To set WEP keys, use "enc" or "key", and no "-" in front of "eth0".
Here is the relevant snippet from "man iwconfig":
Used to manipulate encryption or scrambling keys and security mode.
To set the current encryption key, just enter the key in hex digits as XXXX-XXXX-XXXX-XXXX or
XXXXXXXX. To set a key other than the current key, prepend or append [index] to the key itself
(this won't change which is the active key). You can also enter the key as an ASCII string by
using the s: prefix. Passphrase is currently not supported.
To change which key is the current active key, just enter [index] (without entering any key
off and on disable and reenable encryption.
The security mode may be open or restricted, and its meaning depend on the card used. With most
card, in open mode no authentication is used and the card may also accept non-encrypted ses-
sions, whereas in restricted mode only encrypted sessions are accepted and the card will use
authentication if available.
If you need to set multiple keys, or set a key and change the active key, you need to use multi-
ple key directives. Arguments can be put in any order, the last one will take precendence.
iwconfig eth0 key 0123-4567-89
iwconfig eth0 key s
iwconfig eth0 key  open
iwconfig eth0 key off
iwconfig eth0 key restricted  0123456789
iwconfig eth0 key 01-23 key 45-67  key 
This should help you figure it out. BTW, which of the iwconfig params really
works is entirely dependent on the driver (and the card).