ICSF uses the token validation value (TVV) to verify that a token is valid. The TVV prevents a key token that is not valid or that is overlaid from being accepted by ICSF. It provides a checksum to detect a corruption in the key token.

When an ICSF callable service generates a key token, it generates a TVV and stores the TVV in bytes 60-63 of the key token. When an application program passes a key token to a callable service, ICSF checks the TVV. To generate the TVV, ICSF performs a twos complement ADD operation (ignoring carries and overflow) on the key token, operating on four bytes at a time, starting with bytes 0-3 and ending with bytes 56-59.