Chapter 17

How Encryption Works


Encryption works to protect against many threats to the security of an intranet. There is vulnerability during data transmission when people capture data sent across an intranet, or from the intranet through the Internet. This is a particular problem when transmitting sensitive information. Data is also vulnerable to a variety of threats while stored, including unauthorized access and theft.

When information and data is encrypted, it is altered so that to anyone other than the intended recipient it will look like meaningless garble. Encrypted information needs to be decrypted in order to view it and understand it-that is, turned back to the original message by the recipient, and only by the recipient.

There are several terms you'll need to understand in the encryption process: keys, algorithm, hash function, message digest, and digital fingerprint.

The heart of understanding how cryptosystems work is to understand the concept of keys. There are two basic kinds of encryption: secret-key (symmetric) and public-key (asymmetric) cryptography. Keys are secret values that are used by computers in concert with complex mathematical formulas called algorithms to encrypt and decrypt messages. The idea behind keys is that if someone encrypts a message with a key, only someone with a matching key will be able to decrypt it. Key size is the critical characteristic of encryption systems. Size is counted in bits. DES (Data Encryption Standard) is the most common secret key system. Both the sender and the receiver need to have copies of the same secret key. DES is used by the U.S. government and relies on a 56-bit key. This is the minimum size for effectiveness. DES performs 16 sequential calculations of substitutions on separate halves of the message to derive the encrypted result. DES is a symmetric process, linear calculation, and results in one secret key.

RSA encryption, named after the MIT professors who developed it in 1977 (Ronald Rivest, Adi Shamir, and Leonard Adleman), differs from DES in both technique to derive the result and because RSA uses key pairs instead of one key. The key pairs of RSA are derived by multiplying two large (each a few hundred bits long) prime numbers (factorization) and additional mathematical calculations. The RSA algorithm is the best-known public-key system. In public-key cryptography, a pair of keys are involved: a public key and a private key. Every person has both a public key and a private key. An individual's public key is made freely available, while the private key is exclusively known to each individual. If the public key is used to encrypt a message, only the companion private key can decrypt the message. If someone wanted to send a message to you, for example, he or she would encrypt it with your public key. Only you, with your private key, would be able to decrypt the message and read it. Your public key could not decrypt it. This means that once the message is encrypted, not even the sender can decrypt the message. Conversely, messages encrypted with private keys can only be decrypted with the matching public key. This ensures the authenticity of the sender to the recipient: Only someone with the private key code can encrypt a message that can be decrypted with that public key.

You may have heard about the Clipper chip and the Skipjack method to program a secret key. Skipjack uses an 80-bit key, so would be tougher to crack than DES. The controversy over the Clipper chip is not about the effectiveness of Skipjack, rather it is the fact that the chip contains a "back-door" that would allow others (theoretically only specifically authorized government agents) to get at the secret key, completely defeating the reasons people use encryption, privacy, and security.

PGP (Pretty Good Privacy) is an encryption program that uses a 128-bit key, and furthermore, it uses the RSA algorithm to encrypt the encryption of the 128-bit key. This means that PGP has 2128 possible keys. PGP as an implementation with RSA, uses key pairs, also known as public and private keys.

When a message is run through an encryption algorithm (like RSA) it can also call a hash function. Algorithms are essentially the mathematical method used to generate the keys. The hash function is used as a method to ensure that a message hasn't been altered. For example, if a sent message was 500 words long, but arrived as a message 501 words long, you could tell something had changed in transit. Word count by itself is not sufficient for ensuring that a message hasn't been altered since you could have multiple changes that have a net result of 500 words, and there would be no way to tell that the 500 words contained different words than the original. Hash functions on messages, therefore, are more complex. For example, it might use the number of words and the number of letters as components in the calculation. Because the message is the basis for the algorithm's calculation the result is unique to the message.

This process produces a number known as the message digest. For the purposes of this explanation, think of it as the value of the word count result, 500. The message digest (the 500 value) is then encrypted apart from the message itself, with a sender's private key. Because only the sender has access to this private key, the result is a "digital fingerprint"-a unique number that only the originator with a private key can create and which can only be decrypted with the companion public key.

