> For clean Markdown of any page, append .md to the page URL.
> For a complete documentation index, see https://amer.developers.trustly.com/integrate/api-fundamentals/llms.txt.
> For AI client integration (Claude Code, Cursor, etc.), connect to the MCP server at https://amer.developers.trustly.com/_mcp/server.

# API fundamentals

Trustly's API uses OAuth for authentication and cryptographic signatures for request integrity. Use the sandbox environment to test your integration before going live.

#### [Authentication and OAuth](/integrate/api-fundamentals/authentication-and-oauth)

OAuth login authorization flows for bank-side authentication.

#### [Secure requests and signature validation](/integrate/api-fundamentals/secure-requests-and-signature-validation)

Generate and validate cryptographic signatures for API requests.

#### [Idempotency](/integrate/api-fundamentals/idempotency)

Prevent duplicate operations with unique merchant references.

#### [Testing](/integrate/api-fundamentals/testing)

Use the sandbox environment and demo banks to simulate transactions.

> Authenticate requests, validate signatures, and test your integration in the Trustly sandbox.

## Docs

- [About OAuth authentication](https://amer.developers.trustly.com/integrate/api-fundamentals/authentication-and-oauth.md)
- [Secure requests and validate signatures](https://amer.developers.trustly.com/integrate/api-fundamentals/secure-requests-and-signature-validation.md)
- [Generate request signatures](https://amer.developers.trustly.com/integrate/api-fundamentals/secure-requests-and-signature-validation/generate-request-signatures.md)
- [Validate the redirect signature](https://amer.developers.trustly.com/integrate/api-fundamentals/secure-requests-and-signature-validation/validate-the-redirect-signature.md)
- [Validate the notification signature](https://amer.developers.trustly.com/integrate/api-fundamentals/secure-requests-and-signature-validation/validate-the-notification-signature.md)
- [Encrypt a field value](https://amer.developers.trustly.com/integrate/api-fundamentals/secure-requests-and-signature-validation/encrypt-a-field-value.md)
- [Idempotency](https://amer.developers.trustly.com/integrate/api-fundamentals/idempotency.md)
- [Testing](https://amer.developers.trustly.com/integrate/api-fundamentals/testing.md)
- [Status codes and type definitions](https://amer.developers.trustly.com/integrate/api-fundamentals/status-codes-and-type-definitions.md)