added reference for SPKI encoding
diff --git a/src/crypto.rs b/src/crypto.rs
index 33e6577..ec73de8 100644
--- a/src/crypto.rs
+++ b/src/crypto.rs
@@ -123,7 +123,7 @@
/// # Calculating
/// A `KeyId` is calculated as `sha256(spki(pub_key_bytes))` where `spki` is a function that takes
/// any encoding for a public key an converts it into the `SubjectPublicKeyInfo` (SPKI) DER
-/// encoding.
+/// encoding as defined in [RFC 3280 Appendix A](https://www.ietf.org/rfc/rfc3280.txt).
///
/// Note: Historically the TUF spec says that a key's ID should be calculated with
/// `sha256(cjson(encoded(pub_key_bytes)))`, but since there could be multiple supported data
