SSLClient

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SSLClient adds TLS 1.2 functionality to any network library implementing the Arduino Client interface, including the Arduino EthernetClient and WiFiClient classes. Unlike ArduinoBearSSL, SSLClient is completly self-contained, and does not require any additional hardware (other than a network connection).

SSLClient officially supports SAMD21, SAM3X, ESP32, TIVA C, STM32, and Teensy 4.x; but it should work on any board with at least 110kb flash and 7kb RAM. SSClient does not currently support ESP8266 (see this issue) or AVR due to memory constraints on both platforms.

Overview

Using SSLClient is similar to using any other Arduino-based Client class, as this library was developed around compatibility with EthernetClient. There are a few extra things, however, that you will need to get started:

1. Board and Network Peripheral - Your board should have a lot of resources (>110kb flash and >7kb RAM), and your network peripheral should have a large internal buffer (>7kb). This library was tested with the Adafruit Feather M0 (256K flash, 32K RAM) and the Adafruit Ethernet Featherwing (16kb Buffer), and we still had to modify the Arduino Ethernet library to support larger internal buffers per socket (see the Implementation Gotchas).
2. Trust Anchors - You will need a header containing array of trust anchors (example), which are used to verify the SSL connection later on. This file must generated for every project. Check out TrustAnchors.md on how to generate this file for your project, and for more information about what a trust anchor is.
3. Network Peripheral Driver Implementing Client - Examples include EthernetClient, WiFiClient, and so on—SSLClient will run on top of any network driver exposing the Client interface.
4. Analog Pin - Used for generating random data at the start of the connection (see the Implementation Gotchas).

Once all those are ready, you can create an SSLClient object like this:

BaseClientType baseClientInstance;
SSLClient client(baseClientInstance, TAs, (size_t)TAs_NUM, AnalogPin);

Where:

• BaseClientType - The type of baseClientInstance
• BaseClientInstance - An instance of the class you are using for SSLClient (the class associated with the network interface, from step 3). It is important that this instance be stored outside the SSLClient declaration (for instance, SSLClient(BaseClientType() ...) wouldn't work).
• TAs - The name of the trust anchor array created in step 2. If you generated a header using the tutorial this will probably be TAs.
• TAs_NUM - The number of trust anchors in TAs. If you generated a header using the tutorial this will probably be TAs_NUM.
• AnalogPin - The analog pin to pull random data from (step 4).

For example, if I am using EthernetClient, a generated array of 2 trust anchors, and the analog pin A7, I would declare an SSLClient instance using:

EthernetClient baseClient;
SSLClient client(baseClient, TAs, 2, A7);

Given this client, simply use SSLClient as you would the base client class:

// connect to ardiuino.cc over ssl (port 443 for websites)
client.connect("www.arduino.cc", 443);
// Make a HTTP request
client.println("GET /asciilogo.txt HTTP/1.1");
client.println("User-Agent: AdafruitFeatherM0WiFi");
client.print("Host: ");
client.println(server);
client.println("Connection: close");
client.println();
client.flush();
// read and print the data
...

Note: client.connect("www.arduino.cc", 443) can take 5-15 seconds to finish. This an unavoidable consequence of the SSL protocol, and is detailed in Implementation Notes.

For more information on SSLClient, check out the examples, API documentation, or the rest of this README.

How It Works

SSLClient was created to integrate SSL seamlessly with the Arduino infrastructure, and so it does just that: implementing the brilliant BearSSL as a proxy in front of any Arduino socket library. BearSSL is designed with low flash footprint in mind, and as a result does little verification of improper programming, relying on the developer to ensure the code is correct. Since SSLClient is built specifically for the Arduino ecosystem, most of SSLClient's code adds those programming checks back in, making debugging a fast and simple process.

