The CHECKSUM() is not cryptographically safe, however, if you use it as a one-way hash, it shoudl be unreversible, unless the crackers already have a complete list of the source values.

IE, CHECKSUM(fullname) should not be reversible, unless they already know what all of your "fullname" values could be, and in that context, there is no safe way to hash short text with limited possible values without a variable & secret key.

If you want more protection than CHECKSUM, then you might want to try HASHBYTES().