Distinguished encoding rules
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DER or Distinguished Encoding Rules is a method for encoding a data object, such as an X.509 certificate, to be digitally signed or to have its signature verified.
The Distinguished Encoding Rules of ASN.1 is an International Standard drawn from the constraints placed on BER encodings by X.509. DER encodings are valid BER encodings. DER is the same thing as BER with all but one sender's options removed. For example, in BER a boolean value of true can be encoded in 255 ways, while in DER there is only one way to encode a boolean value of true.
DER is used in cryptography to ensure that a structure that needs to be digitally signed produces a unique serialized representation. Loosely put, DER can be seen as a canonical form of BER (see also Canonical encoding rules).