Proposal: Agent Identity Verification for Agent Cards

## Summary

The A2A protocol defines a robust framework for agent-to-agent communication, but identity verification is currently left to external mechanisms. The Agent Card spec includes metadata fields, yet there is no standardized way for a receiving agent to cryptographically verify *who* it is communicating with.

## Problem

When Agent A discovers Agent B's Agent Card, it has no protocol-level mechanism to verify that the card is authentic. The current spec relies on transport-layer trust (HTTPS, OAuth), which covers *authorization* but not *identity verification* of the agent itself. This matters in:

- Open agent marketplaces where agents from different organizations interact
- Delegation chains where an agent acts on behalf of another
- Audit and compliance contexts requiring proof of which agent performed an action

## Proposal

Add an optional `verifiedIdentity` field to the Agent Card specification:

```json
{
"verifiedIdentity": {
"agentId": "agent_abc123",
"certificate": "",
"issuer": "getagentid.dev",
"verificationEndpoint": "https://getagentid.dev/api/v1/agents/verify"
}
}
```

This allows any A2A-compliant agent to inspect identity claims, verify certificates, and optionally enforce identity verification as a precondition for communication. The field is optional and fully backward-compatible.

## Reference Implementation

[AgentID](https://getagentid.dev) provides the building blocks for this:

- ECDSA P-256 certificates issued per agent
- Verification API for real-time credential validation
- Agent registry for discovery of verified agents
- Existing integrations with CrewAI, LangChain, and MCP

Source: [github.com/haroldmalikfrimpong-ops/getagentid](https://github.com/haroldmalikfrimpong-ops/getagentid)

Happy to submit a sample implementation showing A2A agents exchanging AgentID-signed Agent Cards.

Review the Agent Card specification in the A2A protocol documentation to understand the current structure. Examine the linked AgentID repository for its certificate issuance and verification API. The work involves designing and integrating an optional 'verifiedIdentity' field, ensuring backward compatibility. A successful change includes updating the spec and providing a sample implementation for exchanging signed Agent Cards.

Scritto dal modello di indicizzazione a partire dal testo della issue.