# Developer documentation

- [Protocol description](/developer-documentation/protocol-description.md)
- [Authentication](/developer-documentation/protocol-description/authentication.md)
- [Transactions](/developer-documentation/protocol-description/transactions.md)
- [Challenge](/developer-documentation/protocol-description/challenge.md)
- [HAS for Applications](/developer-documentation/has-for-applications.md)
- [Connection](/developer-documentation/has-for-applications/connection.md)
- [Encryption key (auth\_key)](/developer-documentation/has-for-applications/encryption-key-auth_key.md)
- [Authentication](/developer-documentation/has-for-applications/authentication.md)
- [Authentication request](/developer-documentation/has-for-applications/authentication/authentication-request.md)
- [Authentication payload](/developer-documentation/has-for-applications/authentication/authentication-payload.md)
- [Authentication approval](/developer-documentation/has-for-applications/authentication/authentication-approval.md)
- [Authentication refusal](/developer-documentation/has-for-applications/authentication/authentication-refusal.md)
- [Errors](/developer-documentation/has-for-applications/authentication/errors.md)
- [Expiration](/developer-documentation/has-for-applications/authentication/expiration.md)
- [Transactions](/developer-documentation/has-for-applications/transactions.md)
- [Transaction request](/developer-documentation/has-for-applications/transactions/transaction-request.md)
- [Feedback to user](/developer-documentation/has-for-applications/transactions/feedback-to-user.md)
- [Transaction approval](/developer-documentation/has-for-applications/transactions/transaction-approval.md)
- [Transaction refusal](/developer-documentation/has-for-applications/transactions/transaction-refusal.md)
- [Errors](/developer-documentation/has-for-applications/transactions/errors.md)
- [Expiration](/developer-documentation/has-for-applications/transactions/expiration.md)
- [Challenge](/developer-documentation/has-for-applications/challenge.md)
- [Challenge request](/developer-documentation/has-for-applications/challenge/challenge-request.md)
- [Feedback to user](/developer-documentation/has-for-applications/challenge/feedback-to-user.md)
- [Challenge approval](/developer-documentation/has-for-applications/challenge/challenge-approval.md)
- [Challenge refusal](/developer-documentation/has-for-applications/challenge/challenge-refusal.md)
- [Errors](/developer-documentation/has-for-applications/challenge/errors.md)
- [Expiration](/developer-documentation/has-for-applications/challenge/expiration.md)
- [Attach](/developer-documentation/has-for-applications/attach.md)