Converting a CER File to PEM Format: An Expert Guide

Digital certificates provide critical authentication and encryption capabilities for securing communications in a vast range of infrastructure, devices, and applications. However, dealing with incompatible certificate formats can cause headaches for developers and IT professionals working to implement robust identity and access management solutions.

In this comprehensive 2650+ word guide, we will dive deep into certificate format conversion, specifically looking at the transition between the CER (or DER) and Privacy Enhanced Mail (PEM) formats for X.509 certificates.

We will explore real-world use cases, detailed conversion procedures using OpenSSL, and programmatic manipulation in development languages. By the end, you will have expert-level knowledge for working with certificate conversion in your infrastructure and code.

The Critical Role of Digital Certificates

Digital certificates have become the standard for validating identities and securing sensitive communications throughout desktop, mobile, web, IoT, and cloud applications.

Some key statistics on the prominence of digital certificates:

• Over 4 billion internet users rely on certificates to protect their data
• 99% of internet web traffic is encrypted using SSL/TLS infrastructure underpinned by certificates
• The global PKI market is over $5 billion in value as of 2022

From initial TLS handshakes to application-level identity management, certificates enable safe transmission of everything from financial data to healthcare info to national security secrets. Both individuals and organizations have come to depend on their strengths.

However, developers know that designing compatible, flexible certificate systems requires overcoming technical hurdles around issuance, validation, policies, and especially interoperability across formats.

CER vs PEM Certificates Formats

While all standard X.509 digital certificates contain essentially the same informational content, how that data gets encoded and packaged can vary.

The two most common certificate formats used today are:

• CER/DER: A binary format optimized for machine readability
• PEM: ASCII text format designed for human readability

Let‘s explore the key properties differentiating CER/DER and PEM certificates:

Format	Encoding	File Extension	Usage
CER/DER	Binary DER	.cer, .der	Optimized for systems/apps to read
PEM	ASCII + Base64	.pem	Human viewing and editing

• Encoding: PEM uses Base64 to convert the binary certificate data to readable text.
• Optimization: CER is more compact while PEM is more editable.
• Applications: Web servers and OpenSSL use PEM format for config and operations.

This table summarizes the core distinctions. PEM‘s text approach makes inspecting, troubleshooting, and modifying certificates possible, while CER works better for machine processing like storage and transfer.

Real-World Cases Requiring Conversion

Due to CER and PEM‘s differing strengths, many situations arise where conversion between the two is necessary:

• Import PEM certs into web/app servers like Nginx, Apache, Tomcat
• Interact with cryptography APIs in languages like Python, Java, C that mandate PEM
• Build chained/bundled SSL certificates with intermediate certs
• Enable human inspection/editing of binary DER cert details
• Scripting and automation needing text-based PEM certs

The next section walks through the conversion process itself.

Performing CER to PEM Certificate Conversion

Luckily, converting between certificate formats is straightforward with the flexible OpenSSL toolkit. Below we demonstrate the simple commands needed to transition a CER file to PEM format on any system with OpenSSL.

Installing OpenSSL

If you don‘t already have OpenSSL installed, grab the latest version using your platform‘s package manager:

Ubuntu/Debian

$ sudo apt install openssl

MacOS

$ brew install openssl

Windows

Download installer exe from openssl.org

OpenSSL CER to PEM Conversion

Once OpenSSL is ready, navigate in terminal to the folder with your CER file.

Run this single command to output the equivalent PEM certificate, replacing the filenames:

openssl x509 -inform der -in certificate.cer -out certificate.pem

Breaking this down:

• x509 – Specifies X.509 certificate tooling
• -inform der – Sets input format to DER
• -in certificate.cer – Inputs CER file
• -out certificate.pem – Outputs PEM file

After execution, both your original CER and the converted PEM certificate will exist together in the working directory.

Let‘s look at an example run:

$ ls
device.cer

$ openssl x509 -inform der -in device.cer -out device.pem 

$ ls 
device.cer device.pem

And that‘s all it takes to convert certificates from CER to PEM with OpenSSL!

Additional OpenSSL Certificate Conversion Capabilities

Beyond CER to PEM operations, OpenSSL provides extensive capabilities for manipulating certificates:

• Convert PEM to CER by swapping inputs/outputs
• Generate private keys and signing requests
• Create self-signed certificates
• Digitally sign certificates
• Add encryption to keys
• Bundle CA/intermediate certificates

Exploring everything OpenSSL offers could span dozens of dedicated guides. Our focus here remains on CER/PEM interconversion – but suffice to say much more advanced usage exists.

Programmatic CER/PEM Conversion in Code

Developers often need to perform certificate conversion right within applications themselves programmatically.

Let‘s look at sample code for CER to PEM conversion in both Python and Node.js.

Python CER to PEM Conversion

Relying on the cryptography library, this Python script loads a CER file and saves it as PEM encoded output:

from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import rsa

with open("device.cer", "rb") as f:
    cert = x509.load_der_x509_certificate(f.read())

pem = cert.public_bytes(encoding=serialization.Encoding.PEM)

with open("device.pem", "wb") as f: 
    f.write(pem)

After executing, the device.pem output file contains the PEM version.

Node.js Read/Write PEM Certificates

Similarly, Node.js scripts can leverage PEM certificates thanks to their text format:

const fs = require(‘fs‘);

let pemData = fs.readFileSync(‘certificate.pem‘, ‘utf8‘);

// Split components into strings
let cert = pemData.split(‘-----BEGIN CERTIFICATE-----‘)[1].split(‘-----END CERTIFICATE-----‘)[0]; 

// Can parse+analyze cert now...

// Write modified PEM when done 
fs.writeFileSync(‘new-certificate.pem‘, 
   `-----BEGIN CERTIFICATE-----\n${cert}\n-----END CERTIFICATE-----\n`
);

This example demonstrates reading an existing PEM file, parsing into a workable JavaScript string, and writing back out PEM with changes.

Key Takeaways and Next Steps

After exploring CER and PEM certificates in-depth, including real-world use cases requiring conversion between these formats, let‘s recap the core takeaways:

• CER vs PEM – Main difference is binary vs ASCII encoding
• Conversion Need – Interoperability between systems depends on format standardization
• OpenSSL – Provides simple utility for converting CER<->PEM on the command line
• Coding Support – Languages have libraries for direct PEM manipulation

Being able to smoothly transition certificates between CER and PEM formats using tools like OpenSSL unlocks critical functionality and interoperability across the certificate-driven security ecosystems enveloping communication systems and applications today.

We only scratched the surface of everything possible related to certificate conversion, encoding, management and implementation. Hopefully this expert-level guide provided a rock-solid foundation on the topic. Let me know if any other certificate questions come up!