Where to get security fixings?

[Skiny]

I am fitting some steel bars to some windows and need some security fixings to hold them in place. Obviously I don't want to use regular fixings, because I don't want a thief to easily remove them. However, at the same time, I want the option to remove them in the future, without having to destroy the brickwork as a result.

I thought to use a mixture of 2 or 3 different security heads on whatever screws/bolts. Does anyone know of any that would be suitable?

The mounting holes in the steel frame are around 13m-14mm in diameter. I'd like them to have a minimum length of about 50mm.

One way bolts would be the obvious choice, but as I say, I want to have the option to be able to remove them in the future.

 

[ntb]

http://www.toolstation.com/shop/p58410?searchstr=shear nut

Grind or file flats on them to remove.

 

[Skiny]

I need them to be removable. One way fixings are all that I can find so far.

 

[SFK]

How about these two different types of security bolts:
http://www.alcomet.net/substation-security/alcomet-securitynutsandbolts
Note that I did a google search for "security head bolt m12" and it came up with a few others.

Or security nuts do exist when I found this USA company:
http://www.brycefastener.com/?gclid=CKS08L2HvswCFRDhGwodAucJlw

 

[SFK]

Or a lot cheaper a masonry screw with a Torx head (not security but would get me rummaging around bottom toolbox for a long while looking for the correct screw bit).

http://www.toolstation.com/shop/p12...23&pkw=&pmt=&gclid=CIPc_Y2IvswCFcG6Gwod6DwPRQ