Introduction to Cryptography

S E E M A G O E L

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Introduction to Cryptography

Contents

Basic Terms

Cryptography

The General Goals of Cryptography

Common Types of Attacks

Substitution Ciphers

Transposition Cipher

Steganography- “Concealed Writing”

Symmetric Secret Key Encryption

Types of Symmetric Algorithms

Common Symmetric Algorithms

Asymmetric Secret Key Encryption

Common Asymmetric Algorithms

Public Key Cryptography

Hashing Techniques

Hashing Algorithms

Digital Signatures

Transport Layer Security

Public key infrastructure (PKI)

Basic Terms

Encryption

Scrambling a message or data using a specialized cryptographic algorithm.

Plaintext

The message or data before it gets encrypted.

Ciphertext

The encrypted version of the message.

Cipher

The algorithm that does the encryption.

Decryption

The process of converting ciphertext back to the original plaintext.

Cryptography

Cryptography is the study of

" Secret (crypto-) writing (-graphy)

It can be described as the study of protecting information weather in transit or at rest, by using techniques to render the information unusable to anyone who does not possess the means to decrypt it.

Cryptanalysis is the science of recovering the plaintext from the ciphertext without access to the key.

Plaintext Ciphertext Plaintext

Encryption Decryption

The General Goals of Cryptography

Confidentiality

Assuring that only authorized parties are able to understand the data.

Integrity

Ensuring that when a message is sent over a network, the message that arrives is the same as the message that was originally sent.

Authentication

Ensuring that whoever supplies or accesses sensitive data is an authorized party.

Nonrepudiation

Ensuring that the intended recipient actually received the message & ensuring that the sender actually sent the message.

Common Types of Attacks

Ciphertext-Only Attack

Known-Plaintext Attack

Chosen-Plaintext Attack

Chosen-Ciphertext Attack

Dictionary Attacks

Substitution Ciphers

Caesar’s Cipher

A B C D E F G H I J K L . . . . . . . .

X Y Z A B C D E F G H I J K L . . . . . . .

Atbash Cipher

A B C D E F G H I J K L . . . . . . . .

Z Y X W V U T S R Q P O. . . . . . .

Vigenere Cipher

Polyalphabetic cipher to overcome the shortcomings of simple substitution ciphers

Plaintext

Caesar’s alphabet

Plaintext

Atbash’s alphabet

ATTACKATDAWN………

LEMONLEMONLE……..

LXFOPVEFRNHR……….

Plaintext

Key

Ciphertext

Transposition Cipher

In a transposition cipher, permutation is used, meaning that letters are scrambled. The key determines the positions that the characters are moved to.

Simple substitution and transposition ciphers are vulnerable to attacks that perform frequency analysis.

example text

examp letex

24153 31524

xmepa tlxee

Message

Broken into groups

Key

Ciphertext

Steganography- “Concealed Writing”

It is the art and science of writing hidden messages in an object(wave file, graphic, audio or video) in such a way that no one, apart from the sender and intended recipient, suspects the existence of the message.

The least significant bit of each byte of the image can be replaced with bits of the secret message.

Example of still imagery steganography. Left hand side image is the original cover image,

where as right hand side does embedding a text file into the cover Image make the stego image.

The advantage of steganography, over cryptography alone, is that messages do not attract attention.

Symmetric Secret Key Encryption

With this approach the sender and the receiver use the same secret key to encrypt and decrypt messages.

The strength of symmetric key encryption is fast, bulk encryption.

Major Challenges

Key distribution- It requires a secure mechanism to deliver keys properly.

Scalability- Each pair of users needs a unique pair of keys, so the number of keys grow exponentially

Examples of symmetric algorithms are as follows:

DES (Data Encryption Standard)

3DES

AES (Advanced Encryption Standard)

Types of Symmetric Algorithms

Block Cipher

Operate by encrypting a fixed amount, or “block,” (64 or 128 bit) of data

It is somewhat faster than stream cipher each time n characters executed.

Transmission errors in one cipher text block have no affect on other blocks.

Identical blocks of plaintext produce identical blocks of cipher text.

Block encryption may be more susceptible to cryptanalysis than either stream mode.

Stream cipher

Treats the message as a stream of bits or bytes and performs mathematical functions on them individually

The same plaintext bit or byte will be transformed into a different ciphertext bit or byte each time it is encrypted

Stream cipher is less vulnerable to insertion or deletion.

Transmission error at the nth bit in the stream cipher may lead to incorrect ciphertext thereafter.

Common Symmetric Algorithms

DES

Designed by IBM in the 1970s and adopted by the National Institute for Standards and Technology (NIST)] in 1977 for commercial and unclassified government applications.

DES is a block-cipher employing a 56-bit key that operates on 64-bit blocks.

DES results in a permutation among the 264 possible arrangements of 64 bits, each of which may be either 0 or 1

Triple DES (3DES) is an enhanced version of DES which applies the Data Encryption Standard (DES) cipher algorithm three times to each data block.

