| 6.2 |  | IOS Cryptosystem |
| 6.2.2 |  | Symmetrical encryption |

| Figure shows symmetrical encryption, which is also known as secret key encryption. Symmetrical encryption is used for large volumes of data since asymmetrical encryption is much more CPU intensive. The three encryption algorithms available in the IOS include Digital Encryption Standard (DES), Triple DES (3DES), and Advanced Encryption Standard (AES).

DES is one of the most widely used standards. DES turns clear text into ciphertext via an encryption algorithm. The decryption algorithm on the remote end restores clear text from ciphertext. Keys enable the encryption and decryption. DES is the most widely used symmetric encryption scheme today. It operates on 64-bit message blocks. The algorithm uses a series of steps to transform 64-input bits into 64-output bits. In the standard form, the algorithm uses 64-bit keys. 56 of these 64-bits, are chosen randomly. The remaining 8 bits are parity bits, one for each 7-bit block of the 56-bit random value.

3DES is an alternative to DES that preserves the existing investment in software but makes a brute-force attack more difficult. 3DES takes a 64-bit block of data and performs the operations of encrypt, decrypt, and encrypt. 3DES can use one, two, or three different keys. The advantage of using one key is that 3DES with one key is the same as standard DES for backward compatibility. However, additional processing time is required with one key. Both the DES and 3DES algorithms are in the public domain and freely available. The US Government restricts export of 3DES technology and many other goverments restrict encryption technology within their own boundraries so they may monitor communications.

AES is the newest encryption algorithm. It currently specifies keys with a length of 128, 192, or 256 bits to encrypt blocks with a length of 128, 192, or 256 bits. All nine combinations of key length and block length are possible. AES is now available in the latest Cisco router images that have IPSec DES/3DES functionality.

The most important feature of a cryptographic algorithm is its security against being compromised. The security of a cryptosystem, or the degree of difficulty for an attacker to determine the contents of the ciphertext, is a function of a few variables. In most protocols, the cornerstone to security lies in the secrecy of the key used to encrypt data. Symmetric encryption algorithms are built so that it is extremely difficult for anyone to determine the clear text without having this key. In any cryptosystem, great lengths are taken to protect the secrecy of the encryption key.

| Web Links Books http://williamstallings.com Cryptography http://world.std.com/~franl/crypto/