Transcript
William Stallings Data and Computer Communications Chapter 18 Network Security
Security Requirements Confidentiality Integrity Availability
Passive Attacks Eavesdropping on transmissions To obtain information Release of message contents Outsider learns content of transmission
Traffic analysis By monitoring frequency and length of messages, even encrypted, nature of communication may be guessed
Difficult to detect Can be prevented
Active Attacks Masquerade Pretending to be a different entity
Replay Modification of messages Denial of service Easy to detect Detection may lead to deterrent
Hard to prevent
Security Threats
Conventional Encryption
Ingredients Plain text Encryption algorithm Secret key Cipher text Decryption algorithm
Requirements for Security Strong encryption algorithm Even if known, should not be able to decrypt or work out key Even if a number of cipher texts are available together with plain texts of them
Sender and receiver must obtain secret key securely Once key is known, all communication using this key is readable
Attacking Encryption Crypt analysis Relay on nature of algorithm plus some knowledge of general characteristics of plain text Attempt to deduce plain text or key
Brute force Try every possible key until plain text is achieved
Algorithms Block cipher Process plain text in fixed block sizes producing block of cipher text of equal size Data encryption standard (DES) Triple DES (TDES)
Data Encryption Standard US standard 64 bit plain text blocks 56 bit key
DES Encryption Algorithm
DES Single Iteration
Strength of DES Declared insecure in 1998 Electronic Frontier Foundation DES Cracker machine DES now worthless Alternatives include TDEA
Triple DEA ANSI X9.17 (1985) Incorporated in DEA standard 1999 Uses 3 keys and 3 executions of DEA algorithm Effective key length 168 bit
Location of Encryption Devices
Link Encryption Each communication link equipped at both ends All traffic secure High level of security Requires lots of encryption devices Message must be decrypted at each switch to read address (virtual circuit number) Security vulnerable at switches Particularly on public switched network
End to End Encryption Encryption done at ends of system Data in encrypted form crosses network unaltered Destination shares key with source to decrypt Host can only encrypt user data Otherwise switching nodes could not read header or route packet
Traffic pattern not secure Use both link and end to end
Key Distribution Key selected by A and delivered to B Third party selects key and delivers to A and B Use old key to encrypt and transmit new key from A to B Use old key to transmit new key from third party to A and B
Automatic Key Distribution (diag)
Automatic Key Distribution Session Key Used for duration of one logical connection Destroyed at end of session Used for user data
Permanent key Used for distribution of keys
Key distribution center Determines which systems may communicate Provides one session key for that connection
Front end processor Performs end to end encryption Obtains keys for host
Traffic Padding Produce cipher text continuously If no plain text to encode, send random data Make traffic analysis impossible
Message Authentication Protection against active attacks Falsification of data Eavesdropping
Message is authentic if it is genuine and comes from the alleged source Authentication allows receiver to verify that message is authentic Message has not altered Message is from authentic source Message timeline
Authentication Using Encryption Assumes sender and receiver are only entities that know key Message includes: error detection code sequence number time stamp
Authentication Without Encryption Authentication tag generated and appended to each message Message not encrypted Useful for: Messages broadcast to multiple destinations Have one destination responsible for authentication
One side heavily loaded Encryption adds to workload Can authenticate random messages
Programs authenticated without encryption can be executed without decoding
Message Authentication Code Generate authentication code based on shared key and message Common key shared between A and B If only sender and receiver know key and code matches: Receiver assured message has not altered Receiver assured message is from alleged sender If message has sequence number, receiver assured of proper sequence
Message Authentication Using Message Authentication Code
One Way Hash Function Accepts variable size message and produces fixed size tag (message digest) Advantages of authentication without encryption Encryption is slow Encryption hardware expensive Encryption hardware optimized to large data Algorithms covered by patents Algorithms subject to export controls (from USA)
Using One Way Hash
Secure Hash Functions Hash function must have following properties: Can be applied to any size data block Produce fixed length output Easy to compute Not feasible to reverse Not feasible to find two message that give the same hash
SHA-1 Secure Hash Algorithm 1 Input message less than 264 bits Processed in 512 bit blocks
Output 160 bit digest
Public Key Encryption Based on mathematical algorithms Asymmetric Use two separate keys
Ingredients Plain text Encryption algorithm Public and private key Cipher text Decryption algorithm
Public Key Encryption (diag)
Public Key Encryption Operation One key made public Used for encryption
Other kept private Used for decryption
Infeasible to determine decryption key given encryption key and algorithm Either key can be used for encryption, the other for decryption
Steps User generates pair of keys User places one key in public domain To send a message to user, encrypt using public key User decrypts using private key
Digital Signature Sender encrypts message with their private key Receiver can decrypt using sneders public key This authenticates sender, who is only person who has the matching key Does not give privacy of data Decrypt key is public
RSA Algorithm
RSA Example
IPv4 and IPv6 Security IPSec Secure branch office connectivity over Internet Secure remote access over Internet Extranet and intranet connectivity Enhanced electronic commerce security
IPSec Scope Authentication header Encapsulated security payload Key exchange RFC 2401,2402,2406,2408
Security Association One way relationship between sender and receiver For two way, two associations are required Three SA identification parameters Security parameter index IP destination address Security protocol identifier
SA Parameters Sequence number counter Sequence counter overflow Anti-reply windows AH information ESP information Lifetime of this association IPSec protocol mode Tunnel, transport or wildcard
Path MTU
Transport and Tunnel Modes Transport mode Protection for upper layer protocols Extends to payload of IP packet End to end between hosts
Tunnel mode Protection for IP packet Entire packet treated as payload for outer IP “packet” No routers examine inner packet May have different source and destination address May be implemented at firewall
Required Reading Stallings chapter 18
