User's Manual

16
The authentication data are embedded into a bit sequence which provides a special header and a special footer:
Size Description
Header
4 bit Sequence for recognizing the transmission start and for identifying the device type in connection
and the protocol in use. For the WebOTP protocol the sequence is always 0001.
Data
20*8 bit Data to transmit for authentication. The single bytes are sent starting from the least meaningful
bit.
Footer
2 bit 2 parity bits used for always making the number of key pressing sequence even.
In the recording process it is advisable not to confine oneselves to recording the character sequence though, but to carry
out some special operations for minimizing any error possibility due to interferences from the user's side during
keyboard management.
The algorithm must behave like this:
•
On reception of the fourth bit, if the string does not correspond to 0001 delete the first bit and keep only the
last 3 bits. The sequence always begins with '0001'.
•
If in 200 milliseconds no character is received, consider the sequence as interrupted and restart from the
reception process again.
• After receiving 166 binary codes the transmission is to be deemed concluded.
When reception is completed the received string can be interpreted; if necessary, check the header/footer fields before
using it for authentication.
An example of implementation in J
AVA
S
CRIPT
is to be found in chapter Integration.
6.4.2 WebOTP Alpha communication
Alpha communication has been designed for use in those already-existing contexts where an alphabetical password
input is required.
The transmission is based on the simulation of sixteen special letter codes, where each code is assigned the
representation of a bit group.
The characters in use are:
Character Meaning
B
0000
D
0001
E
0010
F
0011
G
0100
H
0101
I
0110
J
0111
K
1000
L
1001
N
1010
R
1011
T
1100
U
1101
V
1110
Q, A
1111
The letters chosen correspond to keys which have the same position both on the QWERTY, QWERTZ and AZERTY
keyboards, with the only exception of the ‘Q’ letter, which on the AZERTY keyboards will be interpreted as ‘A’. This
choice is fundamental for preventing the generated string from depending on the operating system keyboard
international configuration.
Besides, the characters can be received both in capital letters and in small letters, according to the CAPSLOCK and
SHIFT key status.