Cryptographic Protocols - Codes: The Guide to Secrecy from Ancient to Modern Times - page 192

mechanisms for Section 5.7. We concentrate on some other features to occupy
this section, including some types of protocols that we will not cover in the
succeeding sections, but rather highlight them here as an introductoryfeature
of this chapter.
One aspect, not explicitlydescribed in the following sections of this chap-
ter, but deserving of some preliminarycommentar, is the notion of a sublimi-
nal channel , meaning covert methods for an adversaryto send missives hidden
within a legitimate message. Perhaps the most widely-known such example is
the hiding of bits in a digitized photograph, or commercial digital television
message. Much of this type of subliminal message protocoling is accomplished
via steganography. The reader is referred to the topic [137], which is devoted
entirelyto steganographic methodologies, for such information. Some modern
schemes use nonces, such as the DSA (discussed on page 184), and the ElGa-
mal signature scheme. We will encounter subliminal channels again when we
describe the topic of nuclear test ban treatycompliance in Section 9.6.
Two other intertwined types of protocols with which we end this section are
of value in manysituations. For instance, suppose that Bob is a CIA agent and
he wants to buysecrets from Alice who is a Russian double agent. Bob does
not want to reveal what he knows and what he does not, since the consequences
could range from the merelyembarrassing to the downright dangerous. Hence,
he would like to buysecrets from Alice, but not have her know, in advance, which
ones. In other words, Alice transfers information, containing one or more secrets,
to Bob in such a waythat upon completion of the protocol, Alice does not know
(is “oblivious” as to) which of the secrets Bob received. This mechanism has a
name.

Oblivious Transfer Cryptographic Protocol
We deal with the simplest scenario where onlyone secret is transferred, but
this is easilyextrapolated to numerous secrets. Alice sends Bob two messages,
onlyone of which he receives, and Alice does not know which one. The PKC
and SCK used are RSA and Rijndael, respectively.
1. Alice generates two RSA keypairs and sends the two public keys to Bob.

2. Bob selects a Rijndael key, k , and chooses, at random, one of Alice's public
keys to encrypt it, and sends this to Alice.

3. Alice uses her two private keys to attempt to decipher the cryptogram, only
one of which is successful, but she does not know which one, since she
does not know the secret Rijndael key. So one is binary gibberish and the
other is the legitimate Rijndael key, k .

4. Alice enciphers her two secret messages, one with k , and the other with the
binarygibberish ke, and sends both to Bob. (For the sake of simplicit,
we will assume that the two secret messages are indeed distinct, that is,
that Alice is not trying to cheat Bob.)

5. Bob uses his secret Rijndael keyon both messages, but onlyone is successful
in yielding one of the secrets, and Alice does not know which one.