ASN1 <-> DER encoding with application tag

[Russ Housley]

RFC 2743 shows this structure:
      MechType ::= OBJECT IDENTIFIER
      -- data structure definitions
      -- callers must be able to distinguish among
      -- InitialContextToken, SubsequentContextToken,
      -- PerMsgToken, and SealedMessage data elements
      -- based on the usage in which they occur

      InitialContextToken ::=
      -- option indication (delegation, etc.) indicated within
      -- mechanism-specific token
      [APPLICATION 0] IMPLICIT SEQUENCE {
              thisMech MechType,
              innerContextToken ANY DEFINED BY thisMech
                 -- contents mechanism-specific
                 -- ASN.1 structure not required
              }
The encoded data that you provided dies begin with the [APPLICATION 0] tag, then it if followed by by the { 1 3 6 1 5 5 2 } object identifier.

Russ

> On Nov 4, 2021, at 9:58 AM, Max Larsson <max.larsson at facilityboss.biz> wrote:
> 
> Hi everyone,
>  
> I’m trying to decode and encode Der structure. In my case that are DER encoded GSSAPI structure.
>  
> My DER encoded data looks like this (stripped the pending bytes):
>  
> 0x60 0x7e 0x06 0x06 0x2b 0x06 0x01 0x05 0x05 0x02 0xa0 0x74
>  
> My ANS1 definition in my source look like this:
>  
> typedef struct ContextToken_st {
>     ASN1_OBJECT *mech;
>     ASN1_OCTET_STRING *innerContextToken;
> } GSSAPI_CONTEXTTOKEN;
>  
> DECLARE_ASN1_FUNCTIONS( GSSAPI_CONTEXTTOKEN )
>  
> ASN1_SEQUENCE( GSSAPI_CONTEXTTOKEN ) = {
>     ASN1_SIMPLE( GSSAPI_CONTEXTTOKEN, mech, ASN1_OBJECT ),
>     ASN1_SIMPLE( GSSAPI_CONTEXTTOKEN, innerContextToken, ASN1_OCTET_STRING  )
> } ASN1_SEQUENCE_END( GSSAPI_CONTEXTTOKEN )  
>  
> IMPLEMENT_ASN1_FUNCTIONS( GSSAPI_CONTEXTTOKEN )
>  
> Parsing the above DER data fails, so I decided to encode a own Der structure, to see where the difference is with my setup:
>  
>     . . .
>     negToken = GSSAPI_CONTEXTTOKEN_new();
>     if( negToken != NULL ) {
>         negToken->mech = OBJ_txt2obj( "1.3.6.1.5.5.2",0 );
>         negToken->innerContextToken = ASN1_OCTET_STRING_new();
>  
>         const unsigned char mechToken[] = "\xa0\x74\x30 // … stripped for readability
>  
>         const size_t mechTokenSize = sizeof( mechToken ) - 1;
>         printf( "Size of inner token: %zu\n",mechTokenSize );
>         ASN1_OCTET_STRING_set( negToken->innerContextToken,mechToken,mechTokenSize );
>  
>         buffer = NULL;
>         size_t bufferSize = i2d_GSSAPI_CONTEXTTOKEN( negToken,NULL );
>  
>         printf( "Required buffer size for DER encoding of ASN1 structure: %zu\n",bufferSize );
>  
>         unsigned char *buffer = malloc( bufferSize );
>         unsigned char *p = buffer;
>         i2d_GSSAPI_CONTEXTTOKEN( negToken,&p );
>  
>         for( int len = 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):
>  
> 0x30 0x81 0x80 0x06 0x06 0x2b 0x06 0x01 0x05 0x05 0x02 0x04 0x76 0xa0 0x74
>  
> 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
> 
> Dipl.-Inform. Max Larsson
> Geschäftsleitung
> 
> phone: +49(0)6151/62908-75
> fax: 
> email: max.larsson at facilityboss.biz <mailto:max.larsson at facilityboss.biz>
> web: http://facilityboss.biz <http://facilityboss.biz/>
>  <http://facilityboss.biz/>
> Bad Nauheimer Str. 4
> 64289 Darmstadt
> Germany
> 
> Sitz der Gesellschaft: Darmstadt
> Registergericht: Amtsgericht Darmstadt, HRB 86193
> Geschäftsführer: Dipl.-Inform Max Lars Robert Larsson
> 
>  
> Diese E-Mail enthält unter Umständen vertrauliche und/oder rechtlich geschützte Informationen, die allein für den Adressaten bestimmt sind. Wenn Sie nicht der zutreffende Adressat sind oder diese E-Mail irrtümlich erhalten haben, ist jede Verwendung, Verbreitung, Kopie oder Bezugnahme auf den Inhalt dieser E-Mail verboten. Bitte informieren Sie uns über einen eventuellen Irrtum per Telefon, per Telefax oder E-Mail.
> 
> This e-mail may contain confidential and/or privileged information. If you are not the intended recipient, any disclosure, copying, distribution or reference on the contents of this e-mail is strictly prohibited. If you have received this e-mail in error please notify us by e-mail, facsimile or phone call.
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mta.openssl.org/pipermail/openssl-users/attachments/20211104/4fbb42af/attachment-0001.html>