SHA-1 Certificates & Intermediate CAs

TL;DR

No. While an intermediate CA can issue certificates using SHA-256, it doesn’t prevent it from also issuing certificates with the weaker SHA-1 hash algorithm. However, modern browsers and operating systems are actively phasing out support for SHA-1 certificates due to security vulnerabilities. Issuing SHA-1 certificates is strongly discouraged.

Understanding the Problem

You’re asking if an intermediate Certificate Authority (CA) using a strong hashing algorithm like SHA-256 can still issue end-entity certificates with the older, less secure SHA-1. The answer is technically yes, but it’s a bad practice.

Why It’s Possible

1. Hashing Algorithm per Certificate: Each certificate individually specifies which hashing algorithm was used to sign it. The intermediate CA doesn’t enforce a single hash algorithm for all certificates it issues.
2. CA Configuration: Intermediate CAs are often configured with multiple supported algorithms, including both SHA-256 and SHA-1 (though this is becoming increasingly rare).

Why It’s a Bad Idea

1. Browser Support: Major browsers like Chrome, Firefox, Safari, and Edge have either removed or are planning to remove support for SHA-1 certificates. This means users with these browsers will see security warnings or errors when visiting websites using SHA-1 certificates.
2. Security Risks: SHA-1 has known vulnerabilities that make it possible (though difficult) to create colliding hashes, potentially allowing attackers to forge certificates.
3. Compliance Issues: Many industry standards and regulations require the use of strong cryptography, which excludes SHA-1.

How to Check Certificate Hash Algorithm

You can check the hash algorithm used by a certificate using various tools:

1. OpenSSL: Use the following command in your terminal:

   openssl x509 -noout -text -hash -in your_certificate.pem

   This will output information about the certificate, including its hash algorithm (e.g., SHA256 or SHA1).

2. Web Browser: Most web browsers allow you to view a certificate’s details. Look for the ‘Signature Algorithm’ or similar field in the certificate information.
   • Chrome: Right-click on the padlock icon → Connection is secure → Certificate → Details
   • Firefox: Click on the padlock icon → More Information → View Certificate → Details

Steps to Avoid Issuing SHA-1 Certificates

1. CA Software Configuration: Configure your CA software (e.g., OpenSSL, Microsoft CA) to only support SHA-256 or stronger hashing algorithms for new certificate requests.
2. Template Restrictions: If using certificate templates, ensure they are configured to use SHA-256 or higher.
3. Regular Audits: Regularly audit your CA logs and issued certificates to identify any SHA-1 certificates that may have been inadvertently issued.

Conclusion

While technically possible, issuing end-entity certificates with SHA-1 from an intermediate CA using SHA-256 is strongly discouraged due to security risks, browser incompatibility, and compliance concerns. Focus on issuing certificates exclusively with SHA-256 or stronger algorithms.