diff --git a/src/common/util.c b/src/common/util.c
index 8b010bd320..9d68d2ffc0 100644
--- a/src/common/util.c
+++ b/src/common/util.c
@@ -417,6 +417,15 @@ find_whitespace(const char *s)
   return s;
 }
 
+/** Return true iff the DIGEST_LEN bytes in digest are all zero. */
+int
+tor_digest_is_zero(const char *digest)
+{
+  static char ZERO_DIGEST[] = { 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 };
+
+  return !memcmp(digest, ZERO_DIGEST, DIGEST_LEN);
+}
+
 #define CHECK_STRTOX_RESULT()                           \
   /* Was at least one character converted? */           \
   if (endptr == s)                                      \
diff --git a/src/common/util.h b/src/common/util.h
index eabd4278b9..d9b253dc36 100644
--- a/src/common/util.h
+++ b/src/common/util.h
@@ -111,6 +111,7 @@ const char *hex_str(const char *from, size_t fromlen);
 const char *eat_whitespace(const char *s);
 const char *eat_whitespace_no_nl(const char *s);
 const char *find_whitespace(const char *s);
+int tor_digest_is_zero(const char *digest);
 
 void base16_encode(char *dest, size_t destlen, const char *src, size_t srclen);
 int base16_decode(char *dest, size_t destlen, const char *src, size_t srclen);