diff --git a/include/mbedtls/base64.h b/include/mbedtls/base64.h
index 86fce79..b731885 100644
--- a/include/mbedtls/base64.h
+++ b/include/mbedtls/base64.h
@@ -37,37 +37,39 @@
  * \brief          Encode a buffer into base64 format
  *
  * \param dst      destination buffer
- * \param dlen     size of the buffer
+ * \param dlen     size of the destination buffer
+ * \param olen     number of bytes written
  * \param src      source buffer
  * \param slen     amount of data to be encoded
  *
  * \return         0 if successful, or MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL.
- *                 *dlen is always updated to reflect the amount
+ *                 *olen is always updated to reflect the amount
  *                 of data that has (or would have) been written.
  *
- * \note           Call this function with *dlen = 0 to obtain the
- *                 required buffer size in *dlen
+ * \note           Call this function with dlen = 0 to obtain the
+ *                 required buffer size in *olen
  */
-int mbedtls_base64_encode( unsigned char *dst, size_t *dlen,
+int mbedtls_base64_encode( unsigned char *dst, size_t dlen, size_t *olen,
                    const unsigned char *src, size_t slen );
 
 /**
  * \brief          Decode a base64-formatted buffer
  *
  * \param dst      destination buffer (can be NULL for checking size)
- * \param dlen     size of the buffer
+ * \param dlen     size of the destination buffer
+ * \param olen     number of bytes written
  * \param src      source buffer
  * \param slen     amount of data to be decoded
  *
  * \return         0 if successful, MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL, or
  *                 MBEDTLS_ERR_BASE64_INVALID_CHARACTER if the input data is
- *                 not correct. *dlen is always updated to reflect the amount
+ *                 not correct. *olen is always updated to reflect the amount
  *                 of data that has (or would have) been written.
  *
- * \note           Call this function with *dst = NULL or *dlen = 0 to obtain
- *                 the required buffer size in *dlen
+ * \note           Call this function with *dst = NULL or dlen = 0 to obtain
+ *                 the required buffer size in *olen
  */
-int mbedtls_base64_decode( unsigned char *dst, size_t *dlen,
+int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
                    const unsigned char *src, size_t slen );
 
 /**