Agent-Skills.md

Agent Skills: Implementing End-to-End Encryption for Messaging

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

UncategorizedID: autohandai/community-skills/implementing-end-to-end-encryption-for-messaging

Repository

autohandai/community-skills
autohandaiLicense: Apache-2.0
103

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pnpm dlx add-skill https://github.com/autohandai/community-skills/tree/HEAD/implementing-end-to-end-encryption-for-messaging

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

Skill Metadata

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

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