CryptographySlide 2Conventional Cryptography :Basic DefinitionsBasic DefinitionsSlide 5Classification of Cryptographic systemsBy the way in which the plaintext is processedSlide 8Slide 9Slide 10Slide 11Slide 12Caesar Cipher (A historical note)Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Data Encryption Standard (DES)The strength of DESTRIPLE DEA (Triple Data Encryption Algorithm)Slide 26Slide 27Cipher Block Modes of OperationSlide 29Slide 30Slide 31Slide 32Location Of Encryption DevicesSlide 34Slide 35Slide 36Cryptography•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.2Conventional 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.Basic DefinitionsDecryption algorithm: This is essentially the encryption algorithm run in reverse. It takes the ciphertext and the secret key and produces the origin plaintext.Ciphertext = cryptogramCleartext = plaintext = messageCiphering= encryptionDeciphering = decryption•There are two requirements for secure use of conventional encryption:1. 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.2. 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.Classification of Cryptographic systemsBy the numbers of keys used1. If both sender and receiver use the same key, the system is referred to as symmetric (or single key, secret-key, conventional) cryptosystem1. If the sender and receiver uses a different key, the system is referred to as symmetric or two-key or public-key cryptosystem.By the way in which the plaintext is processedA block cipher processes the input one block of elements at a time, producing an output block for each input block.By the way in which the plaintext is processedA stream cipher processes the input elements continuously, producing output one element at a time, as it goes along.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.•An encryption scheme is computationally secure if the ciphertext generated by the scheme meets one or both of the following criteria:1. The cost of breaking the cipher exceeds the value of the encrypted information.2. 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•A brute-force approach involves trying every possible key until an intelligible translation of the ciphertext into plaintext is obtained.12Assuming 1E12Decryptions / secCaesar 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 partyCiphertext: phhw ph diwhu wkh wrjd sduwbCaesar 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.XOR Operation: Permutations:0 10 0 11 1 0Example1100 0111=1011Example P(0101)=1010Left Circular rotation (or shift) of a Block of Bits :Input: bit 1 bit 2 bit3 bit 4Output: bit2 bit 3 bit 4 bit1Input: bit 1 bit 2 bit3 bit 4Output: bit3 bit 4 bit 1 bit2Basic Operation (i-th round)Li=Ri-1Ri=Li-1 F(Ri-1, Ki)17Virtually 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 roundsFeistel Cipher Structure18(+)(+)(+)•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