What Are the Mainstream SSL Certificate Formats?

The mainstream SSL certificate formats are CRT/CER/PEM, PFX, and JKS, covering core scenarios such as Web servers, Java, and Windows.

Format	Extension	Common Use Cases
CRT / CER / PEM	.crt .cer .pem	Mainstream Web servers including Nginx, Apache, and Tomcat; uses Base64 text encoding and can contain certificates and certificate chains
PFX / PKCS#12	.pfx .p12	Windows IIS servers; a binary format that can contain certificates, private keys, and certificate chains simultaneously
JKS	.jks .keystore	Java ecosystem (Tomcat, Spring Boot, etc.); native Java keystore format

How to choose?

• Nginx / Apache / Caddy: Use the .crt or .pem format
• IIS: Use the .pfx format
• Java applications: Use the .jks keystore, or import the .pfx before use

After downloading the certificate, you can convert it between formats as needed (for example, using OpenSSL or keytool command line tools).