# Meaning of direction in EPS

In EEA (EPS Encryption Algorithm) and EIA (EPS Integrity Algorithm), what exactly does the direction (uplink/downlink) indicate? It's confusing because for both encryption and decryption of a given plaintext, the direction has to be same.

This w.r.t the LTE communication.

For your reference, the section B.1 in 33401-g00.zip at https://www.3gpp.org/ftp/Specs/latest/Rel-16/33_series/

• Can you expand your acronyms to make the question easier to read? Oct 16, 2019 at 19:54
• EPS: Evolved Packet System Oct 21, 2019 at 16:18
• EEA/EIA: EPS Encryption/Integrity Algorithm Oct 21, 2019 at 16:18

The direction bit is set to 0 for uplink direction (from mobile to the network) and set to 1 for downlink direction (from network to the mobile) according to Annex B of TS 33.401. As you stated '0' is used for encryption and decryption in uplink and '1' used for encryption and decryption in downlink.

Taking the example of AES-CTR mode for encryption/decryption (128-EEA2, code name given to this scheme), the counter blocks (T1, T2,..) used for the 128 bit data blocks would be (according to annex B.1.3): UPLINK (Mobile to network)

T1 = COUNT[0]..COUNT[31] | BEARER[0]..BEARER[4] | **0** | 0..0 (26 zeros)| 0..0 (64 zeros)


Tk+1 = Tk++ mod(2^64) // 64 LSBits are incremented for each subsequent block

T1 = COUNT[0]..COUNT[31] | BEARER[0]..BEARER[4] | **1** | 0..0 (26 zeros)| 0..0 (64 zeros)