Cisco Volce over IP

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With CBC, the encryption of each 64-bit block depends on previous blocks. © 2003, Cisco Systems, Inc. All rights reserved. 0—1-1-20 Guidelines Several practical considerations can affect the security of DES-encrypted data: Change keys frequently to prevent brute force attacks. Communicate the DES key from sender to receiver using a secure channel. Consider using DES in CBC mode. With CBC, the encryption of each 64-bit block depends on previous blocks. CBC is the most widely used mode of DES. DES has 4 “weak” keys and 12 “semi-weak” keys.

D = 1(mod(p-1)(q-1)) In other words, d = e-1mod ((p-1)(q-1)) Note 1-1-42 d and n are also relatively prime. The numbers e and n are the public key; the number d is the private key. The two primes, p and q, are no longer needed. They should be discarded, but never revealed. 0 Copyright © 2003, Cisco Systems, Inc. RSA Algorithm—Encryption and Decryption • RSA works on numeric blocks smaller than n. • Encryption of block P: – E = Pe mod n • Decryption of block E: – P = Ed mod n • Factoring n reveals both keys to the attacker, but is extremely difficult.

The encryption of each block therefore depends on previous blocks and the same 64-bit plaintext block can encrypt to different ciphertext blocks. The first block is XORed with an Initialization Vector (IV), which is a public, random value, prepended to each message to bootstrap the chaining process. Note The two blocks of the same color in the first message are encrypted using ECB to produce two blocks of ciphertext, which also share the same color, whereas in CBC mode any patterns of this kind are always hidden by the chaining mechanism.

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