Other Features

Logging

SSLClient also allows for changing the debugging level by adding an additional parameter to the constructor:

EthernetClient baseClient;
SSLClient client(baseClient, TAs, (size_t)2, A7, 1, SSLClient::SSL_INFO);

Logging is always outputted through the Arduino Serial interface, so you'll need to setup Serial before you can view the SSL logs. Log levels are enumerated in ::DebugLevel. The log level is set to SSL_WARN by default.

Errors

When SSLClient encounters an error, it will attempt to terminate the SSL session gracefully if possible, and then close the socket. Simple error information can be found from SSLClient::getWriteError(), which will return a value from the ::Error enum. For more detailed diagnostics, you can look at the serial logs, which will be displayed if the log level is at SSL_ERROR or lower.

Write Buffering

As you may have noticed in the documentation for SSLClient::write, calling this function does not actually write to the network. Instead, you must call SSLClient::available or SSLClient::flush, which will detect that the buffer is ready and write to the network (see SSLClient::write for details).

This was implemented as a buffered function because examples in Arduino libraries will often write to the network like so:

EthernetClient client;
// ...
// connect to ardiuino.cc over ssl (port 443 for websites)
client.connect("www.arduino.cc", 443);
// ...
// write an http request to the network
client.write("GET /asciilogo.txt HTTP/1.1\r\n");
client.write("Host: arduino.cc\r\n");
client.write("Connection: close\r\n");
// wait for response
while (!client.available()) { /* ... */ }
// ...

Notice that every single write() call immediately writes to the network, which is fine with most network clients. With SSL, however, if we are encrypting and writing to the network every write() call, this will result in a lot of small encryption tasks. Encryption takes a lot of time and code, so to reduce the overhead of an SSL connection, SSLClient::write implicitly buffers until the developer states that they are waiting for data to be received with SSLClient::available. A simple example can be found below:

EthernetClient baseClient;
SSLClient client(baseClient, TAs, (size_t)2, A7);
// ...
// connect to ardiuino.cc over ssl (port 443 for websites)
client.connect("www.arduino.cc", 443);
// ...
// add http request to the buffer
client.write("GET /asciilogo.txt HTTP/1.1\r\n");
client.write("Host: arduino.cc\r\n");
client.write("Connection: close\r\n");
// write the bytes to the network, then wait for response
while (!client.available()) { /* ... */ }
// ...

If you would like to trigger a network write manually without using the SSLClient::available, you can also call SSLClient::flush, which will write all data and return when finished.

Session Caching

As detailed in the resources section, SSL handshakes take an extended period (1-4sec) to negotiate. To remedy this problem, BearSSL is able to keep a SSL session cache of the clients it has connected to. If BearSSL successfully resumes an SSL session, it can reduce connection time to 100-500ms.

In order to use SSL session resumption:

• The website you are connecting to must support it. Support is widespread, but you can verify easily using the SSLLabs tool.
• You must reuse the same SSLClient object (SSL Sessions are stored in the object itself).
• You must reconnect to the exact same server.

SSLClient automatically stores an IP address and hostname in each session, ensuring that if you call connect("www.google.com") SSLClient will use the SSL session with that hostname. However, because some websites have multiple servers on a single IP address (github.com being an example), you may find that even if you are connecting to the same host the connection does not resume. This is a flaw in the SSL session protocol — though it has been resolved in TLS 1.3, the lack of widespread adoption of the new protocol prevents it from being used here. SSL sessions can also expire based on server criteria, which will result in a standard 4-10 second connection.

You can test whether or not a website can resume SSL Sessions using the Session Example included with this library. Because of all the confounding factors of SSL Sessions, it is generally prudent while programming to assume the session will always fail to resume.

