# Cryptography

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```					Cryptography

Gerard Klonarides
What is cryptography?

   Symmetric Encryption
   Asymmetric Encryption
   Other cryptography
   Digital signatures
   PKI
What is Cryptography?

   Transforming plaintext to
ciphertext

Hello I love you won’t you tell me your name?


mram?

Cryptography

   Transforming plaintext into
ciphertext
 Substitution
 Transposition
Substitution

   123453452345231
   This = 1234
   Try and crack this one
   This is his hit
   123453452345231
Transposition

   Plaintext elements rearranged
   This is his hit
   ihT sis sih tih
Plaintext is processed

   Block cipher
   A block at a time
   Stream cipher
   Processed continuously
The Cipher Process

   Keys
   Single Key
   Two-key encryption

   Bigger does not mean better
   For example IDEA 128 is better
than RSA 521
   One has to protect the integrity
of the keys
What does 128 bit encryption
mean?
   A 128-bit number has 2128
possible values.
   How big is that?
   218 is how many IPv6 addresses
we have
   2170 is the # of atoms in the earth
   2190 is the # of atoms in the sun
Encryption types

   Single key Encryption
 Conventional
 Symmetric

   Two-key Encryption
 Asymmetric
 Public-key
Symmetric Encryption

A type of encryption where the
same key is used to encrypt and
decrypt the message. This
differs from asymmetric (or
public-key) encryption, which
uses one key to encrypt a
message and another to decrypt
the message.
Asymmetric Encryption
   cryptographic system that uses two keys -- a public key known to everyone
and a private or secret key known only to the recipient of the message.
When John wants to send a secure message to Jane, he uses Jane's
public key to encrypt the message. Jane then uses her private key to
decrypt it.
   An important element to the public key system is that the public and
private keys are related in such a way that only the public key can be used
to encrypt messages and only the corresponding private key can be used
to decrypt them. Moreover, it is virtually impossible to deduce the private
key if you know the public key.
   Public-key systems, such as Pretty Good Privacy (PGP), are becoming
popular for transmitting information via the Internet. They are extremely
secure and relatively simple to use. The only difficulty with public-key
systems is that you need to know the recipient's public key to encrypt a
message for him or her. What's needed, therefore, is a global registry of
public keys, which is one of the promises of the new LDAP technology.
   Public key cryptography was invented in 1976 by Whitfield Diffie and
Martin Hellman. For this reason, it is sometime called Diffie-Hellman
encryption. It is also called asymmetric encryption because it uses two
keys instead of one key (symmetric encryption).
Algorithms

A formula or set of steps for solving a particular
problem. To be an algorithm, a set of rules must be
unambiguous and have a clear stopping point.
Algorithms can be expressed in any language, from
natural languages like English or French to
programming languages like FORTRAN.

   We use algorithms every day. For example, a recipe
for baking a cake is an algorithm. Most programs,
with the exception of some artificial intelligence
applications, consist of algorithms. Inventing elegant
algorithms -- algorithms that are simple and require
the fewest steps possible -- is one of the principal
challenges in programming.
The RSA Algorithm

   Developed by Ron Rivest, Adi
MIT in 1977
   The only widely accepted public-
key algorithm
   A block cipher algorithm
   98%
Authentication

   The ability to verify that the
contents of a message have not
been altered
   The ability to identify the owner
of that message
The Authentication Process

   To create an authenticator
   To check for authenticity
Hash Algorithms

   MD5 -Message Digest Algorithm
   SHA - Secure Hash Algorithm
   DSS – Digital Signature
Standard
DSS FIPS 186-2
   SUMMARY: The Secretary of Commerce approved
Federal Information Processing Standard (FIPS) 186-2,
Digital Signature Standard (DSS), which supersedes
Federal Information Processing Standard (FIPS) 186-1,
Digital Signature Standard (DSS). FIPS 186-2 expands
FIPS 186-1 by specifying an additional voluntary industry
standard for generating and verifying digital signatures.
This action will enable Federal agencies to use the Digital
Signature Algorithm (DSA), which was originally the
single approved technique for digital signatures, as well
as two new ANSI Standards that were developed for the
financial community. These new standards are ANSI
X9.31, Digital Signature Using Reversible Public Key
Cryptography, and ANSI X9.62, Elliptic Curve Digital
Signature Algorithm (ECDSA).
   EFFECTIVE DATE: This standard is effective June 27,
2000.
Diffie-Hellman key agreement

   The Diffie-Hellman key agreement
protocol (also called exponential key
agreement) was developed by Diffie
and Hellman [DH76] in 1976 and
published in the ground-breaking
paper ``New Directions in
Cryptography.'' The protocol allows
two users to exchange a secret key
over an insecure medium without
any prior secrets.

```
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