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Cryptography

Cryptography. Cryptography is the technique of secret writing. A cipher is a method of secret writing. The purpose is to convert an intelligible message, referred to as plaintext, into apparently random nonsense text, referred to as ciphertext.

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Cryptography

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  1. Cryptography • Cryptography is the technique of secret writing. • A cipher is a method of secret writing. • The purpose is to convert an intelligible message, referred to as plaintext, into apparently random nonsense text, referred to as ciphertext. • The encryption process consists of an algorithm and a key. • The algorithm will produce a different output depending on the specific key being used at the time.

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  3. Conventional Cryptography:Basic Definitions • Plaintext: This is the original message or data that is fed into the algorithm as input • Encryption Algorithm: The encryption algorithm performs various substitutions and transformations on the plaintext. • Secret Key: The secret key is also an input to the algorithm. The exact substitutions and transformations performed by the algorithm depend on the key. • Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and on the secret key. For a given message, two different keys will produce two different ciphertexts.

  4. Basic Definitions Decryption algorithm: This is essentially the encryption algorithm run in reverse. It takes the ciphertext and the secret key and produces the origin plaintext. Ciphertext = cryptogram Cleartext = plaintext = message Ciphering= encryption Deciphering = decryption

  5. There are two requirements for secure use of conventional encryption: • The opponent should be unable to decrypt cryptogram or discover the key even if he or she is in possession of a number of cryptograms together with the plaintext that produced each cryptogram. • Sender a receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure. • It is important to note that the security of conventional encryption depends on the secrecy of the key, not the secrecy of the algorithm • The algorithm is supposed to be public.

  6. Classification of Cryptographic systems By the numbers of keys used • If both sender and receiver use the same key, the system is referred to as symmetric (or single key, secret-key, conventional) cryptosystem • If the sender and receiver uses a different key, the system is referred to as symmetric or two-key or public-key cryptosystem.

  7. By the way in which the plaintext is processed A block cipher processes the input one block of elements at a time, producing an output block for each input block.

  8. By the way in which the plaintext is processed A stream cipher processes the input elements continuously, producing output one element at a time, as it goes along.

  9. Cryptanalysis • The process of attempting to discover the plaintext or key is known as cryptanalysis. • The strategy used by the cryptanalyst depends on the nature of the encryption scheme and the information available to the cryptanalyst. • A cipher is breakable if is possible to determine systematically the key (or the plaintext) from pairs plaintext, ciphertext given.

  10. An encryption scheme is computationally secure if the ciphertext generated by the scheme meets one or both of the following criteria: • The cost of breaking the cipher exceeds the value of the encrypted information. • The time required to break the cipher exceeds the useful lifetime of the information. • It is very difficult to estimate the amount of effort required to cryptanalize ciphertext successfully. However, assuming there are no inherent mathematical weaknesses in the algorithm, then a brute-force approach is indicated, and here we can make some reasonable estimates about costs and time

  11. A brute-force approach involves trying every possible key until an intelligible translation of the ciphertext into plaintext is obtained.

  12. Assuming 1E12 Decryptions / sec 12

  13. Caesar Cipher (A historical note) • A substitution cipher is one in which the letters of plaintext are replaced by other letters or by numbers or symbols. • The Caesar cipher involves replacing each letter of the alphabet with the letter standing three places further down the alphabet. For example: • Rule (algorithm) a b c d e f g h i j k l m n o p q r s t u v w x y z d e f g h i j k l m n o p q r s t u v w x y z a b c Message: meet me after the toga party Ciphertext: phhw ph diwhu wkh wrjd sduwb

  14. Caesar Cipher (A historical note) • If we assign a numerical equivalent to each letter (a=0, b=1,.., z=25), then the algorithm can be expressed as follows: C= E(p)= (p+3) modulo 26, Where p is a letter (i.e. a number between 0 and 25) and C=E(P) is the corresponding ciphertext. The decryption algorithm is as follows: p=D(C)=(C-3) modulo 26. The “key space” has 25 elements, i.e. There are 25 possible keys.

