5.7. Choosing a Cryptographic Algorithm
Throughout this chapter we've shown how objects from the Java Security API, such as KeyPairGenerators and Ciphers, can be created by specifying a particular cryptographic algorithm for the implementation of the object. We haven't yet discussed the issues involved in picking one algorithm over another.
5.7.1. Features of Cryptographic Algorithms
Cryptography includes a broad range of techniques under its umbrella. Some of the characteristics that distinguish one technique from another follow.
126.96.36.199. Level of protection
Some encryption techniques provide a virtually unbreakable barrier to information theft; others just require a determined attacker with moderate resources to be broken. One way to compare techniques on this level is to estimate how much CPU time would be required on a machine of a given processing speed to iterate through all the possible keys to the encoded data. For example, "A 128-bit XYZ cryptographic key requires 14.5 months of CPU time on an Acme 24-processor server to be broken." But other issues can affect the level of effort required to break the encrypted data, and make it difficult to objectively compare the security of encryption techniques. For example, if the attacker is not familiar with the format of the data being transmitted, and the data isn't easily interpreted on its own, then it may be tough to tell if an attempt to decode the data has worked or not.
188.8.131.52. Sophistication and complexity
Encryption techniques are usually based upon the mathematical properties of numbers and digital information. The mathematical theories employed in creating encryption techniques vary in their complexity and sophistication; some require poring over many pages of mathematics and statistics journals to be fully understood, while others can be explained using basic concepts of algebra. The resources required to implement and to break a given encryption technique are usually a direct function of its complexity. All other issues being equal, a more complex encryption scheme is normally more expensive to implement, but is also more expensive to break. As an application developer, you'll typically need to trade off efficiency against security. If high throughput is a requirement for your application, you may be willing to use a less complex, and less secure, cryptographic algorithm if it imposes significantly less overhead on your agents.
184.108.40.206. One-, two-, and many-way cryptography
Depending on the nature of your distributed application, there may be situations in which many parties need to be individually or mutually authenticated by one agent. A secure chat room or whiteboard system, for example, may require mutual authentication by all participating parties. Most authentication schemes directly support one- or two-way agent authentication. Few, if any, have any concept of multi-way authenticated communications. Instead, the developer must maintain a set of two-way-authenticated channels at each agent site. One way to deal with multi-way authentication is to define a group of individuals as a group identity with a single key pair and set of certificates. Every person in this group needs to prove ownership by providing a digital signature verifying that they have access to the group's private key, and some kind of certification of their keys. The viability of this approach really lies in the policies of the certification authority being used. If they agree to certify groups, and have defined a policy for verifying membership in certain groups, then most cryptographic algorithms will support a key pair or certificate associated with a named group rather than a named individual.
220.127.116.11. Design issues
As we've already mentioned, modern cryptography involves the use of keys for data signing, encoding, and decoding. Some require the secure distribution of private keys between parties, while others allow the parties to use public keys that can be broadcast openly. In our SecureAgent example, we used an encoding scheme that required the use of secret keys.
Other design issues involve the low-level implementation details of the algorithm. If we're building an agent to be used in an applet context, then we'd like the implementation of an cryptographic algorithm to be done completely in Java without any native method calls, so that we don't have to distribute native libraries to every agent that will talk to our agent.
Another issue that may seem obvious is the level of standardization of a cryptographic algorithm. If an algorithm is standardized, or at least widely used, then there is a better chance that we can use our security-enabled agents with other people's agents without modifying them.
18.104.22.168. Financial and legal issues
An issue of a different sort is the expense of using a cryptographic algorithm. Some of the more sophisticated techniques are patented, and require payment of license fees to be used. Others are simply too involved to be implemented by the average developer, and software implementations can be sold for high prices by those who do have the resources to develop them.
There are also legal and political restrictions to worry about. The United States, for example, has specific restrictions on the types of encryption that can be implemented for export to foreign countries. Currently, there are two separate versions of the Netscape browser: one for international use, and one for use only within the United States. The latter includes a more sophisticated version of the RSA encryption technology, whose export is restricted by the U.S. federal government.
5.7.2. Available Algorithms
There are numerous cryptographic algorithms for data encryption, and a similar number of certification and authentication techniques. Several umbrella security protocols and standards have been developed that incorporate both encryption and authentication facilities. Among them are Netscape's Secure Socket Layer (at the time of this writing, SSL 3.0 is an IETF Internet Draft), the Pretty Good Privacy (PGP) package developed originally by Phil Zimmermann, and the Public Key Cryptography Standard (PKCS) from RSA Laboratories. S-HTTP is a security protocol designed specifically around the HTTP protocol, which differentiates it from these other, more general protocols. The chief motivation behind the development of these packages is to make encryption and authentication technologies easily accessible to developers, and to provide a common protocol for security-enabled applications to interact freely.
