McGraw-Hill The McGraw-Hill Companies, Inc., 2000

Criptografa

Profesora Maria Elena Villapol Maria.villapol@ciens.ucv.ve

INTRODUCCION

Criptografia => escritura secreta. Ciencia y arte de transformar mensajes para

hacerlos seguros e inmunes a ataques.

Componentes criptogrficos

Terminologa

Texto plano Texto cifrado Algoritmo de cifrado Algoritmo de descifrado Clave

Caracteres

Alice => computador/proceso que enva data Bob => computador/proceso que recibe data Eve => computador/proceso que intercepta o cambia data

30.4

Categoras de Criptografa

Criptografa de Clave Simtrica

En criptografa de claves simtricas, la misma clave es usada por el emisor

(para el cifrado) y el receptor (para el descifrado).

La clave es compartida.

Criptografa por clave asimtrica

Claves usadas en criptografa

Comparacin de los esquemas de criptografa

Criptografa de clave simtrica: Cifrado tradicional

Un sistema de cifrado de sustitucin reemplaza un smbolo por otro.

Criptografa de clave simtrica: Cifrado tradicional

Cifrado mono alfabtico.

Cifrado poli alfabtico.

30.13

El algoritmo de cifrado desplaza key caracteres hacia abajo.

El algoritmo de descifrado desplaza key caracteres hacia arriba

Criptografa de clave simtrica: Cifrado tradicional

Cifrado por Desplazamiento: Cifrado Mono alfabtico

Use the shift cipher with key = 15 to encrypt the message HELLO.

Solution We encrypt one character at a time. Each character is shifted 15 characters down. Letter H is encrypted to W. Letter E is encrypted to T. The first L is encrypted to A. The second L is also encrypted to A. And O is encrypted to D. The cipher text is WTAAD.

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.

Use the shift cipher with key = 15 to decrypt the message WTAAD.

Solution We decrypt one character at a time. Each character is shifted 15 characters up. Letter W is decrypted to H. Letter T is decrypted to E. The first A is decrypted to L. The second A is decrypted to L. And, finally, D is decrypted to O. The plaintext is HELLO.

Un cifrado por transposicin reordena (permuta) smbolos en un bloque de

smbolos

Criptografa de clave simtrica: Cifrado tradicional - Cifrado por transposicin

Encrypt the message HELLO MY DEAR, using the key shown in Figure 30.8.

Solution We first remove the spaces in the message. We then divide the text into blocks of four characters. We add a bogus character Z at the end of the third block. The result is HELL OMYD EARZ. We create a three-block ciphertext ELHLMDOYAZER.

Using Example 30.5, decrypt the message ELHLMDOYAZER.

Solution The result is HELL OMYD EARZ. After removing the bogus character and combining the characters, we get the original message HELLO MY DEAR.

Cifrados Modernos Sencillos: Cifrado XOR

Bloque de data a ser cifrado Clave

Cifrados Modernos Sencillos: Cifrado por Rotacin

Key = # rotaciones

Cifrados Modernos Sencillos: Cifrado por Sustitucin: S-box

Cifrados Modernos Sencillos: Cifrado por Permutacin:

P-box

Cifrados Modernos basados en Vueltas

Data Encryption Standard

(DES)

Data Encryption Standard (DES)

Data Encryption Standard (DES): Funcin DES

30.27

Data Encryption Standard (DES): Triple DES

Cifrados Modernos: Advanced Encryption Standard (AES)

30.29

Advanced Encryption Standard (AES)

AES: Estructura de cada Round (Vuelta)

Cifrados Modernos: Modos de Operacin

Cifrados Modernos: Modos de Operacin Electronic Code Book

(ECB)

Electronic Code Book (ECB)

Bloques en texto plano iguales generan iguales textos cifrados.

Reorden de texto plano => reorden texto cifrado.

Los bloques son independientes. Un error en un bloque no es propagado en

otros.

Cifrados Modernos: Modos de Operacin Cipher Block Chainning

(CBC)

Cipher Block Chainning (CBC)

Bloques en texto plano iguales NO generan iguales textos cifrados.

Reorden de texto plano => reorden texto cifrado.

Cada bloque dependiente de otro. Un error en un bloque es propagado en otros.

Cifrados Modernos: Modos de Operacin Cipher feedback(CFB)

Cipher Feedback(CFC)

Si se cambia el IV de un cifrado a otro usando el mismo texto plano => el texto cifrado es diferente.

texto cifrado depende de texto plano y bloque cifrado precedente.

Un error en un bloque es propagado en el prximo.

