TLS is Hard

This is a bit unusual of a post, as it is more of a brain dump than a nicely written prose. I may fill this in over time.

TLS and why it is hard

  • Introduction

    In an ideal magical world, TLS is a thing you drop into a connection and it becomes secure. Both parties just communicate as if they were read/write send/recv packets, but everything is secured.

    The problem is that, although the communication is “secure”, it is crucial that you know who you are talking to. Without this, the encrypted part of the communication is completely meaningless, as you may be communicating with your adversary.

  • How does this work for my browser

    When I connect to https://www.example.com/, the server sends a certificate to me. The things that are important about this certificate:

    • Either “Common Name (CN)” or “Certificate Subject Alt Name” contains the dns domain name of the site looked up. So in this case, the certificate returned must either have CN of “www.example.com”, or it must be one of the entries in the “Certificate Subject Alt Name”.

    • The certificate validity period encompasses the current time.

    • The “Certificate Key Usage” and “Extended Key Usage” indicate this certificate is valid for “TLS Web Server Authentication”, as well as “Signing” and “Key Encipherment”.

    • There is a signature chain from this certificate to a root certificate that is in the browser’s list of trusted root certificates (162 of these at my last check).

    • No certificate in the chain is on a black list of compromised certificates.

    • Other more modern techniques browsers use to only accept valid certificates, especially for popular domains.

    Once these things have been validated, and the rest of the TLS handshake is valid, the client and server will begin communicating over a secure and partially authenticated channel.

    • The client has reasonable assurance that the server is the correct site.

    • The server knows nothing about the client. This is why many sites will then require you to log in. Certificates can be used to authenticate the client, but this is uncommon because this would require users to manage certificates, and this would frequently be done insecurely.

  • What does the browser do (TLS library) to do all of this:

    • Initialize the library, and create a new context.

    • Configure the context to verify the peer, by calling a callback function.

    • Configure options for TLS so that only modern, secure TLS features are used.

    • Load a database of root certificates. For example: /etc/ssl/certs/ca-certificates.crt.

    • Setup the TLS library to use sockets.

    • Configure valid ciphers.

    • Tell the TLS library the hostname it thinks it is talking to.

    • TLS connect (does socket connection).

    • TLS handshake

    • Verify that the server presented a certificate.

    • Verify that the cert chain is trusted

    • Verify that the cert matches the host specified.

    • Whew, communicate with the server.

    The verify callback is optional and allows a particular connection to use different rules than the defaults to state that the chain is valid.

    There is a lot of stuff here, and it is easy to get wrong (often resulting in insecure connections). We can optimize some of this (we shouldn’t allow insecure ciphers, only use the latest TLS, and a lot is going to be common).

    Oh, and it isn’t really quite that simple.

    • TLS allows for renegotiation. Maybe this isn’t needed to IoT usage, but it is part of the spec.

    • TLS/DTLS provide something known as session resumption. Through storing some secret state about a particular communication channel, it can be reestablished later (between the same two parties) with less communication and computational effort. Given the CPU constraints of IoT, session resumption is likely to be quite important.

  • Bigger problems

    • This model of certificate validation doesn’t usually work very well for IoT.

      • The default certificate chain is quite large (e.g. my current one is a 250KB file, which would take up 14 of the flash on my largest IoT SoC).

      • The default algorithms (typically RSA) can use large keys and require lots of RAM.

      • The default TLS record size is 16KB. This can be smaller, but this is an optional TLS extension. By default, the client will need just 32KB of RAM for buffers of TLS data.

      • This is all based on use over TCP. Many IoT protocols are more suited to UDP. The DTLS spec describes the protocol, but support is new, and the connections and usage are different in the use of the TLS library

      • Many IoT devices do not have a secure notion of time (may not even know the time). The certificate validity periods become meaningless, and need to be either ignored, or this problem solved in another way.

    • Work on IoT protocols (specifically LWM2M and Thread) have specific solutions to this (there is also work on how to do this with MQTT).

      • LWM2M gives three options: use certs like the browser does, use PSK, use bare keys.

      • Thread specifies PSK using a specific (and different than LWM2M) ciphersuite.

      • These new features are only newly implemented in TLS libraries.

  • Using these new approaches

    • PSK

      Pre-Shared Keys (PSK) provides a way for the client and server to mutually authenticate using a shared secret. Getting and protecting this secret is a challenging task, and warrants its own study.

      Generally, these keys should be large randomly-generated numbers, to avoid brute-force attacks. The JPAKE specified by Thread allows smaller keys to be used (typically a PIN entered by a user).

    • Certs

      Certificates in IoT will generally only allow a small number of root certs.

      Usually, the client will have to authenticate for this to be meaningful. It is possible to do authentication at a higher level in the protocol, but this is easy to get wrong, and has little benefit.

      Distributing the private keys for these certs to the clients is a similar challenge to distributing PSK.

      Since Certs are more costly computationally than PSK, this is an uncommon approach for these IoT protocols.

    • Bare keys

      Bare keys can be used instead of certificates. This requires both parties to keep track of the valid public key for every entity they communicate with. The benefit is less complexity and computational work, but management of keys becomes more complex.

      TLS extensions to support Bare keys is more preliminary than either Certs or PSK.

  • Handling this in a friendly TLS library:

    • We probably need to be able to handle all three approaches. Connections to public websites will be difficult, unless there is room to store the full cert chain.

    • We can define a context for security that supports these specific ways of doing things (or possibly just Certs, and both ways of doing PSK).

    • The setup will require giving enough information for the library to validate the other side of the connection, as well as to secure itself.

    • Since there are secrets involved, these will likely not be accessible to most code running on the CPU. Secure APIs will need to be used and ways of referencing secrets stored across a secure API.

  • The fundamental problem

    There is a strong argument that we shouldn’t be doing any of this.

    People shouldn’t be writing applications that communicate directly over TLS. Although just connecting and authenticating is complicated, management of secrets (private keys) is also complicated, and very easy to get wrong.

    There is a lot of work going into LWM2M, MQTT, Thread, etc to make the protocols secure, and handle the multitude of management issues. Individuals that write applications directly with TLS will almost certainly get it wrong, resulting in only an illusion of security.

    My suggestion is that we work to get LWM2M, COAPS, Thread, MQTT, and possibly others to use security best practices, likely directly using something such as mbed TLS properly, and that we don’t even advertise having direct TLS/DTLS support.

About David Brown
Software engineer and Jazz musician, currently living in the Denver, CO area.
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