Tags: esoteric-language rev
Rating:
# UDCTF 2023
## Blue Hens
> Last year the feedback was "maybe not so many esolangs". I hear you. So I just wrote one. OK? The linguistic root of the word scholarly is "spare time".
>
> **Blue Hens**
>
> Your input is a file of code.
>
> Each line has the word "blue" some number of times followed by the word "hens" some number of time (separated by spaces).
>
> The number of "blues" on the line is the OPCODE the number of "hens" on that line is the ARG. The line numbers matter and they start at 1.
>
> Our architecture has 2 variables, a register and a counter both start at 0.
>
> OPCODES 1,2,3,4 are arithmetic operations:
>
> 1 is SET: set the register to the ARG
>
> 2 is ADD: add the ARG to the register
>
> 3 is SUB: subtract ARG from the register
>
> 4 is MUL: multiply the register by ARG
>
> OPCODES 5,6,7 are gotos
>
> 5 is GTL: go to line number ARG
>
> 6 is GBL: go back ARG lines
>
> 7 is GUL: go forward ARG lines
>
> OPCODES 8,9,10 are control flow:
>
> 8 is CTA: add ARG to counter
>
> 9 is CTS: subtract ARG from counter
>
> 10 is SKP: skip the next line IF counter is 0
>
> OPCODE 11 is print:
>
> 11 is PRT: print the value of the register (ascii)
>
> [`flag.bluehens`](https://raw.githubusercontent.com/D13David/ctf-writeups/main/udctf23/rev/blue_hens/flag.bluehens)
Tags: _rev_
## Solution
The description basically tells us all, we have to write a small interpreter for a homebrew language called `Blue Hens`. Just implementing what the descriptions tells us we can run the given script and get the flag.
```python
mnem = {
1 : "SET R {0:x}",
2 : "ADD R {0:x}",
3 : "SUB R {0:x}",
4 : "MUL R {0:x}",
5 : "GTL PC {0:x}",
6 : "GBL PC {0:x}",
7 : "GUL PC {0:x}",
8 : "CTA C {0:x}",
9 : "CTS C {0:x}",
10 : "SKP {0:x}",
11 : "PRT R {0:x}"
}
pc = 1
reg = 0
cnt = 0
code = []
lines = open("flag.bluehens", "r").readlines()
code = [(line.count("blue"), line.count("hens")) for line in lines]
code.insert(0, (0, 0))
while pc < len(code):
op, arg = code[pc]
m = mnem[op]
#print(pc, m.format(arg))
if op == 1:
reg = arg
elif op == 2:
reg += arg
elif op == 3:
reg -= arg
elif op == 4:
reg *= arg
elif op == 5:
pc = arg
elif op == 6:
pc -= arg
elif op == 7:
pc += arg
elif op == 8:
cnt += arg
elif op == 9:
cnt -= arg
elif op == 10:
if cnt == 0:
pc += 1
elif op == 11:
print(chr(reg), end="")
if op < 5 or op > 7:
pc += 1
```
Flag `UDCTF{fight_fight_fight_f0r_D3l4war3}`
