An advantages and Dis Advantages of Block and Stream Cipher

In this paper we present a comparison between block and stream ciphers through advantages and disadvantages of both . 1-Introduction: In1999 the Bluetooth specification was published in the documents describing the new short –range link solutions the researchers presented a stream cipher called E0 which was designed to provide the wireless connection with a strong protection against eavesdroppingE0. The E0 encryption system is built around a relatively simple key stream generator which is initialized with a key of at most 128 bit. See [ 1],[2],[5] In this research a brief E0 algorithms is presented with two possible improvements are listed. 2-Some Basic Concepts: 1-Depending on encryption and Decryption Key : A-Symmetric Algorithm C=Ek(M) ,M=DK(C) K is secret single key ,one key algorithm Ie the same key for enc and dec Or the key of dec . can be calculated from enc key BAsymmetric Algorithm :(public key)Algorithm. Two key: one for enc &the second for Dec. i.e C=Ek1 (M) ,M=Dk2(c). normally ,k1, is public ,k2 is secret. 1 2Depending on Cryptographic Techniques: I:Block Cipher Let M be a plaintext message . A block cipher breaks M into successive blocks M1,M2,. ,and enciphers for each M1with the same key k,that is EK(M)=EK(M1)EK(M2). Each block is typically several characters long. EX: 1-play fair cipher: it is a block cipher of size 2 letters . 2-Hill cipher:it is a block cipher of size d letters. 3-DES cipher: it is a block cipher of size 64 bits . 4-Electronic code Book code book. (ECB) Corresponding plain text block to a cipher block Manually: choose 1000-10,000 plain text blocks in a two part book: one for encipherment sorted on plain text the second part sorted on cipher text . Electronic: each key has different code book, book size is bigger than of manual. 5-transposition with period letters. 6-simple substitution with 1 letters. 7-Homophonic substitution with 1 letters . 8-Knapsack of length n bits. II:Stream Cipher ددعلا / 2 ةلجم ةيبرتلا ةيلك ةيساسلأا / لباب ةعماج راذآ / 2202 م يونسلا يملعلا رمتؤملا /صاخ ددع لاثلا ث ةيساسلأا ةيبرتلا ةيلكل 6 / 5 / 2229 م 295 It is breaks the message M into successive characters or bits M1,M2,. ,and enciphers each Mi with the it element ki of a key stream K=K1K2 . That is EK(M)=EK1(M1)EK2(M2) EX:. 1-one time bits (oTP)and Running key ciphers are non periodic . 2-vigenere cipher is periodic because plain text char are enciphered one –by-one and adjacent char are encipher with a different part of the key. 3-Auto key cipher: An Auto key cipher is example on self synchronous such that the key is derived from the message it en cipher in vigeneres first cipher the key is formed by appending the plain text M=m1m2 . to a priming key character ki ,the i-th key character (I 1)starts with k1, next key ki=mi-1 or ci-1. 4-cipher feedback (CFB):it is another example on self synchronous such that plain text is encipher in small units (smaller than block size) . A stream cipher is periodic if the key stream repeats after characters for some fixed d otherwise it is no periodic. III: There are two different approaches to stream encryption , a. Synchronous methods . b. Self-Synchronous methods. For more detail see [3],[4],[6] IV:Linear Feedback Shift Registers An n-stage liner Feedback Shift Registers (LFSR) consists of a shift register R=(rn,rn1,.r1) and a „‟tap‟‟ Sequence T=( tn,tn-1.,t1), where each ri and ti is one binary digit . At each step ,bit ri is appended to the key stearm ,bits rn,. r2 are shifted right ,and a new bit derived from T and R is inserted into he left of the register . Letting R‟ =(Rn,rn-1.ri)denote the next stste of r,we see that the computation of R‟IS THUS: r‟=ri-1 i=1,. n-1 r‟n=TR=∑ti ri, mode 2=ti ri An n-stage LFSR can generate pseudo –random bit strings with a period of 2 -1 . To achieve this, the tap sequence before repeating . This will happen if the polynomial T(x)=tnxn+tn-1+n-1=. tix+1 . Formed the form the elements in the tap sequence plus the constant 1,is primitive . A primitive polynomail of degree n is an irreducible polynomial that device x2n-1+1 , but not xd+1for any that device 2n-1Primitive trinomials of the form T(X) =Xn+Xa +1are particularly appealing ,because only two stages of the feedback register need be taped see[2],[5]. The polynomial T(X) =X4+X+1IS primitive ,so the register will cycle through all 15 nonzero bit combination in GF(23)efor repeating . Strarting R in the initial state 0001 . we have 0 0 0 0 1 0 0 0 1 1 0 0 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1 1 0 1 0 1 1 0 1 0