CS 70
Discrete Mathematics and Probability Theory
Spring 2012
Alistair Sinclair
Note 6
This note is partly based on Section 1.4 of “Algorithms,” by S. Dasgupta, C. Papadimitriou and U. Vazirani,
McGrawHill, 2007.
Public Key Cryptography
In this note, we discuss a very nice and important application of modular arithmetic: the
RSA publickey
cryptosystem
, named after its inventors Ronald Rivest, Adi Shamir and Leonard Adleman.
The basic setting for cryptography is typically described via a cast of three characters: Alice and Bob, who
with to communicate conﬁdentially over some (insecure) link, and Eve, an eavesdropper who is listening in
and trying to discover what they are saying. Let’s assume that Alice wants to transmit a message
x
(written
in binary) to Bob. She will apply her
encryption function E
to
x
and send the encrypted message
E
(
x
)
over
the link; Bob, upon receipt of
E
(
x
)
, will then apply his
decryption function D
to it and thus recover the
original message: i.e.,
D
(
E
(
x
)) =
x
.
Since the link is insecure, Alice and Bob have to assume that Eve may get hold of
E
(
x
)
. (Think of Eve
as being a “sniffer” on the network.) Thus ideally we would like to know that the encryption function
E
is
chosen so that just knowing
E
(
x
)
(without knowing the decryption function
D
) doesn’t allow one to discover
anything about the original message
x
.
For centuries cryptography was based on what are now called
privatekey
protocols. In such a scheme,
Alice and Bob meet beforehand and together choose a secret codebook, with which they encrypt all future
correspondence between them. (This codebook plays the role of the functions
E
and
D
above.) Eve’s only
hope then is to collect some encrypted messages and use them to at least partially ﬁgure out the codebook.
Publickey
schemes, such as RSA, are signiﬁcantly more subtle and tricky: they allow Alice to send Bob
a message without ever having met him before! This almost sounds impossible, because in this scenario
there is a symmetry between Bob and Eve: why should Bob have any advantage over Eve in terms of being
able to understand Alice’s message? The central idea between the RSA cryptosystem is that Bob is able to
implement a
digital lock
, to which only he has the key. Now by making this digital lock public, he gives
Alice (or, indeed, anybody else) a way to send him a secure message which only he can open.
Here is how the digital lock is implemented in the RSA scheme. Each person has a
public key
known to the
whole world, and a
private key
known only to him or herself. When Alice wants to send a message
x
to
Bob, she encodes it using Bob’s public key. Bob then decrypts it using his private key, thus retrieving
x
. Eve
is welcome to see as many encrypted messages for Bob as she likes, but she will not be able to decode them
(under certain simple assumptions explained below).
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 Spring '08
 PAPADIMITROU
 Algorithms, Prime number

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