Python 3, 70 bytes, cracked by @ovs

c=compile(c:=open(0,"rb").read(),"","exec").replace(co_code=c)
exec(c)

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Works in CPython 3.9.6. Takes input from STDIN and should output 42 with a trailing newline to STDOUT.

@ovs has cracked it with a nice solution, but before I reveal mine I'd like to see if anyone can do it without # (byte 0x23)?

Python 3, 70 bytes, cracked by @ovs

c=compile(c:=open(0,"rb").read(),"","exec").replace(co_code=c)
exec(c)

Attempt This Online!

Works in CPython 3.9.6. Takes input from STDIN and should output 42 with a trailing newline to STDOUT.

The challenge was to create something of a polyglot between Python source code and CPython bytecode. The Python source would be compiled (so it needed to be syntactically valid), and then have its bytecode replaced with itself, keeping constants and names.

My solution was:

d,F,d,S,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,44,43,42

Disassembly:

Bytes  Instruction      Argument  Meaning
d,     LOAD_CONST             44  stored integer 42
F,     PRINT_EXPR             44  ignored
d,     LOAD_CONST             44  stored integer 42
S,     RETURN_VALUE           44  ignored
...    [garbage...]

The list of numbers is just to get the Python compiler to store enough constants for , (decimal codepoint 44) to be a valid argument in order to index into the tuple and load 42.

By the way, PRINT_EXPR is an opcode used solely by the Python shell to print the return value of an expression. It's never used in normal code, but it's very useful for hacking around in bytecode-land.

Python 3, 70 bytes

c=compile(c:=open(0,"rb").read(),"","exec").replace(co_code=c)
exec(c)

Attempt This Online!

Works in CPython 3.9.6. Takes input from STDIN and should output 42 with a trailing newline to STDOUT.

Python 3, 70 bytes, cracked by @ovs

c=compile(c:=open(0,"rb").read(),"","exec").replace(co_code=c)
exec(c)

Attempt This Online!

Works in CPython 3.9.6. Takes input from STDIN and should output 42 with a trailing newline to STDOUT.

@ovs has cracked it with a nice solution, but before I reveal mine I'd like to see if anyone can do it without # (byte 0x23)?