Во, не поленился посмотрел спецификацию 802.11i-2004.
http://standards.ieee.org/getieee802/do ... i-2004.pdf
165 страница
Вот цитаты:
The RSNA PSK consists of 256 bits, or 64 octets when represented in hex. It is difficult for a user to cor-
rectly enter 64 hex characters.
Most users, however, are familiar with passwords and pass-phrases and feel
more comfortable entering them than entering keys. A user is more likely to be able to enter an ASCII pass-
word or pass-phrase, even though doing so limits the set of possible keys. This suggests that the best that can
be done is to introduce a pass-phrase to PSK mapping.
<...>
The pass-phrase mapping defined in this subclause uses the PBKDF2 method from PKCS [B16].
PSK = PBKDF2(PassPhrase, ssid, ssidLength, 4096, 256)
Here, the following assumptions apply:
— A pass-phrase is a sequence of between 8 and 63 ASCII-encoded characters. The limit of 63 comes
from the desire to distinguish between a pass-phrase and a PSK displayed as 64 hexadecimal
characters.
—
Each character in the pass-phrase must have an encoding in the range of 32 to 126 (decimal),
inclusive.
— ssid is the SSID of the ESS or IBSS where this pass-phrase is in use, encoded as an octet string used
in the Beacon and Probe Response frames for the ESS or IBSS.
— ssidLength is the number of octets of the ssid.
— 4096 is the number of times the pass-phrase is hashed.
— 256 is the number of bits output by the pass-phrase mapping.
Если с английским плохо - суть в том что в самой процедуре обмена используется PSK, который содержит 256 бит и при записи в hex содержал бы 64 символа. Но так как пользователям сложновато запоминать такие сложные символы используется pass-phrase, которая с помощью хеш функции (PBKDF2) принимающий в виде соли SSID и ещё несколько параметров формирует PSK.
В итоге нигде в спецификации не указано, что pass-phrase не может начинаться с цифры и/или содержать только с цифры. (символы цифр входят в ASCII диапозон 48-57).