The security seems rather redundant - isn't almost all cryptography related to security?
The problem of this tag is that it is used quite unspecific. In the wider sense, almost everything on this site relates to security, thus it is useless here.
In a less wide sense, one often speaks of a $n$-bit security of some primitive or protocol, meaning that an attacker needs at least $2^n$ steps of work to break it. It could be useful this way (and in the past, I left out these usages when removing the tag from other questions), but it would need a good tag wiki for this, as well as some discipline (or regular supervision) to make sure it is not used when not appropriate.
Could we find a better tag name for this second (maybe valid) usecase, so we can ban the general one?
This tag looks useless to me. I've started to detag a few of the questions which have this tag.