cryptography.pptcryptography.pptcryptography.ppt

Computer Security
MIS 604 – IT Solutions to Business Problems
Spring 2002
Secure Communication - Cryptography

• Well established needs for secure communication
– War time communication
– Business transactions
– Illicit Love Affairs
• Requirements of secure communication
1. Secrecy
– Only intended receiver understands the message
2. Authentication
– Sender and receiver need to confirm each others identity
3. Message Integrity
– Ensure that their communication has not been altered, either
maliciously or by accident during transmission
Secure Communication

• Cryptography is the science of secret, or hidden
writing
• It has two main Components:
1. Encryption
– Practice of hiding messages so that they can not be read by
anyone other than the intended recipient
2. Authentication
– Ensuring that users of data/resources are the persons they
claim to be and that a message has not been surreptitiously
altered
Cryptography

• Cipher is a method for encrypting messaged
• Encryption algorithms are standardized & published
• The key which is an input to the algorithm is secret
– Key is a string of numbers or characters
– If same key is used for encryption & decryption the algorithm is called symmetric
– If different keys are used for encryption & decryption the algorithm is called
asymmetric
Encryption - Cipher
Plain Text Encryption
Algorithm
Key A Key B
Cipher Text Plain Text
Decryption
Algorithm

• Algorithms in which the key for encryption and
decryption are the same Cryptography is the
– Example: Caesar Cipher
• Types:
1. Block Ciphers
– Encrypt data one block at a time (typically 64 bits, or 128 bits)
– Used for a single message
2. Stream Ciphers
– Encrypt data one bit or one byte at a time
– Used if data is a constant stream of information
Encryption - Symmetric Algorithms

• Strength of algorithm is determined by the size of the key
– The longer the key the more difficult it is to crack
• Key length is expressed in bits
– Typical key sizes vary between 48bits and 448 bits
• Set of possible keys for a cipher is called key space
– For 40-bit key there are 240
possible keys
– For 40-bit key there are 2128
possible keys
– Each additional bit added to the key length doubles the security
• To crack the key the hacker has to use brute-force
(i.e. try all the possible keys till a key that works is found)
– Super Computer can crack a 56-bit key in 24 hours
– It will take 272
times longer to crack a 128-bit key
(Longer than the age of the universe)
Symmetric Encryption – Key Strength

• Caesar Cipher is a method in which each letter in the
alphabet is rotated by three letters as shown
Symmetric Algorithms – Caesar
Cipher
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
• Let us try to encrypt the message
– Attack at Dawn
Assignment: Each student exchange a secret message with
their closest neighbor about some other person in the
class and the neighbor will decipher it.

Symmetric Algorithms - Caesar
Cipher
Encryption
Plain Text
Message:
Attack at Dawn
Cipher Text
Message:
Dwwdfn Dw Gdyq
Cipher:
Caesar Cipher
Algorithm
Key (3)
Decryption
Plain Text
Message:
Attack at Dawn
Cipher Text
Message:
Dwwdfn Dw Gdyq
Cipher:
Caesar Cipher
Algorithm
Key (3)
How many different keys are possible?

• Any letter can be substituted for any other letter
– Each letter has to have a unique substitute
• There are 26! pairing of letters (~1026
)
• Brute Force approach would be too time consuming
– Statistical Analysis would make it feasible to crack the key
Symmetric Algorithms - Monoalphabetic
Cipher
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
M N B V C X Z A S D F G H J K L P O I U Y T R E W Q
Encrypted
Message:
Nkn, s gktc wky.
mgsbc
Message:
Bob, I love you.
Alice
Cipher:
Monoalphabetic
Cipher
Key

• Developed by Blaise de Vigenere
– Also called Vigenere cipher
• Uses a sequence of monoalpabetic ciphers in tandem
– e.g. C1, C2, C2, C1, C2
• Example
Symmetric Algorithms - Polyalphabetic
Cipher
Encrypted
Message:
Gnu, n etox dhz.
tenvj
Message:
Bob, I love you.
Alice
Cipher:
Monoalphabetic
Cipher
Key
Plain Text A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
C1(k=6) F G H I J K L M N O P Q R S T U V W X Y Z A B C D E
C1(k=20) T U V W X Y Z A B C D E F G H I J K L M N O P Q R S

• Goal of DES is to completely scramble the data and key so
that every bit of cipher text depends on every bit of data
and ever bit of key
• DES is a block Cipher Algorithm
– Encodes plaintext in 64 bit chunks
– One parity bit for each of the 8 bytes thus it reduces to 56 bits
• It is the most used algorithm
– Standard approved by US National Bureau of Standards for
Commercial and nonclassified US government use in 1993
• Cracking the DES code
– In 1997 it was cracked in 140 days
– In 1999 it was cracked in 14 hours
• To improve security Triple-DES has been created
– It uses the DES algorithm multiple times in tandem
Data Encryption Standard (DES)

