When working with digital certificates in Windows environments, you'll frequently encounter both DER and Base64 encoded files with .cer extensions. The key difference:

• DER: Binary format certificate (ASN.1 DER encoded)
• Base64: ASCII text format (PEM-like but with .cer extension)

Here are three reliable approaches to convert your certificates:

Using OpenSSL (Command Line)

The most universal method:

openssl x509 -inform der -in certificate.cer -outform pem -out certificate_base64.cer

Windows Certificate Manager

1. Double-click the .cer file
2. Go to "Details" tab
3. Click "Copy to File"
4. Choose "Base-64 encoded X.509 (.CER)" format

PowerShell Conversion

For batch processing:

$cert = [System.Security.Cryptography.X509Certificates.X509Certificate2]::new("C:\path\to\cert.cer")
[System.Convert]::ToBase64String($cert.RawData) | Out-File "C:\path\to\output.cer"

After conversion, check your file:

----- BEGIN CERTIFICATE -----
[Base64 content]
----- END CERTIFICATE -----

Or use OpenSSL to verify:

openssl x509 -in converted.cer -text -noout

• Web server configurations requiring Base64 format
• Cross-platform certificate sharing
• Embedding certificates in configuration files

For bulk conversions, consider this Bash script:

#!/bin/bash
for file in *.cer; do
    openssl x509 -inform der -in "$file" -outform pem -out "${file%.*}_converted.cer"
done

---

When working with digital certificates, you'll commonly encounter two encoding formats:

• DER (Distinguished Encoding Rules): Binary format commonly used in Windows systems
• PEM/Base64: ASCII-armored format with BEGIN/END headers, widely used in Unix/Linux systems and web servers

Base64-encoded certificates are often required for:

• Apache/nginx web server configurations
• Cross-platform certificate sharing
• Embedding certificates in configuration files
• Certain programming language requirements (like Node.js)

Using OpenSSL (Command Line)

The most reliable method is using OpenSSL:

openssl x509 -inform der -in certificate.cer -out certificate_base64.cer -outform pem

Using Python

For programmatic conversion:

import base64

with open('certificate.cer', 'rb') as der_file:
    der_data = der_file.read()

pem_data = b"-----BEGIN CERTIFICATE-----\n"
pem_data += base64.b64encode(der_data)
pem_data += b"\n-----END CERTIFICATE-----\n"

with open('certificate_base64.cer', 'wb') as pem_file:
    pem_file.write(pem_data)

Using Windows CertMgr

For Windows users:

1. Double-click the .cer file
2. Go to Details tab
3. Click "Copy to File"
4. Choose "Base-64 encoded X.509 (.CER)"

After conversion, check the file contents:

cat certificate_base64.cer

You should see the Base64 content between BEGIN/END CERTIFICATE headers.

• Ensure the original DER file is valid before conversion
• Some systems may require the .pem extension even for Base64 content
• Watch for line endings (CRLF vs LF) when transferring between systems

For converting multiple certificates:

for cert in *.cer; do
    openssl x509 -inform der -in "$cert" -out "${cert%.*}_base64.cer" -outform pem
done

Here's how to use the converted certificate in different scenarios:

Node.js HTTPS Server

const https = require('https');
const fs = require('fs');

const options = {
  key: fs.readFileSync('server.key'),
  cert: fs.readFileSync('certificate_base64.cer')
};

https.createServer(options, (req, res) => {
  res.end('Hello secure world!');
}).listen(443);

Nginx Configuration

server {
    listen 443 ssl;
    server_name example.com;
    
    ssl_certificate /path/to/certificate_base64.cer;
    ssl_certificate_key /path/to/server.key;
    
    # ... other configuration
}