
+  intBuffer = intBuffer.slice(32);
+
+  const flagsBuf = intBuffer.slice(0, 1);
+  intBuffer = intBuffer.slice(1);
+
+  const flags = flagsBuf[0];
+  const counterBuf = intBuffer.slice(0, 4);
+  intBuffer = intBuffer.slice(4);
+
+  const counter = counterBuf.readUInt32BE(0);
+
+  return {
+    rpIdHash,
+    flagsBuf,
+    flags,
+    counter,
+    counterBuf,
+  };
+}
diff --git a/src/types.ts b/src/types.ts
index 59e70c1..0442b31 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -102,3 +102,11 @@ export type SafetyNetJWTPayload = {
 };
 
 export type SafetyNetJWTSignature = string;
+
+export type ParsedAssertionAuthData = {
+  rpIdHash: Buffer,
+  flagsBuf: Buffer,
+  flags: number,
+  counter: number,
+  counterBuf: Buffer,
+};