When communicating securely across the internet, the client (IoT device) and the server must provide proof of their identity
prior to establishing a mutually authenticated TLS connection. In a
infrastructure, digital (or identity) certificates are exchanged to verify each entity's identity. The
X.509 certificate is the most
common digital certificate format and is widely used across the internet and in IoT use cases. The X.509 certificate is
exchanged during the TLS handshake process, making it a critical piece of establishing a TLS
connection. In IoT use cases, data transfer over communication protocols such as HTTPS or
MQTT should occur only after a TLS connection has been established.
In PKI, a signature’s authenticity is established through a key pair: a public key and a private key. Public keys are
disseminated widely, while private keys are known only to the owner; this is done to maintain security across the system.
When data is signed or encrypted with a private key, any recipient of the data can authenticate and/or decrypt the data
using the matching public key. Data encrypted using a public key can only be decrypted by the holder of the private key.
Once a key pair has been generated, a client will apply to a certificate authority for an X.509 certificate, using a
certificate signing request (CSR). The X.509 certificate is either signed by a CA
authority) or is self-signed. In most use cases, the X.509 certificate is only self-signed when it is the
certificate of the root
CA. In IoT use cases, it is more common (and better practice!) for an intermediate CA (instead of the root CA) to sign
each end-entity’s certificate. This prevents the risk of exposing the root certificate. Using intermediate certificates
creates a chain of
trust that can be traced from the root CA to each end-entity.
Additional details can be found here: X.509 RFC5280.
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