diff --git a/ChangeLog b/ChangeLog
index ae9ca35..1469014 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,8 @@
 2012-06-07  Niibe Yutaka  <gniibe@fsij.org>
 
+	* src/openpgp.c (cmd_internal_authenticate): Add check for input
+	length.
+
 	Implement key generation.
 	* src/openpgp.c (cmd_pgp_gakp): Call gpg_do_keygen.
 	* src/openpgp-do.c (proc_key_import): Call with modulus = NULL.
diff --git a/src/openpgp.c b/src/openpgp.c
index c5013af..a46e95e 100644
--- a/src/openpgp.c
+++ b/src/openpgp.c
@@ -744,6 +744,8 @@ cmd_pso (void)
   DEBUG_INFO ("PSO done.\r\n");
 }
 
+
+#define MAX_DIGEST_INFO_LEN 102 /* 40% */
 static void
 cmd_internal_authenticate (void)
 {
@@ -763,6 +765,13 @@ cmd_internal_authenticate (void)
 	  return;
 	}
 
+      if (len > MAX_DIGEST_INFO_LEN)
+	{
+	  DEBUG_INFO ("input is too long.");
+	  GPG_CONDITION_NOT_SATISFIED ();
+	  return;
+	}
+
       r = rsa_sign (apdu.cmd_apdu_data, res_APDU, len,
 		    &kd[GPG_KEY_FOR_AUTHENTICATION]);
       if (r < 0)