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securitycritical PriorityID: cryptographic-key-handling

Cryptographic Standards & Password Hashing Standard

Guardrails for modern cryptography: Argon2id for password hashing (no MD5, SHA-1, or plain bcrypt), AES-256-GCM for symmetric encryption, and constant-time string comparisons.

Prohibited Anti-Pattern: AI uses MD5/SHA256 for password hashing, ECB mode for AES encryption, or standard '===' for secret comparison.

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---
description: Cryptographic Standards & Password Hashing Standard - Guardrails for modern cryptography: Argon2id for password hashing (no MD5, SHA-1, or plain bcrypt), AES-256-GCM for symmetric encryption, and constant-time string comparisons.
globs: **/crypto/**/*.{ts,js,py}, **/auth/**/*.{ts,js,py}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/cryptographic-key-handling"
---

# Cryptographic Standards & Password Hashing Standard

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: AI uses MD5/SHA256 for password hashing, ECB mode for AES encryption, or standard '===' for secret comparison.

## System Constraints
You are an applied cryptography expert. For password hashing, enforce Argon2id (or modern bcrypt with cost >= 12). For symmetric encryption, only use authenticated encryption algorithms (AES-256-GCM or ChaCha20-Poly1305). Always use crypto.timingSafeEqual for checking hashes and API tokens to prevent timing attacks.

## Strict Guidelines
- Never use plain SHA-256 or MD5 for password storage; use Argon2id with memory >= 64MB.
- Use AES-256-GCM with a unique 12-byte initialization vector (IV) per encryption operation.
- Use constant-time comparison (crypto.timingSafeEqual) when validating webhook signatures or secret tokens.

## Bad vs Good Implementations
### Constant-Time Signature Verification against Timing Attacks

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Standard string equality returns early on the first non-matching byte, allowing attackers to measure microsecond latency to forge signatures.
```typescript
// VULNERABLE: Standard string equality is subject to timing attacks
export function verifySignature(userSig: string, expectedSig: string): boolean {
  return userSig === expectedSig; // Leaks character matches via response latency!
}
```

#### Production Standard (Correct)
> **Standard**: timingSafeEqual executes in constant time regardless of where byte discrepancies occur, eliminating timing side-channels.
```typescript
// Production Standard: Constant-time comparison
import crypto from "node:crypto";

export function verifySignature(userSig: string, expectedSig: string): boolean {
  const bufA = Buffer.from(userSig, "hex");
  const bufB = Buffer.from(expectedSig, "hex");
  if (bufA.length !== bufB.length) return false;
  return crypto.timingSafeEqual(bufA, bufB);
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Crypto Audit MCP
Audit cipher suites, IV uniqueness, and key derivation parameters.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

AES-GCM & Argon2 Helper
Generate audited encryption and password hashing helpers.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Cryptographic Standards & Password Hashing Standard

Cryptographic Standards & Password Hashing Standard defines an industrial-grade behavioral standard for security engineering. Guardrails for modern cryptography: Argon2id for password hashing (no MD5, SHA-1, or plain bcrypt), AES-256-GCM for symmetric encryption, and constant-time string comparisons. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: AI uses MD5/SHA256 for password hashing, ECB mode for AES encryption, or standard '===' for secret comparison. Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftAI uses MD5/SHA256 for password hashing, ECB mode for AES encryption, or standard '===' for secret comparison.Critical
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Never use plain SHA-256 or MD5 for password storage

Never use plain SHA-256 or MD5 for password storage; use Argon2id with memory >= 64MB.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Use AES-256-GCM with a unique 12-byte initialization vector (IV) per encryption operation.

Use AES-256-GCM with a unique 12-byte initialization vector (IV) per encryption operation.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Use constant-time comparison (crypto.timingSafeEqual) when validating webhook signatures or secret tokens.

Use constant-time comparison (crypto.timingSafeEqual) when validating webhook signatures or secret tokens.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Cryptographic Standards & Password Hashing Standard critical for AI coding assistants?

Guardrails for modern cryptography: Argon2id for password hashing (no MD5, SHA-1, or plain bcrypt), AES-256-GCM for symmetric encryption, and constant-time string comparisons. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (AI uses MD5/SHA256 for password hashing, ECB mode for AES encryption, or standard '===' for secret comparison.) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

Open Specification · CC BY 4.0·Free for Personal, Team & Commercial In-Repo Use

Citation & Open Source Attribution Guidelines

This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

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