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Chapter 8 Security
Why do we need security? Algorithms
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Who could cause problems?
To get revenge from being fired
Ex employee
To discover a competitorās strategic marketing plan
Businessman
To claim to represent all of Europe, not just Cannes
Sales Rep
To test out someoneās security system, steal data
Cracker
To have fun snooping on peopleās email
Student
Adversary Goal
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Who could cause problems?
Terrorist To steal germ warfare secrets
To learn an enemyās military or industrial secrets
Spy
To steal credit card numbers for sale
Con man
To deny a promise made to a customer by email
Stockbroker
To embezzle money from a company
Accountant
Adversary Goal
Tennenbaum pg 722
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4 main areas
ā¢ Secrecy
ā¢ Authentication
ā¢ Non repudiation
ā¢ Integrity control
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4 main areas
ā¢ Secrecy - encryption
ā¢ Authentication - passwords
ā¢ Non repudiation ā digital signatures
ā¢ Integrity control ā message digest
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Taxonomy of Network Security
Security
Cryptography algorithms
Public key (e.g., RSA)
Secret key (e.g., DES)
Message digest (e.g., MD5)
Security services
Privacy AuthenticationMessage integrity
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Where is security implemented?
- Any or all layers
- Wireless ā link layer
- Ipsec ā network layer
- Needed end to end!
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Terms
- Plaintext : the original message to be sent.
- Encryption : the process of taking a plaintext message and transforming it into a form which cannot be easily read, the ciphertext.
- Ciphertext : the encrypted plaintext
- Decryption : the process of transforming the ciphertext back into the plaintext.
- Key : Many encryption/decryption schemes use the same algorithm but take a parameter which drives the algorithm. This is called a key.
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Cryptography
- Secret Key
- Symmetric
- Participants share a key
- DED Data Encryption Standard
- International Data Encryption Algorithm (IDEA) Plaintext
Encrypt with secret key Ciphertext
Plaintext
Decrypt with secret key
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Cryptography
- Public key
- Asymmetric
- Private key that is not shared
- Public key that is shared so others can use it
- RSA (Rivest, Shamir, Adleman) Plaintext
Encrypt with public key Ciphertext
Plaintext
Decrypt with private key
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Cryptography
- Hash or message digest
- No keys
- Map large message to fixed length number
- A cryptographic checksum
- Computationally infeasible to find 2 messages that hash to the same cryptographic checksum - Message Digest version 5 (MD5) - More efficient than DES or RSA
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Encryption
- 2 techniques
- Substitution : each letter or group of letters is replaced (substituted) with another letter or group of letters.
- Transposition : alter the order in which the characters are sent
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DES
- No published mathematical proof that DES is
secure
- Break it by exhaustively searching all possible
2^56 keys
- (Text Estimate) 1.4X 10^17 microseconds to
break a key (4500 years) using an Alpha that can
do one encryption is 4 microseconds
- 9000 Alphas in parallel ā 6 months
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DES
- Triple DES encryption (uses two keys) The 128
bit composite key should be secure for another
few years at least.
- International Data Encryption Algorithm
(IDEA) (1990) which uses 128 bit keys and 9
iterations of transformations. It can be used in the
same modes as DES.
- Private key algorithms ā how do we exchange the
keys?
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Public Key
- In 1976, Diffie and Helman and Stanford
- Two keys, one used for encryption (the public
key ) and one used for decryption (the private
key ).
- Given one key, it is nearly impossible to figure out
what the other one is.
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RSA
- Rivest, Shamir, and Adelman from MIT
- Asymmetric encryption is very slow in
comparison to symmetric encryption
- Typically used to set up a secure connection and
exchange a secret key to be used for the rest of the
connection
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Public Key (RSA)
- Encryption & Decryption c = me^ mod n m = cd^ mod n
Plaintext
Encrypt with public key Ciphertext
Plaintext
Decrypt with private key
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RSA (cont)
- Choose two large prime numbers p and q (each 256 bits)
- Multiply p and q together to get n
- Choose the encryption key e , such that e and ( p - 1) x ( q - 1) are relatively prime.
- Two numbers are relatively prime if they have no common factor greater than one
- Compute decryption key d such that d = e -1 mod (( p - 1) x ( q - 1))
- Construct public key as ( e , n )
- Construct public key as ( d , n )
- Discard (do not disclose) original primes p and q
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RSA
- 1977, challenge to break a 129 digit RSA message
- Estimated to take 40 quadrillion years
- 1994 (after 17 years) four scientists had broken the code by using a factoring method that required ~ 5000MIP-years
- Done in 8 months by dividing the problem into small pieces and shipping them via email to other computers all over the world ā distributed approach
- If a bad key is chosen, it is easy to break
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Message Digest
- Cryptographic checksum
- just as a regular checksum protects the receiver from accidental changes to the message, a cryptographic checksum protects the receiver from malicious changes to the message.
