Initial commit - 611 cybersecurity skills across all subdomains

skills/implementing-end-to-end-encryption-for-messaging/SKILL.md

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+---
+name: implementing-end-to-end-encryption-for-messaging
+description: End-to-end encryption (E2EE) ensures that only the communicating parties can read messages, with no intermediary (including the server) able to decrypt them. This skill implements a simplified version
+domain: cybersecurity
+subdomain: cryptography
+tags: [cryptography, encryption, e2e, messaging, signal-protocol]
+version: "1.0"
+author: mahipal
+license: MIT
+---
+# Implementing End-to-End Encryption for Messaging
+
+## Overview
+
+End-to-end encryption (E2EE) ensures that only the communicating parties can read messages, with no intermediary (including the server) able to decrypt them. This skill implements a simplified version of the Signal Protocol's Double Ratchet algorithm, using X25519 for key exchange, HKDF for key derivation, and AES-256-GCM for message encryption.
+
+## Objectives
+
+- Implement X25519 Diffie-Hellman key exchange for session establishment
+- Build the Double Ratchet key management algorithm
+- Encrypt and decrypt messages with per-message keys
+- Implement forward secrecy (compromise of current key does not reveal past messages)
+- Handle out-of-order message delivery
+- Implement key agreement using X3DH (Extended Triple Diffie-Hellman)
+
+## Key Concepts
+
+### Signal Protocol Components
+
+| Component | Purpose | Algorithm |
+|-----------|---------|-----------|
+| X3DH | Initial key agreement | X25519 |
+| Double Ratchet | Ongoing key management | X25519 + HKDF + AES-GCM |
+| Sending Chain | Per-message encryption keys | HMAC-SHA256 chain |
+| Receiving Chain | Per-message decryption keys | HMAC-SHA256 chain |
+| Root Chain | Derives new chain keys on DH ratchet | HKDF |
+
+### Forward Secrecy
+
+Each message uses a unique encryption key derived from a ratcheting chain. After a key is used, it is deleted, ensuring that compromise of the current state does not reveal previously sent/received messages.
+
+## Security Considerations
+
+- Delete message keys immediately after decryption
+- Implement message ordering and replay protection
+- Use authenticated encryption (AES-GCM) for all messages
+- Protect identity keys with device-level security
+- Verify identity keys out-of-band (safety numbers)
+
+## Validation Criteria
+
+- [ ] X25519 key exchange produces shared secret
+- [ ] Messages encrypt and decrypt correctly between two parties
+- [ ] Different messages produce different ciphertexts
+- [ ] Forward secrecy: old keys cannot decrypt new messages
+- [ ] Out-of-order messages can be decrypted
+- [ ] Tampered messages are rejected by authentication