Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add pedromneto97/custom-skills --skill crypto-best-practicesgit clone --depth 1 https://github.com/pedromneto97/custom-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/pedromneto97/custom-skills/crypto-best-practices)<a href="https://agentmods.dev/skills/pedromneto97/custom-skills/crypto-best-practices"><img src="https://agentmods.dev/badge/skills/pedromneto97/custom-skills/crypto-best-practices/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/pedromneto97/custom-skills/crypto-best-practices"><img src="https://agentmods.dev/badge/skills/pedromneto97/custom-skills/crypto-best-practices.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00057 | $0.03755 |
| Opus 5 | $0.00028 | $0.01878 |
| Sonnet 5 | $0.00011 | $0.00751 |
| Haiku 4.5 | $0.00006 | $0.00376 |
Grade A, and why
crypto-best-practices scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 12d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 432 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Cryptographic Best Practices
⚠️ Golden Rule
Use a battle-tested library that handles cryptographic composition correctly for you:
- NaCl (by Daniel Bernstein)
- libsodium (NaCl fork by Frank Denis)
- monocypher (libsodium fork by Loup Vaillant)
If you use these libraries, most low-level decisions are already made for you. Only continue reading if forced to make your own choices.
1. Symmetric Encryption
Decision Tree:
- Do you have access to a Key Management System (KMS)? → Use KMS
- Otherwise → Use AEAD (Authenticated Encryption with Associated Data)
Recommended algorithms (in order):
- NaCl/libsodium/monocypher default
- ChaCha20-Poly1305 (faster in software)
- AES-GCM (industry standard, faster with AES-NI)
- AES-CTR with HMAC (fast in software)
❌ Avoid:
- AES-CBC, AES-CTR alone (must use AEAD)
- Block ciphers with 64-bit blocks (Blowfish, 3DES)
- OFB mode
- RC4 (broken)
Why AEAD? Prevents forged ciphertexts. A single byte corruption either fails authentication or reveals nothing.
2. Symmetric Key Length
Recommendations for data protection through 2050:
- Minimum: 128 bits
- Maximum: 256 bits (no harm in using larger)
| Standard | Recommendation | Notes |
|---|---|---|
| NIST | 192 bits | Conservative government standard |
| ECRYPT II | 256 bits | Academic consensus, most conservative |
| ANSSI | 128 bits | European standard, adequate |
| IAD-NSA | 256 bits | Highest security requirement |
If your symmetric key is derived from user input (password):
- The password must provide at least as many bits of entropy as the target key length
- Example: 128-bit AES key needs ≥128 bits of entropy in the password
3. Symmetric Signatures (HMAC/MAC)
Use when authenticating but not encrypting (e.g., API requests, message authentication).
Recommended algorithms (in order):
HMAC variants:
- HMAC-SHA-512/256 (truncated SHA-512)
- HMAC-SHA-512/224 (truncated SHA-512)
- HMAC-SHA-384
- HMAC-SHA-224
- HMAC-SHA-512
- HMAC-SHA-256
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 12d ago First seen · 432 lines · 57 tokens per session scan A 3516c2b75a9a
crypto-best-practices is a skill published in the GitHub repository pedromneto97/custom-skills (2 stars, last pushed 2mo ago), licensed MIT. It adds 57 tokens to every session and 3,755 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.
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