security - How does ASN.1 encode an object identifier?

Question

I am having trouble understanding the basic concepts of ASN.1.

If a type is an OID, does the corresponding number get actually encoded in the binary data?

For instance in this definition:

id-ad-ocsp         OBJECT IDENTIFIER ::= { id-ad 1 }

Does the corresponding 1.3.6.1.5.5.7.48.1 get encoded in the binary exactly like this?

I am asking this because I am trying to understand a specific value I see in a DER file (a certificate), which is 04020500, and I am not sure how to interpret it.

Answer 1

Yes, the OID is encoded in the binary data. The OID 1.3.6.1.5.5.7.48.1 you mention becomes 2b 06 01 05 05 07 30 01 (the first two numbers are encoded in a single byte, all remaining numbers are encoded in a single bytes as well because they're all smaller than 128).

A nice description of OID encoding is found here.

But the best way to analyze your ASN.1 data is to paste in into an online decoder, e.g. http://lapo.it/asn1js/.