LTE:Key Derivation in EPS

Key Derivation in EPS

KDF= (derived key of length kLen) <-- (secret value, [OtherInfo])

KDF= HMAC-SHA-256 (Key, S)

Where S= FC || P0 || L0 || P1 || L1 || P2 || L2 || P3 || L3 ||... || Pn || Ln

FC=0x10-0x1F

Eg.,

K_ASME

S= FC || P0 || L0 || P1 || L1

Where

FC=0x10,
P0=SN id/PLMN ID (viz., MCC+MNC),
L0=length of SN id =0x00 0x03,
P1=SQN xor AK,
L1=length of SQN xor AK=0x00 0x06

Key=CK||IK

Key Hierarchy

Includes following keys: K_eNB, K_NASint, K_NASenc, K_UPenc, K_RRCint and K_RRCenc

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KDF = Key Derivation Function, a one way hash function (SHA256)

• K_ASME = KDF(CK, IK, PLMN Id, SQN xor AK) ->256 bit[FC=0x10,Kin=CK||IK)

• K_eNB = KDF(K_ASME, NAS-UL COUNT) ->256 bit[FC=0x11,Kin= K_ASME]

• NAS Keys->128 bit

– K_NASInt = KDF(K_ASME, NAS-int-alg, algorithm-id)

– K_NASEnc = KDF(K_ASME, NAS-enc-alg, algorithm-id)

• AS Keys->128 bit

– K_RRCInt = KDF(K_eNB, RRC-int-alg, algorithm-id)

– K_RRCEnc = KDF(K_eNB, RRC-enc-alg, algorithm-id)

-K_UPEnc = KDF(K_eNB, UP-enc-alg, algorithm-id)

Note:

Sequence Number (SQN) and the Anonymity Key (AK) is sent to the UE as a part of the Authentication Token (AUTN)

Key	FC	Kin	Length
KASME	0x10	CK||IK	256
KeNB	0x11	KASME	256
NH	0x12	KASME	256
KeNB*	0x13	NH or KeNB	256
NAS,RRC	0x15	KASME or KeNB	128