Uplink SN 4 -- the security context becomes visible, and so does COUNT. Wireshark now prints the full input set it will use for the rest of the connection:

Ciphering Algorithm: NEA2 (AES) (2)
Integrity Algorithm: NIA2 (AES) (2)
BEARER: 0
DIRECTION: Uplink (0)
COUNT: 4
CIPHER KEY: 95F23667A60D91689B31772783E6FDDD

COUNT: 4 is the number to look at. COUNT is 32 bits: a Hyper Frame Number concatenated with the sequence number (TS 38.323). Only the sequence number travels on the wire; the HFN is state each end keeps and increments when the SN wraps. Here the SN has not wrapped, so COUNT and SN read the same -- 4 and 4 -- and they do so on every single PDU in this session. BEARER: 0 is the bearer input to the cipher, and DIRECTION is one bit. Change any one of those four and you get a different keystream, which is precisely why uplink SN 4 and downlink SN 4 are safe to cipher under the same key.

This message, the securityModeComplete, is itself integrity protected -- MAC-I 0x944c9d98 -- but still not ciphered, the mirror of frame 17. Wireshark did not need to decipher it.

Message Details