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Implement a transposition cipher using ARM assembly assignment help

The assignment deals with implementing a cipher based on a transposition scheme using ARM assembly. The program creates a lookup table based on the private key and encrypts or decrypts the message by reading either columnwise or row by row the characters in the lookup table using the input message. The program reads the input message from the standard input and prints the resulting message on the standard output. Here is the solution explained by our ARM assembly assignment helpers.

Cypher Transposition in ARM Assembly 

Implement a transposition cipher using ARM assembly assignment help

Solution @--------------------------------------------------- @ File: cw1.s @ Author: ????????? @ Date: November 12, 2018 @ @ Program to encrypt and decrypt a message from @ the standard input, the output is given in the @ standard output @--------------------------------------------------- .data argerr: .asciz "Error: invalid number of arguments\n" string: .space 1000 @ space to save input string indices: .space 200 @ space to save sorted key indices decstr: .space 1000 @ space to save decrypted string .text .globl main @--------------------------------------------------- @ main procedure @--------------------------------------------------- main: push {lr} @ save LR on the stack ldr r4, [sp, #8] @ get the address of the command line argument ldr r0, [sp, #12] @ get the number of command line arguments cmp r0, #3 @ if the number of arguments is not 3, exit bne error ldr r0, [r4, #8] @ get the private key length bl strlen mov r5, r0 @ save length in r5 ldr r1, =indices @ save indices for the key chars mov r2, #0 @ start in index 0 indloop: str r2, [r1], #4 @ save index in array and post increment add r2, r2, #1 @ increment index cmp r2, r0 @ repeat until we reach the key length blt indloop ldr r0, [r4, #8] @ get the private key string pointer mov r1, r5 @ get string length ldr r2, =indices @ get start address of indices bl sort @ sort the key indices using bubble sort ldr r0, [r4, #4] @ get the encrypt/decrypt option ldrb r6, [r0] @ get the first char, should be 0 or 1 ldr r0, =string @ read the text input from stdin into string buffer mov r1, r5 @ pass key length as row size bl readstr cmp r6, #'0' @ if the first command line option is not '0', decrypt bne dodecrypt doencrypt: mov r1, r0 @ pass number of rows in r1 ldr r0, =string @ pass string to encrypt in r0 ldr r2, =indices @ pass indices table mov r3, r5 @ pass number of indices in r3 bl encrypt @ encrypt string b endprog dodecrypt: mov r1, r0 @ pass number of rows in r1 ldr r0, =string @ pass string to encrypt in r0 ldr r2, =indices @ pass indices table mov r3, r5 @ pass number of indices in r3 bl decrypt @ decrypt string endprog: mov r0,#0 @ return code, no errors b return error: ldr r0,=argerr @ print the error message bl printf mov r0,#1 @ return code to indicate error return: pop {lr} @ restore lr from the stack mov pc,lr @ return to calling function @--------------------------------------------------- @ strlen: get the length of a string @ @ Arguments: r0 = address of string @ Returns: r0 = string length @--------------------------------------------------- strlen: mov r2, #0 @ r2 will count chars, start in zero cntloop: ldrb r1, [r0], #1 @ load a char from the string, increment pointer cmp r1, #0 @ see if we reached the end of the string beq endcnt @ if so, end counting add r2, r2, #1 @ else, increment string length b cntloop @ repeat endcnt: mov r0, r2 @ return the char count mov pc, lr @ return to calling function @--------------------------------------------------- @ reader: reads a string from the standard input, @ strips whitespace and punctuations, changes upper @ to lowercase chars and adds extra characters @ depending on the row size given @ @ Arguments: r0 = address of buffer to save string @ r1 = number of chars in a row (key length) @ Returns: r0 = string length @--------------------------------------------------- readstr: stmfd sp!,{r4-r7, lr} mov r4, r0 @ load input buffer address in r4 mov r5, r1 @ load row length in r5 mov r6, #0 @ r6 will count the columns mov r7, #0 @ r7 will count the rows readin: bl getchar @ read a character from stdin cmp r0, #0 @ if no more input, stop reading ble endread cmp r0, #'A' @ if it's a char between A and Z, transform to lowercase blt