HI Ben,
As per my knowledge this Software dont support 256 bit key size.
Here is the link:http://www.cisco.com/c/en/us/td/docs/wireless/access_point/12-4_3g_JA/configuration/guide/ios1243gjaconfigguide/s43wep.html
another version: 12.3(8)JA supports 256 bit key size to protect data traffic,
http://www.cisco.com/c/en/us/td/docs/wireless/access_point/12-3_8_JA/configuration/guide/1238jasc/s38wep.html
Regards
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