SSL sessions take a lot of memory to store, so by default SSLClient will only store one at a time. You can change this behavior by adding the following to your SSLClient declaration:

EthernetClient baseClient;
SSLClient client(baseClient, TAs, (size_t)2, A7, SomeNumber);

Where SomeNumber is the number of sessions you would like to store. For example this declaration can store 3 sessions:

EthernetClient baseClient;
SSLClient client(baseClient, TAs, (size_t)2, A7, 3);

Sessions are managed internally using the SSLSession::getSession function. This function will cycle through sessions in a rotating order, allowing the session cache to continually overwrite old sessions. In general, it is a good idea to use a SessionCache size equal to the number of domains you plan on connecting to.

If you need to clear a session, you can do so using the SSLSession::removeSession function.

mTLS

As of v1.6.0, SSLClient supports mutual TLS authentication. mTLS is a varient of TLS that verifys both the server and device identities before a connection, and is commonly used in IoT protocols as a secure layer (MQTT over TLS, HTTPS over TLS, etc.).

To use mTLS with SSLClient you will need to a client certificate and client private key associated with the server you are attempting to connect to. Depending on your use case, you will either generate these yourself (ex. Mosquito MQTT setup), or have them generated for you (ex. AWS IoT Certificate Generation). Given this cryptographic information, you can modify the standard SSLClient connection sketch to enable mTLS authentication:

...
/* Somewhere above setup() */

// The client certificate, can be PEM or DER format
// DER format will be an array of raw bytes, and PEM format will be a string
// PEM format is shown below
const char my_cert[] = 
"-----BEGIN CERTIFICATE-----\n"
"MIIDpDCCAowCCQC7mCk5Iu3YmDANBgkqhkiG9w0BAQUFADCBkzELMAkGA1UEBhMC\n"
...
"-----END CERTIFICATE-----\n";

// The client private key, must be the same format as the client certificate
// Both RSA and ECC are supported, ECC is shown below
const char my_key[] = 
"-----BEGIN EC PRIVATE KEY-----\n"
...
"-----END EC PRIVATE KEY-----\n";

// This line will parse and store the above information so SSLClient can use it later
// Replace `fromPEM` with `fromDER` if you are using DER formatted certificates.
SSLClientParameters mTLS = SSLClientParameters::fromPEM(my_cert, sizeof(cert), my_key, sizeof(key));
SSLClient my_client(...);
...
void setup() {
    ...
    /* Before SSLClient connects */

    my_client.setMutualAuthParams(mTLS);
    ...
}
...

Note that both the above client information as well as the correct trust anchors associated with the server are needed for the connection to succeed. Additionally, the certificate must be formatted correctly (according to BearSSL's specification) in order for mTLS to work. If the certificate is improperly formatted, SSLClient will attempt to make a regular TLS connection instead of an mTLS one, and fail to connect as a result. Because of this, if you are seeing errors similar to "peer did not send certificate chain" on your server, check that your certificate and key are formatted correctly (see https://github.com/OPEnSLab-OSU/SSLClient/issues/7#issuecomment-593704969). For more information on SSLClient's mTLS functionality, please see the SSLClientParameters documentation.

Implementation Gotchas

Some ideas that didn't quite fit in the API documentation.

SSLClient with Ethernet

If you are using the Arduino Ethernet library, you will need to modify the library to support the large buffer sizes required by SSL (detailed in resources). You can either modify the library yourself, or use this fork of the Ethernet library with the modification. To use the fork, simply install the library using the "add a .zip library" button in Arduino, and replace #include "Ethernet.h" with #include "EthernetLarge.h" in your sketch. Alternatively if for some reason this solution does not work, you can apply the modification using the instructions below.

