ASN template documentation: adding basics for decoding

First draft of ASN template documentation that helps with writing
parsing code.

wolfcrypt/src/ASN_TEMPLATE.md

@@ -0,0 +1,113 @@
+
+
+# Writing a template
+
+```c
+static const ASNItem <template>[] = {
+/*  <ELEM_0> */ { <depth>, <ASN Type>, <constructed>, <header>, <optional> },
+...
+};
+enum {
+    <TEMPLATE>_<ELEM_0> = 0,
+    <TEMPLATE>_<ELEM_1>,
+    ...
+};
+#define <template>_Length (sizeof(<template>) / sizeof(ASNItem))
+```
+
+## Examples of ASN.1 items
+
+### Sequence
+
+When you have a sequence at depth 0 and want to parse the contents:
+
+```c
+    { 0, ASN_SEQUENCE, 1, 1, 0 },
+```
+
+To skip over the contents of the sequence:
+
+```c
+    { 0, ASN_SEQUENCE, 1, 0, 0 },
+```
+
+### Simple types
+
+An INTEGER at depth 1:
+
+```c
+        { 1, ASN_INTEGER, 0, 0, 1 },
+```
+
+An OCTET_STRING at depth 1 but stop after header in order to parse contents:
+
+```c
+        { 1, ASN_COTET_STRING, 0, 1, 0 },
+```
+
+
+### Optional items
+
+An optional boolean (like criticality of a certificate extension):
+
+```c
+        { 1, ASN_BOOLEAN, 0, 0, 1 },
+```
+
+### Choice
+
+Next ASN.1 item, at depth 2, is one of multiple types:
+
+```c
+            { 2, ASN_TAG_NULL, 0, 0, 2 },
+            { 2, ASN_OBJECT_ID, 0, 0, 2 },
+            { 2, ASN_SEQUENCE, 1, 0, 2 },
+```
+
+Note, use the optional value to uniquely identify the choices.
+
+
+
+# Decoding function outline
+
+An outline of a decoding function:
+
+```c
+#include <wolfssl/wolfcrypt/asn.h>
+
+...
+
+static int Decode<Something>(const byte* input, int sz, <Object Type* <obj>)
+{
+    DECL_ASNGETDATA(dataASN, <template>_Length);
+    int ret = 0;
+    /* Declare variables to parse data into. For example:
+     *   word32 idx = 0;
+     *   byte isCA = 0;
+     */
+
+    CALLOC_ASNGETDATA(dataASN, <template>_Length, ret, <obj>->heap);
+
+    if (ret == 0) {
+        /* Set any variables to be filled in by the parser. For example:
+         *  GetASN_Boolean(&dataASN[BASICCONSASN_IDX_CA], &isCA);
+         *  GetASN_Int8Bit(&dataASN[BASICCONSASN_IDX_PLEN], &<obj>->pathLength);
+         */
+
+        /* Decode the ASN.1 DER. */
+        ret = GetASN_Items(<template>, dataASN, <template>_Length, 1, input,
+                           &idx, (word32)sz);
+    }
+
+    if (ret == 0) {
+        /* Check data in variables is valid. */
+    }
+    if (ret == 0) {
+        /* Put data in variables into object. */
+    }
+
+    FREE_ASNGETDATA(dataASN, <obj>->heap);
+    return ret;
+}
+```
+