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TLS Module

Since Origin / Contributor Maintainer Source
2016-12-15 PhoeniX PhoeniX tls.c

SSL/TLS support

!!! attention The TLS module depends on the net module, it is a required dependency.

NodeMCU includes the open-source version of mbed TLS library.

With the NodeMCU default configuration it supports TLS 1.1 and 1.2 with most common features supported. Specifically, it provides:

  • ciphers: AES, Camellia
  • chaining modes: CBC, CFB, CTR, GCM
  • digest algorithms: RIPEMD-160, SHA1, SHA2
  • signature algorithms: RSA, deterministic ECDSA
  • key exchange algorithms: DHE and ECDHE
  • elliptic curves: secp{256,384}r1, secp256k1, bp{256,384}.

!!! warning

The severe memory constraints of the ESP8266 mean that the `tls` module
is by far better suited for communication with custom, purpose-built
endpoints with small certificate chains (ideally, even self-signed)
than it is with the Internet at large.  By default, our mbedTLS
configuration requests TLS fragments of at most 4KiB and is unwilling
to process fragmented messages, meaning that the entire ServerHello,
which includes the server's certificate chain, must conform to this
limit.  We do not believe it useful or easy to be fully compliant with
the TLS specification, which requires a 16KiB recieve buffer and,
therefore, 32KiB of heap within mbedTLS, even in the steady-state.
While it is possible to slightly raise the buffer sizes with custom
nodeMCU builds, connecting to endpoints out of your control will remain
a precarious position, and so we strongly suggest that TLS connections
be made only to endpoints under your control, whose TLS configurations
can ensure that their ServerHello messages are small.  A reasonable
compromise is to have a "real" computer do TLS proxying for you; the
[socat](http://www.dest-unreach.org/socat/) program is one possible
mechanism of achieving such a "bent pipe" with TLS on both halves.

!!! warning

The TLS glue provided by Espressif provides no interface to TLS SNI.
As such, NodeMCU TLS should not be expected to function with endpoints
requiring the use of SNI, which is a growing fraction of the Internet
and includes, for example, Cloudflare sites using their "universal SSL"
service and other, similar "virtual" TLS servers.  TLS servers to which
you wish NodeMCU to connect should have their own, dedicated IP/port
pair.

!!! warning

The TLS handshake is very heap intensive, requiring between 25 and 30
**kilobytes** of heap, even with our reduced buffer sizes.  Some, but
not all, of that is made available again once the handshake has
completed and the connection is open.  Because of this, we have
disabled mbedTLS's support for connection renegotiation.  You may find
it necessary to restructure your application so that connections happen
early in boot when heap is relatively plentiful, with connection
failures inducing reboots.  LFS may also be of utility in freeing up
heap space, should you wish to attempt re-establishing connections
without rebooting.

!!! tip

If possible, you will likely be much better served by using the ECDSA
signature and key exchange algorithms than by using RSA.  An
increasingly large fraction of the Internet understands ECDSA, and most
server software can speak it as well.  The much smaller key size (at
equivalent security!) is beneficial for NodeMCU's limited RAM.

https://wiki.openssl.org/index.php/Command_Line_Elliptic_Curve_Operations
details how to create ECDSA keys and certificates.

!!! tip

The complete configuration is stored in
[user_mbedtls.h](../../app/include/user_mbedtls.h). This is the file to
edit if you build your own firmware and want to change mbed TLS
behavior.

For a list of possible features have a look at the
[mbed TLS features page](https://tls.mbed.org/core-features).

This module handles certificate verification when SSL/TLS is in use.

tls.createConnection()

Creates TLS connection.

Syntax

tls.createConnection()

Parameters

none

Returns

tls.socket sub module

Example

tls.createConnection()

tls.socket Module

tls.socket:close()

Closes socket.

Syntax

close()

Parameters

none

Returns

nil

See also

tls.createConnection()

tls.socket:connect()

Connect to a remote server.

