Cryptography Reference
In-Depth Information
Further discussion: We stress that (the equivalent) Definitions 5.1
and 5.2 go way beyond saying that it is infeasible to recover the plain-
text from the ciphertext . The latter statement is indeed a minimal
requirement from a secure encryption scheme, but is far from being
a sucient requirement. Typically, encryption schemes are used in
applications where even obtaining partial information on the plain-
text may endanger the security of the application. When designing
an application-independent encryption scheme, we do not know which
partial information endangers the application and which does not. Fur-
thermore, even if one wants to design an encryption scheme tailored to
a specific application, it is rare (to say the least) that one has a precise
characterization of all possible partial information that endanger this
application. Thus, we need to require that it is infeasible to obtain any
information about the plaintext from the ciphertext . Furthermore, in
most applications the plaintext may not be uniformly distributed and
some a-priori information regarding it is available to the adversary. We
require that the secrecy of all partial information is preserved also in
such a case. That is, even in presence of a-priori information on the
plaintext , it is infeasible to obtain any (new) information about the
plaintext from the ciphertext (beyond what is feasible to obtain from
the a-priori information on the plaintext ). The definition of semantic
security postulates all of this. The equivalent definition of indistin-
guishability of encryptions is useful in demonstrating the security of
candidate constructions as well as for arguing about their effect as part
of larger protocols.
Security of multiple messages: Definitions 5.1 and 5.2 refer to the
security of an encryption scheme that is used to encrypt a single plain-
text (per generated key). Since the plaintext may be longer than the
key, 3 these definitions are already non-trivial, and an encryption scheme
satisfying them (even in the private-key model) implies the existence
of one-way functions. Still, in many cases, it is desirable to encrypt
3 Recall that for sake of simplicity we have considered only messages of length n , but the
general definitions refer to messages of arbitrary (polynomial in n ) length. We comment
that, in the general form of Definition 5.1, one should provide the length of the message
as an auxiliary input to both algorithms ( A and B ).
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