CAP(3) CAP(3) NAME cap - capability for changing user name SYNOPSIS bind #¤ dir dir/caphash dir/capuse DESCRIPTION Cap allows a process owned by the host owner (see eve(10.2)) to give another process on the same machine a capability to set its user name to a specified user. The capability is a string of the form: [ 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 pro- cess. 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 CAP(3) CAP(3) owner to prevent its further use. SOURCE /emu/port/devcap.c /os/port/devcap.c SEE ALSO keyring-sha1(2), cons(3), intro(5), eve(10.2) DIAGNOSTICS A write to capuse without a previous write to caphash sets the error string to ``invalid capability''.