Networking / Beginners

# Algorithms and Protocols

Cryptography works on many levels. At the lowest level are cryptographic algorithms. These algorithms describe the steps required to perform a particular computation, typically based around the transformation of data from one format to another. Building on these algorithms, is a protocol. The protocol describes the complete process of executing a cryptographic activity, including explicit information on how to handle any contingency that might arise. Making this distinction is important, because an excellent cryptographic algorithm does not necessarily translate into a strong protocol. The protocol is responsible for more than just the encoding of data; data transmission and key exchange are also properties of a protocol.

Finally, on top of the protocol are the applications. Once again, a strong protocol does not guarantee strong security, as the application itself may lead to further problems. Thus, in order to create a secure solution, a strong protocol is required, as well as a good, robust application implementation.

## Data Encryption

The core of any cryptographic system is encryption, the process of taking a regular set of data, called plaintext, and converting it into an unreadable form, called ciphertext. Encryption allows you to maintain the privacy of sensitive data, even when accessed by unauthorized users. The only way the data can be read is by transforming it back to its original form using a process called decryption. The method of encryption and decryption is called an algorithm or cipher. The concept of encryption, as the message is transported over an insecure public channel, it is encrypted, preventing anyone eavesdropping on the line from being able to understand the data being sent.

Modern algorithms use keys to control the encryption and decryption of data. Once a message has been encrypted, it can only be decrypted by users who have the corresponding key. Key-based algorithms come in two classes: symmetric and asymmetric.

Symmetric algorithms are efficient: They use a single key to encrypt and decrypt all messages. The sender uses the key to encrypt the message and then sends the message to the intended recipient. Once the message is received, the recipient uses the same key to decrypt the message. This type of algorithm works well when there is a safe way to transmit the key between users, such as by meeting before the transmission takes place. Unfortunately a substantial problem arises when exchanging data between loosely related parties, such as an e-commerce Web site and a customer. Exchanging the key is a problem that symmetric encryption itself is incapable of solving; and without a secure method of exchange keys, this method is only useful between private parties.

Symmetric encryption is also referred to as secret-key encryption. The most popular form of this method is the Data Encryption Standard (DES), which was developed in the 1970s. Since then more secure forms of symmetric encryption have been developed; leaders among them include the Advanced Encryption Standard (AES), which is based on the Rijndael algorithm; Triple DES; International Data Encryption Algorithm (IDEA); Blowfish; and the Rivest family of algorithms, RC2, RC4, RC5, and RC6.

Asymmetric encryption addresses the main problem that has plagued symmetric key systems: the use of a single key. For years, cryptologists worked to find a solution to the key distribution problem with little success. Just as many mathematicians started to think that a solution was not possible, Whitfield Diffie and Martin Hellman proved them wrong. In 1975, Diffie and Hellman developed a solution using two separate but related keys: one to encrypt the data and another to decrypt it. The key used to encrypt the data is called the public key. This key can be widely distributed over insecure lines, for general public use. The key used to decrypt the corresponding data is called the private key. This key is never transmitted, as it is only required by the party that needs to decrypt the data. These keys are related in an obscure way using extremely large prime numbers and one-way functions. This technique makes it computationally infeasible to calculate the private key based on the public key. The larger the key is, the more difficult it becomes to break the system. Sixty-four-bit key systems, such as DES, are capable of being attacked by brute force-that is, trying every single key combination until the attacker finds the correct one. The more common 128-bit systems, such as ECC, so far have proven invulnerable to brute-force attacks.

Here is an example of how asymmetric, or public key, cryptography works. Suppose Alice wants to send a secure message to Bob. Alice can use Bob's public key to encrypt the message, since it is publicly available. She then sends the message to Bob. When he receives the message, he uses his private key, to which only he has access, to decrypt the message. Now Alice is able to send a secure message to Bob without having to do a key exchange. If information is to be exchanged in both directions using asymmetric encryption, each party must have his or her own public key and private key combination.

Encrypting a message with the private key and decrypting it with the public key is possible as well, but this has a different objective. It can be used on nonsensitive data simply to prove that the party who encrypted it actually has access to the private key.

