1. OFDM Based Wireless Encryption and
Decryption Message Transfer System for
Military Applications using BlowFish
Algorithm
What is OFDM
Short for Orthogonal Frequency Division Multiplexing, an FDM modulation
technique for transmitting large amounts of digital data over a radio wave. OFDM
works by splitting the radio signal into multiple smaller sub-signals that are then
transmitted simultaneously at different frequencies to the receiver. OFDM reduces
the amount of crosstalk in signal transmissions
Need:
The need for this project arises when we want the data communication to
be protected from others. This is highly essential in the military field and
especially during war times. Because during war periods when a particular
army wants to send a message to remotely located units, which belong to
the same army, in between any person can trap the data when it is
transmitted through wireless.
Our aim of the project is to protect the data from the ensnares.
To accomplish this, we have used a PC as a sender and PIC
microcontroller as a receiver station. The message to be sent is fed in to
the computer for different units. The computer will Encrypt the data using
generating a duplicate Character for a real one.
At the receiving end the PIC microcontroller then Decrypts the
Received data and displays the message on the display. The

2. communication takes place through a Wireless RF Transmitter and
Receiver operating at 433.92 MHz
we can also use CRO at the transmitting point to view the Encrypted data.
To ensure the security level at the receiving end we have a password
protection. Once the Message is received the unit will give an alarm
message so that one can know that the message has arrived. Then he
must enter the password through the keypad. If the password matches
then he is allowed to view the data.
BlowFish Encryption Algorithm
Blowfish is a symmetric block cipher that can be used as a drop-in replacement
for DES or IDEA. It takes a variable-length key, from 32 bits to 448 bits, making it
ideal for both domestic and exportable use. Blowfish was designed in 1993 by
Bruce Schneier as a fast, free alternative to existing encryption algorithms. Since
then it has been analyzed considerably, and it is slowly gaining acceptance as a
strong encryption algorithm. Blowfish is unpatented and license-free, and is
available free for all uses.

3. Blowfish has a 64-bit block size and a variable key length from 32 up to 448 bits
[1]. It is a 16-round Feistel cipher and uses large key-dependent S-boxes. It is
similar in structure to CAST-128, which uses fixed S-boxes.
The Feistel structure of Blowfish
The diagram to the left shows the action of Blowfish. Each line represents 32
bits. The algorithm keeps two subkey arrays: the 18-entry P-array and four 256-
entry S-boxes. The S-boxes accept 8-bit input and produce 32-bit output. One
entry of the P-array is used every round, and after the final round, each half of
the data block is XORed with one of the two remaining unused P-entries.
The diagram to the right shows Blowfish's F-function. The function splits the 32-
bit input into four eight-bit quarters, and uses the quarters as input to the S-
boxes. The outputs are added modulo 232 and XORed to produce the final 32-bit
output.

4. Since Blowfish is a Feistel network, it can be inverted simply by XORing P 17 and
P18 to the ciphertext block, then using the P-entries in reverse order.
Blowfish's key schedule starts by initializing the P-array and S-boxes with values
derived from the hexadecimal digits of pi, which contain no obvious pattern (see
nothing up my sleeve number). The
secret key is then XORed with the P-entries in order (cycling the key if
necessary). A 64-bit all-zero block is then encrypted with the algorithm as it
stands. The resultant ciphertext replaces P1 and P2. The ciphertext is then
encrypted again with the new subkeys, and P3 and P4 are replaced by the new
ciphertext. This continues, replacing the entire P-array and all the S-box entries.
In all, the Blowfish encryption algorithm will run 521 times to generate all the
subkeys - about 4KB of data is processed.

6. Block Diagram
RS232 RF Tx
COMMUNICATION Module1
FOR SERIEL DATA
RF Tx
Module2
PIC
RF Rx Module1 EMBEDDED
315 MHZ MC-16F877
RF Rx Module2
433.92 MHZ
dsPIC 30F4013
ADVANCED PIC
EMBEDDED EMBEDDED
Microcontroller MC-16F877
Alarm Visual
Output(M Alert(MESS
ESSAGE AGE)
4 X 3 Matrix Key Pad.
Password entry
+5v Power +5v Power
Supply Supply

7. Block Diagram
RS232 RF Tx
COMMUNICATION Module1
FOR SERIEL DATA
RF Tx
Module2
PIC
RF Rx Module1 EMBEDDED
315 MHZ MC-16F877
RF Rx Module2
433.92 MHZ
dsPIC 30F4013
ADVANCED PIC
EMBEDDED EMBEDDED
Microcontroller MC-16F877
Alarm Visual
Output(M Alert(MESS
ESSAGE AGE)
4 X 3 Matrix Key Pad.
Password entry
+5v Power +5v Power
Supply Supply

8. Block Diagram
RS232 RF Tx
COMMUNICATION Module1
FOR SERIEL DATA
RF Tx
Module2
PIC
RF Rx Module1 EMBEDDED
315 MHZ MC-16F877
RF Rx Module2
433.92 MHZ
dsPIC 30F4013
ADVANCED PIC
EMBEDDED EMBEDDED
Microcontroller MC-16F877
Alarm Visual
Output(M Alert(MESS
ESSAGE AGE)
4 X 3 Matrix Key Pad.
Password entry
+5v Power +5v Power
Supply Supply

9. Block Diagram
RS232 RF Tx
COMMUNICATION Module1
FOR SERIEL DATA
RF Tx
Module2
PIC
RF Rx Module1 EMBEDDED
315 MHZ MC-16F877
RF Rx Module2
433.92 MHZ
dsPIC 30F4013
ADVANCED PIC
EMBEDDED EMBEDDED
Microcontroller MC-16F877
Alarm Visual
Output(M Alert(MESS
ESSAGE AGE)
4 X 3 Matrix Key Pad.
Password entry
+5v Power +5v Power
Supply Supply