ASN1 <-> DER encoding with application tag

Thu Nov 4 16:29:11 UTC 2021

The data is not correct if it supposed to match RFC 2743.

The first byte is [APPLICATION 0].  That seems fine.

The second byte provides a length for the full SEQUENCE.  It says there are 126 bytes, but you do not have that many.

Russ

> On Nov 4, 2021, at 10:18 AM, Max Larsson <max.larsson at facilityboss.biz> wrote:
> 
> Hi Russ,
>  
> do you mean that the DER data
>  
> 0x60 0x7e 0x06 0x06 0x2b 0x06 0x01 0x05 0x05 0x02 0xa0 0x74
>  
> is wrong?
>  
> If so, that DER data have I captured with wireshark from an smb2 session setup request.
> and that’s even I try to decode with help of openssl. If the case is that that data is wrongly,
> is there a way to get decode with openssl anyway?
>  
> Max
>  
> From: Russ Housley <housley at vigilsec.com <mailto:housley at vigilsec.com>>
> Date: Thursday, 4. November 2021 at 15:08
> To: Max Larsson <max.larsson at facilityboss.biz <mailto:max.larsson at facilityboss.biz>>
> Cc: openssl-users at openssl.org <mailto:openssl-users at openssl.org> <openssl-users at openssl.org <mailto:openssl-users at openssl.org>>
> Subject: Re: ASN1 <-> DER encoding with application tag
> 
> 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 <mailto: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
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