Information Technology Reference
In-Depth Information
(
)
s
CMegg
=
,
0
= (
)
λ
(
)
α
ρ
x
π
()
x
x
Ce gg
,
egg
,
,
x
1
,
x
(3.16)
ρ
Cg
=
x
x
2
,
x
y
ρ
ω
Cg
=
π
()
xx
gx
,
x
3,
x
where π( x ) is mapping from R x to the attribute i that is located at
the corresponding leaf of the access tree.
7. The ciphertext C is sent by the sender (it also includes the access
tree via R matrix):
{
}
CRCCCC
=
,, ,
π
,
,
,
x
(3.17)
0
1
,
x
2
,
x
3
,
x
Decryption by receiver : Receiver U u takes as input ciphertext C , secret
keys { sk i , u }, group G , and outputs message M . It obtains the access
matrix R and maps π from C . It then executes the following steps:
1.
U u calculates the set of attributes {π( x ): x X } ∩ I i that are com-
mon to itself and the access matrix. X is the set of rows of R .
2.
For each of these attributes, it checks if there is a subset X ′ of rows
of R , such that the vector (1, 0, … , 0) is their linear combination.
If  not, decryption is impossible. If yes, it calculates constants
c x
(
)
∈ , such that
c xx
= …
10
,, ,
0
.
q
x
3.
Decryption proceeds as follows:
(
)
()
CeHu C
esk
,
() =
1
,
x
3
,
x
(a)
For each x X ′, decx
(
)
,
C
()
2
,
x
π
xu
,
()
( b)
U u computes MC
=
0 Π
decx
xX
.
None of the above techniques can authenticate users or protect the privacy
of the user. It is just not enough to store the contents securely in the cloud;
it  might also be necessary to ensure the anonymity of the user. However,
the user should be able to prove to the other users that he or she is a valid
user who stored the information without revealing the identity. For exam-
ple, a user would like to store some sensitive information but does not want
to be recognized. The user's privacy needs to be protected when the user
needs to store confidential information but does not reveal his or her identity.
For example, if a user wants to store a controversial record about the employ-
ers, then he or she might want to remain anonymous. The cloud, on the
other hand, must be able to authenticate the user as an authorized person.
Ruj et al. [32] proposed an authentication mechanism that also protects the
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