diff --git a/library/rsa.c b/library/rsa.c
index 2713b5c..210ea46 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -1018,11 +1018,11 @@
          * Digest ::= OCTET STRING
          */
         *p++ = ASN1_SEQUENCE | ASN1_CONSTRUCTED;
-        *p++ = 0x08 + oid_size + hashlen;
+        *p++ = (unsigned char) ( 0x08 + oid_size + hashlen );
         *p++ = ASN1_SEQUENCE | ASN1_CONSTRUCTED;
-        *p++ = 0x04 + oid_size;
+        *p++ = (unsigned char) ( 0x04 + oid_size );
         *p++ = ASN1_OID;
-        *p++ = oid_size;
+        *p++ = oid_size & 0xFF;
         memcpy( p, oid, oid_size );
         p += oid_size;
         *p++ = ASN1_NULL;