Some miscellaneous marshalling helpers.

I'm about to use these in a new piece of code, but they may come in
helpful elsewhere as well. match_ssh_id in particular wraps an idiom
that's quite common in the rest of the codebase.

misc.c

@@ -9,6 +9,7 @@
 #include <ctype.h>
 #include <assert.h>
 #include "putty.h"
+#include "misc.h"
 
 /*
  * Parse a string block size specification. This is approximately a
@@ -1035,3 +1036,36 @@ int smemeq(const void *av, const void *bv, size_t len)
      * we want to return 1, so then we can just shift down. */
     return (0x100 - val) >> 8;
 }
+
+int match_ssh_id(int stringlen, const void *string, const char *id)
+{
+    int idlen = strlen(id);
+    return (idlen == stringlen && !memcmp(string, id, idlen));
+}
+
+void *get_ssh_string(int *datalen, const void **data, int *stringlen)
+{
+    void *ret;
+    int len;
+
+    if (*datalen < 4)
+        return NULL;
+    len = GET_32BIT_MSB_FIRST((const unsigned char *)*data);
+    if (*datalen < len+4)
+        return NULL;
+    ret = (void *)((const char *)*data + 4);
+    *datalen -= len + 4;
+    *data = (const char *)*data + len + 4;
+    *stringlen = len;
+    return ret;
+}
+
+int get_ssh_uint32(int *datalen, const void **data, unsigned *ret)
+{
+    if (*datalen < 4)
+        return FALSE;
+    *ret = GET_32BIT_MSB_FIRST((const unsigned char *)*data);
+    *datalen -= 4;
+    *data = (const char *)*data + 4;
+    return TRUE;
+}

misc.h

@@ -78,6 +78,23 @@ void smemclr(void *b, size_t len);
  * by the 'eq' in the name. */
 int smemeq(const void *av, const void *bv, size_t len);
 
+/* Extracts an SSH-marshalled string from the start of *data. If
+ * successful (*datalen is not too small), advances data/datalen past
+ * the string and returns a pointer to the string itself and its
+ * length in *stringlen. Otherwise does nothing and returns NULL.
+ *
+ * Like strchr, this function can discard const from its parameter.
+ * Treat it as if it was a family of two functions, one returning a
+ * non-const string given a non-const pointer, and one taking and
+ * returning const. */
+void *get_ssh_string(int *datalen, const void **data, int *stringlen);
+/* Extracts an SSH uint32, similarly. Returns TRUE on success, and
+ * leaves the extracted value in *ret. */
+int get_ssh_uint32(int *datalen, const void **data, unsigned *ret);
+/* Given a not-necessarily-zero-terminated string in (length,data)
+ * form, check if it equals an ordinary C zero-terminated string. */
+int match_ssh_id(int stringlen, const void *string, const char *id);
+
 /*
  * Debugging functions.
  *