3.2.1.3 Certificate Storage
The ECC608-TMNGTLS storage is centered around securely holding keys. X.509 certificates tend to be larger than what will fit into a single ECC608-TMNGTLS device slot; therefore, a compressed format is used. This technique may be better called a partial certificate as it stores dynamic certificate information on the device and imposes some limitations. Dynamic information is certificate content that can be expected to change from device to device (e.g., public key, validity dates, etc.). Firmware is expected to have a certificate definition with a template of the full X.509 Certificate containing static information (data that are the same for all certificates) and instructions on how to rebuild the full certificate from the dynamic information in the compressed certificate.
The following application note documents the compressed certificate format: ATECC Compressed Certificate Definition.
The CryptoAuthLib library also contains APIs for working with compressed certificates.
Device Certificate
The device certificate consists of information associated with the actual end unit. For the ECC608-TMNGTLS, the compressed device certificate is stored in Slot #10.
Signer Certificate
The signer certificate consists of the information associated with the signer certificate authority used to sign the device certificate. For the ECC608-TMNGTLS, the compressed signer certificate is stored in Slot #12. The signer public key is also required to rebuild the full signer certificate.
Signer Public Key
The signer public key is the public key needed to verify the signer and the information that is associated with the signer compressed certificate. For the ECC608-TMNGTLS, it is stored in Slot #11.
The following table shows all the slots associated with certificates in the ECC608-TMNGTLS:
| Slot | Description |
|---|---|
| 0 | Primary private key. The public key can be generated at any time using the GenKey command in Mode = 0x00. |
| 10 | Device certificate. This is stored here in a compressed format. |
| 11 | Signer public key. |
| 12 | Signer certificate. This is stored in a compressed format. |
For the ECC608-TMNGTLS, these slots can be overwritten unless the individual slots are locked.