A few notes about how DES is used in the VNC Authentication security type (type 2)
- DES is used in ECB mode.
-
The ECB Key is based upon an ASCII password. The key must be 8 bytes long. The password is either truncated to 8 bytes, or else zeros are added to the end to bring it up to 8 bytes. As an additional twist, each byte in flipped. So, if the ASCII password was "pword"
[0x 70 77 6F 72 64], the resulting key would be[0x 0E EE F6 4E 26 00 00 00]. - The VNC Authentication scheme sends a 16 byte challenge. This challenge should be encrypted with the key that was just described, but DES in ECB mode can only encrypt an 8 byte message. So, the challenge is split into two messages, encrypted separately, and then jammed back together.
password_to_key(Password) ->
Flipped = lists:map(fun flip_byte/1, Password),
Truncated = truncate(Flipped, 8),
pad(Truncated, 8, $\0).
encrypt_challenge(Password, Challenge) ->
Key = password_to_key(Password),
<<High:8/binary, Low:8/binary>> = Challenge,
EncHigh = crypto:des_ecb_encrypt(Key, High),
EncLow = crypto:des_ecb_encrypt(Key, Low),
<<EncHigh/binary, EncLow/binary>>.
3 comments:
I can't speak for other VNC servers, but at least the UltraVNC server (see http://www.uvnc.com/) uses "CBC" and NOT "ECB" ( see http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation#Electronic_codebook_.28ECB.29 for details) mode without padding.
Turns out I wasn't entirely right. The above holds true for UltraVNCs MS-Logon. The ordinary "VNC password" login however uses ECB without an IV (of course).
Pseudo code VNC Authetication:
Read 16byte => $challenge
$A = encrypt( first8bytes( $challenge) , bitflipped( $userpassword), DES/ECB/NoPadding)
$B = encrypt( second8bytes( $challenge), bitflipped( $userpassword), DES/ECB/NoPadding)
send concat( $A, $B)
Pseudo code MS-Logon Authetication:
$key = DiffieHellman( Server).pubKey
$user = encrypt( pad( $username, 256), bitflipped( $key), DES/CBC/NoPadding)
$pwd = encrypt( pad( $userpassword, 64), bitflipped( $key), DES/CBC/NoPadding)
send $user
senr $pwd
You can also mirrored hard secret key of VNC:
vnc_key = [23, 82, 107, 6, 35, 78, 88, 7] in code
vnc_key_for_classic_DES = [232, 74, 214, 96, 196, 114, 26, 224]
If you use common alghorithm of DES encryption (ECB), you should use vnc_key_for_classic_DES without password changing.
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