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Security Threats
Passive attacks Eavesdropping on, or monitoring, transmissions Electronic mail, file transfers, and client/server exchanges
are examples of transmissions that can be monitored
Active attacks Modification of transmitted data Attempts to gain unauthorized access to computer
systems
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Encryption Methods
The essential technology underlying virtually all automated network and computer security applications is cryptography
Two fundamental approaches are in use: conventional encryption, also known as symmetric
encryption public-key encryption, also known as asymmetric
encryption
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Conventional Encryption The only form of encryption prior to late 1970s Five components to the algorithm
Plaintext: The original message or data Encryption algorithm: Performs various substitutions and transformations on
the plaintext. Secret key: Input to the encryption algorithm. Substitutions and
transformations performed depend on this key Ciphertext: Scrambled message produced as output. depends on the plaintext
and the secret key Decryption algorithm: Encryption algorithm run in reverse. Uses ciphertext
and the secret key to produce the original plaintext.
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Conventional Encryption Requirements & Weaknesses
Requirements A strong encryption algorithm Secure process for sender & receiver to obtain secret keys
Methods of Attack Cryptanalysis Brute force
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Data Encryption Standard (DES) Adopted in 1977, reaffirmed for 5 years in 1994, by
NBS/NIST Plaintext is 64 bits (or blocks of 64 bits), key is 56 bits Plaintext goes through 16 iterations, each producing an
intermediate value that is used in the next iteration. DES is now too easy to crack to be a useful encryption
method
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Triple DEA
Alternative to DES, uses multiple encryption with DES and multiple keys
With three distinct keys, TDEA has an effective key length of 168 bits, so is essentially immune to brute force attacks
Principal drawback of TDEA is that the algorithm is relatively sluggish in software
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Public-Key Encryption
Based on mathematical functions rather than on simple operations on bit patterns
Asymmetric, involving the use of two separate keys Misconceptions about public key encryption
it is more secure from cryptanalysis it is a general-purpose technique that has made
conventional encryption obsolete
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Public-Key Encryption Components
Plaintext Encryption algorithm Public key Private key Ciphertext Decryption algorithm
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Characteristics of Public-Key Infeasible to determine the decryption key given
knowledge of the cryptographic algorithm and the encryption key.
Either of the two related keys can be used for encryption, with the other used for decryption.
Slow, but provides tremendous flexibility to perform a number of security-related functions
Most widely used algorithm is RSA
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Location of Encryption Devices Link encryption
Each vulnerable communications link is equipped on both ends with an encryption device.
All traffic over all communications links is secured. Vulnerable at each switch
End-to-end encryption the encryption process is carried out at the two end systems. Encrypted data are transmitted unaltered across the network to the
destination, which shares a key with the source to decrypt the data Packet headers cannot be secured
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Conventional EncryptionKey Distribution
Both parties must have the secret key Key is changed frequently Requires either manual delivery of keys, or a third-
party encrypted channel Most effective method is a Key Distribution Center
(e.g. Kerberos)
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Public-Key EncryptionKey Distribution
Parties create a pair of keys; public key is broadly distributed, private key is not
To reduce computational overhead, the following process is then used:1. Prepare a message.
2. Encrypt that message using conventional encryption with a one-time conventional session key.
3. Encrypt the session key using public-key encryption with recipient’s public key.
4. Attach the encrypted session key to the message and send it.
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Public Key Certificates
1. A public key is generated by the user and submitted to Agency X for certification.
2. X determines by some procedure, such as a face-to-face meeting, that this is authentically the user’s public key.
3. X appends a timestamp to the public key, generates the hash code of the result, and encrypts that result with X’s private key forming the signature.
4. The signature is attached to the public key.
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Web Vulnerabilities
Unauthorized alteration of data at the Web site Unauthorized access to the underlying operating
system at the Web server Eavesdropping on messages passed between a Web
server and a Web browser Impersonation
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Methods for Improving Web Security
Securing the Web site itself install all operating system security patches install the Web server software with minimal system
privileges use a more secure platform
Securing the Web application
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Web Application Security
Secure HyperText Transfer Protocol (SHTTP) Secure Sockets Layer (SSL) Web server packages should incorporate both of
these protocols
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Virtual Private Networks (VPNs) The use of encryption and authentication in the lower
protocol layers to provide a secure connection through an otherwise insecure network, typically the Internet.
Generally cheaper than real private networks using private lines but rely on having the same encryption and authentication system at both ends.
The encryption may be performed by firewall software or possibly by routers.
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IPSec
Can secure communications across a LAN, WANs, and/or the Internet
Examples of use: Secure branch office connectivity over the Internet Secure remote access over the Internet Establishing extranet and intranet connectivity with
partners Enhancing electronic commerce security
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Benefits of IPSec
When implemented in a firewall or router, provides strong security for all traffic crossing the perimeter
IPSec in a firewall is resistant to bypass Runs below the transport layer (TCP, UDP) and so
is transparent to applications Can be transparent to end users Can provide security for individual users if needed
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IPSec Functions
IPSec provides three main facilities authentication-only function referred to as Authentication
Header (AH) combined authentication/encryption function called
Encapsulating Security Payload (ESP) a key exchange function
For VPNs, both authentication and encryption are generally desired
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IPSec Key Management Manual
System administrator manually configures each system with its own keys and with the keys of other communicating systems
Practical for small, relatively static environments
Automated Enables the on-demand creation of keys for SAs and facilitates the
use of keys in a large distributed system Most flexible but requires more effort to configure and requires more
software