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cap - capability for changing user name


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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)

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.




keyring-sha1(2), cons(3), intro(5), eve(10.2)


A write to capuse without a previous write to caphash sets the error string to ``invalid capability''.

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