lib_cap

lib_cap is an XMOS library for validating a set of capabilities against a device identity (MAC address and serial number) and key.

module

Add lib_cap(>=0.0.1) to your module dependencies. There is a single API:

int cap_validate(REFERENCE_PARAM(otp_ports_t, ports),
                 uint64_t vendor_id,
                 const uint8_t capability[72],
                 REFERENCE_PARAM(uint32_t, serial),
                 uint32_t mac_index,
                 uint8_t mac_address[6],
                 REFERENCE_PARAM(uint64_t, capability_flags));

cap_validate returns the serial number and MAC address, for use elsewhere in the application; these are not input parameters. The capability argument is the binary capability signed by the xcaptool command, and encodes both the capability flags and Ed25519 signature. The capability flags are returned in capability_flags.

xcaptool

Source code is in lib_cap/tools.

Examples:

Generating a keypair

Note: keypairs are stored as their raw values and as such are not compatible with the PKCS encodings used by OpenSSL. We can add support for PKCS if required.

xcaptool --command=generate --private-key-file=privkey --public-key-file=pubkey

The public key must be burned into the OTP flash. We will provide a script to do this.

Signing a capability

Signing a capability requires a public/private key pair generated in the preceeding step. Either one or both of the serial number and MAC address must be supplied, to guard against generating a key that could unlock any device.

Serial numbers and capability flags can be provided as decimal or hexadecimal values, the latter prefixed with 0x.

Capability flags are application defined; typically, they will represent a set of licensed features.

Capabilities are output base64-encoded. They should be decoded to binary before storing on the device.

xcaptool --command=sign --private-key-file=privkey --public-key-file=pubkey --serial=0x1234 --mac-address=00:aa:bb:11:cc:ee --capability-flags=0x12345 --vendor-id=0xaabbcc

Verifying a capability

A capability may be verified as follows:

xcaptool --command=verify --private-key-file=privkey --public-key-file=pubkey --serial=0x1234 --mac-address=00:aa:bb:11:cc:ee --capability-flags=0x12345 --vendor-id=0xaabbcc

This logic is similar to that used by the XMOS library.