Ignacio Vazquez-Abrams's answer is correctIgnacio Vazquez-Abrams's answer is correct: files under /proc and /sys are provided by the kernel. If you want to add a file there, write a kernel module. You can find a slightly dated presentation of the /proc programming interface in Linux Device Drivers 3rd ed. ch. 4.
Technically, it is possible to jump through hoops to make a file appear in /proc/sys/crypto (or anywhere else, really). Make a bind mount from /proc/sys/crypto to a staging directory, and make a union mount of the staging directory and an overlay directory back onto /proc/sys/crypto. Here's an example using unionfs-fuse.
# mkdir /tmp/original /tmp/overlay # mount --bind /proc/sys/crypto /tmp/original # unionfs-fuse -o nonempty /tmp/overlay=RW:/tmp/original=RO /proc/sys/crypto # echo hello >/proc/sys/crypto/test # cat /proc/sys/crypto/test hello # umount /proc/sys/crypto # umount /proc/sys/crypto # cat /tmp/overlay/test hello Note: I disclaim any responsibility for system or brain damage caused by experimenting with this stuff. The commands above are perfectly safe, but messing with other areas of /sys and /proc can cause weird behavior.
