Using Cryptography to Secure Information

Overview

Introduction to Cryptography

Using Symmetric Encryption

Using Hash Functions

Using Public Key Encryption / Asymmetric Encryption

Lesson: Introduction to Cryptography

What Is Cryptography?

What Are Algorithms?

What Are Keys?

Common Uses of Cryptography

What Is Cryptography?

Cryptography is a set of techniques used to encode and decode information

Ciphertext hides information in a message while it is transferred from one location to another or while stored

CiphertextCiphertext

hello

EncryptionEncryption

DecryptionDecryptionPlaintextPlaintext

*A%$q

Services

Confidentiality: prevents others from reading the information

Integrity checking: reassuring that the information has not been altered since it was generated

Authentication: verifying someone's (or something's) identity

Terminology

Encryption/Decryption:o encode/decode: translating entire words or phraseso encipher/decipher: translating letters of symbols individually

Plaintext/Ciphertext

Key

Cryptosystem

Cryptography / Cryptanalysis

Cryptology

Terminology

Encryption is the process of encoding a message so that its meaning is not obvious; decryption is the reverse process, transforming an encrypted message back into its normal, original form. we say that we encode, encrypt, or encipher the original message to hide its meaning. Then, we decode, decrypt, or decipher it to reveal the original message. A system for encryption and decryption is called a cryptosystem.

Algorithm (Plaintext) = CiphertextAlgorithm (Plaintext) = Ciphertext

What Are Algorithms?

You can publish algorithms or keep them secret

Multiple users can use the same algorithm to encrypt or decrypt multiple data sources

Algorithm Plaintext Ciphertext

Advance 2 spaces A C

Advance 2 spaces K M

ExamplesExamples

e=f(m) e=f(m)

Algorithms are mathematical formulas to encrypt or decrypt data

Encryption Algorithms

The cryptosystem involves a set of rules for how to encrypt the plaintext and how to decrypt the ciphertext. The encryption and decryption rules, called algorithms, often use a device called a key, denoted by K, so that the resulting ciphertext depends on the original plaintext message, the algorithm, and the key value. We write this dependence as

c = ek ( m )

Essentially, e is a set of encryption algorithms, and the key k selects one specific algorithm from the set. We see later in this chapter that a cryptosystem such as the Caesar cipher is keyless but that keyed encryptions are more difficult to break.An encryption algorithm, or cipher, is a means of transforming plaintext into ciphertext, under the control of a secret key. This process is called encryption or encipherment. given c depends totally on the secrecy of k.

c = ek ( m )m is the plaintext,e is the cipher function,k is the secret key,c is the ciphertext.The reverse process is called decryption or decipherment, and we write

m = dk (c )d is the decipher function,

Note, that the encryption and decryption algorithms e, d are public, the secrecy of m given c depends totally on the secrecy of k.

Symmetric Cryptosytem

the encryption and decryption keys are the same, This form is called symmetric encryption because d and e are mirror-image processes

What Are Keys?

Keys are normally secret Key length and complexity influence

the strength of encryption

A key is a string of bits that is used to vary the results of an algorithm

Algorithm and Key (Plaintext) = Ciphertext

Algorithm and Key (Plaintext) = Ciphertext

ExamplesExamples

Algorithm Key Plaintext Ciphertext

Advance x spaces 3 A D

Advance x spaces 5 A F

Three Techniques in Encryption

Shift Cipher each letter is translated to a letter a fixed number of places after it in the

alphabet. (Caesar Cipher)

Substitution One symbol is exchanged for another, This technique is called a

monoalphabetic cipher or simple substitution.

only one alphabet substitution was used to encrypt the whole alphabet.

Polyalphabetic substitution cipher, encrypt each letter with a different alphabet (Vigenere Cipher)

Transposition (Permutation) The order of the symbols is rearranged

Shift Cipher

The Caesar Cipher:

A shiff of 3:

ci=e(mi)=( mi+3 ) mod 26 Plaintext ABCDEFGHIJKLMNOPQRSTUVWXYZ

Ciphertext defghijklmnopqrstuvwxyzabcUsing this encryption, the message PLEASE CALL ME would be encoded as

PLEASE CALL ME S …….. There are only 26 possible keys, so the time for this exhaustive key

search is very small.

Shift Cipher

i=7

ci=e(mi)=mi+7Using above encryption algorithm, encrypt this message, HELLO