Part 2 of The ABCs of ECDSA (“Sign-Here”) will describe how digital certificates are made and signed. In the previous article (The ABCs of ECDSA: Part 1), we examined the steps of ECDSA performing asymmetric authentication using digital certificates. You may have noticed that both Part 1 and Part 2 are in reverse chronological order; however, it makes better sense to first understand a bit about the actual authentication process before dissecting the details of making the certificate. (Just trust me on that.) Before we get into the nuances of the certificate, let’s recall that authentication is about keeping something real. Such things would be mobile, medical and consumer accessories; embedded firmware; industrial networks; and soon the new platforms of IoT, home automation, and vehicle-to-vehicle communications. Aside from those, there are several others given the fact that the need for authentication is increasing exponentially as more things communicate with each other, and through the cloud, are creating more opportunities for bad actors to apply their mal-intent.
Especially with the increased use of the Internet and the cloud for financial transactions and transmission of confidential personal/medical information, it’s critical to ensure that the sender of information is exactly who they are supposed to be, as well as that the data has not been tampered with. That is where authentication and hardware key storage come in. Here we will focus on asymmetric authentication. Asymmetric authentication using ECDSA is based upon a digital certificate, which in this case, is stored in the ATECC108A device.
So, now let’s go into the chip factory and see how the ECDSA certificate is made and stored in the device. Remember that ECDSA stands for Elliptic Curve Digital Signature Algorithm. The words “Elliptic Curve” are in the name because Elliptic Curve Cryptography (“ECC”) algorithms are used. No mystery there. The benefit of ECC is that it provides extremely strong security with shorter key lengths than other popular algorithms. Bitcoin, for example uses ECC predominantly for that reason. (We won’t go into Bitcoin here.) “DSA” points to the fact that digital signatures are the key element of the process, which is also fairly self evident. The digital signing process is what we describe here, step by step. “Certificate” is the name given to the concept of putting certain types of data together in a prescribed format and then signing that data using hashing algorithms and signing algorithms. (Again, the usage of the certificate is covered in Part 1.)
While we are fully immersed here in cryptographic alphabet soup, we might as well add one more thing to it: The prescribed format used in the ECDSA in the ATECC108A is called ASN.1. ASN.1 basically defines what is what in the certificate, including the serial number, the public key and that sort of thing. It also defines the length of those data elements.
Now, back to building the certificate: The certificate is made and loaded in the key storage device in the chip factory. It is made from two main components:
1. The certificate data
2. The signature
The certificate data is a collection of data from three sources:
1. Static data: Boiler plate type data that does not change, such as the name and address of the manufacturer. (This is the ASN.1 encoded stuff.)
2. Dynamic data: Data from the tester that can change with each device such as time, date, and serial number.
3. Client device’s public key, which has an algorithmic relationship to the client’s private key that will be securely stored in the client device.
The certificate data is formatted according to X.509 specifications (yes, more crypto jargon). X.509 defines the elements and order of the elements in the certificate, such as serial number, public key, subject’s common name (i.e. the name of the certificate), authority ID (normally a SHA-1 hash of the public key), authority common name (i.e. the name of the authority that signs the certificate data), among other things. We will leave more about X.509 for another day.
The certificate data comprises just half of the certificate, the other half is the signature. What is a little tricky to understand at first is that the certificate data do two things: (1) become part of the certificate as it is, and (2) gets hashed and signed to make the signature. Both the certificate data itself and the signature make up the certificate.
The specific steps in order to make the signature begins with a copy of the certificate data being put through a hash algorithm to create a number called a hash value (or digest). ECDSA specifies a 32 byte digest length and SHA256 as the hashing algorithm. Once created, the digest is ready to be signed by the sign module in the factory.
The sign module is a piece of equipment that securely stores the signer’s private key. No one can get access to that key. The sign module uses the ECC sign algorithm to sign the digest of the certificate data with the signer’s private key. The result of that process becomes the “signature.” The signature then joins the original (i.e. unhashed) certificate data to complete the certificate. Note that the signing key is tied to the OEM’s root key to create the root of trust (the notion of root of trust will be addressed in another article).
The certificate is now finished and can be installed into the crypto device. Once the device is finished, it is then shipped to the customer’s factory to be assembled into an accessory, consumable, board or any number of things, i.e. a consumable water filter that later gets installed into refrigerator. In this scenario, when a new filter is installed by the consumer into the refrigerator when the old filter expires, the new filter will be authenticated by the host processor in the refrigerator according to the ECDSA process as described in The ABCs of ECDSA (Part 1).
Below is a video (sorry, no sound) that will visually help walk you through the steps noted above.
Benefits of asymmetric authentication with ECDSA include:
- Increased security because asymmetric authentication does not need secure key storage on the host (only the client)
- No need to update the host with secrets in the field (can update the public key at any time.)
- Uses the advantages of Elliptic Curve Cryptography (high security, short key, less computation)
Atmel CryptoAuthentication™ products such as Atmel’s ATSHA204A, ATECC108A and ATAES132 implement hardware-based storage, which is much stronger then software based storage because of the defense mechanisms that only hardware can provide against attacks. Secure storage in hardware beats storage in software every time. Adding secure key storage is an inexpensive, easy, and ultra-secure way to protect firmware, software, and hardware products from cloning, counterfeiting, hacking, and other malicious threats.
If you have yet to read the first portion of this article, you can find The ABCs of ECDSA (Part 1) here.