# symmetric-cipher-attacks

A playbook for attacking symmetric encryption: CBC padding oracle, ECB cut-and-paste, stream cipher key reuse. Built for CTFs and authorized testing.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 3500 installs · ★ 2547
- Author: yaklang
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/yaklang/hack-skills/tree/HEAD/skills/symmetric-cipher-attacks/SKILL.md
- Updated: 2026-09-13
- HTML: https://affpapa.org/en/nevibecode/skills/symmetric-cipher-attacks

## What it does

The skill tells the agent which technique fits which cipher. A lookup table maps observable behaviour to the likely weakness: identical ciphertext for identical plaintext points at ECB, a distinguishable padding error opens a padding oracle, and a reused stream key collapses into XORing two ciphertexts together.

Python implementations come with it. The padding oracle script walks byte by byte and checks for the 0x02 0x02 false positive; a bit flipping helper rewrites a byte in the following block; ECB detection and a byte-at-a-time walkthrough cover the block mode. Cut-and-paste is spelled out too: align a word into its own block, then swap that block into place. LFSR and LCG state recovery, RC4 biases and meet-in-the-middle round out the set.

Related skills are linked for the cases this one does not cover: `rsa-attack-techniques` when an RSA layer protects the symmetric key, `hash-attack-techniques` for HMAC, `lattice-crypto-attacks` for lattice-based state recovery. Running the scripts needs Python. The padding oracle only works if the server distinguishes a bad padding error — if every response looks the same, that path is closed and you should look elsewhere.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill symmetric-cipher-attacks -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/symmetric-cipher-attacks/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/symmetric-cipher-attacks/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill symmetric-cipher-attacks -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/symmetric-cipher-attacks/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill symmetric-cipher-attacks -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/symmetric-cipher-attacks/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install yaklang/hack-skills symmetric-cipher-attacks
```

2. **Manually: put the files in place**
- File: `.github/skills/symmetric-cipher-attacks/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill symmetric-cipher-attacks -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/yaklang/hack-skills --path skills/symmetric-cipher-attacks
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/symmetric-cipher-attacks/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill symmetric-cipher-attacks -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/yaklang/hack-skills/tree/HEAD/skills/symmetric-cipher-attacks/SKILL.md

## When to use it
- decrypt a CBC block without the key via a padding oracle
- flip a value in the plaintext by editing the previous block
- spot ECB mode from repeated ciphertext blocks
- forge a valid ciphertext by reordering blocks
- recover LCG or LFSR state from predictable output

## Example prompts
- I have an oracle that tells me whether padding is valid. Help me decrypt a CBC block.
- The ciphertext repeats for identical blocks — identify the mode and walk me through cut-and-paste.
- The stream key was reused across two messages. How do I XOR the ciphertexts and recover the plaintext?

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 086278f0b4a3dd0b.

BEGIN THIRD-PARTY FILE 086278f0b4a3dd0b
````text
---
name: symmetric-cipher-attacks
description: >-
  Symmetric cipher attack playbook. Use when exploiting block cipher mode
  weaknesses (CBC padding oracle, ECB cut-and-paste, bit flipping), stream
  cipher key reuse, or meet-in-the-middle attacks.
---

# SKILL: Symmetric Cipher Attacks — Expert Cryptanalysis Playbook

> **AI LOAD INSTRUCTION**: Expert techniques for attacking symmetric encryption in CTF and authorized testing. Covers CBC padding oracle, CBC bit flipping, ECB detection and exploitation, stream cipher key reuse, LFSR/LCG state recovery, RC4 biases, and meet-in-the-middle attacks. Base models often confuse ECB and CBC attack strategies or fail to set up byte-at-a-time ECB decryption correctly.

## 0. RELATED ROUTING

- [rsa-attack-techniques](../rsa-attack-techniques/SKILL.md) when symmetric key is protected by RSA
- [hash-attack-techniques](../hash-attack-techniques/SKILL.md) when HMAC or hash-based authentication is involved
- [lattice-crypto-attacks](../lattice-crypto-attacks/SKILL.md) for LCG/LFSR state recovery via lattice methods

### Advanced Reference

Also load [BLOCK_CIPHER_ATTACKS.md](./BLOCK_CIPHER_ATTACKS.md) when you need:
- Detailed attack scripts with full Python implementations
- Step-by-step byte-at-a-time ECB walkthrough
- PadBuster usage and custom padding oracle scripts
- LCG/LFSR recovery implementation

### Quick attack selection

| Observable Behavior | Likely Weakness | Attack |
|---|---|---|
| Same plaintext → same ciphertext (block-aligned) | ECB mode | Cut-and-paste / byte-at-a-time |
| Padding error distinguishable | CBC padding oracle | Decrypt without key |
| Can modify ciphertext, affects next block | CBC mode, no integrity check | Bit flipping |
| Key reused with XOR/stream cipher | Two-time pad | XOR ciphertexts together |
| Predictable PRNG output | LCG or LFSR | State recovery |
| Double encryption used | 2DES-like | Meet in the middle |

---

## 1. PADDING ORACLE ATTACK (CBC MODE)

### 1.1 Mechanism

CBC decryption: `P_i = D_K(C_i) ⊕ C_{i-1}`

If the server reveals whether padding is valid (PKCS#7), we can decrypt any block by manipulating the previous ciphertext block.

### 1.2 Attack Steps

```
Target: decrypt block C_i (with unknown plaintext P_i)

For byte position b = 15 down to 0 (last byte first):
````
END THIRD-PARTY FILE 086278f0b4a3dd0b

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First 2303 of 14622 bytes. © yaklang · MIT · https://github.com/yaklang/hack-skills/tree/HEAD/skills/symmetric-cipher-attacks/SKILL.md

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---
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