mbedtls/docs/tls13-early-data.md

Writing and reading early or 0-RTT data

An application function to write and send a buffer of data to a server through TLS may plausibly look like:

int write_data( mbedtls_ssl_context *ssl,
                const unsigned char *data_to_write,
                size_t data_to_write_len,
                size_t *data_written )
{
    *data_written = 0;

    while( *data_written < data_to_write_len )
    {
        ret = mbedtls_ssl_write( ssl, data_to_write + *data_written,
                                 data_to_write_len - *data_written );

        if( ret < 0 &&
            ret != MBEDTLS_ERR_SSL_WANT_READ &&
            ret != MBEDTLS_ERR_SSL_WANT_WRITE )
        {
            return( ret );
        }

        *data_written += ret;
    }

    return( 0 );
}

where ssl is the SSL context to use, data_to_write the address of the data buffer and data_to_write_len the number of data bytes. The handshake may not be completed, not even started for the SSL context ssl when the function is called and in that case the mbedtls_ssl_write() API takes care transparently of completing the handshake before to write and send data to the server. The mbedtls_ssl_write() may not been able to write and send all data in one go thus the need for a loop calling it as long as there are still data to write and send.

An application function to write and send early data and only early data, data sent during the first flight of client messages while the handshake is in its initial phase, would look completely similar but the call to mbedtls_ssl_write_early_data() instead of mbedtls_ssl_write().

int write_early_data( mbedtls_ssl_context *ssl,
                      const unsigned char *data_to_write,
                      size_t data_to_write_len,
                      size_t *data_written )
{
    *data_written = 0;

    while( *data_written < data_to_write_len )
    {
        ret = mbedtls_ssl_write_early_data( ssl, data_to_write + *data_written,
                                            data_to_write_len - *data_written );

        if( ret < 0 &&
            ret != MBEDTLS_ERR_SSL_WANT_READ &&
            ret != MBEDTLS_ERR_SSL_WANT_WRITE )
        {
            return( ret );
        }

        *data_written += ret;
    }

    return( 0 );
}

Note that compared to write_data(), write_early_data() can also return MBEDTLS_ERR_SSL_CANNOT_WRITE_EARLY_DATA and that should be handled specifically by the user of write_early_data(). A fresh SSL context (typically just after a call to mbedtls_ssl_setup() or mbedtls_ssl_session_reset()) would be expected when calling write_early_data.

All together, code to write and send a buffer of data as long as possible as early data and then as standard post-handshake application data could plausibly look like:

ret = write_early_data( ssl, data_to_write, data_to_write_len,
                        &early_data_written );
if( ret < 0 &&
    ret != MBEDTLS_ERR_SSL_CANNOT_WRITE_EARLY_DATA )
{
    goto error;
}

ret = write_data( ssl, data_to_write + early_data_written,
                  data_to_write_len - early_data_written, &data_written );
if( ret < 0 )
    goto error;

data_written += early_data_written;

Finally, taking into account that the server may reject early data, application code to write and send a buffer of data could plausibly look like:

ret = write_early_data( ssl, data_to_write, data_to_write_len,
                        &early_data_written );
if( ret < 0 &&
    ret != MBEDTLS_ERR_SSL_CANNOT_WRITE_EARLY_DATA )
{
    goto error;
}

/*
 * Make sure the handshake is completed as it is a requisite to
 * mbedtls_ssl_get_early_data_status().
 */
while( !mbedtls_ssl_is_handshake_over( ssl ) )
{
    ret = mbedtls_ssl_handshake( ssl );
    if( ret < 0 &&
        ret != MBEDTLS_ERR_SSL_WANT_READ &&
        ret != MBEDTLS_ERR_SSL_WANT_WRITE )
    {
        goto error;
    }
}

ret = mbedtls_ssl_get_early_data_status( ssl );
if( ret < 0 )
    goto error;

if( ret == MBEDTLS_SSL_EARLY_DATA_STATUS_REJECTED )
   early_data_written = 0;

ret = write_data( ssl, data_to_write + early_data_written,
                  data_to_write_len - early_data_written, &data_written );
if( ret < 0 )
    goto error;

data_written += early_data_written;

Basically, the same holds for reading early data on the server side without the complication of possible rejection. An application function to read early data into a given buffer could plausibly look like:

int read_early_data( mbedtls_ssl_context *ssl,
                     unsigned char *buffer,
                     size_t buffer_size,
                     size_t *data_len )
{
    *data_len = 0;

    while( *data_len < buffer_size )
    {
        ret = mbedtls_ssl_read_early_data( ssl, buffer + *data_len,
                                           buffer_size - *data_len );

        if( ret < 0 &&
            ret != MBEDTLS_ERR_SSL_WANT_READ &&
            ret != MBEDTLS_ERR_SSL_WANT_WRITE )
        {
            return( ret );
        }

        *data_len += ret;
    }

    return( 0 );
}

with again calls to read_early_data() expected to be done with a fresh SSL context.