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cipher text idea ?

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cipher text idea ?

 
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  #1
May 4th, 2009
this is my code which copies 1st string into 2nd String.

Assembly Syntax (Toggle Plain Text) 
  1. 	AREA	StrCopy1, CODE
  2. 	SWI_WriteC	EQU	&2
  3.  
  4. 	ENTRY		; mark the first instruction
  5. main	
  6. 	ADR	r1, srcstr	; pointer to first string	
  7. 	ADR	r0, dststr	; pointer to second string
  8. 	BL		strcopy	; copy the first into second
  9. 	SWI	0x11		; and exit
  10.  
  11. srcstr	DCB " This is my first (source) string",&0a,&0d,0
  12. dststr	DCB " This is my second (destination) string",&0a,&0d,0
  13.  
  14. 	ALIGN		; realign address to word boundary
  15.  
  16. strcopy
  17. 	LDRB	r2, [r1], #1	; load byte, then update address
  18. 	STRB	r2, [r0], #1	; store byte, then update address
  19. 	CMP	r2, #0		; check for zero terminator
  20. 	BNE	printout		; branch if not equal, to Printout
  21.  
  22.  
  23. printOut
  24. 	SWINE	SWI_WriteC	; Print the content
  25. 	B			strcopy	; branch to strcopy
  26. 	MOV		pc, lr	; return
  27. 	END
*******
Now i want to modify the above code so that it encrypts the dststr using Caesar cipher method and which of course will be printed out on the screen aswell.

Thanks for ur reply and i hope it makes the point clear if not feel free to ask me.

Br.
Last edited by j_cart007; May 4th, 2009 at 3:57 pm.
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Re: cipher text idea ?

 
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  #2
May 5th, 2009
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Now i want to modify the above code so that it encrypts the dststr using Caesar cipher method and which of course will be printed out on the screen aswell.

Thanks for ur reply and i hope it makes the point clear if not feel free to ask me.

Br.
A caesar encryption is just a replacement encryption.
Couple of ways to go about this but I only have time to explain two
One is the simpliest you have two strings
ABCDEFGHI
WLPQSRNXZ
You take each char from the source string and find it's equiv in the top string and use that index to find it's equiv in the encryption key there at the bottom.

An faster way would be to setup your encryption key as the previous one for the first 9 characters...
WLPQSRNXZ

Now I find a character in my source string the first character happens to be an 'A' I check a mask to see if it's upper case or lower case and then depending on that outcome I subtract 'A' from it for upper case characters and 'a' from it for lower case characters.
What's this give me? An offset sir 'A' - 'A' is 0 and a 0 offset into our above string is 'W'
'B' - 'A' is 1 so a 1 offset.
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Re: cipher text idea ?

 
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  #3
May 5th, 2009
thankx for ur reply sir !
whatever i've got so far is in the code if u can pls have a look and correct me:

Assembly Syntax (Toggle Plain Text) 
  1.                 AREA    text, CODE      
  2.  
  3. SWI_WriteC	EQU	&0
  4.  
  5.                 ENTRY                   
  6.  
  7.                 ADR	r4, hello       
  8. 		ADR	r3, alpha
  9. 		ADR	r2, key
  10. loop            
  11.  
  12.                 LDRB    r0, [r4], #1    
  13. 		CMP     r0, #0          
  14. 		BNE	check
  15.  
  16. check           
  17. 		LDRB	r1, [r3], #1
  18. 		CMP	r0, r1
  19. 		BEQ	encrypt
  20.        		B	loop         
  21. encrypt
  22.                 LDRB	r5, [r2], #1
  23. 		SUB	r0,r0,r5
  24. 		ADD	r5,r5,#1
  25. 		STRB	r0,[r5]
  26. 		SWINE	SWI_WriteC		
  27. 		B	loop	
  28.  
  29. endP
  30.  
  31. 	MOV	pc, lr			; return
  32. 	END
  33.  
  34. hello	DCB     "Hello World ",0
  35. alpha	DCB	"ABCDEFGHIJKLMNOPQRSTUVWXYZ",0
  36. key	DCB	"XZYCJBTEKMGH",0
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