# BY MUKTADIUR RAHMAN MAY 06, 2010 INTERODUCTION TO CRYPTOGRAPHY.

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BY MUKTADIUR RAHMAN MAY 06, 2010 INTERODUCTION TO CRYPTOGRAPHY

Contents Introduction Basic Terms Cryptographic Services Symmetric Algorithms Asymmetric Algorithms Hashing Algorithms Digital Signature Public Key Infrastructure Cryptanalysis Attacks

Introduction The study of techniques related to all aspects of data The word "cryptography" is derived from the ancient Greek words "kryptos" (hidden) and "graphia" (writing). Within the field of cryptology one can see two separate divisions:  Cryptography  Cryptanalysis.

Introduction Cryptography  The cryptographer seeks methods to ensure the safety and security of conversations. Cryptanalysis  Cryptanalyst tries to undo the former's work by breaking cryptographer systems.

Basic Terms Plaintext – the original intelligible message Ciphertext – the transformed message Chiper – the algorithm that does the encryption Key – some critical information used by the cipher, known only to the sender receiver. Keyspace – Total number of possible values of keys in a crypto algorithm

Basic Terms Cryptosystem – The combination of algorithm, key, and key management functions used to perform cryptographic operations Cryptology - both cryptography and cryptanalysis Encryption : the process of converting plaintext to cipher text using a cipher and a key Decryption : the process of converting cipher text to plaintext

Cryptographic Services Confidentiality: To help protect a user's identity or data from being read. Data integrity: To help protect data from being changed. Authentication: To ensure that data originates from a particular party. Non-repudiation: To prevent a particular party from denying that they sent a message.

Symmetric Algorithms Symmetric algorithms use a single key shared by two communicating parties. The shared key must remain secret to ensure the confidentiality of the encrypted data. Sharing key is the is the main technological challenge of this kind of encryption Symmetric algorithms are used for:  Confidentiality  Data integrity

Symmetric Algorithms DES((Data Encryption Standard)  Block size is 64 bits  Key size is 56 bits 3DES(Triple Data Encryption Standard)  Using DES twice on a given message  3DES is a viable and popular symmetric block algorithm AES (Advanced Encryption Standard)  NIST( National Institute of Standards and Technology ) replaced DES in 1997 with AES  Uses the Rijndael algorithm  Supports key/block sizes of 128, 192, and 256 bits  Uses 10/12/14 rounds as block size increases

Symmetric Algorithms IDEA (International Data Encryption Algorithm)  Operates on 64 bit blocks in 8 rounds with 128 bit key  Considered stronger than DES and is used in PGP(Pretty Good Privacy) Blowfish  64 bit block cipher with up to 448 bit key and 16 rounds  Designed by Bruce Schneier RC5  Block size 32/64/128 bits  Variable key size up to 2048 bits  Created by Ron Rivest

Asymmetric Algorithms In asymmetric cryptography, each user has two keys: a public key and a private key. The public key is made public. For example, it may be published on a Web site. The private key must be kept secret. It is never shared with anyone. Their main use is in solving the key exchange problem for symmetric cryptography Asymmetric algorithms tend to be very inefficient cause it is very slow, about 100 times slower than DES

Asymmetric Algorithms Diffie-Hellman  First widely known public key cryptography algorithm  Provides means for secure key exchange over insecure channel RSA  Stands for inventors names, Rivest, Shamir, and Adleman  Relies on difficulty of finding prime factorization of large numbers

Hashing Algorithms Cryptographic hashing functions are used to ensure the integrity of data. Cryptographic hashing functions are sometimes called cryptographic checksums or integrity checksums. Hashing functions are also used for digital signatures

Hashing Algorithms MD5  Computes 128-bit hash value  Widely used for file integrity checking SHA-1  Computes 160-bit hash value  NIST approved message digest algorithm

Digital Signature Public key encryption enabled the development of the technology of digital signatures. Digital signatures are somewhat analogous to traditional handwritten signatures. Digital signatures are strongly bound to the document, but weakly bound to the individual. A digital signature is computed, in part, using the contents of the document being signed.

Digital Signature Hash of message encrypted with private key The receiver verifies the signature with sender public key DSS provides Sender authentication Verification of message integrity Non-Repudiation

Public Key Infrastructure A PKI (public key infrastructure) enables users of a basically unsecure public network such as the Internet to securely and privately exchange data and money through the use of a public and a private cryptographic key pair that is obtained and shared through a trusted authority. The public key infrastructure provides for a digital certificate that can identify an individual or an organization and directory services that can store and, when necessary, revoke the certificates. public keydigital certificate

Public Key Infrastructure A public key infrastructure consists of:  A certificate authority (CA) that issues and verifies digital certificate. A certificate includes the public key or information about the public keyCAdigital certificate  A registration authority (RA) that acts as the verifier for the certificate authority before a digital certificate is issued to a requestorRA  One or more directories where the certificates (with their public keys) are held  A certificate management system

Cryptanalysis Attacks Brute force  Trying all key values in the keyspace Frequency Analysis  Guess values based on frequency of occurrence Dictionary Attack  Find plaintext based on common words

Cryptanalysis Attacks Replay Attack  Repeating previous known values Factoring Attacks  Find keys through prime factorization Known Plaintext  Format or content of plaintext available Known cipher attacks  The attacker has the ciphertext and tries to decrypt the message by generating all possible keys

Cryptanalysis Attacks Chosen Plaintext  Attack can encrypt chosen plaintext Chosen Ciphertext  Decrypt known ciphertext to discover key Differential Power Analysis  Side Channel Attack  Identify algorithm and key length

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