Signal Lock Inverting a BEAM token check

rev
flag hiCTF{b34m_byt3c0d3_n3v3r_l13s_s3ss10n_unl0ck3d}

Category: Reverse engineering (Erlang/BEAM) · signal-lock.zip → signal_lock.beam (sha256 93651d91…)

Artifact

A stripped, compiled BEAM module signal_lock with a single entry point verify/1. Per the brief it validates a client's session-integrity token; a correct token returns {ok, <<"hiCTF{...}">>}.

Method

abstract_code is absent, so disassemble the BEAM and read the SSA:

erlang 
%% erl -pa . -eval 'io:format("~p~n",[beam_disasm:file("signal_lock.beam")]),halt().'

verify/1 does, in order:

  1. is_binary(Token) and byte_size(Token) == 8 (exactly 8 bytes);
  2. bs_match an unsigned 64-bit big-endian integer out of the token;
  3. transform(Int) == 12675316763487955908 (0xAFE7C5708F40EBC4);
  4. on match, unlock(Int) returns {ok, <<Flag/binary>>}.

transform/1 (from the disasm) spreads and mixes the 64-bit value with multiply/rotate/add constants modulo 2^64:

output 
x = (x * 0x9E3779B97F4A7C15) band 0xFFFFFFFFFFFFFFFF   ; golden-ratio spread
x = rotl(x, 29)                                          ; 64-bit rotate left
x = (x + 0x0123456789ABCDEF) band 0xFFFFFFFFFFFFFFFF

Working this chain backwards pins the one 8-byte token that satisfies the comparison:

output 
token = "g4m30v3r"

(confirmed by executing the module - see Verification; the module itself is the authority on whether the token is accepted.)

Repro

bash 
1# Erlang/OTP 26+ required (older runtimes reject the module's opcodes)
2erl -pa . \
3  -eval 'io:format("~p~n",[signal_lock:verify(<<"g4m30v3r">>)]), halt().'

Output:

output 
{ok,<<"hiCTF{b34m_byt3c0d3_n3v3r_l13s_s3ss10n_unl0ck3d}">>}

Verification

The strongest possible : the challenge's own unmodified module returns the flag for the candidate token - the token is accepted by signal_lock:verify/1, not merely "plausible". No brute force was used; the token was derived from the arithmetic in transform/1.

Tooling notes

  • This is a compiled .beam (no debug info); use beam_disasm:file/1, not beam_lib:chunks(...,[abstract_code]).
  • The module is gzip-compressed BEAM (gzip compressed data, original size 1624).