The first, and best-known, asymmetric key algorithm was released in 1977 by Ron Rivest, Adi Shamir, and Leonard Adelman; who are commonly known as RSA. Other popular algorithms include Elliptic Curve Cryptography (ECC) and Diffie-Hellman (DH). RSA is being challenged in the mobile space by ECC, which is much less expensive in terms of processing power and key size, which are essential attributes in mobile computing.

Asymmetric ciphers are not, however, a perfect solution. Choosing a private key is not trivial, as a poor choice can lead to an easily broken scheme. Also, asymmetric ciphers provide a solution to the key distribution problem by using a public key and a private key, but they also are much more complicated, and therefore computationally slower than symmetric ciphers. For large sets of data, this can be problematic. In these cases, a combination of symmetric and asymmetric systems is an ideal solution. This allows you to take advantage of the higher performance of symmetric algorithms, by sending the secret key over insecure channels using public key systems. Once all parties have the secret key, the remainder of the data for that session can be encrypted and decrypted using symmetric algorithms. This is the basis for public key cryptography as used by many of today's leading protocols.

## Digital Certificates

Digital certificates provide a way to guarantee that a public key belongs to the party it represents. For this to be successful, the certificate itself also has to be verified to ensure that it represents the claimed entity (a person or organization). This is accomplished using a trusted third party called a certificate authority (CA). The most common certificate authorities include VeriSign, Entrust, and Certicom. Users can purchase a digital certificate from the CA and use this certificate for authentication and to circulate their private key. Once a recipient has their private key and they are confident the recipient is who he or she claims to be (which is largely based on the trust of the CA), they can then send encrypted messages to this person, knowing they can decrypt them using their private key. Digital certificates typically contain the following:

• The name of the holder, as well as other information that uniquely identifies the holder. Additional information can include the URL of a Web server using the certificate or an email address.
• The holder's public key.
• The name of the CA that issued the certificate.
• The lifetime that the certificate if valid for (usually a start and end date).
• A digital signature from the CA to make it easy to detect if the transmission has been tampered with.

Certified users also have the option of self-signing a digital certificate, thereby becoming a CA themselves. This additional party can be considered trustworthy if he or she was signed by another trusted key. In this way, you can continue to navigate to the root CA, thereby determining who provided the initial certificate. If the root CA is not recognized or trusted, then each certificate in the chain is considered invalid.

The leading format for digital certificates is X.509, an industry standard for authentication. These certificates are prominent in Internet applications. In the wireless space, another form of digital certificate, called a WAP Server WTLS Certificate, is available. These certificates, commonly shortened to WTLS certificates, are a simpler version of X.509, created because X.509 certificates were too big for wireless applications. The WTLS certificates are primarily used in WAP applications where the microbrowser wants to authenticate the identity of a WAP server and encrypt information using the Wireless Transport Layer Security (WTLS). (There is more information on WTLS later in this tutorial.)

## Digital Signatures

Digital signatures are used to verify that a message really came from the claimed sender. It is based upon the notion that only the creator of the signature has the private key and that it can be verified using a corresponding public key. The digital signature is created by computing the message digest of a document, then concatenating it with information about the signer, a timestamp, and any other required information. A message digest is a function that takes arbitrary-sized input data (the message) and generates a fixed-size output, called a digest. This set of information is then encrypted using the private key of the sender using a suitable asymmetric algorithm. The resulting encrypted block of information is the digital signature.

The message digest that was calculated is a bit value that represents the current state of a document. If the document changes in any way, the message digest will also change. By incorporating the message digest into the digital signature, it is possible for the recipient of the document to easily detect if the document has been altered since the digital signature was created.

## Public Key Infrastructure

Public key infrastructure (PKI) is the term used to describe a complete organization of systems and rules defining a single security system. The Internet Engineering Task Force (IETF) X.509 Working Group (www.ietf.org/internet-drafts/draft-ietf-pkix-roadmap-09.txt) defines PKI as "the set of hardware, software, people, and procedures needed to create, manage, store, distribute, and revoke certificates based on public key cryptography."

The components of PKI include the following:

• Certificate authorities responsible for issuing and revoking certificates
• Registration authorities responsible for binding between public keys and the identities of their holders
• Certificate holders who have been issued certificates that they can use to sign digital documents
• Repositories that store certificates as well as certificate revocation lists
• Security policy that defines an organization's top-level direction on security