Add HMAC to glossary

Added a new glossary entry for HMAC, since the abbreviation is used in another entry.

glossary.asciidoc

@@ -221,6 +221,10 @@ hardware wallet::
 hash::
     A digital fingerprint of some binary input.
 
+hash-based message authentication code (HMAC)::
+    An HMAC is a message authentication code method for verifying the integrity and authenticity of a message based on a hash function and a cryptographic key.
+    It is used in onion routing to ensure the integrity of a packet at each hop, as well as within the Norse protocol variant used for message encryption.
+
 hash function::
     A cryptographic hash function is a mathematical algorithm that maps data of arbitrary size to a bit string of a fixed size (a hash) and is designed to be a one-way function, that is, a function which is infeasible to invert.
     The only way to recreate the input data from an ideal cryptographic hash function's output is to attempt a brute-force search of possible inputs to see if they produce a match, or use a rainbow table of matched hashes.