• Identity interception - Somebody might steal your identity and use it as their own.
• Masquerading          - If you send your username and password in cleartext, somebody might grab it from the network.
• Replay attack           - Capturing a request and replaying it.
• Data interception and manipulation
• Repudiation
• Denial of service 

• Link encryption:       Each vulnerable communication link is equipped on both ends with an encryption device.  It has the following                                                          disadvantages:
• It is only effective only if all potential weak links from source to destination are secured. 
• It violates end-to-end argument.
  

• End-to-end encryption:  Data is encrypted only at the source node and decrypted at the destination node.
• We cannot encrypt the header, so data is secure but the traffic pattern is not secure. Here traffic pattern refers to
1. Identities of partners
2. How frequently do two partners communicate.
3. Message patterns. Eg: length, time.

• Network layer encryption:    Examples: IPSEC,VPN,SKIP

• Transport layer security:     Examples: SSL,PCT(Private communication technology).

• Application layer security:   Examples: Privacy enhanced mail, Pretty good privacy, SHTTP

• Monoalphabetic cipher: letters of the plaintext are mapped into unique letters.
• Polyalphabetic cipher:   Letters of the plaintext are mapped into ciphertext space depending on their position on the text.
  
• Stream cipher:          A key stream is generated and is used to encrypt data. Encryption/Decryption is done one symbol at a time.
• Block cipher:            Encryption/Decryption is done on fixed size blocks.

The art of breaking ciphers is known as cryptanalysis.

Methods of cryptanalysis:

•     Ciphertext-Only :   The opponent possesses a string of cipher text y.
•      Known plaintext:    The opponent possesses a string of plaintext x, and the corresponding ciphertext string y.
•      Chosen plaintext:    The opponent can choose a plaintext string x and construct corresponding ciphertext string y : he has                                                       obtained access to encryption machinery.
•      Chosen ciphertext:  The opponent has obtained access to decryption machinery. He can chose ciphertext string y and construct                                             the corresponding plaintext string x.
  

There are two fundamentally different ways ciphers may be secure.

•     Undonditional security:

  No matter how much computing power is available, the cipher cannot be broken.

•     Computational security:

It may one one of the following:

1. Given limited computing resources (e.g., time needed for  calculations is greater than time of universe), the cipher cannot be broken.
2. Provide evidence of computational security by reducing the  security of cryptosystem to some well-studied problem thought to be difficult (e.g, factoring). Such systems are called Provably secure.
   

Secret-key algorithms:

In secret-key algorithm, a k-bit ``secret key'' is shared by two users, who use it to transform plaintext inputs to an encoded cipher.By carefully designing transformation algorithms, each bit of output can be made to depend on every bit of the input. With such an arrangement, a key of 128 bits used for encoding results in a key space of two to the 128th (or about ten to the 38th power).

            Example algorithms: DES,TripleDES,AES.

Public-key algorithms:

• In public-key algorithm, we use a pair of keys : one public the otherone private. Only private key need to be kept secret.
• The pair of keys is produced by a mathematical algorithm.
• One key is used for encryption and the other is used for decryption.
  

These algorithms solve the problem of secure key exchange.

            Example algorithms: RSA,Galois Field system.

•      The main problem with secret-key encryption establishing a secret key between the two parties in communication. Any two users      who wished to communicate could agree on a key in advance, but in practice for many users this would require secure storage and       organization of a awkwardly large database of agreed-on keys. A possible solution is to agree on a key at the time of                           communication. But this has the problem that anybody can eavesdrop on the transmission line before the two parties establish             the key. In the cryptography literature this is referred to as the key distribution problem.

            Approaches to solving key distribution problem:

    One method for solving the key distribution problem is appointing a central key distribution center. Every potential communicating party must register with the server and establish a shared secret key. If party A wishes to establish a secret key with party B this
request is sent to the central server. The server can then inform B that A wishes to communicate, and re-encrypt and re-transmit a
key B has sent. ``The problem with this approach is that key distribution center should be a trusted party agreeable to everyone``.

    A secret key can be agreed upon even without a central server. For example, the Diffie-Hellman key exchange is a protocol for
agreeing on a secret key based on publicly-discussed very large prime numbers. Its security is based on the assumed difficulty of  
taking discrete logarithms modulo very large prime numbers.

•      In general public key systems are slower compared to private key algorithms.
  

•     Public key algorithms have the problem of key validation. If A wishes to send encrypted data to a person , B and has obtained what is known as B's public key. How does A know it really is B's public key? What if a third party, C were to publish a public key in B's name? If C knew B's public key, he can reencrypt A's message to B using the real public key and deliver it to B as if it had come directly from A. B would then decrypt it with his private key and read a message as if it had come directly from A. 

•  Key management and distribution
• Public key is easier to distribute than the secret key.
  
•  Performance
• Secret key scheme is much faster
  
•  Security Services possible
• Digital signing is only possible with public key
  
•  Suitability to intranet and internet
• Public key is better suited to both.
  
•  Encryption algorithms used are different.
•  Key length may vary.