To compile the above programs at prompt use.
On Lab debian VM
$cc -o caesar caesar.c

On Ubuntu 10.04 or above
$gcc -o caesar caesar.c

On Mac OS X Mountain Lion
$gcc -o caesar caesar.c 

------------------------
Usage for Caesar Cipher.
------------------------

Program accepts three arguments. 
The first one is the plain text
the second one is the Key
the third one is the mode.

'e' runs the algorithm in encrpyt mode
'd' runs the algorithm in decrypt mode

For the above program, The key is specified as a letter. If the key is 'A' There is no change even after encryption as the shift of the letters will be 0

Plaintext that is longer than a word has to be enclosed in qoutes to escape spaces. 
Only alpanumeric characters are encryped using caesars cipher. all non-alphanumeric characters are left in the ciphertext as they are in the plain text(including spaces)

For example: 
# ./caesar HELLO A e
Mode: e
Input: HELLO
Output : HELLO

# ./caesar "THIS IS A REALLY LONG STRING BUT NOT THAT LONG" B e
Mode: e
Input: THIS IS A REALLY LONG STRING BUT NOT THAT LONG
Output : UIJT JT B SFBMMZ MPOH TUSJOH CVU OPU UIBU MPOH

To decrypt
# ./caesar HELLO A d
Mode: d
Input: HELLO
Output : HELLO

# ./caesar "UIJT JT B SFBMMZ MPOH TUSJOH CVU OPU UIBU MPOH" B d
Mode: d
Input: UIJT JT B SFBMMZ MPOH TUSJOH CVU OPU UIBU MPOH
Output : THIS IS A REALLY LONG STRING BUT NOT THAT LONG


--------------------------
Usage for Vigenere Cipher.
--------------------------

Program accepts three arguments. 
The first one is the plain text
and the second one is the Key.
the third one is the mode.

'e' runs the algorithm in encrpyt mode
'd' runs the algorithm in decrypt mode

For the above program, The key is specified as a letter. If the key is 'A' There is no change even after encryption as the shift of the letters will be 0

Plaintext that is longer than a word has to be enclosed in qoutes to escape spaces. 
Only alpanumeric characters are encryped using vigenere cipher. all non-alphanumeric characters are left in the ciphertext as they are in the plain text(including spaces)

The Key for this implementation cannot contanin spaces.
For example: 

# ./vigenere HELLO ABC e
Mode: e
Input: HELLO
Output : HFNLP

# ./vigenere "THIS IS A REALLY LONG STRING" ABC e
Mode: e
Input: THIS IS A REALLY LONG STRING
Output : TIKS KS C SGAMNY NOOI TVRJPG

# ./vigenere "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG" STUVWXYZ e
Mode: e
Input: THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
Output : LAY MRGBC VMKTL XHR FRKOK IQAO SZX GWWW VHA

To Decrypt
# ./vigenere HFNLP ABC d
Mode: d
Input: HFNLP
Output : HELLO

# ./vigenere "TIKS KS C SGAMNY NOOI TVRJPG" ABC d
Mode: d
Input: TIKS KS C SGAMNY NOOI TVRJPG
Output : THIS IS A REALLY LONG STRING

# ./vigenere "LAY MRGBC VMKTL XHR FRKOK IQAO SZX GWWW VHA" STUVWXYZ d
Mode: d
Input: LAY MRGBC VMKTL XHR FRKOK IQAO SZX GWWW VHA
Output : THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG