n
Then receiver computes message digest of sent file if hash is the same it has not been modified
Digital Signal Standard (DSS) and Secure Hash Standard (SHS)
n
Enables use of RSA digital signature algorithm or DSA –Digital Signature Algorithm (based on El Gamal)
n
Both use The Secure Hash Algorithm to compute message digest then processed by DSA to verify the signature.
Message digest is used instead of the longer message because faster.
SHA1
 Secure Hash Algorithm produces 160 bit digest if message is less than 2^64 bits.
n
It is computationally infeasible to find message from message digest
n
It is computationally infeasible to find to different messages with same message digest
n
Padding bits are added to message to make it a multiple of 512
MD5
n
Developed by Ronald Rivest in 1991
n
Produces 128 bit message digest
Hashed Message Authentication Code (HMAC)
n
Uses key to generate a Message Authentication Code which is used as a checksum
Birthday Attack
n
You in a room with better than 50/50 chance of another person having your birthday?
Need 253 people
n
You in a room with better than 50/50 chance of two people having the same birthday?
Need 23 people
Cryptographic Attack
Brute Force Attack
 try every possible combination
Known Plain Text
– attacker has copy of plain text and the associated ciphertext of several messages
Chosen Plain Text
– chosen plain text is encrypted. The attacker has the plaintext and ciphertext and can choose
the plaintext that gets encrypted.
Adaptive Chosen Plain Text
– selection of plain text is altered based on previous results
Ciphertext Only
– only ciphertext is known.
The attacker has the ciphertext of several messages. Each of the
messages has been encrypted using the same encryption algorithm.
Chosen Ciphertext
– Portions of the cipher text are selected for trial decryption while having access to plain text.
The attacker can choose the ciphertext to be decrypted and has access to the resulting decrypted plaintext.
Adaptive Chosen Ciphertext 
Chosen cipher text are selected for trial decryption where selection is based on
previous results
Birthday Attack
– the probability of two different messages having same message digest or finding two different
messages that have the same message digest
Meet in the Middle
– For attacking double encryption from each end and comparing in the middle
Man in the Middle
– intercepting messages and forwarding on modified versions
Differential Cryptanalysis
– Private key cryptography looking at text pairs after encryption looking for differences
Linear Cryptanalysis –
using plain text and cipher text to generate a linear approximation of a portion of the key
Differential Linear Cryptanalysis –
using both linear and differential approaches
Factoring
– using mathematics to determine the prime factors of large numbers
Statistical
– exploiting the lack of randomness in key generation
Public Key Certification Systems
n
A source could post a public key under the name of another individual
n
Digital certificates counter this attack, a certificate can bind individuals to their key
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview.
Sign up
to
access the rest of the document.
 Fall '12
 Hurst
 Cryptography

Click to edit the document details