1. Efficient Time-Bound Hierarchical Key Assignment Scheme
Source: IEEE Transactions on Knowledge
and Data Engineering, Vol. 16,
No. 10, Oct. 2004, pp
Authors: Hung-Yu Chien
Speaker: Chia-Lin Kao
Date: 2004/09/22

2. Outline
Introduction
Proposed scheme
Conclusions

3. Introduction A B C F D E

4. Notations Role: Ci, Cj, TA(Trusted Agent)
Data in Cards: IDi, ki, ht1(a), hz-t2(b), X
ki: secret key for Ci
a, b: random secret values at TA
z: max time periods
hm(x) = h(h(…(h(x))…))
Ki,t: key for Ci at time t
h(): a secure one-way hash function

5. Proposed Scheme Initialization:
TA randomly selects n secret key ki, 1≤i≤n, and two random secret values a and b.
TA publishes a public value rij on an authenticated public board.
X is TA’s secret key
rij=h(X||IDi||IDj||ki) ⊕kj

6. Proposed Scheme User Registration:
TA distributes ki to Ci through a secure channel.
TA also issues the user a tamper-rsistant device in which TA’s secret key X, and the identity of Ci, IDi, ht1(a), hz-t2(b) are stored.

7. Proposed Scheme Encrypting Key Gneration:
At time t (t1≤t≤t2), the data belonging to Cj would be encrypted by the key Kj,t
Kj,t=h(kj⊕ht(a)⊕hz-t (b))

8. Proposed Scheme Decrypting Key Derivation: Ci inputs rij, IDj, and ki
deivce computes kj by kj=rij⊕h(X||IDi||IDj||ki)
ht(a)=ht-t1(ht1(a))
hz-t(b)=ht2-t(hz-t2(b))
Kj,t=h(kj⊕ht(a)⊕hz-t (b))
rij=h(X||IDi||IDj||ki) ⊕kj
t1≤t≤t2

9. Conclusions
New time-bound key assignment scheme based on a low-cost tamper-resistant device.
Without public key cryptography, our scheme greatlyreduces the computational load and the implementation cost.