Network Securitymicl-easj.dk/IT Security/Overheads/Conventional... · 2018-04-20 · “rail-fence” cipher disgruntled employee. d r l e o . i g u t e m l y e. s n d p e. drleoigute

Network Security

Conventional Encryption

Selected slides fromCSC290 Hofstra University andVitaly Shmatikov University of Texas

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Caesar Cipher

cipher: PHHW PH DIWHU WKH WRJD SDUWBplain: MEET ME AFTER THE TOGA PARTY

plain: abcdefghijklmnopqrstuvwxyz

key: defghijklmnopqrstuvwxyzabc

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Basic Types of Ciphers

Transposition ciphers – rearrange bits or characters in the dataSubstitution ciphers – replace bits, characters, or blocks of characters with substitutes

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“Rail-Fence” CipherDISGRUNTLED EMPLOYEE

D R L E O

I G U T E M L Y E

S N D P E

DRLEOIGUTE MLYESNDPE

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Encryption MethodsThe essential technology underlying virtually all automated network and computer security applications is cryptographyTwo fundamental approaches are in use:

Conventional Encryption, also known as symmetric encryptionPublic-key Encryption, also known as asymmetric encryption

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Conventional Encryption Model

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Conventional EncryptionThe only form of encryption prior to late 1970sLong historyMost widely used

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Conventional EncryptionFive components to the algorithm

Plaintext: The original message or dataEncryption algorithm: Performs various substitutions and transformations on the plaintextSecret key: Input to the encryption algorithm. Substitutions and transformations performed depend on this keyCiphertext: Scrambled message produced as output. depends on the plaintext and the secret keyDecryption algorithm: Encryption algorithm run in reverse. Uses ciphertext and the secret key to produce the original plaintext

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Conventional EncryptionMore rigorous definitionFive components to the algorithm

A Plaintext message space, MA family of enciphering transformations, EK:M → C, where K∈KA key space, KA ciphertext message space, CA family of deciphering transformations, DK: C → M, where K∈K

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Conventional EncryptionM EK DK MC

EK defined by an encrypting algorithm EDK defined by an decrypting algorithm D

For given K, DK is the inverse of EK, i.e.,DK(EK(M))=Mfor every plain text message M

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Requirements & Weaknesses

RequirementsA strong encryption algorithmSecure process for sender & receiver to obtain secret keys

Methods of AttackCryptanalysisBrute force

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CryptanalysisThe process of attempting to discover the plaintext or key

Alan Turing broke the Enigma Code in WWII

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Cryptanalysis

Security depends on the key......NOT the secrecy of the algorithmLow cost chips are possiblePrincipal security problem is maintaining the secrecy of the key!

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Cryptographic SystemsType of Transformation – substitution and/or transposition; no information must be lost, i.e., reversibleNumber of Keys Used – symmetric, single key, conventional; asymmetric, two-key, public-key encryptionPlaintext Processing – block or streamcipher

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Attacks On Encrypted Msgs

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Computationally Secure

Cost of breaking cipher exceeds value of encrypted informationTime to break cipher exceeds useful lifetime of the information

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Exhaustive Key Search

Brute Force with massively parallel processors

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English Redundancy

Delete vowels and double letters

mst ids cn b xprsd n fwr ltrs, bt th xprnc s mst nplsnt

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Simple Cryptanalysis

UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZVUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSXEPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ

CIPHERTEXT:

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Letter Frequency In the English Language

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Simple Cryptanalysis

IT WAS DISCLOSED YESTERDAY THAT SEVERALINFORMAL BUT DIRECT CONTACTS HAVE BEEN MADEWITH POLITICAL REPRESENTATIVES OF THE VIET CONG IN MOSCOW

PLAINTEXT:

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20th Century Encryption

20’s & 30’s bootleggers made heavy use of cryptographyFBI create an office for code-breakingJapanese Purple MachineGerman Enigma MachineNavajo Code Talkers - Windtalkers

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Hedy Lamarr1941, Lamarr and composer George Antheil received a patent for their invention of a classified communication system that was especially useful for submarinesIt was based on radio frequencies changed at irregular periods that were synchronized between the transmitter and receiverSpread Spectrum – wireless devices

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Feistel Cipher StructureHorst Feistel of IBM, 1973Input is plaintext block of length 2w bits (usually 64) and a key KBlock is divided into two halves, L0 and R0Each round i has inputs Li-1 and Ri-1, derived from the previous round, along with subkey KiSubstitution is performed on the left half of the dataRound function F applied to right half and then XOR’d with left

