| CPC H04L 9/0869 (2013.01) [H04L 63/06 (2013.01); H04W 12/041 (2021.01); H04L 2209/80 (2013.01); H04L 2463/061 (2013.01); H04W 36/0011 (2013.01); H04W 88/14 (2013.01)] | 28 Claims |
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1. A communication system, comprising:
a base station of a source network; and
a mobility management entity of the source network, wherein:
the base station is configured to send, in a first handover process from the source network to a target network, a handover required message to the mobility management entity; and
the mobility management entity is configured to:
receive the handover required message from the base station;
obtain a first non-access stratum (NAS) downlink count value in the first handover process;
derive, according to a key derivation function (KDF), a root key, the first NAS downlink count value, and a first key comprising a ciphering key and an integrity key;
send at least a portion of the first NAS downlink count value to a user equipment in the first handover process;
after deriving the first key, obtain, in the first handover process, a second NAS downlink count value by incrementing a value to the first NAS downlink count value, wherein the second NAS downlink count value is obtained in the absence of the mobility management entity sending a NAS message; and
after the first handover process fails, derive a second key in a second handover process according to the KDF, the root key, and the second NAS downlink count value.
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