Question

. Write a program which encodes any string using the XOR instruction.  Test it using your <first name last name> in the data segment to produce cipher text and then decode using the program to get plain text.  Use the last two digits of your student id as the key.  Print plane text from the data segment, print the cipher text, and then print the plain text upon execution.  Submit the asm/list file and screenshots that shows the output of your code.

What are the strengths and weaknesses of this encryption method (25% of points, Typewritten answer required)?

Answer 1

The program for the above problem with the output window screenshot with two different string is given below-

encryptDecrypt(char*):

push rbp

mov rbp, rsp

sub rsp, 32

mov QWORD PTR [rbp-24], rdi

mov BYTE PTR [rbp-5], 82

mov rax, QWORD PTR [rbp-24]

mov rdi, rax

call strlen

mov DWORD PTR [rbp-12], eax

mov DWORD PTR [rbp-4], 0

.L3:

mov eax, DWORD PTR [rbp-4]

cmp eax, DWORD PTR [rbp-12]

jge .L4

mov eax, DWORD PTR [rbp-4]

movsx rdx, eax

mov rax, QWORD PTR [rbp-24]

add rax, rdx

movzx eax, BYTE PTR [rax]

mov edx, DWORD PTR [rbp-4]

movsx rcx, edx

mov rdx, QWORD PTR [rbp-24]

add rdx, rcx

xor al, BYTE PTR [rbp-5]

mov BYTE PTR [rdx], al

mov eax, DWORD PTR [rbp-4]

movsx rdx, eax

mov rax, QWORD PTR [rbp-24]

add rax, rdx

movzx eax, BYTE PTR [rax]

movsx eax, al

mov edi, eax

call putchar

add DWORD PTR [rbp-4], 1

jmp .L3

.L4:

nop

leave

ret

.LC0:

.string "Encrypted String: "

.LC1:

.string "Decrypted String: "

main:

push rbp

mov rbp, rsp

sub rsp, 32

movabs rax, 2334393380863435085

movabs rdx, 7306916077306274156

mov QWORD PTR [rbp-32], rax

mov QWORD PTR [rbp-24], rdx

mov BYTE PTR [rbp-16], 0

mov edi, OFFSET FLAT:.LC0

mov eax, 0

call printf

lea rax, [rbp-32]

mov rdi, rax

call encryptDecrypt(char*)

mov edi, 10

call putchar

mov edi, OFFSET FLAT:.LC1

mov eax, 0

call printf

lea rax, [rbp-32]

mov rdi, rax

call encryptDecrypt(char*)

mov eax, 0

leave

ret

__static_initialization_and_destruction_0(int, int):

push rbp

mov rbp, rsp

sub rsp, 16

mov DWORD PTR [rbp-4], edi

mov DWORD PTR [rbp-8], esi

cmp DWORD PTR [rbp-4], 1

jne .L9

cmp DWORD PTR [rbp-8], 65535

jne .L9

mov edi, OFFSET FLAT:_ZStL8__ioinit

call std::ios_base::Init::Init() [complete object constructor]

mov edx, OFFSET FLAT:__dso_handle

mov esi, OFFSET FLAT:_ZStL8__ioinit

mov edi, OFFSET FLAT:_ZNSt8ios_base4InitD1Ev

call __cxa_atexit

.L9:

nop

leave

ret

_GLOBAL__sub_I_encryptDecrypt(char*):

push rbp

mov rbp, rsp

mov esi, 65535

mov edi, 1

call __static_initialization_and_destruction_0(int, int)

pop rbp

ret

The strength is that it can encrypt the string which is impossible for brute force method and it is done with XOR instruction. The weakness is that by using the same key repeatedly the XOR cipher can be broken.