ASN1 <-> DER encoding with application tag

[Max Larsson]

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

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