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    1 
    2 OpenSSL ASN1 Revision
    3 =====================
    4 
    5 This document describes some of the issues relating to the new ASN1 code.
    6 
    7 Previous OpenSSL ASN1 problems
    8 =============================
    9 
   10 OK why did the OpenSSL ASN1 code need revising in the first place? Well
   11 there are lots of reasons some of which are included below...
   12 
   13 1. The code is difficult to read and write. For every single ASN1 structure
   14 (e.g. SEQUENCE) four functions need to be written for new, free, encode and
   15 decode operations. This is a very painful and error prone operation. Very few
   16 people have ever written any OpenSSL ASN1 and those that have usually wish
   17 they hadn't.
   18 
   19 2. Partly because of 1. the code is bloated and takes up a disproportionate
   20 amount of space. The SEQUENCE encoder is particularly bad: it essentially
   21 contains two copies of the same operation, one to compute the SEQUENCE length
   22 and the other to encode it.
   23 
   24 3. The code is memory based: that is it expects to be able to read the whole
   25 structure from memory. This is fine for small structures but if you have a
   26 (say) 1Gb PKCS#7 signedData structure it isn't such a good idea...
   27 
   28 4. The code for the ASN1 IMPLICIT tag is evil. It is handled by temporarily
   29 changing the tag to the expected one, attempting to read it, then changing it
   30 back again. This means that decode buffers have to be writable even though they
   31 are ultimately unchanged. This gets in the way of constification.
   32 
   33 5. The handling of EXPLICIT isn't much better. It adds a chunk of code into 
   34 the decoder and encoder for every EXPLICIT tag.
   35 
   36 6. APPLICATION and PRIVATE tags aren't even supported at all.
   37 
   38 7. Even IMPLICIT isn't complete: there is no support for implicitly tagged
   39 types that are not OPTIONAL.
   40 
   41 8. Much of the code assumes that a tag will fit in a single octet. This is
   42 only true if the tag is 30 or less (mercifully tags over 30 are rare).
   43 
   44 9. The ASN1 CHOICE type has to be largely handled manually, there aren't any
   45 macros that properly support it.
   46 
   47 10. Encoders have no concept of OPTIONAL and have no error checking. If the
   48 passed structure contains a NULL in a mandatory field it will not be encoded,
   49 resulting in an invalid structure.
   50 
   51 11. It is tricky to add ASN1 encoders and decoders to external applications.
   52 
   53 Template model
   54 ==============
   55 
   56 One of the major problems with revision is the sheer volume of the ASN1 code.
   57 Attempts to change (for example) the IMPLICIT behaviour would result in a
   58 modification of *every* single decode function. 
   59 
   60 I decided to adopt a template based approach. I'm using the term 'template'
   61 in a manner similar to SNACC templates: it has nothing to do with C++
   62 templates.
   63 
   64 A template is a description of an ASN1 module as several constant C structures.
   65 It describes in a machine readable way exactly how the ASN1 structure should
   66 behave. If this template contains enough detail then it is possible to write
   67 versions of new, free, encode, decode (and possibly others operations) that
   68 operate on templates.
   69 
   70 Instead of having to write code to handle each operation only a single
   71 template needs to be written. If new operations are needed (such as a 'print'
   72 operation) only a single new template based function needs to be written 
   73 which will then automatically handle all existing templates.
   74 
   75 Plans for revision
   76 ==================
   77 
   78 The revision will consist of the following steps. Other than the first two
   79 these can be handled in any order.
   80  
   81 o Design and write template new, free, encode and decode operations, initially
   82 memory based. *DONE*
   83 
   84 o Convert existing ASN1 code to template form. *IN PROGRESS*
   85 
   86 o Convert an existing ASN1 compiler (probably SNACC) to output templates
   87 in OpenSSL form.
   88 
   89 o Add support for BIO based ASN1 encoders and decoders to handle large
   90 structures, initially blocking I/O.
   91 
   92 o Add support for non blocking I/O: this is quite a bit harder than blocking
   93 I/O.
   94 
   95 o Add new ASN1 structures, such as OCSP, CRMF, S/MIME v3 (CMS), attribute
   96 certificates etc etc.
