About
.CER files are PEM file with the cer extension to indicate that they hold a certificate.
They have the public key, they don't have the private key in a Public Key Cryptography model
The format is defined in the x509 specification.
This is one of the many crypto files.
Example
-----BEGIN CERTIFICATE-----
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UjEOMAwGA1UEBxMFUGFyaXMxHDAaBgNVBAoTE1BTQSBQZXVnZW90IENpdHJvZW4xFzAVBgNVBAsT
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b2dyYW1zX1BhcnRuZXJzLnBkZjAdBgNVHSUEFjAUBggrBgEFBQcDAgYIKwYBBQUHAwEwDgYDVR0P
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hlFodHRwOi8vaW5mb2NlcnQucHNhLXBldWdlb3QtY2l0cm9lbi5jb20vUFNBX1BldWdlb3RfQ2l0
cm9lbl9Qcm9ncmFtc19QYXJ0bmVycy5jcnQwDQYJKoZIhvcNAQELBQADggEBAEPoyPVt+OqGlzwy
XJ2zSFDFmVn66RAsnnk15+KaJezXPfu1V03gBVIib3eTmJbnNpHXDAKeuvvFjnB+SoaXbEXNbRLD
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-----END CERTIFICATE-----