CODEWITHSUNDEEP
copensslasn.1
tagirkaZ
tagirkaZ

Reputation: 469

OpenSSL how ASN1_TYPE and ASN1_ITEM are related

I'm trying to digitally sign a pdf document using OpenSSL library. Now I want to get a digest from the pdf document content. I only found method which create the digest from ASN1_ITEM. But what I have is ASN1_OCTET_STRING object which I can convert to ASN1_TYPE if needed. I suppose that there should be some way to convert ASN1_TYPE or ASN1_OCTET_STRING into ASN1_ITEM. But I don't understand how ASN1_TYPE and ASN1_TYPE are related.

P.S. Would be grateful if someone share some kind of proper documentation of OpenSSL. What I could find are mostly source files without proper explanation.

Upvotes: 1

Views: 539

Answers (1)

Reinier Torenbeek
Reinier Torenbeek

Reputation: 17383

Since you write "what I have is ASN1_OCTET_STRING object", the following snippet, found under the NOTES section in the documentation of the ASN1_STRING utilities, seems useful:

Almost all ASN1 types in OpenSSL are represented as an ASN1_STRING structure. Other types such as ASN1_OCTET_STRING are simply typedef'ed to ASN1_STRING and the functions call the ASN1_STRING equivalents.

(You can verify that this happens in ossl_typ.h, it is actually not a typedef but a #define)

That same documentation page gives a function ASN1_STRING_to_UTF8() that you can use to get a representation of the data as an unsigned char * as well as its length. Once you have that, you can calculate a digest to your liking.

This is not an answer to your question "how ASN1_TYPE and ASN1_ITEM are related" but it seems to cover the problem that you describe. If that is not the case, please clarify.

Upvotes: 2

Related Questions