TCAT X.509 certificates generation

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TCAT uses X.509 Certificate Extensions to provide permissions with certificates.

Extensions

Extensions were introduced in version 3 of the X.509 standard for certificates. They allow certificates to be customised to applications by supporting the addition of arbitrary fields in the certificate. Each extension, identified by its OID (Object Identifier), is marked as “Critical” or “Non-Critical”, and includes the extension-specific data.

Certificates generation (by script)

The directory auth-generate contains example scripts and a Makefile to generate TCAT Commissioner certificates and TCAT Device certificates. The scripts can also handle multiple CAs, and provide the most detailed view on how to generate these certificates.

To generate all certificates:

cd auth-generate
make

This will create an output directory with subdirectories for each of the created identities. Each subdirectory can be used as a value for the BBTC Commissioner --cert_path argument, if needed.

NOTE: the directory auth-generate/ca contains an example CA certificate and private key (for signing). Other CAs can be added in here. This CA is not the same CA used for the TCAT Commissioner and Device identities in the auth and auth-cert directories! The CA for the latter is privately maintained by Thread Group.

Certificates generation (manually)

Thread TCAT uses Elliptic Curve Cryptography (ECC), so we use the ecparam openssl argument to generate the keys.

Root certificate

1. Generate the private key:

openssl ecparam -genkey -name prime256v1 -out ca_key.pem

2. We can then generate the .csr (certificate signing request) file, which will contain all the parameters of our final certificate:

openssl req -new -sha256 -key ca_key.pem -out ca.csr

3. Finally, we can generate the certificate itself:

openssl req -x509 -sha256 -days 365 -key ca_key.pem -in ca.csr -out ca_cert.pem

4. See the generated certificate using

openssl x509 -in ca_cert.pem -text -noout

Commissioner (client) certificate

1. Generate the key:

openssl ecparam -genkey -name prime256v1 -out commissioner_key.pem

2. Specify additional extensions when generating the .csr (see sample configuration):

openssl req -new -sha256 -key commissioner_key.pem -out commissioner.csr -config commissioner.cnf

3. Generate the certificate:

openssl x509 -req -in commissioner.csr -CA ca_cert.pem -CAkey ca_key.pem -out commissioner_cert.pem -days 365 -sha256 -copy_extensions copy

4. View the generated certificate using:

openssl x509 -in commissioner_cert.pem -text -noout

5. View parsed certificate extensions using:

openssl asn1parse -inform PEM -in commissioner_cert.pem

Configurations

file: commissioner.cnf (line 1.3.6.1.4.1.44970.3 = DER:21:01:01:01:01 specifies permissions (all)) See scripts in auth-generate directory for more details.

[ req ]
default_bits           = 2048
distinguished_name     = req_distinguished_name
prompt                 = no
req_extensions         = v3_req

[ req_distinguished_name ]
CN                     = Commissioner

[v3_req]
1.3.6.1.4.1.44970.3 = DER:04:05:21:01:01:01:01
authorityKeyIdentifier = none
subjectKeyIdentifier = none