The following program is a CaesarCipher encryption algorithm written in MIPS assembly language

CaesarCipher.asm

.data
 prompt: .asciiz "Encrypt(E) or Decrypt(D) ?"
 indata: .space 20
 plaintext: .asciiz "Enter Plain text: "
 ciphertext: .asciiz "Enter Cipher text: "
 data: .space 40
 Key: .asciiz "Key ?"
 WrongKeyword: .asciiz "\nPlease enter a valid character"
.text
main:
start:
 # Encryption or Decryption
 la $a0,prompt
 li $v0,4
 syscall
 # read the input
 la $a0,indata
 la $a1,5
 li $v0,8
 syscall 
 lb $t2,0($a0)
 # Encryption 
 beq $t2,69,Plaintext
 # Decryption
 beq $t2,68,Ciphertext
 # Wrong Key word Enetred
 la $a0,WrongKeyword
 li $v0,4
 syscall
 # Terminated Program if a wrong key word is entered
 li $v0,10
 syscall
 
Plaintext:
 # Read Plain text value
 la $a0,plaintext
 li $v0,4
 syscall
 la $a0,data
 la $a1,40  		# Maximum 40 characters can be read ( This value can be changed as prefer)
 li $v0,8
 syscall
 la $t0,($a0) 		# Entered string is stored in the t0 register
 li $t1,0 		# string length
 # Read Key
 la $a0,Key
 li $v0,4
 syscall
 li $v0,5
 syscall
 move $t3,$v0 		# stores the key value
 
Encrypt:
 lb $t4, 0($t0)  	 	# The first character is read
 beq $t4,10,end 	 	# Terminate program on the \n 
 beqz $t4,end  			# Terminate Program when the end of the string is reached
 jal islower 
  			# Check if the character is lower case
Encrypt2:
 beq $v0,1,EncryptLower   	# EncryptLowercase characters
 beq $v0,0,EncryptUpper   	# EncryptUppercase characters
 move $a0, $t4   		# if the character is not upper or lower

PrintEncryptChar:
 li $v0,11 			# print the encrypted character
 syscall
 add $t0,$t0,1 			# Points to the next character
 add $t1,$t1,1 			# length is incremented
 j Encrypt
 
Ciphertext:
 # Read Cipher text value
 la $a0,ciphertext
 li $v0,4
 syscall
 la $a0,data
 la $a1,40
 li $v0,8
 syscall
 la $t0,($a0) 		# Entered string is stored in the t0 register
 li $t1,0	 	# string length
 # Read Key
 la $a0,Key
 li $v0,4
 syscall
 li $v0,5
 syscall
 move $t3,$v0 		# stores the key value
 
Decrypt:
 lb $t4, 0($t0)  	# First character is read
 beq $t4,10,end  	# Terminate program when \n is pressed
 beqz $t4,end   	# Terminate program when the end of the string is reached 
 jal Decryptislower  	# Check for lower case characters

Decrypt2:
 beq $v0,1,DecryptLower 	# Decrypt lower case characters
 beq $v0,0,DecryptUpper 	# Decrypt Upper case characters
 move $a0, $t4  		# If not upper case or lower
 
PrintDecryptChar:
 li $v0,11 			# Print the Decrypted character
 syscall
 add $t0,$t0,1 			# Points to the next character
 add $t1,$t1,1 			# length is incremented
 j Decrypt
 
end:
 li $v0,10
 syscall
 
# For the use of Encryption

islower:
 bgt $t4,122,NotlowerOrupper	 	# if the character is not lower case or upper case
 blt $t4,97,IsupperORanyother 		# if the character is not lower case 
 li $v0,1   				# store value 1 in register V0 if the character is a lower case character
 jr $ra    	
 			# return back to the return address
NotlowerOrupper:
 li $v0,2   				# store value 2 in register VO if the character is not lower or upper
 j Encrypt2  
  				# jump back
IsupperORanyother:
 blt $t4,65,NotlowerOrupper		# if the character is not upper or lower
 bgt $t4,91,NotlowerOrupper 		# if character is not upper or lower
 li $v0,0   				# store value 0 in register V0 if the character is upper case
 j Encrypt2
 
EncryptLower:
 li $t5,26   				# Encrypt lower case characters
 sub $t4,$t4,97
 add $t4, $t4, $t3
 div $t4,$t5
 mfhi $a0
 addi $a0,$a0,97
 j PrintEncryptChar
 
EncryptUpper:
 li $t5,26   				# Encrypt Upper case characters
 sub $t4,$t4,65
 add $t4, $t4, $t3
 div $t4,$t5
 mfhi $a0
 addi $a0,$a0,65
 j PrintEncryptChar
 
#For the use of Decryption
Decryptislower:
 bgt $t4,122,Decrypt_NotlowerOrupper 	# if the character is not upper or lower case
 blt $t4,97,Decrypt_IsupperORanyother 	# if the character is not lower case
 li $v0,1    				# store value 1 in register V0 if the decrypt character is lower case
 jr $ra
 
Decrypt_NotlowerOrupper:
 li $v0,2   				# store calue 2 in register V0 if the decrypt character is not upper or lower case
 j Decrypt2
 
Decrypt_IsupperORanyother:
 blt $t4,65,NotlowerOrupper 		# if the character is not upper or lower case
 bgt $t4,91,NotlowerOrupper 		# if the character id not upper or lower case
 li $v0,0   				# store value 0 in the v0 register if the character is upper case
 j Decrypt2
 
DecryptLower:
 li $t5,26   				# Decrypt lower case characters
 sub $t4,$t4,97
 sub $t4, $t4, $t3
 sub $t6,$zero,$t4
 li $t7,1
 
 ModFornegative:
 mul $s0,$t5,$t7
 bgt $s0,$t6,ModCal
 addi $t7,$t7,1
 j ModFornegative
 
 ModCal:
 add $t4,$s0,$t4
 div $t4,$t5
 mfhi $a0
 addi $a0,$a0,97
 j PrintDecryptChar
 
DecryptUpper:
 li $t5,26   				# Decrypt Upper caes characters
 sub $t4,$t4,65
 sub $t4, $t4, $t3
 sub $t6,$zero,$t4
 li $t7,1
 
 ModFornegativeUpper:
 mul $s0,$t5,$t7
 bgt $s0,$t6,ModCalUpper
 addi $t7,$t7,1
 j ModFornegativeUpper
 
 ModCalUpper:
 add $t4,$s0,$t4
 div $t4,$t5
 mfhi $a0
 addi $a0,$a0,65
 j PrintDecryptChar