euiccSignature1 [APPLICATION 55] OCTET STRING, --EUICC_Sign1, tag 5F37
the above is what I have in an ASN.1 data object. I'm going to DER encode it. can somebody please tell me what I have to do with that "tag 5F37" and what is inside that square bracket?
Check out the BER tag encoding rules first, then DER is just a subset of it, for example here. Work the bits together and you will get 0x5F37. The square brackets are the encoding format for an Application, the its an application specific Octet String and has a certain meaning according to the application it is used in. For more infos on the notation look here