- One-way function
- given a cryptographic checksum for a message, it is virtually impossible to figure out what message produced that checksum; it is not computationally feasible to find two messages that hash to the same cryptographic checksum.
- Relevance
- if you are given a checksum for a message and you are able to compute exactly the same checksum for that message, then it is highly likely this message produced the checksum you were given.
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Authentication Protocols
Client Server ClientId, E(x, CHK)
E(y + 1, CHK)
E(SK, SHK)
E(x + 1, SHK), E(y, SHK)
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- Trusted third party (Kerberos)
S A B
E((T, L, K, B), K (^) A), E((A, T), K),
E((T, L, K, A), K (^) B)
A, B
E(T + 1, K)
E((T, L, K, A), K (^) B)
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- Public key authentication
A B
E(x, Public (^) B)
x
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Authentication Techniques
- Three way handshake
- Trusted third party
- Public/private key
- Can use your own private key to encrypt a small portion of a message (digest)
- Receiver uses your public key to decrypt the digest
- This authenticates you
- Use DES or RSA (described in pgs. 585-
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CAās
- Certificate Authorities are trusted servers who maintain the certificates.
- The CAs verify through external means (telephone, mail, etc.) that you are who you say you are.
- They will then maintain your X.509 certificate.
- When someone wants your public key, they contact the CA.
- The CA digitally signs your public key using its own private key.
- The requester uses the CAās public key to decrypt the message get your public key 38
Key Distribution (cont)
- Chain of Trust
- if X certifies that a certain public key belongs to Y , and Y certifies that another public key belongs to Z , then there exists a chain of certificates from X to Z
- someone that wants to verify Z ās public key has to know X ās public key and follow the chain
- Certificate Revocation List
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SSL
- Secure Sockets Layer (SSL) ā (originated by
Netscape) as of ā98 controlled by the IETF who
renamed it Transport Layer Security (TSL)
- Supports the X.509 digital certificate standard.
- All web browsers and servers support this.
- It is a standard for authentication and encryption
and also allows message integrity checks.
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SSL
- Implements security at the transport layer
- Web servers using this will run at port 443 instead
of 80
- SSL handshake protocol is completed before an
application can transmit or receive information
- The two sides must authenticate each other and
negotiate an encryption algorithm.
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SSL
- public key authentication and encryption
- The domestic version uses a 128-bit key.
- Secure server URLs use https:// We know we are
connected to one if we see the lock on the bottom
of our browser or a dialog box comes up.
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HTTPS
- Security enhanced version of HTTP
- developed by EIT and released in 1994
- This protocol adds application layer encryption
and authentication on top of the socket based
communication.
- Client and server communicate in normal HTTP
fashion then negotiate security requirements.
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HTTPS
- Functions HTTPS supports
- Authentication, checks for certificate revocation, supports certificate hierarchies, digital signatures, various security levels, and secured communications through firewalls.
- Supports security at the document level.
- Uses public key encryption.
Do we need both SSL and HTTPS?
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IPSec
- IP Security (IPSec) is the IETFs new Ipv
security protocol that provides authentication and
encryption and key management at the IP level.
- It has also been added on to Ipv4.
- It has the potential to speed things up and be a
better security channel if the protocol is
implemented in hardware on IP routers and
switches.
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IPSec
- Authentication algorithms (AH):
- Encryption algorithms (ESP):
- DES (56 bit) and triple DES (168 bit) or AES (128, 192, and 256 bit) based on Rijndael algorithm created by Rijman and Daemen - see http://csrc.nist.gov/encryption/aes
- Key Exchange (IKE):
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Firewalls
- Packet filtering or
- Proxy-based application gateways.
Rest of the Internet Firewall Local site
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Firewalls
- Packet Filter
- Implemented in routers
- Tables indicate which communication protocols are allowed in and out of the network (e.g no telnet)
- Packets are dropped rejected or permitted based on application port number, source and destination IP address
- Disadvantage: can mimic a trusted IP
Rest of the Internet Firewall Local site
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Firewalls
- Proxy Firewall - Also called an application firewall
- Proxy processes are run for various types of application traffic (ie. Mail proxy)
- The proxy can evaluate packets based on context, authorization, and authentication instead of just IP addresses
- A proxyās IP address is the only address visible to the outside world ā hides internal IP addresses. End user must direct messages directly to the proxy
- Transparent proxy: user doesnāt know his messages are traveling to the proxy first
Rest of the Internet Firewall Local site
