Where can I read about DER certificate structure - fields and its registered values? Where is an official documentation for ASN1 structure and values of x509 certs?
Using OpenSSL command like openssl asn1parse -in server.der -inform DER I can see how the cert is structured, but, for example, value OBJECT :sha256WithRSAEncryption is HEX 2A 86 48 86 F7 0D 01 01 0B, or countryName identifier is 55 04 06 - where are all this values documented?
ASN.1 is defined by the International Telecommunication Union (the ITU). ASN.1 stands for Abstract Syntax Notation One, and is a schema definition language. DER, as well as its base form BER, are serialization formats of the data defined by the ASN.1-specified schema.
ASN.1 and DER/CER/BER are specified in ITU-T-X series standards, available at the official ITU website. Information about OIDs are available at http://oid-info.com/