diff --git a/src/or/buffers.c b/src/or/buffers.c
index 8981fd283b..58ce5733eb 100644
--- a/src/or/buffers.c
+++ b/src/or/buffers.c
@@ -281,6 +281,7 @@ buf_pullup(buf_t *buf, size_t bytes)
 }
 
 #ifdef TOR_UNIT_TESTS
+/* Return the data from the first chunk of buf in cp, and its length in sz. */
 void
 buf_get_first_chunk_data(const buf_t *buf, const char **cp, size_t *sz)
 {
@@ -292,6 +293,53 @@ buf_get_first_chunk_data(const buf_t *buf, const char **cp, size_t *sz)
     *sz = buf->head->datalen;
   }
 }
+
+/* Write sz bytes from cp into a newly allocated buffer buf.
+ * Returns NULL when passed a NULL cp or zero sz.
+ * Asserts on failure: only for use in unit tests.
+ * buf must be freed using buf_free(). */
+buf_t *
+buf_new_with_data(const char *cp, size_t sz)
+{
+  /* Validate arguments */
+  if (!cp || sz <= 0) {
+    return NULL;
+  }
+
+  tor_assert(sz < SSIZE_T_CEILING);
+
+  /* Allocate a buffer */
+  buf_t *buf = buf_new_with_capacity(sz);
+  tor_assert(buf);
+  assert_buf_ok(buf);
+  tor_assert(!buf->head);
+
+  /* Allocate a chunk that is sz bytes long */
+  buf->head = chunk_new_with_alloc_size(CHUNK_ALLOC_SIZE(sz));
+  buf->tail = buf->head;
+  tor_assert(buf->head);
+  assert_buf_ok(buf);
+  tor_assert(buf_allocation(buf) >= sz);
+
+  /* Copy the data and size the buffers */
+  tor_assert(sz <= buf_slack(buf));
+  tor_assert(sz <= CHUNK_REMAINING_CAPACITY(buf->head));
+  memcpy(&buf->head->mem[0], cp, sz);
+  buf->datalen = sz;
+  buf->head->datalen = sz;
+  buf->head->data = &buf->head->mem[0];
+  assert_buf_ok(buf);
+
+  /* Make sure everything is large enough */
+  tor_assert(buf_allocation(buf) >= sz);
+  tor_assert(buf_allocation(buf) >= buf_datalen(buf) + buf_slack(buf));
+  /* Does the buffer implementation allocate more than the requested size?
+   * (for example, by rounding up). If so, these checks will fail. */
+  tor_assert(buf_datalen(buf) == sz);
+  tor_assert(buf_slack(buf) == 0);
+
+  return buf;
+}
 #endif
 
 /** Remove the first <b>n</b> bytes from buf. */
diff --git a/src/or/buffers.h b/src/or/buffers.h
index 52b21d5885..08d9471fb7 100644
--- a/src/or/buffers.h
+++ b/src/or/buffers.h
@@ -64,7 +64,10 @@ void assert_buf_ok(buf_t *buf);
 #ifdef BUFFERS_PRIVATE
 STATIC int buf_find_string_offset(const buf_t *buf, const char *s, size_t n);
 STATIC void buf_pullup(buf_t *buf, size_t bytes);
+#ifdef TOR_UNIT_TESTS
 void buf_get_first_chunk_data(const buf_t *buf, const char **cp, size_t *sz);
+buf_t *buf_new_with_data(const char *cp, size_t sz);
+#endif
 STATIC size_t preferred_chunk_size(size_t target);
 
 #define DEBUG_CHUNK_ALLOC