# HTTP Message Signatures (RFC 9421)

**Canonical:** https://apis.io/apis/agent-readiness/rfc-9421-http-message-signatures/  
**Provider:** Agent Readiness — https://apis.io/providers/agent-readiness/  
**Base URL:** https://www.rfc-editor.org/rfc/rfc9421.html  
**Documentation:** https://www.rfc-editor.org/rfc/rfc9421.html

HTTP Message Signatures (RFC 9421) is one of 15 APIs that [Agent Readiness](https://apis.io/providers/agent-readiness/) publishes on the [APIs.io](https://apis.io/) network. Tagged areas include Identity, Bot Auth, Agent Readiness, and Standards. The published artifact set on APIs.io includes API documentation.

A cryptographic signature scheme for HTTP messages, used by the emerging web-bot-auth profile to let agents authenticate themselves to origins. Provider support for verifying or surfacing RFC 9421 signatures is a forward-looking agent-readiness signal.

## Machine-readable artifacts (1)

- **Documentation** — https://www.rfc-editor.org/rfc/rfc9421.html

## Other Agent Readiness APIs (12)

- [Model Context Protocol (MCP)](https://apis.io/apis/agent-readiness/model-context-protocol/)
- [OpenAPI Specification](https://apis.io/apis/agent-readiness/openapi-initiative/)
- [AsyncAPI Specification](https://apis.io/apis/agent-readiness/asyncapi-initiative/)
- [JSON Schema](https://apis.io/apis/agent-readiness/json-schema/)
- [Agent Skills](https://apis.io/apis/agent-readiness/agent-skills/)
- [/.well-known/api-catalog (RFC 9727)](https://apis.io/apis/agent-readiness/well-known-api-catalog/)
- [Web Bot Auth (draft)](https://apis.io/apis/agent-readiness/web-bot-auth/)
- [Content-Usage / AIPREF (IETF AIPREF WG)](https://apis.io/apis/agent-readiness/content-usage-aipref/)
- [Cloudflare Content Signals](https://apis.io/apis/agent-readiness/cloudflare-content-signals/)
- [APIs.json](https://apis.io/apis/agent-readiness/apis-json/)
- [OpenID Connect](https://apis.io/apis/agent-readiness/openid-connect/)
- [Stripe API (reference provider)](https://apis.io/apis/agent-readiness/stripe-api/)

## Tags

Identity, Bot Auth, Agent Readiness, Standards

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Profiled by [API Evangelist](https://apievangelist.com) and published on [APIs.io](https://apis.io/apis/agent-readiness/rfc-9421-http-message-signatures/). The API's provider profile, Kin Score and agent-readiness rating are at https://apis.io/providers/agent-readiness/.