MyTutorialON Cryptography.ppt
- 1. 1/17/2024 Tutorial on Network Security: Sep 2003 1 Network Security Bijendra Jain (bnj@cse.iitd.ernet.in)
- 2. 1/17/2024 Tutorial on Network Security: Sep 2003 2 Lecture 1: Introduction
- 3. 1/17/2024 Tutorial on Network Security: Sep 2003 3 Top-level issues Safety, security and privacy Security policy – threats, both external and internal – economic gains – cost of securing resources – cryptographic methods vs. physical security Information security: – nature of resources (HW, SW, information) – during storage, access and communication – limited to a single computer vs. network security – various layers (physical through application layers)
- 4. 1/17/2024 Tutorial on Network Security: Sep 2003 4 Security threats Intentional vs. accidental Various forms of violations: – Non-destructive – Destructive – Repudiation – Denial of service Threat techniques: – crypt-analysis – snooping – masquerading – replay attacks – virus, worms – etc.
- 5. 1/17/2024 Tutorial on Network Security: Sep 2003 5 Security services Services (or functions) vs. mechanisms Security functions: – confidentiality – authentication – integrity – non-repudiation – access control – availability
- 6. 1/17/2024 Tutorial on Network Security: Sep 2003 6 Security mechanisms Physical controls Audit trails Fraud detection (data mining) Steganography Encryption: – private-key vs. public-key encryption – key generation, exchange, and management – certification Firewalls etc.
- 7. 1/17/2024 Tutorial on Network Security: Sep 2003 7 Lecture 2: Symmetric-key encryption
- 8. 1/17/2024 Tutorial on Network Security: Sep 2003 8 Cryptographic systems Symmetric vs. asymmetric encryption Number of keys used Key lengths Block vs. stream cipher Crypt-analysis (assume algorithm is known) – ciphertext (only) – plaintext + ciphertext – chosen plaintext + ciphertext – chosen ciphertext + plaintext Key size Possible no. of keys Time to crack (1 encryption/microsec) Time to crack (106 encryptions/microsec) 32 109 36 min 2. msec 56 1016 1100 years 10 hrs 128 1038 5 x 1024 years 5 x 1018 years 26 character permutation 1026 6 x 1012 years 6 x 106 years
- 9. 1/17/2024 Tutorial on Network Security: Sep 2003 9 Symmetric cryptographic system Symmetric encryption – Plaintext, X – Ciphertext, Y – Secret keys for encryption, decryption, K Secret key, K Encrypt EK(X) Decrypt DK(X) Crypt- analysis X Y X K K Secure channel Insecure channel
- 10. 1/17/2024 Tutorial on Network Security: Sep 2003 10 Asymmetric cryptographic system Asymmetric encryption – Plaintext, X – Ciphertext, Y – Two keys K1, and K2. One is secret, other is public – One of them (secret or public) is used to encrypt, the other for decryption – Helps with confidentiality, digital signatures Key generation, management Encrypt EK(X) Decrypt DK(X) Crypt- analysis X Y X K1 K2 Insecure channel
- 11. 1/17/2024 Tutorial on Network Security: Sep 2003 11 Symmetric encryption Substitution cipher Transposition cipher DES Triple DES Blowfish, RC5, RC4, etc.
- 12. 1/17/2024 Tutorial on Network Security: Sep 2003 12 Substitution cipher Ceasar cipher – encrypt C (p+k) mod n – decrypt p (C-k) mod n – assumes set of n characters – easily breakable in n-1 steps Substitute using n x n table – encrypt Ci lookup_encrypt(pi) – decrypt pj lookup_decrypt(Cj) – 26! Different keys – may be broken using known “relative frequency” of each character – To counter: use multiple symbols to substitute substitute multiple symbols at a time – e.g. two letter strings at a time
- 13. 1/17/2024 Tutorial on Network Security: Sep 2003 13 Transposition cipher Transposition example: To make it more secure: – transposition it multiple times – combine it with substitution ciphers Key 4 3 1 2 5 6 7 Plaintext a t t a c k p o s t p o n e d u n t I l t w o a m x y z Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ
- 14. 1/17/2024 Tutorial on Network Security: Sep 2003 14 DES Combination of several substitution and transposition ops – Applied to each block of size 64 bits – Key is 56 bits – Uses portions of key at different steps – Uses techniques referred to by “diffusion and confusion” Developed by IBM 1971-73, accepted by NBS (USA) as a standard in 1977 Primarily a block cipher Decrypt DK(X) P1 K C1 Encypt EK(X) C1 K P1
- 15. 1/17/2024 Tutorial on Network Security: Sep 2003 15 DES encryption algorithm Initial permutation Round 1 Round 2 Round 16 32-bit swap Inverse permute K1 K2 K16 Permuted key Permuted key Permuted key Left circular shift Left circular shift Left circular shift Permuted key 64-bit plaintext 64-bit ciphertext 56-bit key
- 16. 1/17/2024 Tutorial on Network Security: Sep 2003 16 Cipher Block Chaining Encrypt EK(X) C1 IV K + P1 Encrypt EK(X) C2 + P2 K Decrypt DK(X) P1 IV K + C1 P2 C2 Decrypt DK(X) K + Primarily a block cipher –May be used in “block chaining mode”
- 17. 1/17/2024 Tutorial on Network Security: Sep 2003 17 Strength of DES Key size of 56 bits appears to be too small – In 1993 Weiner developed HW device for $100K with 5760 search engines to break it in 35 hours – In 1997, 70,000 systems on Internet discovered the key in less than 96 days (part of plaintext is given) – Automating the process is difficult, unless plaintext is known Perhaps breakable by studying and exploiting weakness – Differential cryptanalysis – Linear cryptanalysis Trapdoor – US Govt changed the original design Continues to enjoy wide acceptibility – Particularly with triple-DES (used in PGP)
