## On Chosen Ciphertext Security of Multiple Encryptions

#### Webpage for a paper by Goldreich, Lustig and Naor

#### Abstract

We consider the security of multiple and possibly related
plaintexts in the context of a chosen ciphertext attack.
That is the attacker in addition and concurrently to obtaining encryptions
of multiple plaintexts under the same key,
may issue encryption and decryption queries and
partial information queries.
Loosely speaking, an encryption scheme is considered
secure under such attacks if all that the adversary can learn from
such attacks about the selected plaintexts can be obtained from the
corresponding partial information queries.
The above definition extends the definition of semantic security
under chosen ciphertext attacks (CCAs),
which is also formulated in this work.
The extension is in considering the security of multiple plaintexts
rather than the security of a single plaintext. We prove that both these
formulations are equivalent to the standard formulation of CCA,
which refers to indistinguishability of encryptions. The good news
is that any encryption scheme that is secure in the standard CCA
sense is in fact secure in the extended model.

The treatment holds both for public-key and private-key
encryption schemes.

#### Material available on-line

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