Cifrados Modernos: Modos de Operacin Output Feedback(OFB)

Output Feedback(OFB)

Si se cambia el IV de un cifrado a otro usando el mismo texto plano => el texto cifrado es diferente.

texto cifrado depende de texto plano. Un error en un bloque NO es propagado en

prximos textos cifrados.

ASYMMETRIC-KEY CRYPTOGRAPHY: Rivest, Shamir and Adleman (RSA)

ASYMMETRIC-KEY CRYPTOGRAPHY: Rivest, Shamir and Adleman (RSA)

30.42

In RSA, e and n are announced to the public; d and are kept secret.

30.44

Bob chooses 7 and 11 as p and q and calculates n = 7 11 = 77. The value of = (7 1) (11 1) or 60. Now he chooses two keys, e and d. If he chooses e to be 13, then d is 37. Now imagine Alice sends the plaintext 5 to Bob. She uses the public key 13 to encrypt 5.

30.45

Bob receives the ciphertext 26 and uses the private key 37 to decipher the ciphertext:

The plaintext 5 sent by Alice is received as plaintext 5 by Bob.

30.46

Jennifer creates a pair of keys for herself. She chooses p = 397 and q = 401. She calculates n = 159,197 and = 396 400 = 158,400. She then chooses e = 343 and d = 12,007. Show how Ted can send a message to Jennifer if he knows e and n.

Solution Suppose Ted wants to send the message NO to Jennifer. He changes each character to a number (from 00 to 25) with each character coded as two digits. He then concatenates the two coded characters and gets a four-digit number. The plaintext is 1314. Ted then uses e and n to encrypt the message. The ciphertext is 1314343 = 33,677 mod 159,197. Jennifer receives the message 33,677 and uses the decryption key d to decipher it as 33,67712,007 = 1314 mod 159,197. Jennifer then decodes 1314 as the message NO. Figure 30.25 shows the process.

ASYMMETRIC-KEY CRYPTOGRAPHY: Rivest, Shamir and Adleman (RSA)

Let us give a realistic example. We randomly chose an integer of 512 bits. The integer p is a 159-digit number.

The integer q is a 160-digit number.

We calculate n. It has 309 digits:

We calculate . It has 309 digits:

We choose e = 35,535. We then find d.

Alice wants to send the message THIS IS A TEST which can be changed to a numeric value by using the 0026 encoding scheme (26 is the space character).

The ciphertext calculated by Alice is C = Pe, which is.

Bob can recover the plaintext from the ciphertext by using P = Cd, which is

The recovered plaintext is THIS IS A TEST after decoding.

CriptografaINTRODUCCIONComponentes criptogrficosTerminologaCategoras de CriptografaCriptografa de Clave SimtricaSlide Number 7Criptografa por clave asimtricaClaves usadas en criptografaComparacin de los esquemas de criptografaCriptografa de clave simtrica: Cifrado tradicionalSlide Number 12Criptografa de clave simtrica: Cifrado tradicionalCriptografa de clave simtrica: Cifrado tradicionalSlide Number 15Slide Number 16Slide Number 17Criptografa de clave simtrica: Cifrado tradicional - Cifrado por transposicinSlide Number 19Slide Number 20Cifrados Modernos Sencillos: Cifrado XORCifrados Modernos Sencillos: Cifrado por RotacinCifrados Modernos Sencillos: Cifrado por Sustitucin: S-boxCifrados Modernos Sencillos: Cifrado por Permutacin: P-boxCifrados Modernos basados en VueltasData Encryption Standard (DES)Data Encryption Standard (DES): Funcin DESData Encryption Standard (DES): Triple DESCifrados Modernos: Advanced Encryption Standard (AES)Advanced Encryption Standard (AES)AES: Estructura de cada Round (Vuelta)Cifrados Modernos: Modos de Operacin Cifrados Modernos: Modos de Operacin Electronic Code Book (ECB)Electronic Code Book (ECB)Cifrados Modernos: Modos de Operacin Cipher Block Chainning (CBC)Cipher Block Chainning (CBC)Cifrados Modernos: Modos de Operacin Cipher feedback(CFB)Cipher Feedback(CFC)Cifrados Modernos: Modos de Operacin Output Feedback(OFB)Output Feedback(OFB)ASYMMETRIC-KEY CRYPTOGRAPHY: Rivest, Shamir and Adleman (RSA)ASYMMETRIC-KEY CRYPTOGRAPHY: Rivest, Shamir and Adleman (RSA)Slide Number 43Slide Number 44Slide Number 45Slide Number 46Slide Number 47ASYMMETRIC-KEY CRYPTOGRAPHY: Rivest, Shamir and Adleman (RSA)Slide Number 49Slide Number 50Slide Number 51Slide Number 52