• Any exposure to the secret key compromises
secrecy of ciphertext
• A key needs to be delivered to the recipient of
the coded message for it to be deciphered
– Potential for eavesdropping attack during transmission
of key
Symmetric Encryption – Limitations

• Uses a pair of keys for encryption
– Public key for encryption
– Private key for decryption
• Messages encoded using public key can only be decoded by the
private key
– Secret transmission of key for decryption is not required
– Every entity can generate a key pair and release its public key
Asymmetric Encryption
Plain Text
Cipher
Public Key Private Key
Cipher Text Plain Text
Cipher

• Two most popular algorithms are RSA & El Gamal
– RSA
• Developed by Ron Rivest, Adi Shamir, Len Adelman
• Both public and private key are interchangable
– El Gamal
• Developed by Taher ElGamal
• Potential for eavesdropping attack during transmission of key
• Efficiency is lower than Symmetric Algorithms
– A 1024-bit asymmetric key is equivalent to 128-bit symmetric key
Asymmetric Encryption

• Slow compared to symmetric Encryption
• It is problematic to get the key pair generated for the encryption.
• Vulnerable to man-in-the-middle attack
– Hacker could generate a key pair, give the public key away and tell everybody,
that it belongs to somebody else. Now, everyone believing it will use this key for
encryption, resulting in the hacker being able to read the messages. If he
encrypts the messages again with the public key of the real recipient, he will not
be recognized easily.
Asymmetric Encryption -
Weaknesses
Bob
David
Bob’s
Message
+ Public key
Cipher
David’s
Public Key
Trudeau
(Middle-man)
Trudeau’s
Message
+ public key
Cipher
Trudeau’s
Public Key
Bob’s
Encrypted
Message
Trudeau’s
Encrypted
Message
David’s
Message
+ public key
Cipher
Trudeau’s
Encrypted
Message
Bob’s
Public Key
Trudeau’s
New Message
+ public key
Cipher
Trudeau’s
Encrypted
Message
David’s
Public Key

• Used to improve efficiency
– Symmetric key is used for encrypting data
– Asymmetric key is used for encrypting the symmetric key
Asymmetric Encryption – Session-Key
Encryption
Plain Text Cipher
(DES)
Session Key
Recipient’s Public Key
Cipher Text
Encrypted
Key
Cipher
(RSA)
Send to Recipient

• Pretty Good Privacy (PGP)
– Used to encrypt e-mail using session key encryption
– Combines RSA, TripleDES, and other algorithms
• Secure/Multipurpose Internet Mail Extension (S/MIME)
– Newer algorithm for securing e-mail
– Backed by Microsoft, RSA, AOL
• Secure Socket Layer(SSL) and Transport Layer Socket(TLS)
– Used for securing TCP/IP Traffic
– Mainly designed for web use
– Can be used for any kind of internet traffic
Asymmetric Encryption – Encryption
Protocols

• Key agreement is a method to create secret key by exchanging only
public keys.
• Example
– Bob sends Alice his public key
– Alice sends Bob her public key
– Bob uses Alice’s public key and his private key to generate a session key
– Alice uses Bob’s public key and her private key to generate a session key
– Using a key agreement algorithm both will generate same key
– Bob and Alice do not need to transfer any key
Asymmetric Encryption – Key Agreement
Cipher
(DES)
Session Key
Cipher
(DES)
Bob’s
Public Key
Alice’s
Public Key
Bob’s
Private Key
Alice’s
Private Key
Alice and Bob
Generate Same
Session Key!

Diffie-Hellman Mathematical Analysis
Bob & Alice
agree on non-secret
prime p and value a
Generate Secret
Random Number x
Compute Public Key
ax
mod p
Compute Public Key
(ay
)x
mod p
Generate Secret
Random Number y
Compute Public Key
ay
mod p
Compute Public Key
(ax
)y
mod p
Bob Alice
Identical Secret Key
Bob & Alice
exchange
public keys

• Diffie-Hellman is the first key agreement algorithm
– Invented by Whitfield Diffie & Martin Hellman
– Provided ability for messages to be exchanged securely without
having to have shared some information previously
– Inception of public key cryptography which allowed keys to be
exchanged in the open
• No exchange of secret keys
– Man-in-the middle attack avoided
Asymmetric Encryption – Key Agreement
contd.

