Building Lists

Essentially container ASN.1 type is an array of the asn1_list type. So let us examine how we convert a simple container to code these routines can use.

RSAKey ::= SEQUENCE {    N        INTEGER,   E        INTEGER} 

First, we will begin with defining a list.

asn1_list RSAKey[2];

Next, we need two integers to hold the values. Keep in mind that these are the C long type and not a BigNum as we actually require for secure RSA. This is for demonstration purposes only.

long N, E;

Now we must actually assign the elements of the RSAKey.

RSAKey[0].type   = ASN1_DER_INTEGER;<a name="176"></a><a name="IDX-84"></a>RSAKey[0].length = 1;RSAKey[0].data   = &N;

This triplet of code is fairly awkward and makes ASN.1 coding a serious pain. A simpler solution to this problem is to define a macro to assign the values in a single line of C.

asn1.h:144   #define asn1_set(list, index, Type, Length, Data) \ 145   do {                                               \ 146      asn1_list     *ABC_list;                       \ 147      int            ABC_index;                      \ 148                                                     \ 149      ABC_list   = list;                             \ 150      ABC_index  = index;                            \ 151                                                     \ 152      ABC_list[ABC_index].type   = Type;             \ 153      ABC_list[ABC_index].length = Length;           \ 154      ABC_list[ABC_index].data   = Data;             \ 155   }  while (0);

The macro for those unfamiliar with the C preprocessor looks fairly indirect. First, we make a copy of the list and index parameters. This allows them to be temporal. Consider the invocation as follows.

asn1_set(mylist, i++, type, length, data);

If we did not first make a copy of the index, it would be different in every instance it was used during the macro. Similarly, we could...