1. Introduction
Link keys are used to encrypt/decrypt the communication between each two devices. The network security depends on the robustness of the link keys. Default global preconfigured link key Zigbee 09 is generally used in home automation applications. The problem is that the same link key is used for all the nodes, and this is a security leak. For maximizing security, Zigbee 3.0 provides install code out of band process to create preconfigured link keys that are unique between the trust center and each joining device.
2. Install code
To enter the Zigbee network and obtain the network key, an installation code is needed. During the manufacturing process, an install code is created for each node. This code is shareable between the joining node and the trust center (out-of-band) and it permits to derive the key at both sides using Matyas-Meyer-Oseas (MMO) hash function.
Install code is made up of 6,8, 12 or 16 bytes hexadecimal number with 2 bytes CRC. The CRC must be appended to the installation code in little endian format. Zigbee 3.0 base device behavior specification requires the use of 16-byte install code.
Joining network process using install code |
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3. Trust center (ZC) install code process
The trust center should decide whether a joining node must use the default link key or an install code link key depending on the value of bdbJoinUsesInstallCodeKey.
If bdbJoinUsesInstallCodeKey is FALSE, the Trust Center permits a node to join its network without having a corresponding install code link key. If bdbJoinUsesInstallCodeKey is TRUE, the trust center only permits a node to join its network if a corresponding install code link key associated with the node has been preinstalled before the node joins. The trust center must add an entry into its link key table based on joining device EUI64 address.
To improve Zigbee network security, it is better to form Zigbee network with a NULL preconfigured link key. (const uint8_t sec_key_null [ZB_SEC_KEYSIZE] = {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};)
As a result, only devices for which an entry was added to TC link key table could join the network.
/* Attempt to join a Zigbee network */
ZbStartupConfigGetProDefaults(&config);
/* Set the centralized network */
APP_DBG("Network config : APP_STARTUP_CENTRALIZED_COORDINATOR");
config.startupControl = ZbStartTypeForm;
/* Using the NULL preconfigured Link Key */
memcpy(config.security.preconfiguredLinkKey, sec_key_null, ZB_SEC_KEYSIZE);
config.channelList.count = 1;
config.channelList.list[0].page = 0;
config.channelList.list[0].channelMask = 1 << CHANNEL; /*Channel in use */
/* Using ZbStartupWait (blocking) */
status = ZbStartupWait(zigbee_app_info.zb, &config);
Enable install code:
uint32_t tcPolicy = 0;
ZbApsGet(zigbee_app_info.zb, ZB_APS_IB_ID_TRUST_CENTER_POLICY, &tcPolicy, sizeof(tcPolicy));
tcPolicy |= (ZB_APSME_POLICY_IC_SUPPORTED | ZB_APSME_POLICY_TCLK_UPDATE_REQUIRED);
ZbApsSet(zigbee_app_info.zb, ZB_APS_IB_ID_TRUST_CENTER_POLICY, &tcPolicy, sizeof(tcPolicy));
Add dedicated preconfigured link key for joining device with EUI64 address ieeeAddr:
struct ZbApsmeAddKeyReqT addKeyReq;
struct ZbApsmeAddKeyConfT addKeyConf;
memset(&addKeyReq, 0, sizeof(addKeyReq));
addKeyReq.keyType = ZB_SEC_KEYTYPE_TC_LINK;
addKeyReq.keySeqNumber = 0;
addKeyReq.partnerAddr = ieeeAddr;
memcpy(addKeyReq.key, XXX_LinkKey, ZB_SEC_KEYSIZE);
ZbApsmeAddKeyReq(zigbee_app_info.zb, &addKeyReq, &addKeyConf);
Joining Device Install Code Steps |
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4. Joining device install code process
Configure joining device with the same preconfigured link key:
/* Attempt to join a zigbee network */
ZbStartupConfigGetProDefaults(&config);
/* Set the centralized network */
APP_DBG("Network config : APP_STARTUP_CENTRALIZED_ROUTER");
config.startupControl = ZbStartTypeJoin;
/* Using the same preconfigured link key as the one configured on trust center */
memcpy(config.security.preconfiguredLinkKey, XXX_LinkKey, ZB_SEC_KEYSIZE);
config.channelList.count = 1;
config.channelList.list[0].page = 0;
config.channelList.list[0].channelMask = 1 << CHANNEL; /*Channel in use */
/* Using ZbStartupWait (blocking) */
status = ZbStartupWait(zigbee_app_info.zb, &config);
Node install code steps |
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