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Feistel Cipher Structure

Things to consider:-Block size (64)-Key Size (128)-# of rounds (16)-SubKey Generation-Round function

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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 bitsPlaintext goes through 16 iterations, each producing an intermediate value that is used in the next iterationDES is now too easy to crack to be a useful encryption method

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Strength of DES

Concerns about the algorithm itself

Concerns about 56-bit key – this is the biggest worry

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Strength of DESDES is the most studied encryption algorithm in existenceNo one has succeeded in discovering a fatal weakness1998, DES Cracker from Electronic Frontier Foundation, built for $250,000Solution: Use a bigger key

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Triple DESC = EK3

[DK2[EK1

[P ]]]

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Triple DESAlternative to DES, uses multiple encryption with DES and multiple keysWith three distinct keys, 3DES has an effective key length of 168 bits, so it is essentially immune to brute force attacksBackward compatible with DESPrincipal drawback of DES is that the algorithm is relatively sluggish in software

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Advanced Encryption Standard

NIST call for proposals in 1997Nov, 2001 – Rijndael [rain´ dow]Symmetric block cipher (128 bits) and key lengths 128, 192, 256Two Flemish cryptographers: Joan Daeman and Vincent Rijmen

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Overview of AES

4Transformations:Substitute BytesShift RowsMix ColumnsAdd Round Key

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AES URLS

http://csrc.nist.gov/CryptoToolkit/aes/rijndael/ -NIST AES

http://www.esat.kuleuven.ac.be/~rijmen/rijndael/ - Rijndael Home Page

http://www.esat.kuleuven.ac.be/~rijmen/rijndael/Rijndael_Anim.zip - Great Animation

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IDEAInternational Data Encryption Algorithm

1991 by Swiss Federal Institute of TechnologyUses 128-bit keyComplex functions replace S-boxesHighly resistant to cryptanalysisUsed in PGP

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Blowfish

1993 by Bruce SchneierEasy to implement; high execution speedVariable key length up to 448 bitsUsed in a number of commercial applications

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RC5

1994 by Ron Rivest, one of the inventors of RSA algorithmDefined in RFC2040Suitable for hardware and softwareSimple, fast, variable length key, low memory requirementsHigh security

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CAST-128

1997, Entrust TechnologiesRFC 2144Extensively reviewedVariable key length, 40-128 bitsUsed in PGP

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Conventional Encryption Algorithms

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Encrypting a Large MessageSo, we’ve got a good block cipher, but our plaintext is larger than 128-bit block sizeElectronic Code Book (ECB) mode

Split plaintext into blocks, encrypt each one separately using the block cipher

Cipher Block Chaining (CBC) modeSplit plaintext into blocks, XOR each block with the result of encrypting previous blocks

Also various counter modes, feedback modes, etc.

ECB Mode

Identical blocks of plaintext produce identical blocks of ciphertextNo integrity checks: can mix and match blocks

plaintext

ciphertext

blockcipher

blockcipher

blockcipher

blockcipher

blockcipher

CBC Mode: Encryption

Identical blocks of plaintext encrypted differentlyLast cipherblock depends on entire plaintext

plaintext

ciphertext

blockcipher

blockcipher

blockcipher

blockcipher

⊕Initializationvector(random) ⊕ ⊕ ⊕

Sent with ciphertext(preferably encrypted)

CBC Mode: Decryptionplaintext

ciphertext

decrypt decrypt decrypt decrypt

⊕Initializationvector ⊕ ⊕ ⊕

Cipher Block Chaining Mode

Input to algorithm is the XOR of current plaintext block and preceding ciphertext blockRepeating patterns are not exposed

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ECB vs. CBC (due to Bart Preneel)

AES in ECB mode AES in CBC mode

Similar plaintextblocks producesimilar ciphertextblocks (not good!)

Location of Encryption Devices

Link EncryptionEach vulnerable communications link is equipped on both ends with an encryption deviceAll traffic over all communications links is securedVulnerable at each switch

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Location of Encryption Devices

End-to-end EncryptionThe encryption process is carried out at the two end systemsEncrypted data are transmitted unaltered across the network to the destination, which shares a key with the source to decrypt the dataPacket headers cannot be secured

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Location of Encryption Devices

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Key DistributionBoth parties must have the secret keyKey is changed frequentlyRequires either manual delivery of keys, or a third-party encrypted channelMost effective method is a Key Distribution Center (e.g. Kerberos)

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Key Distribution

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