Amf/nas/nas_security/security.go "ToDo: use real mac calculate"

voje · December 30, 2020, 3:40am

Hi,

I’ve been trying to simulate my own UE and gNB as a standalone devices that could connect to a running Free5gc core. I’m currently debugging the integrity algorithm and I’ve come across a curious ToDo in AMF code:

// ToDo: use real mac calculate
mac32, err := NasMacCalculate(ue.IntegrityAlg …)

github.com

free5gc/amf/blob/23e47d309c4f1fd3fde3e1c3505c83136bec0644/nas/nas_security/security.go#L166


		receivedMac32 := securityHeader[2:]
		// remove security Header except for sequece Number
		payload = payload[6:]

		// Caculate ul count
		if ue.ULCountSQN > sequenceNumber {
			ue.ULCountOverflow++
		}
		ue.ULCountSQN = sequenceNumber
		if integrityProtected {
			// ToDo: use real mac calculate
			mac32, err := NasMacCalculate(ue.IntegrityAlg, ue.KnasInt, ue.GetSecurityULCount(), context.SECURITY_BEARER_3GPP,
				context.SECURITY_DIRECTION_UPLINK, payload)
			if err != nil {
				ue.MacFailed = true
				return nil, err
			}
			if !reflect.DeepEqual(mac32, receivedMac32) {
				logger.NasLog.Warnf("NAS MAC verification failed(0x%x != 0x%x)", mac32, receivedMac32)
				ue.MacFailed = true
			}

What exactly do you mean by ‘real mac calculate’? Is the current one a non-standard development version? For example if I try to connect some third party simulator, will the integrity check fail?

Thanks in advance!

free5GC · December 30, 2020, 3:40am

Hi,

This TODO is just we did not clean it clear. Sorry about that.

The MAC calculate is correct.