| CPC H04L 9/0861 (2013.01) [H04L 9/0833 (2013.01); H04L 63/205 (2013.01); H04W 12/02 (2013.01); H04W 12/033 (2021.01); H04L 63/162 (2013.01); H04L 69/22 (2013.01); H04L 2209/80 (2013.01); H04W 12/04 (2013.01)] | 16 Claims |

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1. A method of transmitting data in a device to device communication system, the method comprising:
determining whether a security feature is applied to a packet data convergence protocol (PDCP) service data unit (SDU); and
transmitting a PDCP protocol data unit (PDU) based on the determination to a receiving user equipment (UE),
wherein a PDCP header of the PDCP PDU includes a PDCP sequence number (SN) and a first key identification (ID),
wherein, in case that the security feature is applied to the PDCP SDU, the first key ID is set to a value for identifying a first key, the first key being used to generate an encryption key for securing the PDCP SDU, and
wherein, in case that the security feature is not applied to the PDCP SDU, the first key ID is set to a zero value.
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