Next, a new, random key is generated to encrypt the actual message and the digital signature. The recipient will need a copy of this random key in order to decrypt the message. This random key is the only key in the world that can decrypt the message and it is solely in the possession of the sender. This means the random key must now be sent, maintaining its secrecy, to the recipient, so the message can be decrypted. To allow for secure sending of the random key, it too is encrypted, this time with the recipient's public key. The encrypted random key is referred to as the digital envelope. Only the recipient will be able to decrypt the random key since it was encrypted with his or her public key-and so only his or her private key can decrypt it.

The result of this process is an encrypted confidential message, an encrypted signature, and the encrypted digital envelope. When the recipient gets the message, he or she decrypts the digital envelope with the private key, which results in the random key used to encrypt the message. The recipient then uses the random key to decrypt the actual message. However, at this stage there is no way to check that the message hasn't been altered en route-or that the message is authentic; that is, sent by the person it claims to be sent by. The recipient now uses the sender's public key to decrypt his or her encrypted digital signature. The recipient then gets the message digest-the message's "digital fingerprint."

By running the digital fingerprint message through the same algorithm-the hash function-a new message digest is generated. If authentic, this new message digest should match the original message digest precisely. If they don't match, either someone else composed the message, or the message was altered by someone after it was written.

In the process described above, a public-key system was crucial to the flow. Private key (or secret key) cryptosystems are not feasible to be used widely on the Internet or intranets for things such as electronic commerce. For a company to conduct business over the Internet or intranets with a private key system would mean creating millions of different private keys-one for each person who wanted to do business-and then figuring out some way to send those private keys securely over the Internet, which is not really possible. In secret key cryptography, only one key is used to encrypt and decrypt messages. With a public-key system, a business only needs to create a single public/private key combination. The business would post the public key for anyone to use to encrypt information-but only the business itself, with the private key, would be able to decrypt the data.

How Encryption Works

One means of securing an intranet is to use encryption-altering data so that only someone with access to specific decryption codes can understand the information. Encryption is used for storing and sending passwords to make sure that no snoopers can understand them. Encryption is used as well when data is sent between intranets on Very Secure Private Networks (VSPNs). Encryption is also used to conduct commerce on the Internet to protect credit card information during transmission.

  1. Keys are the heart of encryption. Keys are complex mathematical formulas (algorithms), that are used to encrypt and decrypt messages. If someone encrypts a message, only someone with the proper key will be able to decrypt the message. There are two basic key systems, secret-key and public-key cryptography.
  2. An algorithm is used to perform a hash function. This process produces a message digest unique to the message. The message digest is encrypted with the sender's private key which results in a digital fingerprint.
  3. Data Encryption Standard (DES) is a secret-key (symmetric) system; there is no public key component. Both the sender and the receiver know the secret code word. This method is not feasible for conducting business over the Internet.
  4. RSA is a public-key (asymmetric) system. RSA uses key pairs to encrypt and decrypt messages. Each person has a public key, available to anyone on a public key ring, and a private key, kept only on their computer. Data encrypted with someone's private key can only be decrypted with their public key; and data encrypted with their public key can only be decrypted with their private key. Therefore, RSA requires an exchange of public keys; this can be done without a need for secrecy since the public key is useless without the companion private key.
  5. PGP, Pretty Good Privacy, a program invented by Philip Zimmermann, is a popular method used to encrypt data. It uses MD5 (message-digest 5) and RSA cryptosystems to generate the key pairs. PGP is a popular program that can run on UNIX, DOS, and Macintosh platforms. It offers some variations of functionality, like compression, that other cryptosystems do not. Multiple key pairs can be generated and placed on public and private key rings.

How Cryptosystems Work

Because of the open nature of the Internet, it is easy for people to intercept messages that travel across it-making it difficult to send confidential messages or financial data, such as credit card in-formation. To solve the problem, cryptosystems have been developed. A popular one, called RSA, uses keys to encrypt and decrypt messages so that only the sender and receiver can understand the messages. The system requires that each person have a public key that is made available to anyone, and a private key that they keep only on their computer. Data encrypted with someone's private key can only be decrypted with their private key. This illustration is an example of how a public-key system works. In it, Gabriel and Mia want to exchange a confidential message. They have already exchanged public keys.