Network Security

Security Threats

Intercept Interrupt Modification Fabrication

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

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

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.

Conventional Encryption Operation

Conventional Encryption Requirements & Weaknesses

Requirements A strong encryption algorithm Secure process for sender & receiver to obtain secret keys

Methods of Attack Cryptanalysis Brute force

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

Public-Key Encryption Components

Plaintext Encryption algorithm Public key Private key Ciphertext Decryption algorithm

Public-Key Encryption Operation

Public-Key Signature Operation

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

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

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)

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.

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.