• Authentication is the process of determining the authenticity
of a message or user.
• Two types of authentication:
– Authentication of the identity presented by a remote or application
participating in a session
– Authentication of the sender’s identity is presented along with a
message.
Authentication

• Use of secret character string only known to user
and server
• Problems with password based authentication:
– Attacker learns password by social engineering
– Attacker cracks password by brute-force and/or
guesswork
– Eavesdrops password if it is communicated
unprotected over the network
– Replays an encrypted password back to the
authentication server
Authentication – Password Based

• Set of rules that governs the communication of data related to authentication
between the server and the user
• Techniques used to build a protocol are
– Transformed password
• Password transformed using one way function before transmission
• Prevents eavesdropping but not replay
– Challenge-response
• Server sends a random value (challenge) to the client along with the
authentication request. This must be included in the response
• Protects against replay
– Time Stamp
• The authentication from the client to server must have time-stamp embedded
• Server checks if the time is reasonable
• Protects against replay
• Depends on synchronization of clocks on computers
– One-time password
• New password obtained by passing user-password through one-way function n
times which keeps incrementing
• Protects against replay as well as eavesdropping
Authentication Protocols

• Personal Tokens are hardware devices that generate
unique strings that are usually used in conjunction with
passwords for authentication
• Different types of tokens exist
– Storage Token: A secret value that is stored on a token and is
available after the token has been unlocked using a PIN
– Synchronous one-time password generator: Generate a new
password periodically (e.g. each minute) based on time and a
secret code stored in the token
– Challenge-response: Token computes a number based on a
challenge value sent by the server
– Digital Signature Token: Contains the digital signature private
key and computes a computes a digital signature on a supplied
data value
• A variety of different physical forms of tokens exist
– e.g. hand-held devices, Smart Cards, PCMCIA cards, USB tokens
Authentication – Personal Tokens

• Uses certain biological characteristics for
authentication
– Biometric reader measures physiological indicia and
compares them to specified values
– It is not capable of securing information over the
network
• Different techniques exist
– Fingerprint Recognition
– Voice Recognition
– Handwriting Recognition
– Face Recognition
– Retinal Scan
– Hand Geometry Recognition
Authentication – Biometrics

• Probability of two irises producing exactly the same
code: 1 in 10 to the 78th power
• Independent variables (degrees of freedom)
extracted: 266
• IrisCode record size: 512 bytes
• Operating systems compatibility: DOS and Windows
(NT/95)
• Average identification speed (database of 100,000
IrisCode records): one to two seconds
Authentication – Iris Recognition
The scanning process takes advantage of
the natural patterns in people's irises,
digitizing them for identification purposes
Facts

• A message digest is a fingerprint for a document
• Purpose of the message digest is to provide proof that a document has not been tampered with.
• Hash functions used to generate message digests are one way functions that have following
properties
– It must be computationally infeasible to reverse the function
– It must be computationally infeasible to construct two messages which which hash to the same digest
• Some of the commonly used hash algorithms are
– MD5 – 128 bit hashing algorithm by Ron Rivest of RSA
– SHA & SHA-1 – 162 bit hashing algorithm developed by NIST
Authentication – Message Digests

• A digital signature is a data item which accompanies or is logically
associated with a digitally encoded message.
• It has two goals
– A guarantee of the source of the data
– Proof that the data has not been tampered with
• A digital signature is created with a persons private key and verified
by their public key
Authentication – Digital Signatures
Message
Sent to
Receiver
Digest
Algorithm
Digital
Signature
Sent to
Receiver
Message
Digest
Sender’s
Private Key
Sender’s
Public Key
Message
Digest
Signature
Algorithm
Signature
Algorithm
Digest
Algorithm
Message
Digest
Sender Receiver
Same?

• A digital certificate is a signed statement by a trusted party that another
party’s public key belongs to them.
– This allows one certificate authority to be authorized by a different authority
(root CA)
• Top level certificate must be self signed
• Any one can start a certificate authority
– Name recognition is key to some one recognizing a certificate authority
– Verisign is industry standard certificate authority
Authentication – Digital Certificates
Identity
Information
Certificate
Authority’s
Private Key
Sender’s
Public Key
Signature
Algorithm
Certificate

• Chaining is the practice of signing a certificate with ainother private
key that has a certificate for its public key
– Similar to the passport having the seal of the government
• It is essentially a person’s public key & some identifying information
signed by an authority’s private key verifying the person’s identity
• The authorities public key can be used to decipher the certificate
• The trusted party is called the certificate authority
Authentication – Certificate Chaining
Certificate
Authority’s
Private Key
Signature
Algorithm
New Certificate
Certificate

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