cipher text idea ?
 

this is my code which copies 1st string into 2nd String.

        AREA        StrCopy1, CODE

        SWI_WriteC        EQU        &2

 

        ENTRY                ; mark the first instruction

main        

        ADR        r1, srcstr        ; pointer to first string        

        ADR        r0, dststr        ; pointer to second string

        BL                strcopy        ; copy the first into second

        SWI        0x11                ; and exit



srcstr        DCB " This is my first (source) string",&0a,&0d,0

dststr        DCB " This is my second (destination) string",&0a,&0d,0



        ALIGN                ; realign address to word boundary



strcopy

        LDRB        r2, [r1], #1        ; load byte, then update address

        STRB        r2, [r0], #1        ; store byte, then update address

        CMP        r2, #0                ; check for zero terminator

        BNE        printout                ; branch if not equal, to Printout

        



printOut

        SWINE        SWI_WriteC        ; Print the content

        B                        strcopy        ; branch to strcopy

        MOV                pc, lr        ; return

        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.

Re: cipher text idea ?
 

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.

Re: cipher text idea ?
 

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:

                AREA    text, CODE      



SWI_WriteC        EQU        &0



                ENTRY                   



                ADR        r4, hello       

                ADR        r3, alpha

                ADR        r2, key

loop            

               

                LDRB    r0, [r4], #1    

                CMP     r0, #0          

                BNE        check



check           

                LDRB        r1, [r3], #1

                CMP        r0, r1

                BEQ        encrypt

                       B        loop         

encrypt

                LDRB        r5, [r2], #1

                SUB        r0,r0,r5

                ADD        r5,r5,#1

                STRB        r0,[r5]

                SWINE        SWI_WriteC                

                B        loop        



endP



        MOV        pc, lr                        ; return

        END



hello        DCB     "Hello World ",0

alpha        DCB        "ABCDEFGHIJKLMNOPQRSTUVWXYZ",0

key        DCB        "XZYCJBTEKMGH",0