.SRL File Extension
OpenSSL Security Certificate Serial Number
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An SRL file contains a serial number generated while signing an OpenSSL certificate. It is used to uniquely identify a signed certificate. SRL files use the same base filename as a certificate's .PEM, .CER, and .KEY files.
More Information
To certify that users' information is being handled securely, web server owners must install an SSL certificate. OpenSSL allows users to create their own SSL certificates. As part of installing an OpenSSL certificate, users must "sign" the certificate. Signing the certificate produces an SRL file, which contains the certificate's unique serial number.
Every time a user creates a signed certificate, they must create a new SRL file for that certificate. When a user creates their first OpenSSL certificate, they use the -CAcreateserial command to create that certificate's SRL file. When a user creates subsequent certificates, they use the -CAserial command and specify the path to their original SRL file to create a new SRL file that contains an incremented serial number. The serial numbers that SRL files contain are written using the hexadecimal system.
NOTE: You can view a website's SSL certificate serial number by clicking the Lock icon that appears next to the website's URL and then viewing the website's security certificate details.
How to open an SRL file
You can open an SRL file in any text editor, such as Microsoft Notepad (Windows), Apple TextEdit (Mac), or GitHub Atom (cross-platform). However, you should not edit the text your SRL file contains. Editing your SRL file may cause your SSL security certificate to function incorrectly.