Cryptographic Co-Processor ATECC508A (Qwiic) Hookup Guide

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Contributors: QCPete
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Cryptographic Authentication

Cryptography is a large subject matter. There many different types of cryptographic processes, each with their own different purpose and type of math. In this tutorial, we are focusing on cryptographic authentication. On a very high level, this usually involves data, digital signatures and verification. First, we must understand what a digital signature is and how it is created. Second we must understand verification which includes the use of a private/public key pair. Let's jump into digital signatures!

Digital signatures

A digital signature is quite similar to a hand-written signature. After you write a letter, you add your signature at the bottom. Just like the hand-written signature, a digital signature is added to your data. When we say "added" we mean that it is included as a separate chunk of data. So when sending some data with a digital signature, you would send the data as one piece of data, and then you would also send the digital signature as a separate piece of data. So this is the first difference to understand: digital signatures are like hand-written signatures, except they don't live on the same piece of paper (they are a separate chunk of data).

Another big difference: to create a digital signature, you need a bit more than a pen. You actually need three things:

  1. Data: This is the data you want to sign. For this co-processor, we are going to sign 32 bytes of data. This is essentially your "letter without a signature".
  2. A Key: Is simply a unique chunk of data. The key lives on a protected part of the co-processors memory. It was created during configuration. We will never have access to it. The idea of a single digital key is pretty straight forward. It parallels the physical door key quite well. The chunk of data is unique, just like the grooves and cuts of a physical key are unique.
  3. An Algorithm: The complex math that we will offload to the co-processor. It is a particular type of cryptographic math called Elliptic Curve Cryptography (ECC). Using our data and key, this algorithm will output a digital signature.

One more thing: digital signatures will be different when you change your message. This is pretty different than the traditional idea of a signature. When we think of hand-written signatures, we think that the entire purpose is for it to be exactly the same every time. The author should be the only one who can hold the pen like so and make that unique signature. With digital signatures we need to think a bit differently. Remember that we are sending our message (and a key) into an algorithm. This will result in a new and different signature if the message or key changes. In most cases, a digital signature is always unique from all other previous digital signatures.

So that was easy enough, right? Good. Digital signatures aren't so scary now. But what would we want to do with one of these? Verify it.

Verification

Verification of a digital signature is quite similar verifying a hand-written signature. In both cases, we want to determine if the signature is authentic. With a hand-written signature, you might just use your eyes to see if it looks correct. With a digital signature we need to use a bit more than our eyes. We need the data, the signature, a key, and an algorithm (sound familiar?). Simple enough, right? Well this is where it starts to get slightly more complex. We need to understand one more concept: the idea of key pairs.

Up until this point, we have called the "key" simply a "key", but to be more accurate, this should be called the private key. And it is part of a key pair. When we configure the co-processor it actually creates a key pair which includes two keys: one private key and one public key. These two keys work together, and they allow us to sign and verify signatures. We use the private key to sign the data, and we use the public key to verify data. This is also known as an asymetric cryptographic process.

Now that we're talking private and public key pairs, let's clearly define what we need to create a digital signature, and what we need to verify a digital signature:

To create a digital signature we need: To verify a digital signature we need:
1. Data
2. Private Key
3. Algorithm
1. Data
2. Signature
3. Public Key
4. Algorithm
Algorithm output: a new digital signature Algorithm output: yes or no

From Alice to Bob

Although you can accomplish a few neat things using only one of these chips (signing data, verifying signatures, creating high-quality random numbers, etc.), the truly powerful communication security starts when you have two of them (we will refer to them as Alice and Bob).

Our Arduino library utilizes the old story of Alice, Bob and Eve. If you read up on cryptography, you will often come across these names. Alice and Bob are always trying to have a private conversation, but Eve keeps eavesdropping (and sometimes trying to impersonate Alice). So Alice and Bob start using cryptography to keep Eve out of the mix.

Side note: In a lot of traditional examples, Alice and Bob are encrypting and decrypting their messages. I want to highlight here again, that this chip utilizes a different cryptographic tool: authentication. With the ATECC508A, we can create and verify signatures. The message is still sent out in the open/public for Eve to clearly read. However, Eve will never have Alice's private key and could never create a valid signature (to authenticate the message). Therefore, Eve can never pretend to be Alice!

In our examples, Alice is sending a message to Bob. Alice can use her own co-processor to create a digital signature. She can then send her message and signature to Bob. Bob can then use his own separate co-processor to verify the message and signature.

Alice and Bob example
An example demonstrating the functionality of an asymmetric key pair (Alice and Bob).

If you are brand new to cryptography, take a minute to watch the above gif loop a couple times. It shows the basic steps necessary to have an authenticated message from Alice to Bob. The use of a private/public key pair can be a bit daunting at first sight, but after going through this tutorial, we hope it can become a piece of cake.