diff --git a/include/polarssl/net.h b/include/polarssl/net.h
index 725babef6..c02dbd286 100644
--- a/include/polarssl/net.h
+++ b/include/polarssl/net.h
@@ -143,7 +143,7 @@ int net_recv( void *ctx, unsigned char *buf, size_t len );
  *                 or a non-zero error code; POLARSSL_ERR_NET_WANT_WRITE
  *                 indicates write() is blocking.
  */
-int net_send( void *ctx, unsigned char *buf, size_t len );
+int net_send( void *ctx, const unsigned char *buf, size_t len );
 
 /**
  * \brief          Gracefully shutdown the connection
diff --git a/include/polarssl/ssl.h b/include/polarssl/ssl.h
index 1108b8b79..5a699ccba 100644
--- a/include/polarssl/ssl.h
+++ b/include/polarssl/ssl.h
@@ -231,7 +231,7 @@ struct _ssl_context
     int  (*f_rng)(void *);
     void (*f_dbg)(void *, int, const char *);
     int (*f_recv)(void *, unsigned char *, size_t);
-    int (*f_send)(void *, unsigned char *, size_t);
+    int (*f_send)(void *, const unsigned char *, size_t);
     int (*f_vrfy)(void *, x509_cert *, int, int);
 
     void *p_rng;                /*!< context for the RNG function     */
@@ -450,7 +450,7 @@ void ssl_set_dbg( ssl_context *ssl,
  */
 void ssl_set_bio( ssl_context *ssl,
         int (*f_recv)(void *, unsigned char *, size_t), void *p_recv,
-        int (*f_send)(void *, unsigned char *, size_t), void *p_send );
+        int (*f_send)(void *, const unsigned char *, size_t), void *p_send );
 
 /**
  * \brief          Set the session callbacks (server-side only)
diff --git a/library/net.c b/library/net.c
index e1d91e4c7..fcfe4d796 100644
--- a/library/net.c
+++ b/library/net.c
@@ -317,7 +317,7 @@ int net_recv( void *ctx, unsigned char *buf, size_t len )
 /*
  * Write at most 'len' characters
  */
-int net_send( void *ctx, unsigned char *buf, size_t len )
+int net_send( void *ctx, const unsigned char *buf, size_t len )
 {
     int ret = write( *((int *) ctx), buf, len );
 
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 7f82fa0a1..7cb625865 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1770,7 +1770,7 @@ void ssl_set_dbg( ssl_context *ssl,
 
 void ssl_set_bio( ssl_context *ssl,
             int (*f_recv)(void *, unsigned char *, size_t), void *p_recv,
-            int (*f_send)(void *, unsigned char *, size_t), void *p_send )
+            int (*f_send)(void *, const unsigned char *, size_t), void *p_send )
 {
     ssl->f_recv     = f_recv;
     ssl->f_send     = f_send;