readin cmp r0, #'Z' bgt iflow add r0, r0, #32 @ convert to lowercase b savechar @ save the character iflow: cmp r0, #'a' @ if it's a char between a and z, save blt readin cmp r0, #'z' bgt readin savechar: strb r0, [r4], #1 @ save character in the buffer add r6, r6, #1 @ increment column cmp r6, r5 @ if we reached the key length blt readin mov r6, #0 @ restart column to 0 add r7, r7, #1 @ increment row number b readin endread: cmp r6, #0 @ see if we need padding beq nopad @ if not, return mov r0, #'x' @ padding char padloop: strb r0, [r4], #1 @ save character in the buffer add r6, r6, #1 @ increment column cmp r6, r5 @ see if we reached the key length blt padloop @ if not, repeat add r7, r7, #1 @ increment row number nopad: mov r0, r7 @ return number of rows ldmfd sp!,{r4-r7, lr} mov pc, lr @ return to calling function @--------------------------------------------------- @ sort: sorts the characters in a string and saves the @ sorted indices in the array given as argument @ @ Arguments: r0 = address of string to sort @ r1 = string length @ r2 = pointer to array to save sorted @ indices @--------------------------------------------------- sort: stmfd sp!,{r4-r8, lr} add r3, r1, #-1 @ save length-1 in r3 mov r4, #0 @ index i for the loop l1: mov r5, #0 @ index j for the loop l2: ldrb r6, [r0, r5] @ load string[j] add r7, r5, #1 ldrb r8, [r0, r7] @ load string[j + 1] cmp r6, r8 @ if string[j] <= string[j+1], go to next ble skip strb r8, [r0, r5] @ swap string[j] and string[j+1] strb r6, [r0, r7] ldr r6, [r2, r5, lsl #2] @ swap indices[j] and indices[j+1] add r7, r5, #1 ldr r8, [r2, r7, lsl #2] str r8, [r2, r5, lsl #2] str r6, [r2, r7, lsl #2] skip: add r5, r5, #1 @ j++ cmp r5, r3 blt l2 add r4, r4, #1 @ i++ cmp r4, r1 blt l1 ldmfd sp!,{r4-r8, lr} mov pc, lr @ return to calling function @--------------------------------------------------- @ encrypt: encrypts a string and prints in standard @ output @ @ Arguments: r0 = string to encrypt @ r1 = number of rows in string @ r2 = pointer to array of key indices @ r3 = number of key indices @--------------------------------------------------- encrypt: stmfd sp!,{r4-r10, lr} mov r7, r1 @ save number of rows in r7 mov r8, r3 @ save number of indices in r8 mov r9, r0 @ put string address in r9 mov r10, r2 @ put index address in r10 mov r4, #0 colloop: ldr r6, [r10, r4, lsl #2] @ get index of column to use add r6, r6, r9 @ get start address in string MOV r5, r7 @ repeat the inner loop the number of rows row loop: LDR r0, [r6] @ load character from string bl putchar @ output to stdout add r6, r6, r8 @ advance to next row subs r5, r5, #1 @ repeat for all rows in string bgt rowloop add r4, r4, #1 @ advance to next column cmp r4, r8 @ repeat for all columnns in key blt colloop mov r0, #10 @ print an ending newline bl putchar ldmfd sp!,{r4-r10, lr} mov pc, lr @ return to calling function @--------------------------------------------------- @ decrypt: decrypts a string and prints the result @ in the standard output @ @ Arguments: r0 = string to decrypt @ r1 = number of rows in string @ r2 = pointer to array of key indices @ r3 = number of key indices @--------------------------------------------------- decrypt: stmfd sp!,{r4-r11, lr} mov r7, r1 @ save number of rows in r7 mov r8, r3 @ save number of indices in r8 mov r9, r0 @ put string address in r9 mov r10, r2 @ put index address in r10 ldr r11, =decstr @ point to decrypt buffer with r11 mov r4, #0 dcolloop: ldr r6, [r10, r4, lsl #2] @ get index of column to use add r6, r6, r11 @ get start address in decrypted string mov r5, r7 @ repeat the inner loop the number of rows drowloop: ldrb r0, [r9], #1 @ load character from string strb r0, [r6] @ save in decrypt buffer add r6, r6, r8 @ advance to next row subs r5, r5, #1 @ repeat for all rows in string bgt drowloop add r4, r4, #1 @ advance to next column cmp r4, r8 @ repeat for all columnns in key blt dcolloop ldr r4, =decstr @ point to decrypt buffer with r4 mul r5, r7, r8 @ get number of chars in r5 = columns*rows print: ldrb r0, [r4], #1 @ load a character from the decrypted string cmp r0, #'x' @ compare it with the padding character beq endprint @ if equal, end printing bl putchar @ else, print char on stdout subs r5, r5, #1 @ decrement number of characters to print bgt print @ repeat while not zero endprint: mov r0, #10 @ print an ending newline bl putchar ldmfd sp!,{r4-r11, lr} mov pc, lr @ return to calling function