What is .cer?

.cer files typically contain X.509 digital certificates that associate a public key with a domain, organization, or user identity. They are used in TLS/SSL, VPNs, code signing, and other secure communication scenarios, and may be encoded as either binary DER or text-based PEM.

This quick guide explains when to use .cer files, how to open them on any device, and how to share them instantly with FileXhost.

When to use .cer files

You are installing or distributing SSL/TLS certificates for websites, APIs, or services.
You need to import a certificate into an operating system, browser, or application trust store.
You are working with certificate chains from a certificate authority that ships files with the .cer extension.
You want to share a public certificate so clients or devices can validate secure connections.

How to open .cer files

CER files can be opened with certificate viewers and management tools such as OpenSSL, Keychain Access on macOS, the Windows Certificate Manager, or Linux utilities like openssl and certutil. Double-clicking a CER file on many systems opens a GUI viewer that shows the certificate subject, issuer, validity period, and key usage. When you upload CER files to FileXhost, they can be shared as downloadable certificate artifacts for operators and developers.

Algorithm details

CER files store X.509 certificates encoded using ASN.1 and serialized as DER (binary) or PEM (Base64 with header/footer lines). The certificate structure includes the subject, issuer, public key, validity dates, serial number, signature algorithm, and extensions that describe how the certificate may be used (such as server authentication or code signing). Private keys are not included in CER files and must be stored separately.

Browser & platform support

Desktop: Browsers use certificates internally for HTTPS but do not execute CER files. Users can import CER files into browser or OS trust stores to trust specific CAs or certificates.
Mobile: Mobile platforms allow importing CER files into system certificate stores for enterprise or custom CA trust.
OS: Windows, macOS, and Linux offer certificate managers and command-line tools to inspect and manage CER files.

Format comparison

Feature	Details
CER vs CRT	CER and CRT are largely interchangeable naming conventions for certificate files; either may be PEM or DER encoded, depending on the issuing platform.
CER vs PEM	PEM describes a text-based container format, while CER is a file extension. A CER file can internally use PEM encoding or raw binary DER.
CER vs KEY	CER holds public certificate data, whereas KEY files store private keys that must be kept secret and never shared publicly.
CER vs PFX/P12	PFX/P12 bundles certificates and private keys together in a password-protected container; CER typically contains only the certificate.

How to create cer files

Certificate Authorities: Public and private CAs issue CER files as part of the certificate enrollment process.
Enterprise PKI: Internal PKI systems generate and distribute CER files for corporate devices, VPNs, and services.
OpenSSL: Use OpenSSL to generate CSRs and receive CER files from CAs, or to export certificates from existing stores.
Development Tools: Local development environments may create self-signed CER files for testing secure endpoints.

How to convert cer files

FileXhost: Store and distribute CER files via FileXhost so infrastructure and security teams can download and install them where needed.
OpenSSL CLI: Convert between CER, CRT, PEM, and DER encodings or package certificates into PFX/P12 containers.
Certificate Managers: Use OS-specific tools to import CER files and export to other formats or keystores.
Online Tools: Some web utilities can parse and display CER contents, but avoid uploading sensitive or private material.

Advantages & disadvantages

Advantages

Standard way to distribute public X.509 certificates for many platforms
Recognized by operating systems, web servers, and certificate management tools
Works alongside PEM, CRT, and PFX in typical TLS and PKI workflows

Disadvantages

Extension and encoding differences (CER, CRT, PEM, DER) can be confusing in practice
Incorrect installation or missing intermediate certificates can lead to trust warnings
Managing renewals, chains, and deployment requires careful operational processes

Tools & software

CLI Tools

OpenSSL, certutil, keytool, CFSSL

Servers/Proxies

Nginx, Apache HTTP Server, HAProxy, Envoy, Traefik

OS & Browsers

Windows Certificate Manager, macOS Keychain Access, Linux trust stores, browser certificate inspectors

Frequently asked questions

Is a CER file the same as a PEM file?

A CER file is a certificate file that may use PEM or DER encoding. PEM refers specifically to a Base64 text wrapper with header and footer lines; some CER files use this encoding, while others are raw binary DER.

Can I use CER and CRT interchangeably?

Often yes. Many systems accept either extension as long as the underlying encoding is correct. The difference is mostly naming convention rather than strict format.

Does a CER file contain my private key?

No. CER files normally contain only public certificate data. Private keys are stored in separate KEY or PEM files and must be protected carefully.

How do I install a CER file?

Use your server or operating system's certificate management tools to import the CER into the appropriate store (for example, web server configuration, system trust store, or application keystore), then reload or restart services as needed.

Technical specs

File type: Security
Extension: .cer
MIME type: application/x-x509-ca-cert, application/pkix-cert, application/octet-stream
Compression: Uncompressed
Max file size on FileXhost: Up to 25 MB per file on the free plan and up to 1 GB on Pro FileXhost accounts.

Upload your .cer file to FileXhost to get a clean, shareable URL in seconds. View the file in a modern browser, protect access with optional settings, and let others download it without any confusing ads or cluttered file pages.

Upload .cer file