ASN1 <-> DER encoding with application tag

[Max Larsson]

Hi Matt,

Your suggestion helped a lot for the initial application tag.

I must confess I haven’t jet googled the method ANS1_item_ex yet what exactly it does
and how it behave and can be used.

But now I’m wondering how I can “cascade” using this method to influence the encoding
to avoid the writing of the  additional bytes after the OID gest encoded and before
the innerToken is encoded:

….. 0x05 0x05 0x02 0x04 0x76 0xa0 0x74 ….(here are a lot of bytes omitted)

Best regards

Max


From: openssl-users <openssl-users-bounces at openssl.org> on behalf of Matt Caswell <matt at openssl.org>
Date: Thursday, 4. November 2021 at 17:14
To: openssl-users at openssl.org <openssl-users at openssl.org>
Subject: Re: ASN1 <-> DER encoding with application tag


On 04/11/2021 13:58, Max Larsson wrote:
>          i2d_GSSAPI_CONTEXTTOKEN( negToken,&p );
>

You can tell i2d to encode using "application" tagging like this:

         ASN1_item_ex_i2d((const ASN1_VALUE **)&negToken, &p,
                          ASN1_ITEM_rptr(GSSAPI_CONTEXTTOKEN), 0,
                          ASN1_TFLG_APPLICATION);

Matt




> for( intlen = 0;len < bufferSize;len++ ) {
>
> if( ( len % 8) == 0)
>
>                  printf( "  ");
>
> if( ( len % 16) == 0)
>
>                  printf( "\n\t\t");
>
>              printf( " 0x%02x",(short)buffer[ len ] );
>
>          }
>
>          printf( "\n");
>
>      . . .
>
> The code above output the following DER encoded structure (the
> difference marled in bold):
>
> *0**x**3**0**0**x**81 0x80*0x060x060x2b0x060x010x050x050x02*0x04
> 0x76*0xa00x74
>
> The google result, which I found seems to point into the direction to
> use application tags to encode.
>
> But I haven’t found any example or how to how to achieve this with
> openssl, can anyone give me sone hints?
>
> Best regards
>
> Max Larsson
>
> Mit freundlichen Grüßen
> Best regards
>
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