dir/caphash
dir/capuse

[ fromuser@ ] touser@key

where fromuser is a process's current user name, touser is its new user name, and key is a string of random characters (eg, produced by security-random(2)).

Caphash is a write-only file that can only be opened by the host owner. A process enables the use of a capability by writing the keyed hash of fromuser@touser to caphash. The hash is computed using Keyring->hmac_sha1 as follows:

kr := load Keyring Keyring->PATH;
IPint: import kr;
users := sys->sprint("%s@%s", fromuser, touser);
cap := sys->sprint("%s@%s", users, key);
digest := array[Keyring->SHA1dlen] of byte;
ausers := array of byte users;
kr->hmac_sha1(ausers, len ausers, array of byte key, digest, nil);
if(sys->write(caphashfd, digest, len digest) < 0)
	error();

The capability (eg, cap in the example) can then be passed to another process.

Capuse is a write-only file that can be opened by any process. It can then write a capability string to change its user name, provided that capability has previously been enabled by the host owner via caphash, and if the capability included a fromuser, the writing process currently has that user name. After a successful write, the writing process will be owned by touser. Any capability can be used at most once.

A capability enabled by caphash has a limited lifetime, on the order of 30 seconds. Caphash can be removed by the host owner to prevent its further use.