An ASCII string agreed upon by two systems for establishing a security association. Has limit of 128 characters. Pre-shared keys allow users to share a secret key (a string of ASCII characters in this implementation) for the purpose of authentication; i.e., by using the pre-shared key, a principal's identify may be checked. When it is time to establish a connection to a remote site, the pre-shared key is used for encrypting a random piece of data; at the remote site, the remote key for the originator is used, since it is shared, to decrypt the data. A match in the data after decryption means successful authentication.