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BY MUKTADIUR RAHMANMAY 06, 2010
INTERODUCTION TO CRYPTOGRAPHY
Contents
IntroductionBasic TermsCryptographic ServicesSymmetric AlgorithmsAsymmetric AlgorithmsHashing AlgorithmsDigital SignaturePublic Key InfrastructureCryptanalysis 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 keyThe receiver verifies the signature with
sender public keyDSS 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 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 key
A registration authority (RA) that acts as the verifier for the certificate authority before a digital certificate is issued to a requestor
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
THANK YOU
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