- 18. 1/17/2024 Tutorial on Network Security: Sep 2003 18 Double-DES Two stages of encryption, using two different keys Decrypt EK2(X) X K2 Encypt EK1(X) C P K1
- 19. 1/17/2024 Tutorial on Network Security: Sep 2003 19 Double-DES “two stages cannot be reduced to one stage”: – for given K1, K2, there is no K s.t. EK2(EK1(P)) = EK(P) Meet-in-the-middle attack – Let C = EK2(EK1(P)), and X = EK1(P) = DK2(C) – Let known P and C – Search for K1 and K2 such that X = EK1(P) = DK2(C) – Complexity is O(256 + 256), not O(2128)
- 20. 1/17/2024 Tutorial on Network Security: Sep 2003 20 Triple-DES Three stages of encryption, using two different keys Decrypt EK2(X) X1 K2 Encypt EK1(X) C P K1 X2 Decrypt EK3(X) K3
- 21. 1/17/2024 Tutorial on Network Security: Sep 2003 21 IDEA International data encryption algorithm (IDEA) developed in 1991, gaining ground block cipher better understood US government has had no role in its design design principle: – block size 64 bits – key length 128 bits – more emphasis on “diffusion” and “confusion” uses three operations: – “exclusive-OR”, “addition”, “multiplication” – some effort to make HW implementation easier
- 22. 1/17/2024 Tutorial on Network Security: Sep 2003 22 RC5 developed by Rivest, in 1994 suitable for HW or SW implementation on microprocessors – simple – different word length – low memory high level of security – simpler determination of strength – variable no. of “rounds”, key length
- 23. 1/17/2024 Tutorial on Network Security: Sep 2003 23 Blowfish Developed in 1993 block cipher up to 448 bit keys no known attacks simple, fast and compact algorithm cycles/"round" No. of rounds cycles/byte encrypted Blowfish 9 16 18 RC5 12 16 23 DES 18 16 45 IDEA 50 8 50 Triple-DES 18 48 108
- 24. 1/17/2024 Tutorial on Network Security: Sep 2003 24 Summary: symmetric key encryption Since the same key is used to encrypt and decrypt, the system is also know as private-key encryption Symmetric key encryption – uses shared secret keys – also known as “private-key” encryption Primarily used for purpose of confidentiality – but may be used to authenticate as well, but may be “repudiated” Key sharing or management is an issue – particularly when the no. of clients sharing the key is “large”
- 25. 1/17/2024 Tutorial on Network Security: Sep 2003 25 Application to confidentiality Private-key encryption may be used to provide confidentiality of messages during transfer over LANs and/or WANs At issue: – what information: User data vs. headers Identity of correspondents vs. node/route identity – in what layer, and between what points Link-layer vs. end-to-end vs. application level Assumption: data over physical network is accessible – Wireless links – Employee of the network service provider – Your own colleagues
- 26. 1/17/2024 Tutorial on Network Security: Sep 2003 26 Link-level vs. end-to-end confidentiality Host A Host B R R R Link-level enrypt/ decrypt End-to-end enrypt/ decrypt
- 27. 1/17/2024 Tutorial on Network Security: Sep 2003 27 Link-level vs. end-to-end confidentiality Link-level encryption End-to-end encryption Security within nodes, hosts Exposed in intermediate nodes Exposed in end hosts Encrypted in intermediate nodes Encrypted/Decrypted by end hosts Role of end devices, intermediate nodes Intermediate nodes require encryption One key for each link Done in hardware Only end hosts need encryption One key per session/connection Perhaps done in software
- 28. 1/17/2024 Tutorial on Network Security: Sep 2003 28 Traffic confidentiality Issues: – Identity of communicating entities – Identity of hosts, routers – Traffic volumes, patterns Link-level encryption offers better confidentiality Padding may be used to “hide” patterns and volumes
- 29. 1/17/2024 Tutorial on Network Security: Sep 2003 29 Key distribution Secret key must be distributed between the communicating entities, say A and B Link level encryption requires L number of keys to be distributed, one for each device at the end of a link Host-to-host encryption requires N*(N-1)/2 keys to be distributed Two techniques: – Physical delivery (works only in a very limited environs) A delivers it to B A trusted third party C delivers the key to A and to B – Electronic delivery using an established secure connection or session A delivers it to B after suitably encrypting it A trusted third party C delivers the key to A and to B using secure channels to A and to B.
- 30. 1/17/2024 Tutorial on Network Security: Sep 2003 30 Key distribution Electronic distribution by B to A, though process initiated by A Above: – N1 and N2 are “nonce”, – MKm is the “master key” used by A and B – KS is the new “session key” – F is a well-known function, such as ADD 1
- 31. 1/17/2024 Tutorial on Network Security: Sep 2003 31 Key distribution Electronic distribution by trusted third party C to A and to B
- 32. 1/17/2024 Tutorial on Network Security: Sep 2003 32 Key distribution Above: – KA and KB are keys used by A and B, respectively, to communicate with C – IDA identifies entity A
- 33. 1/17/2024 Tutorial on Network Security: Sep 2003 33 Key distribution Secure operation of these schemes, against: – Masquerade – replay attacks Other issues: – Hierarchy of keys – Lifetime of a session key – Generation of Nonce or Random numbers
- 34. 1/17/2024 Tutorial on Network Security: Sep 2003 34 Thanks