CSR Validation: Automated Checks

TL;DR

Automatically check your Certificate Signing Request (CSR) before submitting it to avoid delays and rejections. This guide shows you how to validate common CSR issues using simple tools and scripts.

1. Understand the Basics

A CSR contains information about your organisation and the domain you want to secure. Common problems include:

• Incorrect Domain Name: The domain in the CSR doesn’t match what you’re requesting a certificate for.
• Invalid Characters: Using unsupported characters in fields like Organisation or City.
• Missing Information: Required fields are left blank (e.g., Country Code).
• Incorrect Format: The CSR itself isn’t properly formatted according to the X.509 standard.

2. Manual Inspection

You can open a CSR file in a text editor. It will look like a long string of encoded text. While you can’t easily read it directly, you can identify basic issues:

• File Extension: Ensure the file has a .csr extension.
• Length: A typical CSR is several kilobytes in size. Very short or excessively long files are suspicious.

3. Using OpenSSL for Validation

OpenSSL is a powerful command-line tool available on most Linux systems and can be installed on Windows (e.g., via Chocolatey). Here’s how to use it:

Step 1: Extract CSR Information

Use the following command to decode the CSR and view its contents:

openssl req -text -noout -in your_csr.csr

Replace your_csr.csr with the actual filename.

Step 2: Check Domain Name

Examine the output for the Subject Alternative Name (SAN) field. This is where domain names are listed. Verify that your intended domain(s) are present and correctly spelled:

openssl req -text -noout -in your_csr.csr | grep 'Subject Alternative Name'

Step 3: Check Common Name

The Common Name (CN) field should contain the primary domain name:

openssl req -text -noout -in your_csr.csr | grep 'Subject:'

Step 4: Check Country Code

Verify that the Country Name (C) is a valid two-letter ISO country code:

openssl req -text -noout -in your_csr.csr | grep 'Country Name'

4. Online CSR Validators

Several websites offer free CSR validation tools. These can quickly identify common errors without requiring command-line knowledge:

• SSL Checker: https://www.sslchecker.com/csr-decoder
• DigiCert CSR Decoder: https://www.digicert.com/help/ (search for ‘CSR decoder’)

Simply upload your CSR file to the website and review the results.

5. Scripting Validation (Advanced)

For automated systems, you can create scripts to validate CSRs programmatically. Here’s a basic Python example using the cryptography library:

import cryptography.x509 as x509
from cryptography.hazmat.backends import default_backend

try:
    with open('your_csr.csr', 'rb') as f:
        csr = x509.load_pem_x509_csr(f.read(), default_backend())

    subject = csr.subject
    common_name = subject.get_attributes_for_oid(x509.NameOID.COMMON_NAME)[0].value
    country_code = subject.get_attributes_for_oid(x509.NameOID.COUNTRY_NAME)[0].value

    print(f'Common Name: {common_name}')
    print(f'Country Code: {country_code}')

except Exception as e:
    print(f'CSR validation failed: {e}')

Note: This is a simplified example. You’ll need to install the cryptography library (pip install cryptography) and handle potential errors more robustly in a production environment.

6. Common Fixes

• Incorrect Domain Name: Regenerate the CSR with the correct domain name using your web server or certificate authority’s instructions.
• Invalid Characters: Remove any unsupported characters from the Organisation, City, or other fields when regenerating the CSR.
• Missing Information: Ensure all required fields are filled in correctly during CSR generation.