É uma chave hexadecimal ou uma frase secreta?
Ao usar uma frase secreta, então, de acordo com Escolhendo uma senha para redes que usam Wired Equivalent Privacy (WEP) :
Choosing and using a WEP password can be a challenge, because WEP is an older security protocol that was developed before members of the Wi-Fi Alliance had agreed on a uniform way to treat passwords. The result is that a WEP password may not work for all computers in a multiplatform environment.
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For a 40 bit WEP network, always choose a 5-character password.
For a 128 bit WEP network, always choose a 13-character password.
[..]
Products that allow plain language (ASCII) password entry may not force the network administrator to create 5- or 13-character passwords that would result in the required 40 (5*8) and 104 (13*8) bit-length passwords. Instead, they use a "hashing" routine to convert an odd-length password to the correct bit length. The multiplatform issue arises when the hashing results of products from two different manufacturers don't match each other. The resulting "network equivalent passwords," which are always given in hexadecimal format, are different.
Eu não sei o que usar para uma rede de 64 bits, mas talvez você possa usar uma chave hexadecimal?
(E, como você provavelmente sabe: o WEP é antigo e inseguro. Use o WPA2, se possível.)