diff --git a/lib/libpam/openpam_ctype.h b/lib/libpam/openpam_ctype.h
index f78574d..96389f8 100644
--- a/lib/libpam/openpam_ctype.h
+++ b/lib/libpam/openpam_ctype.h
@@ -38,6 +38,14 @@
 #define is_digit(ch)				\
 	(ch >= '0' && ch <= '9')
 
+/*
+ * Evaluates to non-zero if the argument is a hex digit.
+ */
+#define is_xdigit(ch)				\
+	((ch >= '0' && ch <= '9') ||		\
+	 (ch >= 'a' && ch <= 'f') ||		\
+	 (ch >= 'A' && ch <= 'F'))
+
 /*
  * Evaluates to non-zero if the argument is an uppercase letter.
  */