PEM Format
The PEM format is the most common format that Certificate Authorities issue certificates in. PEM certificates usually have extentions such as .pem, .crt, .cer, and .key. They are Base64 encoded ASCII files and contain "-----BEGIN CERTIFICATE-----" and "-----END CERTIFICATE-----" statements. Server certificates, intermediate certificates, and private keys can all be put into the PEM format.
Apache and other similar servers use PEM format certificates. Several PEM certificates, and even the private key, can be included in one file, one below the other.
DER Format
The DER format is simply a binary form of a certificate instead of the ASCII PEM format. It sometimes has a file extension of .der but it often has a file extension of .cer so the only way to tell the difference between a DER .cer file and a PEM .cer file is to open it in a text editor and look for the BEGIN/END statements. All types of certificates and private keys can be encoded in DER format. DER is typically used with Java platforms.
HTTPS только на ассиметричных ключахречи никакой не было, надо таки внимательнее читать. Речь шла ровно про то, что если вместо DH использовать, например RSA для key exchange, то для расшифровки будет достаточно обладать закрытым ключом.
Где-то в среднем раз в полгода происходит авария одной из сотен железок, полного понимания причин которой нет