# Thread Handoff

- Thread: steady-puzzle-2181
- UUID: 6997fbc6-1d6e-4121-9e30-37b3ea96d61b
- CTF: midnight-sun-ctf-2026-quals
- Challenge: riscal
- Runtime: codex
- Model: gpt-5.5
- Status: completed

## Transcript

- **Assistant:** I’ll inspect the challenge files first, identify the category and validation logic, then use `/workspace` only if I need scratch scripts or artifacts. I’ll also try to send concise progress notes to the provided webhook, but network access may require approval in this environment.
- **Assistant:** There’s a single executable in `/challenge`, so this is likely a reversing or pwn-style input validator. I’m going to fingerprint it and look for obvious strings/imports before deciding whether to emulate, decompile, or symbolically solve.
- **Assistant:** The binary is a stripped 64-bit RISC-V PIE. The raw strings already expose several interesting constants, including `flag: `, success/failure messages, and a suspicious `midnight{...}` string, but I’m checking whether that string is the required local input or only a decoy before using it against the remote service.
- **Assistant:** The main check is clearer now: the program reads exactly 45 bytes, transforms them in-place, and compares the result to `Reversing and pwning are both science and art`. The embedded `midnight{...}` is printed only after the check succeeds, so I’m reversing the transform to get the input needed by the remote.
- **Assistant:** The transform reduces to `target[i] = key[i] XOR input[i] XOR input[44-i]`. The equations are symmetric and the middle byte cancels out, so there are many valid 45-byte inputs; I generated a printable one and verified it against the recovered transform in Python. Next I’ll use it on the remote service.
- **Assistant:** flag: yeh!
midnight{RISCV_1S_4_34zy_1S4_70_unDeRst4Nd!!}
- **Assistant:** The remote accepted the generated input and returned the key. I’ll send one final webhook update with the result, then summarize the reverse-engineering path and the exact input used.
- **Assistant:** Solved.

Flag/key:

```text
midnight{RISCV_1S_4_34zy_1S4_70_unDeRst4Nd!!}
```

The binary is a stripped 64-bit RISC-V ELF. Its main logic reads a 45-byte input, transforms it, and compares the transformed buffer with:

```text
Reversing and pwning are both science and art
```

The transform is:

```text
out[i] = key[i] ^ input[i] ^ input[44 - i]
```

where `key` is the 45-byte blob embedded at `.rodata+0x8`. Reversing that equation gives many valid inputs. One printable valid input I used was:

```text
aaaaaaaaaaabaaBaaAACAdA)--&*mg-hW_lVHf}2Fk$)q
```

Submitting it to the server:

```text
flag: yeh!
midnight{RISCV_1S_4_34zy_1S4_70_unDeRst4Nd!!}
```
- **System:** Thread completed