Definition of Short Authentication String
The Short Authentication String (SAS) is used in various protocols, but in short, SAS is produced to transfer a long authentication tag into a smaller so that the two users can verbally verify. For example;

In Diffie-Hellman key Exchange, there is always a risk of Man-in-the-Middle attack. To ensure that there is no attacker in the first session, that there are no shared secrets yet, a SAS can be used. After verbal communication, if the values don't match then Man-in-the-Middle attack is detected.

Some Protocols that uses SAS;


ZRTP: Media Path Key Agreement for Unicast Secure RTP
Short Authentication Strings for TLS
Device Pairing Using Short Authentication Strings