wpa_passphrase

WPA-PSK uses pre-shared key as a passphrase of 8 to 63 printable ASCII characters or as a string of 64 hexadecimal digits.

If ASCII characters are used, the 256 bit key is calculated by applying the PBKDF2 key derivation function to the passphrase, using the SSID as the salt and 4096 iterations of HMAC-SHA1.

wpa_passphrase -- utility for generating a 256-bit pre-shared WPA key from an ASCII passphrase. You can download wpa_passphrase.tar.gz from here.

# tar xzvf wpa_passphrase.tar.gz
# make
# ./wpa_passphrase humble mypassword
network={
        ssid="humble"
        #psk="mypassword"
        psk=aa382e1c4ac62580c25ee2b33a1cf6179176baad4a5cffc43be9c8d2b103f4aa
}

AP can be configured with ASCII passphrase or HEX digits
Which enables the windows client to uses either of the above key

For home and small office networks its preferred to use WPA2-personal authentication method which also uses CCMP, AES based encryption.
Each wireless network device authenticates with the access point using the same 256-bit key.

Reference:
http://www.hautespot.net/wpapsk.html
http://fuse4bsd.creo.hu/localcgi/man-cgi.cgi?wpa_passphrase+8