Before everyone goes yelling: "NEVER DO YOUR OWN CRYPTO", I haven't, technically (I'm still half way), but apart from that I am making a TLS-like protocol, but way lighter.

I needed to secure communication between several embedded systems with very little RAM/ROM and processor power (well, it has 100 MHz, but a lot of it is in constant use and cannot be interrupted).

What I am going to do is use AES-128 with GCM (or CBC + HMAC) to secure the communication between the devices. Some devices can only use GCM, others use CBC + HMAC so (still working on it) I have to make a TLS-like structure to agree which one to use (device communicate through a server, not peer-to-peer, so having only 1 of the 2 isn't a problem; the server just needs both).

I've been doing a lot of research these past months (2 1/2), and I would like to say I have covered the basics. I'm now starting with implementing everything I have found (usage of keys, IVs, nonces, encrypt then MAC, etc.).

Now comes the question: when I'm finished I want to have the code I created checked. Obviously the deep level scrutiny TLS has gone through won't be achievable, but what ways are there? Are there sites where I can post my code for review? Are there companies which can 'certify' it? What would normally happen when a company created a secure communication and want to verify if it protects against the things they envisioned?

And yes, I know I'm not using a PKI. The hardware is in a secure environment, and no new devices can be added to the server (the server keeps a list of devices which can connect and monitors if one ever goes offline), so the company said PKI might be something they'd consider later on, but for a first version would not be necessary.

EDIT: I want to clarify (I'm taking back a few words). I will be using TLS to create this protocol, and the amount of TLS usages is of yet still unclear. When possible, I will simply try to remove everything I don't need from TLS and create it that way, I might take pieces of it (encrypt-then-mac, adding IV to ciphertext, etc.) and use that.

But either way, it is not TLS and even only removing stuff from TLS source code might introduce weaknesses so an 'audit' is still necessary.

  • 9
    You have to understand: you are STILL trying to write your own crypto. This is bad. As @GZBK implied, you'll have to have your code audited by a third party in order to assess how well it fits your expectations but the fact that you're writing your own crypto will make that assessment waaaay more expensive.
    – Stephane
    Commented Apr 20, 2015 at 9:49
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    @Stephane the joke was I haven't written it yet (i'm in the process of writing it). and at least I'm not trying to create my own crypto primitive. Maybe i should have mentioned the primitives i do use are from libraries so I did not implement that my self. But yes, I am stitching it together my self. But sometimes you just have to which is why i need a good way to verify what I did is correct.
    – Vincent
    Commented Apr 20, 2015 at 10:02
  • 4
    It's simple: auditing code for security is expensive so the more you use already verified code the cheapest it'll be. In that context, not using a standard protocol like TLS is usually hard to justify
    – Stephane
    Commented Apr 20, 2015 at 10:05
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    And I get it even Bruce Schneier makes mistakes But with sufficient prep someone should be able to combine a few crypto primitives to create a protocol. @Bob We have tens of thousands of devices in the fields, we cannot possibly afford to upgrade those, even if we wanted many customers won't approve of us opening up everything again to install a new one (customer would have to pay for this). So yes like you said, it's not simple if not impossible when taking into account the customer
    – Vincent
    Commented Apr 20, 2015 at 10:24
  • 2
    @VincentAdvocaat Bruce Schneier does not make mistakes. He makes, and then others miss take. It is true that sometimes one must stitch together a high-level procedure from existing low(er) level crypto libraries. Audits are more about looking for loopholes in procedure and ways to obviate the protocol you have created, than checking whether a crypto algorithm is in the unprovable category of "cryptologically strong enough" or that an implementation of it is correct. "How to audit" is hard to answer; as formal audits are themselves of questionable utility.
    – zxq9
    Commented Apr 20, 2015 at 14:13

3 Answers 3


Define a specification

If you are writing/developing a new protocol before you cut code, you should start by writing a specification like a Request For Comments (RFC).

Even if you never intend to publish your protocol the process is the important part. The RFC would be the basis of any assessment that you want to have done in the future.

In addition to how the protocol works you would want to decribe the reasons why you are developing a new protocol/suite and why someone should implement your protocol.


To validate your specification you could consider a combination of:

  • Security research organisations e.g. SANS
  • Universities
  • Security community through the publishing research papers
  • Third party risk assessment

Please consider that developing robust protocol takes time, and you will likely need to version it, so make provision for this. As you have mentioned, SSL/TLS have undergone a lot of review and have been versioned a number of times.


After you have defined a specification that has had recieved review you will need to develop a reference implementation that can be complete code review and then pentration testing.

For the code review you would be looking for an assessor with specialist security skills in the programming langauge you are using and a background in penetration testing. They will be assessing your code against the specification.

For the pentration testing you would want to find an assessor who had the capability to develop they own custom test tools, and have reconised certifications such as ISO 27001 and Certified Ethical Hacker (CEH).

Other points

  • Obviously before commiting such a large time and money expense you would need to consider if it is really worth it for the potential lifetime of the system that you are developing.

  • Like others have said using an existing protocol implementation generally cost less and reduces security risks.

  • While you have a limited comput capacity there may be other ways to address this (instead of implenting a new security stack). Do you really need message privacy (in a controlled secure environment) or just message integrity checking? Or can you just encrypt/decrypt the envelope.

Anyway hope that helps.


There are IT security companies which provide services like code auditing (most often along with other services like penetration testing, etc.). You contact them, describing your needs, then if this fall under their competences they will send you an estimate.

  • 1
    I take it google is my friend in finding these companies? i know of FoxIT (dutch company) which does this and already has done this for a different project of ours, though that was more of a pen test. Do you have any other recommendations?
    – Vincent
    Commented Apr 20, 2015 at 10:04
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    Putting companies name here would be more advertisement than advices. Yes, I think that using Google to list available companies, their location, check what services they provide, what kind of customers they usually target (a SME targeting company will not have the same pricing than one targeting worldwide corporations... but on the other side they will not have the same level of expertise either), their certifications (employee + possibly company level certifications) and know what these certs actually imply... In other words, get a view of the market and make your choice :) Commented Apr 20, 2015 at 10:15
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    You know, asking how best to evaluate a company for code auditing might not be a bad question
    – Stephane
    Commented Apr 20, 2015 at 12:26
  • You are fully right, there are several actions taken from public and private groups in order to provide evaluation criteria for IT security auditing companies and reduce some abuse... but that would be a totally different question. Feel free to ask it :) Commented Apr 20, 2015 at 14:21
  • @VincentAdvocaat Meanwhile, if this answer fits your needs, do not forget to mark it as accepted, thanks :) Commented Apr 21, 2015 at 13:10

Depending on language and environment:

US DHS and a third party host SWAMP (software assurance marketplace). You must apply for access and membership, but it's worth looking into.


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