Nyandesuka??

solved by treeindustry

People told me to make my code more accessible, so I decided to make my latest project in Scratch! Check it out: https://scratch.mit.edu/projects/851085916/

Notes

This challenge was originally at https://scratch.mit.edu/projects/845244451/ but was made simpler so people would solve it.

Also, please forgive my poor paint skills

Code

Sprite code:

Nothing much to see here.

The stage code has 4 segments, the first segment is the initial setup:

Notable variables:

• sp is 1020

• STACK has 1024 0s (.rodata has nothing)
• Label Stack and TTY are initially empty

getchar gets the value from the next character from our input based on ascii:

index_of is the index of a character in a string:

Then we have the main segment which is the code we have to reverse engineer:

Not mentioned is putchar, which just outputs a character.

Strategy

Read the code and step through it

Observations

Label Stack is never used.

winning needs to be 4 for us to solve the challenge.

Item sp + 4 (which is 1024) of STACK is accessed in many of the if blocks:

• First, it is set to 4

• If it is greater than 0, it decreases by 1 every iteration
• If it is 0, the program exits
• Different blocks are run if it is 4, 3, 2 or 1, and winning can only be increased in one of these blocks

Every iteration, getchar is called 3 times.

Values in the stack

Items sp + 3 (1023) to sp - 10 (1010) are only modified once per iteration. Let the values of the 3 characters from our input be x, y and z respectively. The values in the stack are:

Item	Code	Value
1023	replace item sp+3 of STACK with return value	x
1022	replace item sp+2 of STACK with return value	y
1021	replace item sp+1 of STACK with return value	z
1020	replace item sp of STACK with item sp+3 of STACK	x
1019	replace item sp-1 of STACK with item sp of STACK	x
1018	replace item sp-2 of STACK with (item sp-1 of STACK - 48) * 100	100(x - 48)
1017	replace item sp-3 of STACK with item sp+2 of STACK	y
1016	replace item sp-4 of STACK with item sp-3 of STACK	y
1015	replace item sp-5 of STACK with (item sp-4 of STACK - 48) * 10	10(y - 48)
1014	replace item sp-6 of STACK with item sp+1 of STACK	z
1013	replace item sp-7 of STACK with item sp-6 of STACK	z
1012	replace item sp-8 of STACK with (item sp-2 of STACK - 48)	100(x - 48) - 48
1011	replace item sp-9 of STACK with (item sp-8 of STACK + item sp-5 of STACK)	100(x - 48) + 10(y - 48) - 48
1010	replace item sp-10 of STACK with (item sp-9 of STACK + item sp-7 of STACK)	100(x - 48) + 10(y - 48) + z - 48

Now we get a hint from one of the organizers:

‘0’ equals forty eight

This means that item 1010 100(x - 48) + 10(y - 48) + z - 48 is just the 3 digit number we input (or other unicode characters if we need a value greater than 999)

Now back to item 1024. Depending on its value, different if blocks are run, which all increase winning by 1, but those if blocks also need item 1009 of the stack to be 0.

Additionally, item 1009 has different values depending on which segment of the code is being run.

item 1024	item 1009
4	item 1010 - 119
3	item 1010 - 80
2	item 1010 - 67
1	item 1010 - 119

item 1024 starts at 4, so the first 3 characters are 119, the next 3 are 080, then 067, and 119
The passcode is 119080067119

Untitled[Source]

 
WOW! good job making it this far, seriously. ~ scuffed 
 
blahaj{1ch1_n1_54n_ny4!_4r16470}

blahaj{1ch1_n1_54n_ny4!_4r16470}

Leat'eq - Tokyo