Compaq WL110 User Manual page 98

These keys can be used to smooth the transition from the usage of one key to
usage of a next key. The general requirement for two cards to transmit encrypted
between each other is that they share a common key value at the same key-index
number in the 4-key area at the moment of transmission. The key-index of the
key that was used for encryption is transmitted in clear-text in the header of the
message, and will be used at the receiving side to determine which of the 4 keys
to use for decryption.
It is not mandatory that both sides (typically Access Point and 11Mbps Wireless
LAN station) have the same active set of 4 keys. As long as there is one key in
common, they can communicate, provided they both use that common key.
Note: The 802.11 standard also defines the possibility for having a unique key
per Station, tied to the station's MAC Address. 11Mbps Wireless LAN
currently does not support that feature of the standard WEP function.
When planning the usage of different keys over time a number of aspects have to
be considered:
the length of time one key stays in use;
this is a direct trade-off between security level (= the chance of someone
finding out what the key value is) and operational overhead (= the efforts to
reconfigure Access Point and stations)
the requirements for smooth transition from one key to another
the minimization of end user exposure to key values
The key roll-over possibilities built in the 802.11 standard and offered by
11Mbps Wireless LAN allow for a number of scenarios, each with different
values for the above aspects.
The sequence of key configuration settings at Access Point (shown as
AP=Access Point) and Station (shown as STA) over time is shown in a number
of tables below. Each table reflects a certain key roll-over strategy. Notice that
the column "Outward Key" shows which key is used to encrypt traffic from AP
to STA and the column "Inward Key(s)" indicates the key(s) that are allowed
and possibly used to encrypt traffic from STA to AP. The WEP Keys that are
configured are shown in order of index number 1-2-3-4; the column "Tx" is the
index number configured for transmission. The key values are shown by capital
letters to indicate a real key or by zero to indicate a non-configured index.
The column "Keys 1-2-3-4" shows an equal sign (=) when the value does not
change from the previous period. This is particularly relevant when it concerns
the stations keys, since it is envisaged that knowledge of the key values is
typically not transferred to the end users, so they have to return their station
equipment to an IT department to get the key values changed. It is envisaged that
changing the Txkey Index is an action that can be done by end users, since it
does not reveal secret information.
Three key roll-over strategies are distinguished:
Single Key – No Transition (page 7-17),
Single Key – Transition Period (page 7-17), and
Alternative Schemes (page 7-18).
Security
7-16