AES

AES was announced by National Institute of Standards and Technology on November 26, 2001.

AES is a block cipher with a block length of 128 bits.

It allows for three different key lengths: 128, 192, or 256 bits.

Encryption consists of 10 rounds of processing for 128-bit keys, 12 rounds for 192-bit keys, and 14 rounds for 256-bit keys.

Asymmetric Secret Key Encryption

Asymmetric encryption uses a key pair (Public key and Private key) .

The two different asymmetric keys are mathematically related but cannot be derived from each other.

Each key type can be used to encrypt and decrypt. If data is encrypted with a private key, it must be decrypted with the corresponding public key and vice versa.

Better key distribution and scalability than symmetric systems.

Works much slower than symmetric systems.

Examples of asymmetric key algorithms:

RSA

Elliptic Curve Cryptosystem (ECC)

Common Asymmetric Algorithms

RSA (Ron Rivest, Adi Shamir and Leonard Adleman)

Developed in 1978 at MIT

RSA gets its security from the difficulty of factoring large numbers

Best known & widely used

Each user generates a public/private key pair by applying the RSA algorithm to two large primes at random say p and q

One advantage of using RSA is that it can be used for encryption and digital signatures

RSA is used in many Web browsers with the Secure Sockets Layer (SSL) protocol

Common Asymmetric Algorithms (contd.)

Elliptic Curve Cryptosystems (ECCs)

ECC was introduced by Victor Miller and Neal Koblitz in 1985

For elliptic-curve-based protocols, it is assumed that finding the discrete logarithm of a random elliptic curve element with respect to a publicly known base point is infeasible

The size of the elliptic curve determines the difficulty of the problem.

ECC requires significantly smaller key size with same level of security as compared to the key size for RSA : faster computations, need less storage space

ECC ideal for constrained environments : Pagers , PDAs , Cellular Phones and Smart Cards

Public Key Cryptography

It is a hybrid use of two different algorithms: asymmetric and symmetric

Public key cryptography uses two keys (public and private) generated by an asymmetric algorithm for protecting encryption keys and key distribution, and a secret key is generated by a symmetric algorithm and used for bulk encryption.

Bill sends a message

Hashing Techniques

Cryptographic hashing functions are used to ensure the integrity of data using an integrity checksum.

Hashing functions are one-way functions. This means that the ciphertext (i.e., the checksum) cannot be used to reconstruct the plaintext.

The checksum (the ciphertext) is much smaller than the plaintext.

Hashing functions provide a kind of digital fingerprint.

The security of the hashing function is related to the size of the resulting checksum (in bits)

Examples of Hashing Algorithms:

MD5 (Message-Digest algorithm 5)

SHA (Secure Hash Algorithm)

Hashing Algorithms

MD5

128- bit hash value typically expressed as a 32-digit hexadecimal number.

MD5 processes a variable-length message into a fixed-length output of 128 bits.

It is easy to compute.

It is infeasible to modify a message without changing its hash.

No two messages have the same hash.

SHA

SHA-1 produces a 160-bit hash value.

SHA-256 uses 32-bit words.

SHA-512 uses 64-bit words.

The collision ratio for SHA is far less than the collision ratio MD5.

Digital Signatures

Goals

Itshould be proof of authenticity and should be impossible to forge.

It should be impossible to alter the signed document without detection.

It should be impossible to transplant the signature to another document.

Technology

A hash function to help generate the digital signature, S.

Symmetric (secret key) cryptography to encrypt the message, M.

Public key cryptography to share the secret key used to encrypt and decrypt the message, M.

Public key cryptography to encrypt and decrypt the digital signature, S.

Transport Layer Security

It provides communication security over the Internet.

Encrypt the segments of network connections at the Application Layer using asymmetric cryptography for key exchange, symmetric encryption for privacy, and message authentication codes for message integrity.

Widespread use in applications such as web browsing, electronic mail, Internet faxing, instant messaging etc.

The TLS protocol is made up of two layers.

The record protocol is designed to protect confidentiality

The handshake protocol allows authentication

TLS is application protocol-independent.

A vulnerability (CVE-2011-3389) was reported in The SSL protocol which allows attackers to obtain plaintext HTTP headers via a blockwise chosen-boundary attack on an HTTPS session.

Public key infrastructure (PKI)

It consists of programs, data formats, procedures, communication protocols, security policies, and public key cryptographic mechanisms working in a comprehensive manner to enable a wide range of dispersed people to communicate in a secure and predictable fashion.

PKI is an ISO authentication framework that uses public key cryptography and the X.509 standard protocols

PKI provides authentication, confidentiality, nonrepudiation, and integrity of the messages exchanged

PKI is a hybrid system of symmetric and asymmetric key algorithms

Each person who wants to participate in a PKI requires a digital certificate

Thank You!!