Manual Modification

First find the location of the library in the directory where Arduino is installed (C:\Program Files (x86)\Arduino on Windows). Inside of this directory, navigate to libraries\Ethernet\src (C:\Program Files (x86)\Arduino\libraries\Ethernet\src on Windows). Modify Ethernet.h to replace these lines:

...
// Configure the maximum number of sockets to support.  W5100 chips can have
// up to 4 sockets.  W5200 & W5500 can have up to 8 sockets.  Several bytes
// of RAM are used for each socket.  Reducing the maximum can save RAM, but
// you are limited to fewer simultaneous connections.
#if defined(RAMEND) && defined(RAMSTART) && ((RAMEND - RAMSTART) <= 2048)
#define MAX_SOCK_NUM 4
#else
#define MAX_SOCK_NUM 8
#endif

// By default, each socket uses 2K buffers inside the Wiznet chip.  If
// MAX_SOCK_NUM is set to fewer than the chip's maximum, uncommenting
// this will use larger buffers within the Wiznet chip.  Large buffers
// can really help with UDP protocols like Artnet.  In theory larger
// buffers should allow faster TCP over high-latency links, but this
// does not always seem to work in practice (maybe Wiznet bugs?)
//#define ETHERNET_LARGE_BUFFERS
...

With this:

...
// Configure the maximum number of sockets to support.  W5100 chips can have
// up to 4 sockets.  W5200 & W5500 can have up to 8 sockets.  Several bytes
// of RAM are used for each socket.  Reducing the maximum can save RAM, but
// you are limited to fewer simultaneous connections.
#define MAX_SOCK_NUM 2

// By default, each socket uses 2K buffers inside the Wiznet chip.  If
// MAX_SOCK_NUM is set to fewer than the chip's maximum, uncommenting
// this will use larger buffers within the Wiznet chip.  Large buffers
// can really help with UDP protocols like Artnet.  In theory larger
// buffers should allow faster TCP over high-latency links, but this
// does not always seem to work in practice (maybe Wiznet bugs?)
#define ETHERNET_LARGE_BUFFERS
...

You may need to use sudo or administrator permissions to make this modification. We change MAX_SOCK_NUM and ETHERNET_LARGE_BUFFERS so the Ethernet hardware can allocate a larger space for SSLClient, however a downside of this modification is we are now only able to have two sockets concurrently. As most microprocessors barely have enough memory for one SSL connection, this limitation will rarely be encountered in practice.

Seeding Random Data

The SSL protocol requires that SSLClient generate some random bits before connecting with a server. BearSSL provides a random number generator but requires a some entropy for a seed. Normally this seed is generated by taking the microsecond time using the internal clock, however since most microcontrollers are not build with this feature another source must be found. As a simple solution, SSLClient uses a floating analog pin as an external source of random data, passed through to the constructor in the analog_pin argument. Before every connection, SSLClient will take the bottom byte from 16 analog reads on analog_pin, and combine these bytes into a 16 byte random number, which is used as a seed for BearSSL. To ensure the most random data, it is recommended that this analog pin be either floating or connected to a location not modifiable by the microcontroller (i.e. a battery voltage readout).

Certificate Verification

SSLClient uses BearSSL's minimal x509 verification engine to verify the certificate of an SSL connection. This engine requires the developer create a trust anchor array using values stored in trusted root certificates. Check out this document for more details on this component of SSLClient.

BearSSL also features a known certificate validation engine, which only allows for a single domain in exchange for a significantly reduced resource usage (flash and CPU time). This functionality is planned to be implemented in the future.

Resources

The SSL protocol recommends a device support many different encryption algorithms, as well as protocols for SSL itself. The complexity of both of those components results in many medium sized components forming an extremely large whole. Additionally, most embedded processors lack the sophisticated math hardware commonly found in a modern CPU, and as a result require more instructions to create the encryption algorithms SSL requires. This not only increases size but makes the algorithms slow and memory intensive.

To illustrate this, I will run some tests on various domains below. I haven't yet, but I will.

If flash footprint is becoming a problem, there are numerous debugging strings (~3kb estimated) that can be removed from SSLClient.h, SSLClientImpl.h, and SSLClientImpl.cpp. I have not figured out a way to configure compilation of these strings, so you will need to modify the library to remove them yourself.

