openssl-selfsigned-cert

---
name: openssl-selfsigned-cert
description: Guides the creation of self-signed SSL/TLS certificates using OpenSSL, including key generation, certificate creation, combined PEM files, and verification scripts. This skill should be used when tasks involve generating self-signed certificates, creating SSL certificate infrastructure, or writing certificate verification scripts.
license: MIT
---

# OpenSSL Self-Signed Certificate Creation

This skill provides guidance for creating self-signed SSL/TLS certificates using OpenSSL command-line tools, including proper verification and scripting approaches.

## Core Workflow

### Step 1: Create Directory Structure

Establish the output directory before generating any files:

```bash
mkdir -p /path/to/certs
```

### Step 2: Generate Private Key

Generate an RSA private key (2048-bit minimum, 4096-bit recommended for production):

```bash
openssl genrsa -out /path/to/certs/server.key 2048
```

### Step 3: Create Self-Signed Certificate

Generate the certificate using the private key:

```bash
openssl req -new -x509 -key /path/to/certs/server.key -out /path/to/certs/server.crt -days 365 -subj "/CN=localhost"
```

Adjust the `-subj` parameter as needed for the use case. Common fields:
- `/CN=` - Common Name (domain or hostname)
- `/O=` - Organization
- `/OU=` - Organizational Unit
- `/C=` - Country (2-letter code)
- `/ST=` - State/Province
- `/L=` - Locality/City

### Step 4: Create Combined PEM File (if required)

Combine the key and certificate into a single PEM file:

```bash
cat /path/to/certs/server.key /path/to/certs/server.crt > /path/to/certs/combined.pem
```

### Step 5: Verify Generated Files

Verify the certificate and key are valid and matching:

```bash
# Verify certificate
openssl x509 -in /path/to/certs/server.crt -text -noout

# Verify key
openssl rsa -in /path/to/certs/server.key -check -noout

# Verify key matches certificate (modulus should match)
openssl x509 -noout -modulus -in /path/to/certs/server.crt | openssl md5
openssl rsa -noout -modulus -in /path/to/certs/server.key | openssl md5
```

## Writing Verification Scripts

When creating Python scripts for certificate verification, follow these critical guidelines:

### Prefer Standard Library Over External Dependencies

**Avoid external dependencies** like `cryptography` unless absolutely necessary. The script must work in the target execution environment without relying on virtual environments or pip-installed packages.

**Recommended approaches (in order of preference):**

1. **Use `subprocess` to call OpenSSL commands** - Most reliable, no dependencies:

```python
import subprocess

def verify_certificate(cert_path):
    """Verify certificate using OpenSSL subprocess calls."""
    result = subprocess.run(
        ["openssl", "x509", "-in", cert_path, "-text", "-noout"],
        capture_output=True,
        text=True
    )
    return result.returncode == 0, result.stdout
```

2. **Use Python's built-in `ssl` module** - Standard library, always available:

```python
import ssl

def load_certificate(cert_path):
    """Load and parse certificate using ssl module."""
    context = ssl.create_default_context()
    context.load_cert_chain(certfile=cert_path)
    return True
```

3. **If external libraries are required**, install system-wide (not in virtual environment):

```bash
pip install cryptography  # Not: uv add, pip install in venv
```

### Script Execution Environment

**Critical consideration:** Test scripts the same way they will be executed in the final environment.

- If the test runs `python /path/to/script.py`, verify with exactly that command
- Do NOT rely on `uv run python` or virtual environment activation
- System Python must have access to all required modules

### Complete Python Script Template

```python
#!/usr/bin/env python3
"""Certificate verification script using only standard library."""

import subprocess
import sys
import os

def verify_certificate(cert_path):
    """Verify a certificate file exists and is valid."""
    if not os.path.exists(cert_path):
        return False, f"Certificate file not found: {cert_path}"

    result = subprocess.run(
        ["openssl", "x509", "-in", cert_path, "-text", "-noout"],
        capture_output=True,
        text=True
    )

    if result.returncode != 0:
        return False, f"Invalid certificate: {result.stderr}"

    return True, result.stdout

def verify_key(key_path):
    """Verify a private key file exists and is valid."""
    if not os.path.exists(key_path):
        return False, f"Key file not found: {key_path}"

    result = subprocess.run(
        ["openssl", "rsa", "-in", key_path, "-check", "-noout"],
        capture_output=True,
        text=True
    )

    if result.returncode != 0:
        return False, f"Invalid key: {result.stderr}"

    return True, "Key is valid"

def verify_key_cert_match(key_path, cert_path):
    """Verify that a key and certificate match."""
    key_modulus = subprocess.run(
        ["openssl", "rsa", "-noout", "-modulus", "-in", key_path],
        capture_output=True,
        text=True
    )

    cert_modulus = subprocess.run(
        ["openssl", "x509", "-noout", "-modulus", "-in", cert_path],
        capture_output=True,
        text=True
    )

    if key_modulus.stdout == cert_modulus.stdout:
        return True, "Key and certificate match"
    return False, "Key and certificate do not match"

if __name__ == "__main__":
    # Example usage - adjust paths as needed
    cert_path = "/path/to/server.crt"
    key_path = "/path/to/server.key"

    success, msg = verify_certificate(cert_path)
    print(f"Certificate: {'PASS' if success else 'FAIL'} - {msg[:100] if success else msg}")

    success, msg = verify_key(key_path)
    print(f"Key: {'PASS' if success else 'FAIL'} - {msg}")

    success, msg = verify_key_cert_match(key_path, cert_path)
    print(f"Match: {'PASS' if success else 'FAIL'} - {msg}")
```

## Common Pitfalls

### 1. Virtual Environment Isolation

**Problem:** Installing dependencies in a virtual environment (venv, uv) that won't be available when the script runs in the test/production environment.

**Solution:** Either use standard library only, or install dependencies system-wide with `pip install` (not `uv add` or `pip install` inside an activated venv).

### 2. Incomplete File Writes

**Problem:** File write operations may be truncated or incomplete.

**Solution:** Always verify file contents after writing critical files:
```bash
cat /path/to/file  # Verify contents
wc -l /path/to/file  # Verify line count
```

### 3. Testing in Wrong Environment

**Problem:** Running `uv run python script.py` succeeds but `python script.py` fails.

**Solution:** Always test with the exact command that will be used in production/testing. If tests run `python /app/script.py`, verify with exactly that command.

### 4. Assuming OpenSSL Availability

**Problem:** Script assumes OpenSSL is installed and in PATH.

**Solution:** Check for OpenSSL availability at script start:
```python
import shutil
if not shutil.which("openssl"):
    sys.exit("Error: OpenSSL not found in PATH")
```

## Verification Checklist

Before declaring the task complete:

1. All required files exist and have correct content
2. Certificate is valid: `openssl x509 -in cert.crt -text -noout` succeeds
3. Key is valid: `openssl rsa -in key.key -check -noout` succeeds
4. Key and certificate modulus match
5. Combined PEM contains both key and certificate (if required)
6. Python script runs successfully with system Python (not venv)
7. All file paths in scripts match actual file locations