initial commit

internal/auth/hmac.go

@@ -0,0 +1,20 @@
+package auth
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+)
+
+// sha256HMAC computes an HMAC-SHA256 tag.
+//
+// It is a thin wrapper so that the session store and the OIDC state store
+// cannot accidentally diverge on how they sign. The stdlib hmac package already
+// performs a constant-time internal comparison; the constant-time requirement
+// in this codebase is about how *we* compare the resulting tags.
+func sha256HMAC(key, body []byte) [32]byte {
+	mac := hmac.New(sha256.New, key)
+	mac.Write(body)
+	var out [32]byte
+	copy(out[:], mac.Sum(nil))
+	return out
+}