TABLE OF CONTENTS Now we will consider how to implement ASN.1 encoders and decoders. Fortunately, most ASN.1 types are primitive and fairly simple to process. The constructed types are slightly harder to develop if the intent is to have a user-friendly API. In our case, we re going to strive for maximum effort while writing the ASN.1 routines such that the resulting code has the maximal amount of use.
All of the ASN.1 routines are found in the ch2 directory of the source code repository. There is a collection of C source files, a single H header file to gather up the prototypes, and a GNU Makefile that will build the collection into an archive using GCC.
The first routines we examine deal with getting, reading, and encoding the length of ASN.1 encodings. The logic is shared by all other ASN.1 types, and as such, makes sense to re-use the code where possible.
The first routine simply returns the length of an encoding, including the header, length bytes, and payload.
der_length.c:001 #include asn1.h"002 unsigned long der_length(unsigned long payload)003 { 004 unsigned long x;005006 if (payload > 127) { 007 x = payload;008 while (x) { 009 x >>= 8;010 ++payload;011 } 012 } 013014 return payload + 2;015 } The function accepts as input the payload length and returns the size of the eventual encoding. This function is suitable for all types where the payload length is known in advance; that is, primitive (nonconstructed) types. Note that for the BIT STRING type the calling function will have to add in the padding counter byte to the payload length for this to work.
This function is useful for encoders, as it allows the caller...