Read Buffer Overflow

SSL is a buffered protocol, and since most microcontrollers have limited resources (see Resources), SSLClient is limited in the size of its buffers. A common problem I encountered with SSL connections is buffer overflow, caused by the server sending too much data at once. This problem is caused by the microcontroller being unable to copy and decrypt data faster than it is being received, forcing some data to be discarded. This usually puts BearSSL in an unrecoverable state, forcing SSLClient to close the connection with a write error. If you are experiencing frequent timeout problems, this could be the reason why.

In order to remedy this problem, the device must be able to read the data faster than it is being received, or alternatively have a cache large enough to store the entire payload. Since SSL's encryption forces the device to read slowly, this means we must increase the cache size. Depending on your platform, there are a number of ways this can be done:

• Sometimes your communication shield will have an internal buffer, which can be expanded through the driver code. This is the case with the Arduino Ethernet library (in the form of the MAX_SOCK_NUM and ETHERNET_LARGE_BUFFERS macros), however the library must be modified for the change to take effect.
• SSLClient has an internal buffer SSLClientImpl::m_iobuf, which can be expanded. BearSSL limits the amount of data that can be processed based on the stage in the SSL handshake, and so this will change will have limited usefulness.
• In some cases, a website will send so much data that even with the above solutions, SSLClient will be unable to keep up (a website with a lot of HTML is an example). In these cases you will have to find another method of retrieving the data you need.
• If none of the above are viable, it is possible to implement your own Client class which has an internal buffer much larger than both the driver and BearSSL. This would require in-depth knowledge of programming and the communication shield you are working with, as well as a microcontroller with a significant amount of RAM.

Cipher Support

By default, SSLClient supports only TLS1.2 and the ciphers listed in this file under suites[], and the list is relatively small to keep the connection secure and the flash footprint down. These ciphers should work for most applications, however if for some reason you would like to use an older version of TLS or a different cipher, you can change the BearSSL profile being used by SSLClient to an alternate one with support for older protocols. To do this, edit SSLClientImpl::SSLClientImpl to change these lines:

br_client_init_TLS12_only(&m_sslctx, &m_x509ctx, m_trust_anchors, m_trust_anchors_num);
// comment the above line and uncomment the line below if you're having trouble connecting over SSL
// br_ssl_client_init_full(&m_sslctx, &m_x509ctx, m_trust_anchors, m_trust_anchors_num);

to this:

// br_client_init_TLS12_only(&m_sslctx, &m_x509ctx, m_trust_anchors, m_trust_anchors_num);
// comment the above line and uncomment the line below if you're having trouble connecting over SSL
br_ssl_client_init_full(&m_sslctx, &m_x509ctx, m_trust_anchors, m_trust_anchors_num);

If for some unfortunate reason you need SSL 3.0 or SSL 2.0, you will need to modify the BearSSL profile to enable support. Check out the BearSSL profiles documentation and I wish you the best of luck.

Known Issues

• In some drivers (Ethernet), calls to Client::flush will hang if internet is available but there is no route to the destination. Unfortunately SSLClient cannot correct for this without modifying the driver itself, and as a result the recommended solution is ensuring you choose a driver with built-in timeouts to prevent freezing. More information here.
• When using PubSubClient on the ESP32, a stack overflow will occur if the user does not flush the buffer immediately after writing. The cause of this issue is under active investigation. More information in issue https://github.com/OPEnSLab-OSU/SSLClient/issues/9.
• Previous to SSLClient v1.6.7, calls to SSLClient::stop would sometimes hang the device. More information in issue https://github.com/OPEnSLab-OSU/SSLClient/issues/13.
• Previous to SSLClient v1.6.6, calls to SSLClient::connect would fail if the driver indicated that a socket was already opened (Client::connected returned true). This behavior created unintentional permanent failures when Client::stop would fail to close the socket, and as a result was downgraded to a warning in v1.6.6.
• Previous to SSLClient v1.6.3, calling SSLClient::write with more than 2Kb of total data before flushing the write buffer would cause a buffer overflow.