matter_fabric.h
the operational identity a commissioner installs.
Overview
the operational identity a commissioner installs. Attestation ends with the commissioner holding a public key this node proved it owns. What follows is the commissioner handing back an identity built on that key: AddTrustedRootCertificate trust this root AddNOC and here is who you are underneath it Both certificates arrive as MATTER TLV, not X.509. The spec defines a compressed form precisely so a constrained node can read one without an ASN.1 decoder, and this file is that reader. It reads three things and ignores the rest: the subject's node id, its fabric id, and the public key. Validity dates, key usage and the signature are what a node checks when VERIFYING a certificate somebody else presents, which is CASE's job. A commissioner has no reason to lie to itself about a NOC it just minted, and this node cannot check the signature anyway without the issuer's key -- which, for the NOC, is the root it was told to trust one command earlier and has no independent reason to believe.
depends on matter_status.h · used by matter_clusters.h matter_case.c matter_fabric.c
API
Cstruct matter_cert_info
What matter_cert_parse() found. Absent fields leave their have_* flag false.
Cstruct matter_fabric
One fabric's worth of operational identity. Held in RAM and nothing more. A fabric is supposed to survive a reboot, and this one does not; there is no settings backend on this port yet, and the fail-safe would roll an incomplete commissioning back regardless. What it does have to survive is the gap between AddNOC and CASE, which is the same boot. The trusted root is kept as a public key rather than as the certificate it arrived in: verifying a peer's NOC chain needs the key, and nothing this node does needs the other 300-odd bytes. The ICAC is kept whole because CASE has to send it back out.
Cstruct matter_icac_slot
The one intermediate certificate this node can hold, and whose it is.
owner_index- the fabric index holding it, or 0 when free.
##define MATTER_CERT_MAX 400u
kMaxCHIPCertLength (credentials/CHIPCert.h:54).
##define MATTER_NOC_MAX 320u
The largest OPERATIONAL certificate this node stores per fabric. Smaller than MATTER_CERT_MAX because a NOC is not a root: it carries one subject, one issuer and one key, where a root may carry extensions this node never parses. Apple's is 253 bytes; this leaves room for a longer one without reserving 400 bytes per fabric on a part with 128 KB of RAM in total. An oversized NOC is REFUSED by AddNOC, never truncated -- a certificate missing its last bytes fails signature verification with nothing to say why.
##define MATTER_IPK_LEN 16u
The identity protection key: one AES-128 key, shared by a whole fabric.
##define MATTER_FABRIC_PUBKEY_LEN 65u
Uncompressed P-256 point, 0x04 || X || Y.
##define MATTER_COMPRESSED_FABRIC_LEN 8u
Compressed fabric identifier: 64 bits (crypto/CHIPCryptoPAL.cpp:796-833).
##define MATTER_INSTANCE_NAME_LEN 34u
The operational instance name, "%08X%08X-%08X%08X" plus its NUL -- compressed fabric id, hyphen, node id, 32 uppercase hex digits in total (lib/dnssd/ServiceNaming.cpp, MakeInstanceName).
Fint matter_cert_parse(const uint8_t *cert, size_t len, struct matter_cert_info *out)
Read the interesting fields out of a Matter operational certificate. @return MATTER_OK if the TLV parsed, whatever the certificate turned out to contain -- the caller decides which fields it needed. An error means the bytes were not a well-formed certificate at all.
Fint matter_fabric_compressed_id(const uint8_t root_pub[MATTER_FABRIC_PUBKEY_LEN], uint64_t fabric_id, uint8_t out[MATTER_COMPRESSED_FABRIC_LEN])
Derive the compressed fabric identifier (crypto/CHIPCryptoPAL.cpp:796-833). HKDF-SHA256(ikm = root public key WITHOUT its 0x04 prefix, salt = fabric id, 8 bytes big-endian, info = "CompressedFabric", len = 8) Two fabrics can share a fabric id -- it is chosen by whoever built them -- so this mixes in the root public key to get something that identifies a fabric on the wire without being guessable from its number alone. The 0x04 is dropped because it says only that the point is uncompressed; it is the same byte for every key and carries nothing to derive from. @param root_pub uncompressed, and refused if it does not start with 0x04. @return MATTER_OK, or MATTER_E_INVAL.
Fint matter_fabric_instance_name(const struct matter_fabric *fabric, char *out, size_t cap)
Write the DNS-SD instance name a commissioner looks this node up by. "<compressed-fabric-id>-<node-id>", 16 uppercase hex digits each, which is the instance part of <name>._matter._tcp.local. @param out at least MATTER_INSTANCE_NAME_LEN bytes; NUL-terminated. @return MATTER_OK, or MATTER_E_INVAL.