diff --git a/defs.h b/defs.h
index c406b3e9..b2079572 100644
--- a/defs.h
+++ b/defs.h
@@ -34,4 +34,10 @@ typedef uint32_t uint32;
 #define TYPECHECK(to_check, to_return)                  \
     (sizeof(to_check) ? (to_return) : (to_return))
 
+/* Return a pointer to the object of structure type 'type' whose field
+ * with name 'field' is pointed at by 'object'. */
+#define FROMFIELD(object, type, field)                                  \
+    TYPECHECK(object == &((type *)0)->field,                            \
+              ((type *)(((char *)(object)) - offsetof(type, field))))
+
 #endif /* PUTTY_DEFS_H */
diff --git a/misc.c b/misc.c
index 08198004..c85adfbd 100644
--- a/misc.c
+++ b/misc.c
@@ -466,44 +466,66 @@ char *dupprintf(const char *fmt, ...)
     return ret;
 }
 
-struct strbuf {
-    char *s;
-    int len, size;
+struct strbuf_impl {
+    int size;
+    struct strbuf visible;
 };
+
+#define STRBUF_SET_PTR(buf, ptr)                                \
+    ((buf)->visible.s = (ptr),                                  \
+     (buf)->visible.u = (unsigned char *)(buf)->visible.s)
+
+char *strbuf_append(strbuf *buf_o, size_t len)
+{
+    struct strbuf_impl *buf = FROMFIELD(buf_o, struct strbuf_impl, visible);
+    char *toret;
+    if (buf->size < buf->visible.len + len + 1) {
+        buf->size = (buf->visible.len + len + 1) * 5 / 4 + 512;
+        STRBUF_SET_PTR(buf, sresize(buf->visible.s, buf->size, char));
+    }
+    toret = buf->visible.s + buf->visible.len;
+    buf->visible.len += len;
+    buf->visible.s[buf->visible.len] = '\0';
+    return toret;
+}
+
 strbuf *strbuf_new(void)
 {
-    strbuf *buf = snew(strbuf);
-    buf->len = 0;
+    struct strbuf_impl *buf = snew(struct strbuf_impl);
+    buf->visible.len = 0;
     buf->size = 512;
-    buf->s = snewn(buf->size, char);
-    *buf->s = '\0';
-    return buf;
+    STRBUF_SET_PTR(buf, snewn(buf->size, char));
+    *buf->visible.s = '\0';
+    return &buf->visible;
 }
-void strbuf_free(strbuf *buf)
+void strbuf_free(strbuf *buf_o)
 {
-    sfree(buf->s);
+    struct strbuf_impl *buf = FROMFIELD(buf_o, struct strbuf_impl, visible);
+    if (buf->visible.s) {
+        smemclr(buf->visible.s, buf->size);
+        sfree(buf->visible.s);
+    }
     sfree(buf);
 }
-char *strbuf_str(strbuf *buf)
+char *strbuf_to_str(strbuf *buf_o)
 {
-    return buf->s;
-}
-char *strbuf_to_str(strbuf *buf)
-{
-    char *ret = buf->s;
+    struct strbuf_impl *buf = FROMFIELD(buf_o, struct strbuf_impl, visible);
+    char *ret = buf->visible.s;
     sfree(buf);
     return ret;
 }
-void strbuf_catfv(strbuf *buf, const char *fmt, va_list ap)
+void strbuf_catfv(strbuf *buf_o, const char *fmt, va_list ap)
 {
-    buf->s = dupvprintf_inner(buf->s, buf->len, &buf->size, fmt, ap);
-    buf->len += strlen(buf->s + buf->len);
+    struct strbuf_impl *buf = FROMFIELD(buf_o, struct strbuf_impl, visible);
+    STRBUF_SET_PTR(buf, dupvprintf_inner(buf->visible.s, buf->visible.len,
+                                         &buf->size, fmt, ap));
+    buf->visible.len += strlen(buf->visible.s + buf->visible.len);
 }
-void strbuf_catf(strbuf *buf, const char *fmt, ...)
+void strbuf_catf(strbuf *buf_o, const char *fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);
-    strbuf_catfv(buf, fmt, ap);
+    strbuf_catfv(buf_o, fmt, ap);
     va_end(ap);
 }
 
diff --git a/misc.h b/misc.h
index 8075423b..e6a5bf3b 100644
--- a/misc.h
+++ b/misc.h
@@ -37,9 +37,14 @@ char *dupprintf(const char *fmt, ...)
 char *dupvprintf(const char *fmt, va_list ap);
 void burnstr(char *string);
 
+struct strbuf {
+    char *s;
+    unsigned char *u;
+    int len;
+    /* (also there's a surrounding implementation struct in misc.c) */
+};
 strbuf *strbuf_new(void);
 void strbuf_free(strbuf *buf);
-char *strbuf_str(strbuf *buf);         /* does not free buf */
 char *strbuf_to_str(strbuf *buf); /* does free buf, but you must free result */
 void strbuf_catf(strbuf *buf, const char *fmt, ...);
 void strbuf_catfv(strbuf *buf, const char *fmt, va_list ap);