115 lines
3.3 KiB
Python
115 lines
3.3 KiB
Python
import random
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import secrets
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import sys
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def eprint(*args, **kwargs):
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print("[Codegen]", *args, file=sys.stderr, **kwargs)
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def custom_op_xor(value, amount):
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# x[rd] = ((x[rs1] << (i32)imm) ^ 0xFF) >> (i32)imm;
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return (((value << amount) ^ 0xFF) >> amount)
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manglers = [
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(lambda value: custom_op_xor(value, 5), lambda value: f"trunc_xor({value}, 5)"),
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(lambda value: custom_op_xor(value, 3), lambda value: f"trunc_xor({value}, 3)"),
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(lambda value: custom_op_xor(value, 2), lambda value: f"trunc_xor({value}, 2)"),
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(lambda value: custom_op_xor(value, 1), lambda value: f"trunc_xor({value}, 1)"),
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(lambda value: value + 1, lambda value: f"{value} + 1"),
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(lambda value: value + 128, lambda value: f"{value} + 128"),
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(lambda value: value ^ 0xAB, lambda value: f"{value} ^ 0xAB"),
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# This is a trivial test mangler, do not use
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# (lambda value: value, lambda value: f"{value}")
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]
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def mangle(value):
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result = b""
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c_manglers = []
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for c in value:
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mangler, c_mangler = random.choice(manglers)
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# Mangle the input (str -> int)
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# With overflow (mod 255)
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result += (mangler((ord(c))) % 255).to_bytes()
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c_manglers.append(c_mangler)
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if len(result) != len(value):
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eprint("WE GOT AN OVERFLOW")
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return (result, c_manglers)
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def get_check_code(flag_part, user_part, c_mangler):
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return f"""if(({flag_part}) != ({c_mangler(user_part)})){{
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return false;
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}}
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"""
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def main():
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if len(sys.argv) != 3:
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eprint(f"Usage: {sys.argv[0]} <flag_format> <flag_length>")
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exit(1)
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# Flag prefix is not used, currently the flag is just used as-is
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flag_format = sys.argv[1]
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flag_len = int(sys.argv[2])
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# Generate a secret
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secret = secrets.token_urlsafe(flag_len)
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secret = flag_format.replace("FLAG_GOES_HERE", secret)
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eprint(f"Generated Flag: {secret}")
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# Change the bytes of the secret
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mangled_secret, c_manglers = mangle(secret)
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# Shuffle the secret, but keep track of the order
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# We need this so we know the real location of the characters to compare
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secret_shuffleorder = list(range(len(secret)))
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random.shuffle(secret_shuffleorder)
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# Shuffle the mangled secret according to the order
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mangled_secret = b"".join([mangled_secret[x].to_bytes() for x in secret_shuffleorder])
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# the mangler for secret[0] is no longer at c_manglers[0!]
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c_manglers = [c_manglers[x] for x in secret_shuffleorder]
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# Force binary representation
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mangled_secret = "".join(f'\\x{b:02x}' for b in mangled_secret)
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checks = []
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for i, c_mangler in enumerate(c_manglers):
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checks.append(get_check_code(f"MANGLED[{i}]", f"userflag[{secret_shuffleorder[i]}]", c_mangler))
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random.shuffle(checks)
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check = "\n".join(checks)
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# Just add one level of indent to make the generated code easier to read
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check = check.replace("\n", "\n ")
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print(f"""// This file is automatically generated, do not modify!
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// The generated secret flag is "{secret}"
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#pragma once
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#include <stddef.h>
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#include "custom_instructions.h"
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static volatile uint8_t MANGLED[] = "{mangled_secret}";
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bool check(char* userflag, uint8_t len) {{
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if (len != {len(secret)}) {{
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return false;
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}}
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{check}
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return true;
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}}
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""")
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if __name__ == "__main__":
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main()
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