What is CER?

CER files are X.509 certificates - identical content to CRT, just different extension. Contains public key, subject information, issuer (Certificate Authority), validity dates, and digital signature. CER is Microsoft's preferred extension (Windows Certificate Manager uses .cer). Can be encoded as: DER format (binary, preferred by Windows) or PEM format (Base64 ASCII text). Double-clicking .cer on Windows opens Certificate Import Wizard. Used interchangeably with .crt on many systems.

CER files are prevalent in Windows environments - Active Directory Certificate Services (AD CS), IIS web servers, Windows certificate stores. Export certificates from Windows as .cer (DER or Base64). Used for: SSL/TLS (HTTPS), code signing (Authenticode for .exe/.dll), email encryption (S/MIME), document signing, VPN authentication. CER vs CRT distinction is mostly conventional: Unix/Linux prefers .crt (PEM), Windows prefers .cer (DER or PEM). Both are X.509 certificates - functionally equivalent. Certificate Authorities issue both extensions interchangeably.

Did you know? CER and CRT are the same format - just different file extensions!

History

CER emerged as Microsoft's preferred extension for X.509 certificates in Windows environments, becoming standard for Windows certificate management.

Key Milestones

  • 1988: X.509 standard established
  • 1995: Windows certificate stores
  • 2000: IIS SSL certificate support
  • 2006: Code signing (Authenticode)
  • 2010s: Enterprise PKI adoption
  • Present: Universal Windows standard

Key Features

Core Capabilities

  • Windows Integration: Native support
  • DER/PEM Encoding: Binary or text
  • Certificate Store: Import/export
  • Code Signing: Authenticode support
  • SSL/TLS: IIS, Windows servers
  • S/MIME: Email encryption

Common Use Cases

Windows SSL

IIS web servers

Code Signing

Authenticode (EXE/DLL)

Enterprise PKI

Active Directory CS

Email

S/MIME encryption

Advantages

  • Native Windows support (double-click import)
  • Identical to CRT (universal compatibility)
  • DER binary format (compact)
  • Certificate Manager integration
  • Code signing (Authenticode)
  • Enterprise PKI standard
  • IIS web server native format

Disadvantages

  • Extension confusion with CRT
  • DER format not human-readable
  • Primarily Windows-centric
  • Unix/Linux prefer .crt extension
  • May require conversion for some tools
  • Same limitations as CRT (renewal, CA trust)

Technical Information

Format Specifications

Specification Details
File Extension .cer
Alternative .crt (identical content)
MIME Type application/x-x509-ca-cert
Standard ITU-T X.509
Encoding DER (binary) or PEM (Base64)
Platform Windows preferred, cross-platform

Common Tools

  • Windows: Certificate Manager (certmgr.msc), IIS Manager
  • Import: Double-click (Windows), certutil (CLI)
  • Conversion: OpenSSL (CER ↔ CRT, DER ↔ PEM)
  • Issuance: DigiCert, Let's Encrypt, AD CS