When working with secure systems, a common scenario involves Server A obtaining a certificate signed by Server B (acting as a Certificate Authority, or CA). But if the ultimate goal is to exchange public keys so that JWTs (JSON Web Tokens) signed by Server A can be verified by Server B, you might wonder: Do I really need to use a CSR (Certificate Signing Request)? Can’t I just send my public key directly?
The answer is: it depends on your use case and trust model. Here’s a breakdown of why you might still want to use a CSR, and when you can skip it.
What Is a CSR?
A CSR (Certificate Signing Request) is a formal request for a certificate that includes:
- A Public Key: The key for which the certificate is requested.
- Metadata: Information such as the organization name, domain, and other identifying details.
- A Digital Signature: The CSR is signed using the private key corresponding to the public key, proving ownership of the key pair.
The CA (Server B in this case) validates the CSR and signs it, issuing a certificate that binds the public key to the metadata and ensures trustworthiness.
Key Differences: CSR vs Direct Public Key Exchange
1. CSR Purpose
A CSR isn’t just about sending a public key. It’s a mechanism to:
- Verify the ownership of the private key.
- Attach additional identity information (e.g., the name or domain of the entity requesting the certificate).
- Enable a trusted certificate chain, which simplifies verification in distributed systems.
2. Direct Public Key Exchange
You can directly send a public key to Server B without using a CSR. In this approach:
- Server B can use the received public key to verify JWTs signed by Server A’s private key.
- There’s no built-in identity validation, certificate lifecycle management, or chain of trust.
While this works in certain scenarios, it has limitations and security trade-offs.
When to Use a CSR
Advantages of Using a CSR
Identity Validation
- The CSR ensures that the entity requesting the certificate (Server A) owns the corresponding private key and has been validated by a trusted CA (Server B).
Certificate Lifecycle Management
- Certificates can be renewed, revoked, and managed systematically. In contrast, a direct public key exchange lacks this infrastructure.
Chain of Trust
- A certificate issued via a CSR establishes a trusted relationship between Server A and Server B. This is especially important in environments with multiple parties or external systems.
Security Best Practices
- Using certificates aligns with widely accepted security standards, such as TLS and mutual authentication.
Use Case
If Server A and Server B are communicating in a public or distributed environment, or if Server B needs to validate Server A’s identity, using a CSR and signed certificate is the better choice.
When a Direct Public Key Exchange Is Enough
In simpler or controlled environments, you might not need the full CSR process. For example:
Closed Systems
- If Server A and Server B are part of the same internal system, and there’s no need for third-party validation, you can directly exchange public keys.
Simplified Trust
- If Server B already trusts Server A, sending the public key directly might be sufficient. Server B can store and use the key to verify JWTs without additional steps.
No Metadata Needed
- If no identity metadata (e.g., domain name or organization) is required, the CSR adds little value.
Trade-Offs
- No Verification: Server B must blindly trust the received public key.
- No Revocation or Renewal: Directly exchanged public keys lack lifecycle management.
- No Chain of Trust: There’s no mechanism to validate the authenticity of the public key.
Summary: To CSR or Not to CSR?
| Scenario | CSR Required? | Why |
|---|---|---|
| Public or distributed systems | Yes | Need for identity validation, lifecycle management, and chain of trust. |
| Closed or internal systems | No | Trust is already established, and the CSR process adds unnecessary overhead. |
| Need for lifecycle management (renewal, etc.) | Yes | Certificates provide systematic management of trust material. |
| Minimalistic trust exchange | No | Direct public key exchange is simpler and sufficient for closed systems. |
Conclusion
While skipping the CSR and directly exchanging public keys can work in simpler or internal systems, the CSR process provides essential security, validation, and lifecycle benefits in more complex or public-facing environments. The decision ultimately depends on the trust model, security requirements, and operational context of your system.
For secure systems, especially those that involve public networks or external parties, using a CSR to obtain a signed certificate is the more robust and widely accepted approach.