Question 1

What is the difference between .cer, .crt and .pem?

Accepted Answer

The extensions are habits, not formats. Any of them can hold a certificate as text (PEM, starting with "-----BEGIN CERTIFICATE-----") or as binary (DER). This page reads either, whatever the file is called.

Question 2

Does it check that the certificate is genuine?

Accepted Answer

No. It shows what the certificate says about itself; it does not check its signature against a trusted authority or whether it has been revoked. Compare its SHA-256 fingerprint with one you got from the owner by another route to be sure it is the right one.

Question 3

My .pem file has a private key in it. Is that safe here?

Accepted Answer

The key is never shown or used; the page only says that the file holds one. Nothing is uploaded. The file should still be kept secret, since anyone who has the private key can pose as the certificate's owner.

Question 4

Can it open a .pfx or .p12 file?

Accepted Answer

No. Those bundle a certificate with its private key behind a password, and opening one here would mean typing that password into a web page. Use your operating system's certificate manager or OpenSSL for those.

Question 5

Is my certificate uploaded?

Accepted Answer

No. The file is read by this browser tab on your own device, and the fingerprints are worked out there too.

Free online certificate viewer: open CER, CRT and PEM files