I wonder why there is a limit of just 63 characters for the passphrase of WPA2-PSK. It's not even a power of two and looks very unusual to me, but surely there's some deeper meaning to this number.
The PSK is derived from the passphrase using PBKDF2 key derivation function with SHA1 as the pseudo random function. The passphrase is an 8-63 character ASCII encoded string.
PSK = PBKDF2(PassPhrase, ssid, ssidLength, 4096, 256)
The PSK is 32 bytes (256 bits), often displayed as 64 hex characters.
According to the 802.11i specification:
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.
So the difference is just to distinguish a 64 hex character PSK from a 8-63 character ASCII passhprase.