GCHQ Stage 1 Solution

; GCHQ canyoucrackit.co.uk stage 1 solution

; Dr Gareth Owen, School of Engineering, University of Greenwich, England
; g.h.owen@gre.ac.uk

; Explanation: http://gchqchallenge.blogspot.com
; To run:
; nasm -f elf p1-complete.asm
; gcc -o p1 p1-complete.asm
; ./p1

global main
main:

jmp start

; unused - code for part 3
scasd
ret 0xa3bf
; unused - end code for part 3

start:
sub esp,0x100    ; 4096 bytes
xor ecx,ecx

; loop through 256 bytes of memory and set values to equal offset (e.g. pos 1 = val 1)
; RC4 KSA initialisation
ksa_part1_loop:
mov [esp+ecx],cl
inc cl
jnz ksa_part1_loop

xor eax,eax

; key for RC4 algorithm
mov edx,0xdeadbeef

ksa_key_loop:   ; cl used as j (loop counter)
add al,[esp+ecx];
add al,dl   ; take cur byte from key
ror edx,0x8     ; next byte in key (rotate 8 bits)
mov bl,[esp+ecx]
mov bh,[esp+eax]
mov [esp+eax],bl
mov [esp+ecx],bh
inc cl
jnz ksa_key_loop

jmp label1

decrypt:
mov ebx,esp
add ebx,0x4 ; keystream location

pop esp     ; loads end of program address into ESP (using location from earlier call which placed it on stack )

; sanity check
pop eax         ; ensure we're at end of program (see last four instructions)
cmp eax,0x41414141
jnz myexit

; esp = beginning of data from image
; check valid data
pop eax
cmp eax,0x42424242
jnz myexit

; begin decryption
pop edx     ; get num bytes ( = 50/32h)
mov ecx,edx
mov esi,esp ; stack pointer into esi
mov edi,ebx ; destination is old esp + 4
sub edi,ecx ; move back down stack 32 bytes
rep movsb   ; move ECX bytes from ESI to EDI
mov esi,ebx
mov ecx,edx
mov edi,ebx
sub edi,ecx
xor eax,eax
xor ebx,ebx
xor edx,edx

decrypt_loop:           ; main decrypt loop
inc al
add bl,[esi+eax]
mov dl,[esi+eax]
mov dh,[esi+ebx]
mov [esi+eax],dh
mov [esi+ebx],dl
add dl,dh
xor dh,dh
mov bl,[esi+edx]
mov dl,[edi]
xor dl,bl           ; xor with keystraem
mov [edi],dl            ; store result
inc edi
dec ecx
jnz decrypt_loop

myexit:
mov eax, 4 ; sys_write
mov ebx, 1 ; stdout
mov edx, 0x32 ; length
lea ecx, [edi-0x32] ; location
int 0x80

xor ebx,ebx  ; clear ebx
mov eax,ebx  ; clear eax
inc al       ; set eax to 1 (syscall for exit)
int 0x80     ; make syscall

label1:
nop
nop
call decrypt    ; CALL SO THAT WE GET THIS LOCATION ONTO THE STACK (e.g. end of file for later for decryption)

; 0x41414141 (to check we're in right place later)
dd 0x41414141
;inc ecx
;inc ecx
;inc ecx
;inc ecx

; INSERT STUFF TO BE DECRYPTED HERE
db  042h
db  042h
db  042h
db  042h
db  032h
db  00h
db  00h
db  00h
db  091h
db  0d8h
db  0f1h
db  06dh
db  070h
db  020h
db  03ah
db  0abh
db  067h
db  09ah
db  0bh
db  0c4h
db  091h
db  0fbh
db  0c7h
db  066h
db  0fh
db  0fch
db  0cdh
db  0cch
db  0b4h
db  02h
db  0fah
db  0d7h
db  077h
db  0b4h
db  054h
db  038h
db  0abh
db  01fh
db  0eh
db  0e3h
db  08eh
db  0d3h
db  0dh
db  0ebh
db  099h
db  0c3h
db  093h
db  0feh
db  0d1h
db  02bh
db  01bh
db  011h
db  0c6h
db  011h
db  0efh
db  0c8h
db  0cah
db  02fh