   97 
   98 Description of major changes
   99 ============================
  100 
  101 The BOOLEAN type now takes three values. 0xff is TRUE, 0 is FALSE and -1 is
  102 absent. The meaning of absent depends on the context. If for example the
  103 boolean type is DEFAULT FALSE (as in the case of the critical flag for
  104 certificate extensions) then -1 is FALSE, if DEFAULT TRUE then -1 is TRUE.
  105 Usually the value will only ever be read via an API which will hide this from
  106 an application.
  107 
  108 There is an evil bug in the old ASN1 code that mishandles OPTIONAL with
  109 SEQUENCE OF or SET OF. These are both implemented as a STACK structure. The
  110 old code would omit the structure if the STACK was NULL (which is fine) or if
  111 it had zero elements (which is NOT OK). This causes problems because an empty
  112 SEQUENCE OF or SET OF will result in an empty STACK when it is decoded but when
  113 it is encoded it will be omitted resulting in different encodings. The new code
  114 only omits the encoding if the STACK is NULL, if it contains zero elements it
  115 is encoded and empty. There is an additional problem though: because an empty
  116 STACK was omitted, sometimes the corresponding *_new() function would
  117 initialize the STACK to empty so an application could immediately use it, if
  118 this is done with the new code (i.e. a NULL) it wont work. Therefore a new
  119 STACK should be allocated first. One instance of this is the X509_CRL list of
  120 revoked certificates: a helper function X509_CRL_add0_revoked() has been added
  121 for this purpose.
  122 
  123 The X509_ATTRIBUTE structure used to have an element called 'set' which took
  124 the value 1 if the attribute value was a SET OF or 0 if it was a single. Due
  125 to the behaviour of CHOICE in the new code this has been changed to a field
  126 called 'single' which is 0 for a SET OF and 1 for single. The old field has
  127 been deleted to deliberately break source compatibility. Since this structure
  128 is normally accessed via higher level functions this shouldn't break too much.
  129 
  130 The X509_REQ_INFO certificate request info structure no longer has a field
  131 called 'req_kludge'. This used to be set to 1 if the attributes field was
  132 (incorrectly) omitted. You can check to see if the field is omitted now by
  133 checking if the attributes field is NULL. Similarly if you need to omit
  134 the field then free attributes and set it to NULL.
  135 
  136 The top level 'detached' field in the PKCS7 structure is no longer set when
  137 a PKCS#7 structure is read in. PKCS7_is_detached() should be called instead.
  138 The behaviour of PKCS7_get_detached() is unaffected.
  139 
  140 The values of 'type' in the GENERAL_NAME structure have changed. This is
  141 because the old code use the ASN1 initial octet as the selector. The new
  142 code uses the index in the ASN1_CHOICE template.
  143 
  144 The DIST_POINT_NAME structure has changed to be a true CHOICE type.
  145 
  146 typedef struct DIST_POINT_NAME_st {
  147 int type;
  148 union {
  149     STACK_OF(GENERAL_NAME) *fullname;
  150     STACK_OF(X509_NAME_ENTRY) *relativename;
  151 } name;
  152 } DIST_POINT_NAME;
  153 
  154 This means that name.fullname or name.relativename should be set
  155 and type reflects the option. That is if name.fullname is set then
  156 type is 0 and if name.relativename is set type is 1.
  157 
  158 With the old code using the i2d functions would typically involve:
  159 
  160 unsigned char *buf, *p;
  161 int len;
  162 /* Find length of encoding */
  163 len = i2d_SOMETHING(x, NULL);
  164 /* Allocate buffer */
  165 buf = OPENSSL_malloc(len);
  166 if(buf == NULL) {
  167     /* Malloc error */
  168 }
  169 /* Use temp variable because &p gets updated to point to end of
  170  * encoding.
  171  */
  172 p = buf;
  173 i2d_SOMETHING(x, &p);
  174 
  175 
  176 Using the new i2d you can also do:
  177 
  178 unsigned char *buf = NULL;
  179 int len;
  180 len = i2d_SOMETHING(x, &buf);
  181 if(len < 0) {
  182     /* Malloc error */
  183 }
  184 
  185 and it will automatically allocate and populate a buffer with the
  186 encoding. After this call 'buf' will point to the start of the
  187 encoding which is len bytes long.