Impossibility to refuse authorship .Sender needn’t have possibility no refuse his authorship.

3.Attack. . Type of attacks. Threats to computer systems.Threat Models. Threat TreesAn attempted cryptanalysis is called ATTACK

Given: C1 = Ek(P1), C2 = Ek(P2),...Ci = Ek(Pi)

Deduce: Either P1, P2,...Pi; k; or an algorithm to infe from = Ek( )

1. Known-plaintext attack.

His job is to deduce the key (or keys)

used to encrypt the messages or an algorithm to decrypt any new messages encrypted with the

same key (or keys).

Given: P1, C1 = Ek(P1), P2, C2 = Ek(P2),...Pi, Ci = Ek(Pi)

Deduce: Either k, or an algorithm to infer from Ci+1 = Ek( )

2. Chosen-plaintext attack. The cryptanalyst not only has access to the ciphertext and

associated plaintext for several messages, but he also chooses the plaintext that gets

encrypted.

3. Adaptive-chosen-plaintext attack. This is a special case of a chosen-plaintext attack. Not

only can the cryptanalyst choose the plaintext that is encrypted, but he can also modify his

choice based on the results of previous encryption.

4.Chosen-ciphertext attack. The cryptanalyst can choose different ciphertexts to be decrypted and has access to the decrypted plaintext. For example, the cryptanalyst has access

to a tamperproof box that does automatic decryption. His job is to deduce the key.

Given: C1, P1 = Dk(C1), C2, P2 = Dk(C2),...Ci, Pi = Dk(Ci)

Deduce: k

5. Chosen-key attack. This attack doesn’t mean that the cryptanalyst can choose the key; it

means that he has some knowledge about the relationship between different keys.

Threat model is a description of a set of security aspects; that is, when looking at a piece of software (or any computer system), one can define a threat model by defining a set of possible attacks to consider. It is often useful to define many separate threat models for one computer system. Each model defines a narrow set of possible attacks to focus on. A threat model can help to assess the probability, the potential harm,

Active threats

    Input manipulation, Program alteration,  Direct file alteration

Passive threats

    System faults (power, communications, hardware), Disasters (water, fire, wind, earthquake)

4.Basic methods of encryption.Classification of encryption methods.Examples of simple methods.

In cryptography, a substitution cipher is a method of encryption by which units of plaintext are replaced with ciphertext, according to a regular system; the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing an inverse substitution.

For example Caesar: A Caesar cipher with key 5:

plaintext alphabet 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

ciphertext alphabet F G H I J K L M N O P Q R S T U V W X Y Z A B C D E

In classical cryptography, a permutation cipher is a transposition cipher in which the key is a permutation.

To apply a cipher, a random permutation of size e is generated (the larger the value of e the more secure the cipher). The plaintext is then broken into segments of size e and the letters within that segment are permuted according to this key.

EXAMPLE: I LIVE IN SEMEY

Lets rewrite it into table, consisting of 3 ROW and 4 COLUMNS

ILI

VEI

NSE

MEY

We get encrypted text: ivnm lese iiey

In cryptography, a stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream (keystream). In a stream cipher each plaintext digit is encrypted one at a time with the corresponding digit of the keystream, to give a digit of the cyphertext stream. An alternative name is a state cipher, as the encryption of each digit is dependent on the current state. In practice, a digit is typically a bit and the combining operation an exclusive-or (xor).

An example of a self-synchronising stream cipher is a block cipher in cipher feedback (CFB) mode.

5.Caesar’s cipher generalized. Other monoalphabeticciphers.The problems.

CAESAR CIPHER

• This is the earliest known example of a substitution cipher.

 Each character of a message is replaced by a character three position down in the alphabet.

plaintext: are you ready

ciphertext: DUH BRX UHDGB

 

• If we represent each letter of the alphabet by an integer that corresponds to its position in the alphabet, the formula for replacing each character ’p’ of the plaintext with a character ’C’ of the ciphertext can be expressed as

C = E( 3, p ) = (p + 3) mod 26

• A more general version of this cipher that allows for any degree of shift would be expressed by

C = E( k, p ) = (p + k) mod 26

 

• The formula for decryption would be

p = D( k, C ) = (C - k) mod 26

• In these formulas, ’k’ would be the secret key. The symbols ’E’ and ’D’ represent encryption and decryption.

Monoalphabetic Ciphers

Here is an example of a plaintext – ciphertext alphabet pair for each type ofcipher we have seen thus far.

• 1. A Caesar cipher with key 5:

plaintext alphabet 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

ciphertext alphabet F G H I J K L M N O P Q R S T U V W X Y Z A B C D E

• 2. A Decimation Cipher modulo 26 with key 21:

plaintext alphabet 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

ciphertext alphabet U P K F A V Q L G B W R M H C X S N I D Y T O J E Z

• 3. A Linear Cipher modulo 26 with key 7m + 9:

plaintext alphabet 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

ciphertext alphabet P W D K R Y F M T A H O V C J Q X E L S Z G N U B I

These are all monoalphabetic ciphers, ciphers in which the same plaintextletters are always replaced by the same ciphertext letters. Mono, meaning one, indicates that each letter has a single substitute

In cryptanalysis, frequency analysis is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking classical ciphers.Frequency analysis is based on the fact that, in any given stretch of written language, certain letters and combinations of letters occur with varying frequencies.. For instance, given a section of English language, E, T, A and O are the most common, while Z, Q and Xare rare.

6.One-Time Pad cipher.

One-Time Pads

Classically, a one-time pad is nothing more than a large nonrepeating set of truly random key letters, written on sheets of paper, and glued

together in a pad. In its original form, it was a one-time tape for teletypewriters. The sender uses each key letter on the pad to encrypt exactly one plaintext character. Encryption is the addition

modulo 26 of the plaintext character and the one-time pad key character.

Each key letter is used exactly once, for only one message. The sender encrypts the message and then destroys the used pages of the pad or used section of the tape. The receiver has an identical pad

and uses each key on the pad, in turn, to decrypt each letter of the ciphertext. The receiver destroys the same pad pages or tape section after decrypting the message. New message—new key letters. For example, if the message is:

ONETIMEPAD

and the key sequence from the pad is

TBFRGFARFM

then the ciphertext is

IPKLPSFHGQ

because

O + T mod 26 = I

N + B mod 26 = P

E + F mod 26 = K

etc.

Assuming an eavesdropper can’t get access to the one-time pad used to encrypt the message, this scheme is perfectly secure. A given ciphertext message is equally likely to correspond to any

possible plaintext message of equal size.

Since every key sequence is equally likely (remember, the key letters are generated randomly), an adversary has no information with which to cryptanalyze the ciphertext. The key sequence could just

as likely be:

POYYAEAAZX

which would decrypt to:

SALMONEGGS

or

BXFGBMTMXM

which would decrypt to:

GREENFLUID