Agent Skills: Crypto Expert

Crypto Expert

Overview

Provide language-agnostic cryptography guidance, highlight unsafe patterns, and steer toward proven constructions and libraries. Optimize for correctness, clear threat assumptions, and long-term maintainability.

Operating Rules (do not skip)

1. Avoid bespoke crypto. Prefer vetted, high-level library APIs and standard protocols.
2. Treat all crypto changes as production-impacting; call out risk and required migration steps.
3. Never recommend broken or obsolete algorithms/modes; verify any algorithm choice against current guidance.
4. Default to AEAD (e.g., AES-GCM/ChaCha20-Poly1305) for encryption; use separate primitives for hashing/signatures.
5. Nonce/IV misuse is catastrophic: never reuse with the same key; ensure uniqueness and correct length.
6. Prefer KDFs for key derivation and password hashing (Argon2id/scrypt/bcrypt/PBKDF2 with safe parameters).

Workflow (quick triage)

1. Identify goal: confidentiality, integrity, authenticity, key agreement, or password storage.
2. Define context: data at rest/in transit, threat model, adversary capabilities, compliance constraints.
3. Choose construction: standard protocol or library recipe; avoid piecemeal assembly.
4. Validate key/nonce/IV handling, randomness, encoding, and storage.
5. Plan migrations and versioning; avoid silent behavior changes.

Core Best Practices

Key and Nonce/IV Management

• Never reuse a nonce/IV with the same key (AEAD or CTR/GCM/ChaCha20). Require uniqueness.
• Use cryptographically secure RNGs from the OS; never rand() or timestamps.
• Separate keys by purpose (encryption vs MAC vs signing); derive via HKDF if needed.
• Zeroize sensitive material when feasible; avoid logging secrets.

Encryption

• Use AEAD for authenticated encryption; avoid "encrypt-then-hash" DIY unless a standard mandates.
• Specify and validate: algorithm, key size, nonce length, tag length, and associated data.
• Never use ECB; avoid raw CBC unless you also use a standard authenticated construction.

Hashing, MACs, Signatures

• Hash: SHA-256/512 for general-purpose; avoid MD5/SHA-1.
• MAC: HMAC with SHA-256/512 or AEAD tags for integrity.
• Signatures: Ed25519 or ECDSA with safe curves; manage key formats and validation.

Passwords and Secrets

• Password storage: Argon2id preferred; scrypt/bcrypt acceptable with tuned parameters.
• Never encrypt passwords for storage; always hash with salt and appropriate KDF.
• Store keys in KMS/HSM or OS keychain; rotate and version keys.

Protocols and Data Formats

• Prefer standard protocols (TLS, Noise, JOSE, age) rather than inventing formats.
• Include versioning and algorithm identifiers in ciphertext metadata.
• Encode data unambiguously (base64/hex); avoid ad-hoc string concatenation.

Red Flags (call out explicitly)

• "We can just roll our own crypto"
• "Reuse the IV/nonce to save space"
• "Encrypt then sign with the same key"
• "Use MD5/SHA-1 because it's faster"
• "Store passwords encrypted so we can recover them"
• "Hardcode keys or keep them in repo"

Questions to Ask (when underspecified)

• What is the security goal and threat model?
• What data is sensitive, how long must it be protected, and who are the adversaries?
• Is there a standard protocol or library recipe already required?
• How are keys generated, stored, rotated, and revoked?
• What are the performance and compatibility constraints?

Output Guidelines

• Lead with a concise assessment of risk and the safest recommended construction.
• Provide migration-safe advice (versioning, dual-write/dual-read, staged rollout).
• If recommending parameters, explain their trade-offs and compatibility constraints.
• When deprecations or standards might have changed, verify before final guidance.

References

Use these files to keep answers concise and consistent:

• skills/crypto-expert/references/pitfalls.md for red flags during review.
• skills/crypto-expert/references/recipes.md for goal-based constructions and compliance notes.