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-rw-r--r--src/or/onion_tap.c12
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diff --git a/src/or/onion_tap.c b/src/or/onion_tap.c
index abe779351f..2769300945 100644
--- a/src/or/onion_tap.c
+++ b/src/or/onion_tap.c
@@ -9,10 +9,22 @@
* \brief Functions to implement the original Tor circuit extension handshake
* (a.k.a TAP).
*
+ * The "TAP" handshake is the first one that was widely used in Tor: It
+ * combines RSA1024-OAEP and AES128-CTR to perform a hybrid encryption over
+ * the first message DH1024 key exchange. (The RSA-encrypted part of the
+ * encryption is authenticated; the AES-encrypted part isn't. This was
+ * not a smart choice.)
+ *
* We didn't call it "TAP" ourselves -- Ian Goldberg named it in "On the
* Security of the Tor Authentication Protocol". (Spoiler: it's secure, but
* its security is kind of fragile and implementation dependent. Never modify
* this implementation without reading and understanding that paper at least.)
+ *
+ * We have deprecated TAP since the ntor handshake came into general use. It
+ * is still used for hidden service IP and RP connections, however.
+ *
+ * This handshake, like the other circuit-extension handshakes, is
+ * invoked from onion.c.
**/
#include "or.h"