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CISSP For Dummies, 4th Edition by Peter Gregory, Lawrence Miller

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Not Quite the Metric System: Symmetric and Asymmetric Key Systems

Cryptographic algorithms are broadly classified as either symmetric or asymmetric key systems.

Symmetric key cryptography

Symmetric key cryptography, also known as symmetric algorithm, secret key, single key, and private key cryptography, uses a single key to both encrypt and decrypt information. Two parties (for our example, Thomas and Richard) can exchange an encrypted message by using the procedure in Lab 8-2:

Lab 8-2 Exchanging an Encrypted Message with Symmetric Key Cryptography

1. The sender (Thomas) encrypts the plaintext message with a secret key known only to the intended recipient (Richard).

2. The sender then transmits the encrypted message to the intended recipient.

3. The recipient decrypts the message with the same secret key to obtain the plaintext message.

In order for an attacker (Harold) to read the message, he must guess the secret key (by using a brute-force attack, for example) or intercept the secret key during the initial exchange.

The following list includes the main disadvantages of symmetric systems:

check.png Distribution: Secure distribution of secret keys is absolutely required either through out-of-band methods or by using asymmetric systems.

check.png Scalability: A different key is required for each ...

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