Syntax

connect(port, ip|domain)

Parameters

  • port port number
  • ip IP address or domain name string

Returns

nil

See also

tls.socket:on()

tls.socket:getpeer()

Retrieve port and ip of peer.

Syntax

getpeer()

Parameters

none

Returns

  • ip of peer
  • port of peer

tls.socket:hold()

Throttle data reception by placing a request to block the TCP receive function. This request is not effective immediately, Espressif recommends to call it while reserving 5*1460 bytes of memory.

Syntax

hold()

Parameters

none

Returns

nil

See also

tls.socket:unhold()

tls.socket:on()

Register callback functions for specific events.

Syntax

on(event, function())

Parameters

  • event string, which can be "dns", "connection", "reconnection", "disconnection", "receive" or "sent"
  • function(tls.socket[, string]) callback function. The first parameter is the socket. If event is "receive", the second parameter is the received data as string. If event is "reconnection", the second parameter is the reason of connection error (string). If event is "dns", the second parameter will be either nil or a string rendering of the resolved address.

Returns

nil

Example

srv = tls.createConnection()
srv:on("receive", function(sck, c) print(c) end)
srv:on("connection", function(sck, c)
  -- Wait for connection before sending.
  sck:send("GET / HTTP/1.1\r\nHost: google.com\r\nConnection: keep-alive\r\nAccept: */*\r\n\r\n")
end)
srv:connect(443,"google.com")

!!! note The receive event is fired for every network frame! See details at net.socket:on().

See also

tls.socket:send()

Sends data to remote peer.

Syntax

send(string)

Parameters

  • string data in string which will be sent to server

Returns

nil

Note

Multiple consecutive send() calls aren't guaranteed to work (and often don't) as network requests are treated as separate tasks by the SDK. Instead, subscribe to the "sent" event on the socket and send additional data (or close) in that callback. See #730 for details.

See also

tls.socket:on()

tls.socket:unhold()

Unblock TCP receiving data by revocation of a preceding hold().

Syntax

unhold()

Parameters

none

Returns

nil

See also

tls.socket:hold()

tls.cert Module

tls.cert.verify()

Controls the vertificate verification process when the Nodemcu makes a secure connection.

Syntax

tls.cert.verify(enable)

tls.cert.verify(pemdata)

Parameters

  • enable A boolean which indicates whether verification should be enabled or not. The default at boot is false.
  • pemdata A string containing the CA certificate to use for verification.

Returns

true if it worked.

Can throw a number of errors if invalid data is supplied.

Example

Make a secure https connection and verify that the certificate chain is valid.

tls.cert.verify(true)
http.get("https://example.com/info", nil, function (code, resp) print(code, resp) end)

Load a certificate into the flash chip and make a request. This is the IdenTrust DST Root CA X3 certificate; it is used, for example, by letsencrypt, for their intermediate X3 authority; letsencrypt is one option for obtaining your own SSL certificates free of cost.

tls.cert.verify([[
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
]])

http.get("https://letsencrypt.org/", nil, function (code, resp) print(code, resp) end)

Notes

The certificate needed for verification is stored in the flash chip. The tls.cert.verify call with true enables verification against the value stored in the flash.

The certificate can be loaded into the flash chip in two ways -- one at firmware build time, and the other at initial boot of the firmware. In order to load the certificate at build time, just place a file containing the CA certificate (in PEM format) at server-ca.crt in the root of the nodemcu-firmware build tree. The build scripts will incorporate this into the resulting firmware image.

The alternative approach is easier for development, and that is to supply the PEM data as a string value to tls.cert.verify. This will store the certificate into the flash chip and turn on verification for that certificate. Subsequent boots of the nodemcu can then use tls.cert.verify(true) and use the stored certificate.

tls.setDebug function

mbedTLS can be compiled with debug support. If so, the tls.setDebug function is mapped to the mbedtls_debug_set_threshold function and can be used to enable or disable debugging spew to the console. See mbedTLS's documentation for more details.