CMP, SCEP, EST, or ACME? Choosing the Right Certificate Management Protocol

SCEP – The Old Reliable Workhorse

Simple Certificate Enrollment Protocol (SCEP) RFC 8894 was originally developed in 2000 for Cisco by Verisign to simplify the certificate enrollment process for network devices. It became widely used due to its ease of deployment and support for automated certificate issuance. Initially, it lacked strong security mechanisms, but improvements over time have enhanced its security model. Today, it remains a popular choice for routers, firewalls, and VPN devices requiring a straightforward certificate enrollment process. However, a significant limitation of SCEP is that it only supports RSA and does not support ECDSA, making it less adaptable to modern cryptographic standards. Additionally, SCEP does not support certificate revocation, which is a major shortcoming in its certificate lifecycle management. A key advantage of SCEP is that Microsoft InTune supports the protocol, making it useful for enterprise device management.

EST – The Modern Upgrade to SCEP

Enrollment over Secure Transport (EST) was introduced in RFC 7030 (2013) as a successor to SCEP, addressing its security weaknesses. EST leverages TLS-based authentication for secure certificate enrollment and renewal, making it a preferred option for enterprise PKI environments that demand strong security. It has gained adoption in cloud environments, enterprise security systems, and IoT deployments due to its improved authentication model and enhanced security measures. EST supports multiple authentication methods, including HTTP Basic/Digest Authentication, Client Authentication, and Certificate-Less TLS Authentication. Additionally, EST supports both RSA and ECDSA based keys, offering greater cryptographic flexibility. However, EST does not support certificate revocation, which is a limitation in environments requiring full certificate lifecycle management.

ACME – The Web’s Favorite Certificate Bot

Automated Certificate Management Environment (ACME) was introduced by the Internet Security Research Group (ISRG) and became widely recognized when Let’s Encrypt adopted it to provide free SSL/TLS certificates. It was standardized in RFC 8555 (2019) and is designed to automate the issuance and renewal of digital certificates for web servers. ACME has become a crucial protocol for securing websites, reducing the burden of manual certificate management, and ensuring continuous encryption across the internet.

Commonalities among CMP, SCEP, EST & ACME

Despite their differences, these protocols share common characteristics:

• Certificate Enrollment & Management: All four protocols are designed to automate certificate issuance, renewal, and revocation but not all protocol provide all of these features

• PKI Integration: They interact with Certificate Authorities (CAs) to issue and manage X.509 certificates.

• Security Measures: They employ cryptographic mechanisms, such as digital signatures and encryption, to ensure secure certificate transactions.

• Automation Capabilities: They reduce manual certificate management efforts, enhancing efficiency and security.

Understanding Each Protocol

1. Certificate Management Protocol (CMP)

Purpose: CMP is a robust protocol designed for complex PKI environments, supporting extensive certificate lifecycle operations, including enrollment, renewal, revocation, and key recovery.

Key Features:

• Supports both initial certificate enrollment, revocation and renewal.

• Uses strong authentication mechanisms (e.g., digital signatures, shared secrets).

• Can work with multiple PKI components, such as Registration Authorities (RAs) and CAs.

Pros:

✔ Highly secure with multiple authentication mechanisms.
✔ Flexible and supports complex PKI deployments.
✔ Supports key recovery and multiple transport mechanisms (HTTP, TCP, email).
✔ Used in 3GPP LTE frameworks and railway communication networks for secure certificate management.

Cons:

✖ More complex to implement compared to other protocols.
✖ Requires extensive configuration and management.

2. Simple Certificate Enrollment Protocol (SCEP)

Purpose: SCEP was developed to provide a simplified and automated certificate enrollment process, mainly for network devices such as routers, switches, and VPN gateways.

Key Features:

• Uses HTTP-based transport for certificate requests.

• Primarily designed for network devices.

• Relies on shared secrets for authentication.

Pros:

✔ Easy to implement and widely supported.
✔ Lightweight and ideal for constrained devices.
✔ Works well in large-scale enterprise environments.
✔ Microsoft InTune supports SCEP protocol.

Cons:

✖ Weaker security model compared to newer protocols (e.g., EST, CMP).
✖ Lacks support for modern authentication methods.
✖ Does not provide robust certificate lifecycle management.
✖ Only supports RSA and does not support ECDSA, limiting cryptographic flexibility.
✖ No support for certificate revocation, making it unsuitable for environments requiring strict lifecycle management.

3. Enrollment over Secure Transport (EST)

Purpose: EST is designed as a modern replacement for SCEP, offering improved security and flexibility in certificate enrollment over HTTPS.

Key Features:

• Uses TLS-based mutual authentication for certificate requests.

• Supports strong authentication with certificates instead of shared secrets.

• Provides additional security features like re-enrollment and CA certificates retrieval.

• Supports HTTP Basic/Digest Authentication, Client Authentication, and Certificate-Less TLS Authentication.

• Supports RSA and ECDSA based keys.

Pros:

✔ Stronger security model than SCEP (uses TLS authentication).
✔ More suitable for enterprise environments requiring secure automation.
✔ Supports both client and server-side authentication.

Cons:

✖ More complex than SCEP, requiring certificate-based authentication.
✖ Not as widely adopted as SCEP in legacy systems.
✖ No support for certificate revocation.

4. Automated Certificate Management Environment (ACME)

Purpose: ACME is designed for automating certificate issuance and renewal for web servers, making it a widely used protocol for securing internet communications.

Key Features:

• Developed by ISRG and widely adopted by Let’s Encrypt.

• Uses HTTP and DNS challenges to validate domain ownership.

• Fully automated certificate issuance and renewal.

• Designed primarily for web server certificates.

Pros:

✔ Completely automated, reducing administrative effort.
✔ Open-source and widely adopted.
✔ Secure validation process using HTTP/DNS challenges.

Cons:

✖ Primarily designed for web servers, limiting broader PKI applications.
✖ Not suitable for client-side or device certificate enrollment.
✖ Limited customization compared to CMP or EST.