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+
+OpenSSL ASN1 Revision
+=====================
+
+This document describes some of the issues relating to the new ASN1 code.
+
+Previous OpenSSL ASN1 problems
+=============================
+
+OK why did the OpenSSL ASN1 code need revising in the first place? Well
+there are lots of reasons some of which are included below...
+
+1. The code is difficult to read and write. For every single ASN1 structure
+(e.g. SEQUENCE) four functions need to be written for new, free, encode and
+decode operations. This is a very painful and error prone operation. Very few
+people have ever written any OpenSSL ASN1 and those that have usually wish
+they hadn't.
+
+2. Partly because of 1. the code is bloated and takes up a disproportionate
+amount of space. The SEQUENCE encoder is particularly bad: it essentially
+contains two copies of the same operation, one to compute the SEQUENCE length
+and the other to encode it.
+
+3. The code is memory based: that is it expects to be able to read the whole
+structure from memory. This is fine for small structures but if you have a
+(say) 1Gb PKCS#7 signedData structure it isn't such a good idea...
+
+4. The code for the ASN1 IMPLICIT tag is evil. It is handled by temporarily
+changing the tag to the expected one, attempting to read it, then changing it
+back again. This means that decode buffers have to be writable even though they
+are ultimately unchanged. This gets in the way of constification.
+
+5. The handling of EXPLICIT isn't much better. It adds a chunk of code into 
+the decoder and encoder for every EXPLICIT tag.
+
+6. APPLICATION and PRIVATE tags aren't even supported at all.
+
+7. Even IMPLICIT isn't complete: there is no support for implicitly tagged
+types that are not OPTIONAL.
+
+8. Much of the code assumes that a tag will fit in a single octet. This is
+only true if the tag is 30 or less (mercifully tags over 30 are rare).
+
+9. The ASN1 CHOICE type has to be largely handled manually, there aren't any
+macros that properly support it.
+
+10. Encoders have no concept of OPTIONAL and have no error checking. If the
+passed structure contains a NULL in a mandatory field it will not be encoded,
+resulting in an invalid structure.
+
+11. It is tricky to add ASN1 encoders and decoders to external applications.
+
+Template model
+==============
+
+One of the major problems with revision is the sheer volume of the ASN1 code.
+Attempts to change (for example) the IMPLICIT behaviour would result in a
+modification of *every* single decode function. 
+
+I decided to adopt a template based approach. I'm using the term 'template'
+in a manner similar to SNACC templates: it has nothing to do with C++
+templates.
+
+A template is a description of an ASN1 module as several constant C structures.
+It describes in a machine readable way exactly how the ASN1 structure should
+behave. If this template contains enough detail then it is possible to write
+versions of new, free, encode, decode (and possibly others operations) that
+operate on templates.
+
+Instead of having to write code to handle each operation only a single
+template needs to be written. If new operations are needed (such as a 'print'
+operation) only a single new template based function needs to be written 
+which will then automatically handle all existing templates.
+
+Plans for revision
+==================
+
+The revision will consist of the following steps. Other than the first two
+these can be handled in any order.
+ 
+o Design and write template new, free, encode and decode operations, initially
+memory based. *DONE*
+
+o Convert existing ASN1 code to template form. *IN PROGRESS*
+
+o Convert an existing ASN1 compiler (probably SNACC) to output templates
+in OpenSSL form.
+
+o Add support for BIO based ASN1 encoders and decoders to handle large
+structures, initially blocking I/O.
+
+o Add support for non blocking I/O: this is quite a bit harder than blocking
+I/O.
+
+o Add new ASN1 structures, such as OCSP, CRMF, S/MIME v3 (CMS), attribute
+certificates etc etc.
+
+Description of major changes
+============================
+
+The BOOLEAN type now takes three values. 0xff is TRUE, 0 is FALSE and -1 is
+absent. The meaning of absent depends on the context. If for example the
+boolean type is DEFAULT FALSE (as in the case of the critical flag for
+certificate extensions) then -1 is FALSE, if DEFAULT TRUE then -1 is TRUE.
+Usually the value will only ever be read via an API which will hide this from
+an application.
+
+There is an evil bug in the old ASN1 code that mishandles OPTIONAL with
+SEQUENCE OF or SET OF. These are both implemented as a STACK structure. The
+old code would omit the structure if the STACK was NULL (which is fine) or if
+it had zero elements (which is NOT OK). This causes problems because an empty
+SEQUENCE OF or SET OF will result in an empty STACK when it is decoded but when
+it is encoded it will be omitted resulting in different encodings. The new code
+only omits the encoding if the STACK is NULL, if it contains zero elements it
+is encoded and empty. There is an additional problem though: because an empty
+STACK was omitted, sometimes the corresponding *_new() function would
+initialize the STACK to empty so an application could immediately use it, if
+this is done with the new code (i.e. a NULL) it wont work. Therefore a new
+STACK should be allocated first. One instance of this is the X509_CRL list of
+revoked certificates: a helper function X509_CRL_add0_revoked() has been added
+for this purpose.
+
+The X509_ATTRIBUTE structure used to have an element called 'set' which took
+the value 1 if the attribute value was a SET OF or 0 if it was a single. Due
+to the behaviour of CHOICE in the new code this has been changed to a field
+called 'single' which is 0 for a SET OF and 1 for single. The old field has
+been deleted to deliberately break source compatibility. Since this structure
+is normally accessed via higher level functions this shouldn't break too much.
+
+The X509_REQ_INFO certificate request info structure no longer has a field
+called 'req_kludge'. This used to be set to 1 if the attributes field was
+(incorrectly) omitted. You can check to see if the field is omitted now by
+checking if the attributes field is NULL. Similarly if you need to omit
+the field then free attributes and set it to NULL.
+
+The top level 'detached' field in the PKCS7 structure is no longer set when
+a PKCS#7 structure is read in. PKCS7_is_detached() should be called instead.
+The behaviour of PKCS7_get_detached() is unaffected.
+
+The values of 'type' in the GENERAL_NAME structure have changed. This is
+because the old code use the ASN1 initial octet as the selector. The new
+code uses the index in the ASN1_CHOICE template.
+
+The DIST_POINT_NAME structure has changed to be a true CHOICE type.
+
+typedef struct DIST_POINT_NAME_st {
+int type;
+union {
+	STACK_OF(GENERAL_NAME) *fullname;
+	STACK_OF(X509_NAME_ENTRY) *relativename;
+} name;
+} DIST_POINT_NAME;
+
+This means that name.fullname or name.relativename should be set
+and type reflects the option. That is if name.fullname is set then
+type is 0 and if name.relativename is set type is 1.
+
+With the old code using the i2d functions would typically involve:
+
+unsigned char *buf, *p;
+int len;
+/* Find length of encoding */
+len = i2d_SOMETHING(x, NULL);
+/* Allocate buffer */
+buf = OPENSSL_malloc(len);
+if(buf == NULL) {
+	/* Malloc error */
+}
+/* Use temp variable because &p gets updated to point to end of
+ * encoding.
+ */
+p = buf;
+i2d_SOMETHING(x, &p);
+
+
+Using the new i2d you can also do:
+
+unsigned char *buf = NULL;
+int len;
+len = i2d_SOMETHING(x, &buf);
+if(len < 0) {
+	/* Malloc error */
+}
+
+and it will automatically allocate and populate a buffer with the
+encoding. After this call 'buf' will point to the start of the
+encoding which is len bytes long.