diff --git a/pkcs1.c b/pkcs1.c
index f88936916019327a717da6465c01e3fcf0392a24..74d7cfc03129607cc5b78c84dad5571f60308cbe 100644
--- a/pkcs1.c
+++ b/pkcs1.c
@@ -34,6 +34,12 @@
 
 #include "pkcs1.h"
 
+/* Formats the PKCS#1 padding, of the form
+ *
+ *   0x01 0xff ... 0xff 0x00 id
+ *
+ * where the 0xff ... 0xff part consists of at least 8 octets.
+ */
 void
 pkcs1_signature_prefix(unsigned length,
 		       uint8_t *buffer,