22.214.171.124. Encryption techniques
We've already discussed some of the differences between public key encryption and secret key techniques. Some commonly used secret key encryption algorithms are the Data Encryption Standard (DES), and the International Data Encryption Algorithm (IDEA).
Some common forms of public key encryption in use today are Diffie-Hellman, derived from the original paper describing public key systems; RSA, licensed by RSA Laboratories; and the Digital Signature Algorithm (DSA), developed by the National Institute of Standards and Technology (NIST). Diffie-Hellman uses an encryption algorithm based on factoring prime numbers, while DSA is based on an algorithm involving discrete logarithms. These two algorithms are believed by many cryptographers to be comparably hard to crack. RSA is based upon a combination of their own public key encryption scheme with other secret key algorithms, such as DES block ciphers.
126.96.36.199. Certificates and authentication techniques
As we've already seen, certification and authentication schemes are typically founded on an existing public key encryption technique. The RSA public key cryptography system, for example, can be used in combination with a hashing technique to implement an authentication scheme. One party "signs" a message by running it through a known hashing algorithm, and then encrypts the hashed message with their private key to generate a digital signature. The signature is then sent along with the original message to the other party. The receiving party can then decrypt the signature using the sender's public key, and hash the clear message using the same hashing algorithm used to generate the signature. If the hashed message is equal to the decrypted signature, then the receiving party can assume that the sender is the person that owns the public key. In order to impersonate some other party, we would have to know their private key. Otherwise, we wouldn't be able to generate both a clear message and an encrypted, hashed version of the message that check out against each other.
All authentication systems use certificates, which minimally contain an identifier for a party and the party's public key. The most commonly used standard format for digital certificates is the ITU X.509 standard certificate format. SSL, PKCS, and S-HTTP all offer X.509-compliant certification methods within their protocols. RSA can also be used in conjunction with X.509 certificates. Many cryptographic systems, however, resort to nonstandard certificate formats based on their own binary formatting schemes, where a public key, an identifier, and several other parameters, such as expiration times and serial numbers, are serialized and encrypted before being transmitted.
In most cases, the power of a certificate to authenticate a party rests on the certifying authority (CA) that issues and vouches for the certificate. VeriSign, BBN, and even the United States Postal Service offer CA services. Certificate authorities usually provide several levels of certification and require various types of proof of identity. In some cases an email verification is sufficient; other certificates require verification by a notary public before being issued. A certificate issued from a CA must be installed somehow on your host computer in order to be properly broadcast by your network applications when secure transmissions are attempted. The Netscape browser, for example, will use your personal certificate to establish secure HTTP connections when a remote HTTP server requests it.
5.7.3. General Security Protocols
We've already mentioned some of the more common general security protocols in use today. Support for them in the Java environment is becoming more broadly available, but implementation of the cryptographic algorithms underlying these protocols is a complex task that typically requires the backing of a software development company. If public-domain or shareware versions of these protocols do become available they will undoubtedly be few and far between.
188.8.131.52. Secure Socket Layer (SSL)
The Secure Socket Layer, originally put forth by Netscape and now an Internet Draft of the IETF, defines a general security protocol for private, authenticated communications. SSL is a protocol that layers on top of a network transport protocol (such as TCP/IP), but below application protocols (such as HTTP) or security APIs (such as the Java Security API). The protocol allows agents to choose from a suite of encryption algorithms and authentication schemes, including DES ciphers and RSA cryptography.
In addition to providing communications security with existing cryptographic techniques, SSL also provides a means to extend the protocol to include new encryption methods that may be developed. SSL also provides a session caching facility, where key pairs associated with a connection to a particular party can be retrieved and used to reinstate secure connections in sessionless application protocols, like HTTP.
184.108.40.206. Pretty Good Privacy (PGP)
PGP was originally developed by Phil Zimmermann as an effort to get effective privacy and authentication tools into the hands of the general technology community. There has been a shroud of controversy surrounding PGP since its public release by Zimmermann, involving United States export laws concerning cryptographic technology. We won't discuss the sordid details here, except to say that there are essentially two versions of PGP: one for use within the United States and Canada, and another for international use, which does not use certain implementations of RSA encryption algorithms.
In terms of the families of encryption algorithms that we discussed in an earlier section, PGP is a hybrid technique that uses public key methods to distribute private keys between agents. The reason for this is efficiency: public key methods are computationally very expensive, and can significantly reduce your data throughput if all data is transmitted with public key encryption. PGP tries to circumvent this by using RSA public key encryption to transmit a pair of random private keys between the two parties securely. Once the agents have each other's private keys, they can proceed to encrypt their messages using faster, private key encryption. Messages are encrypted using a block cipher called IDEA. Note that, while SSL allows us to specify what kind of encryption we would like to use, the encryption scheme used by PGP is fixed.
Authentication in PGP can be implemented with digital signatures, using a method similar to that used by RSA for authentication. An MD5 hashing function and RSA encryption is used to generate the signature.
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