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.
|CAP(3 )||Rev: Thu Feb 15 14:43:36 GMT 2007|