  15. Example 1100 0111=1011 XOR Operation: Permutations: Example P(0101)=1010

  16. Left Circular rotation (or shift) of a Block of Bits : Input: bit 1 bit 2 bit3 bit 4 Output: bit2 bit 3 bit 4 bit1 Input: bit 1 bit 2 bit3 bit 4 Output: bit3 bit 4 bit 1 bit2 Basic Operation (i-th round) Li=Ri-1 Ri=Li-1F(Ri-1, Ki)

  17. Feistel Cipher Structure • Virtually all conventional block ciphers have a structure first described by H. Feistel of IBM in 1973. • Parameters • Block size: larger block sizes mean greater security (all other things being equal) but reduce encryption/decryption speed. A block size is a reasonable tradeoff and is nearly universal in block cipher design. • Key Size: Larger key size means greater security but may decrease encryption/decryption speed. The most common key length in modern algorithms is 128 bits. • Number of rounds: The essence of the Feistel cipher is that a single round offers inadequate security but that multiple rounds offer increasing security. A typical size is 16 rounds 17

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  19. Feistel Cipher Structure • Subkey generation algorithm: Greater complexity in this algorithm lead to greater difficulty of cryptanalysis. • Round Function: Again, greater complexity generally means greater resistance to cryptanalysis. • Decryption Process • The decryption process is as follows: use the ciphertext as input to the algorithm, but use the subkeys Ki in reverse order. That is, use Kn in the first round, Kn-1 in the second, and so on until K1 is used in the last round. 19

  20. Data Encryption Standard (DES) • The most widely used encryption scheme is defined in the data encryption standard (DES) adopted in 1977 by National Institute of Standards and Technology (NIST), as a Federal Information Processing Standard 46 (FIPS PUB 46). In 1994, NIST reaffirmed DES for federal use for another five years in FIPS PUB46-2. • Block cipher (64 bits) • Key (64 bits, but 8 bits are used as parity bits) • DES has a Feistel cipher structure with 16 rounds 20

  21. Data Encryption Standard (DES) • The process of decryption with DES is essentially the same as the encryption process. The rule is as follows: use the ciphertext as input to the DES algorithm, but use the keys in reverse order. That is, use K16 in the first iteration, K15 in the second iteration, and so on until K1 is used o0n the sixteenth and last iteration.

  22. The strength of DES Concerns about the strength of DES fall in two categories: • Concerns about the design of the algorithm: Despite numerous approaches, no one has so far succeeded in discovering a fatal weakness in DES. • Concerns about the use of a 56-bit key: a 56-bit key is too small!

  23. TRIPLE DEA(Triple Data Encryption Algorithm) • TDEA uses three executions of the DES algorithm. • C=EK3 [DK2 [EK1[P]]] C= ciphertext P=plaintext EK[X]= encryption of X using key K DK[X]=decryption of Y using key K • Decryption is simply the same operation with the keys reversed P=DK1 [EK2 [DK3[C]]] • C=EK1 [DK1 [EK1[P]]]=?

  24. With three different keys, TDEA has an effective key length of 168 bits. Other Symmetric Block Ciphers • IDEA • Blowfish • RC5 • CAST-128

  25. Cipher Block Modes of Operation • A symmetric block cipher processes one bit block of data at a time. Operation Modes • Electronic Code Book (ECB): In this case each block plaintext is encrypted using the same key. • Typical application: secure transmission of single values (e.g. an encryption key)

  26. With ECB, if the same 64-bit block of plaintext appears more than once in the message, it always produces the same ciphertext. Because of this, for lengthy messages, the ECB mode may be no secure.

  27. Cipher Block Chaining Mode (CBC) • Typical application: General-purpose block-oriented transmission Cipher Feedback Mode (CBC) • The DES scheme is essentially a block cipher technique that uses 64-bit blocks. It is possible to convert DES into a stream cipher, using the cipher feedback mode (CFB). • Typical application: General-purpose block-oriented transmission

  28. Location Of Encryption Devices • The most powerful, and most common, approach to countering the threats to network security is encryption. • In order to use encryption, it is necessary to decide what to encrypt and where the encryption process will be located. • There are two fundamental alternatives: • Link encryption • End-to end encryption

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  30. Link encryption In this case there is a encryption device in each side of each vulnerable link. • All traffic over all communications links is secured. • One disadvantage of this approach is that the message must be decrypted each time it enters a packet switch. This is necessary because the switch must read the address in the packet header to route the packet. Thus the message is vulnerable in each switch. • End-End encryption The encryption process is carried out at the two end systems.

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