diff --git a/appl/telnet/libtelnet/kerberos.c b/appl/telnet/libtelnet/kerberos.c
index 67c9933fe..a1ccde880 100644
--- a/appl/telnet/libtelnet/kerberos.c
+++ b/appl/telnet/libtelnet/kerberos.c
@@ -111,13 +111,13 @@ static int unpack_cred(unsigned char *buf, int len, CREDENTIALS *cred);
 
 
 static int
-Data(Authenticator *ap, int type, void *d, int c)
+Data(Authenticator *ap, int type, const void *d, int c)
 {
     unsigned char *p = str_data + 4;
-    unsigned char *cd = (unsigned char *)d;
+    const unsigned char *cd = (const unsigned char *)d;
 
     if (c == -1)
-	c = strlen((char *)cd);
+	c = strlen((const char *)cd);
 
     if (auth_debug_mode) {
 	printf("%s:%d: [%d] (%d)",
diff --git a/appl/telnet/libtelnet/misc-proto.h b/appl/telnet/libtelnet/misc-proto.h
index e9ace347d..c610c5efa 100644
--- a/appl/telnet/libtelnet/misc-proto.h
+++ b/appl/telnet/libtelnet/misc-proto.h
@@ -61,7 +61,7 @@
 void auth_encrypt_init (char *, char *, char *, int);
 void auth_encrypt_user(char *name);
 void auth_encrypt_connect (int);
-void printd (unsigned char *, int);
+void printd (const unsigned char *, int);
 
 char** genget (char *name, char **table, int stlen);
 int isprefix(char *s1, char *s2);
diff --git a/appl/telnet/libtelnet/misc.c b/appl/telnet/libtelnet/misc.c
index 9b1862943..6bbdebe63 100644
--- a/appl/telnet/libtelnet/misc.c
+++ b/appl/telnet/libtelnet/misc.c
@@ -81,7 +81,7 @@ auth_encrypt_connect(int cnt)
 }
 
 void
-printd(unsigned char *data, int cnt)
+printd(const unsigned char *data, int cnt)
 {
     if (cnt > 16)
 	cnt = 16;