diff --git a/src/common/crypto.c b/src/common/crypto.c
index f3268fe183..6761fd71f3 100644
--- a/src/common/crypto.c
+++ b/src/common/crypto.c
@@ -656,6 +656,18 @@ crypto_pk_key_is_private(const crypto_pk_env_t *key)
   return PRIVATE_KEY_OK(key);
 }
 
+/** Return true iff <b>env</b> contains a public key whose public exponent
+ * equals 65537.
+ */
+int
+crypto_pk_check_key_public_exponent(crypto_pk_env_t *env)
+{
+  tor_assert(env);
+  tor_assert(env->key);
+
+  return BN_is_word(env->key->e, 65537);
+}
+
 /** Compare the public-key components of a and b.  Return -1 if a\<b, 0
  * if a==b, and 1 if a\>b.
  */
diff --git a/src/common/crypto.h b/src/common/crypto.h
index 576c03dc30..0fcd067ae1 100644
--- a/src/common/crypto.h
+++ b/src/common/crypto.h
@@ -92,6 +92,7 @@ size_t crypto_pk_keysize(crypto_pk_env_t *env);
 crypto_pk_env_t *crypto_pk_dup_key(crypto_pk_env_t *orig);
 crypto_pk_env_t *crypto_pk_copy_full(crypto_pk_env_t *orig);
 int crypto_pk_key_is_private(const crypto_pk_env_t *key);
+int crypto_pk_check_key_public_exponent(crypto_pk_env_t *env);
 
 int crypto_pk_public_encrypt(crypto_pk_env_t *env, char *to, size_t tolen,
                              const char *from, size_t fromlen, int padding);