8.1 Choosing an Authentication Method

8.1.1 Problem

You need to perform authentication, and you need to choose an appropriate method.

8.1.2 Solution

The correct method depends on your needs. When a server needs to be authenticated, and the client does not, SSL/TLS is a popular solution. When mutual authentication is desirable, there are a whole bevy of options, such as tunneling a traditional protocol over SSL/TLS or using a dedicated protocol. The best dedicated protocols not only perform mutual authentication but also exchange keys that can then be used for encryption.

8.1.3 Discussion

An authentication factor is some thing that contributes to establishing an identity. For example, a password is an authentication factor, as is a driver's license. There are three major categories of authentication factors:

Things you know

This category generally refers to passwords, PIN numbers, or passphrases. However, there are systems that are at least partially based on the answers to personal questions (though such systems are low on the usability scale; they are primarily used to reset forgotten passwords without intervention from customer service people, in order to thwart social engineering attacks).

Things you have

ATM cards are common physical tokens that are often implicitly used for authentication. That is, when you go to an ATM, having the card is one factor in having the ATM accept who you are. Your PIN by itself is not going to allow someone to get money out in your name.

Things you are

This category generally refers to biometrics such as fingerprints or voice analysis. It includes things you have that you are not going to lose. Of course, an attacker could mimic your information in an attempt to impersonate you.

No common authentication factors are foolproof. Passwords tend to be easy to guess. While cryptography can help keep properly used physical tokens from being forged, they can still be lost or stolen. And biometric devices today have a significant false positive rate. In addition, it can be simple to fool biometric devices; see http://www.puttyworld.com/thinputdeffi.html.

In each of these major categories, there are many different technologies. In addition, it is easy to have a multifactor system in which multiple technologies are required to log in (supporting the common security principle of defense in depth). Similarly, you can have "either-or" authentication to improve usability, but that tends to decrease security by opening up new attack vectors.

Clearly, choosing the right technology requires a thorough analysis of requirements for an authentication system. In this chapter, we'll look at several common requirements, then examine common technologies in light of those requirements.

However, let us first point out that it is good to build software in such a way that authentication is implemented as a framework, where the exact requirements can be determined by an operational administrator instead of a programmer. PAM (Pluggable Authentication Modules) lets you do just that, at least on the server side, in a client-server system. SASL (Simple Authentication and Security Layer) is another such technology that tries to push the abstraction that provides plugability off the server and into the network. We find SASL a large mess and therefore do not cover it here. PAM is covered in Recipe 8.12.

There are several common and important requirements for authentication mechanisms. Some of these may be more or less important to you in your particular environment:

Practicality of deployment

This is the reason that password systems are so common even though there are so many problems with them. Biometrics and physical tokens both require physical hardware and cost money. When deploying Internet-enabled software, it is generally highly inconvenient to force users to adopt one of these solutions.

Usability

Usability is a very important consideration. Unfortunately, usability often trades off against good security. Passwords are a good example: more secure mechanism would require public keys to establish identity. Often, the user's private key will be password-protected for defense in depth, but that only protects against local attacks where an attacker might get access to steal the key—a well-designed public key-based protocol should not be vulnerable to password-guessing attacks.

Another common usability-related requirement is that the user should not have to bring any special bits with him to a computer to be able to log in. That is, many people want a user to be able to sit down at an arbitrary computer and be able to authenticate with data in his head (e.g., a password), even if it means weaker security. For others, it is not unreasonable to ask users to carry a public key around.

When passwords are used, there are many different mechanisms to improve security, but most of them decrease usability. You can, for example, expire passwords, but users hate that. Alternatively, you can enforce passwords that seem to have sufficient entropy in them (e.g., by checking against a dictionary of words), but again, users will often get upset with the system. In many cases, adding something like a public key mechanism adds more security and is less burdensome than such hacks turn out to be.

Use across applications

For some people, it is important to manage authentication centrally across a series of applications. In such a situation, authentication should involve a separate server that manages credentials. Kerberos is the popular technology for meeting this requirement, but a privately run public key infrastructure can be used to do the same thing.

Patents

Many people also want to avoid any algorithms that are likely to be covered by patent.

Efficiency

Other people may be concerned about efficiency, particularly on a server that might need to process many connections in a short period of time. In that situation, it could be important to avoid public key cryptography altogether, or to find some other way to minimize the impact on the server, to prevent against denial of service.

Common mechanism

It may also be a requirement to have authentication and key exchange be done by the same mechanism. This can improve ease of development if you pick the right solution.

Economy of expression

An authentication protocol should use a minimal number of messages to do work. Generally, three messages are considered the target to hit, even when authentication and key exchange are combined. This is usually not such a big deal, however. A few extra messages generally will not noticeably impact performance. Protocol designers like to strive to minimize the number of messages, because it makes their work more elegant and less ad hoc. Of course, simplicity should be a considered requirement, but then again, we have seen simple five-message protocols, and ridiculously complex three-message protocols!

Security

Security is an obvious requirement at the highest level, but there are many different security properties you might care about, as we'll describe in the rest of this section.

In terms of the security of your mechanism, you might require a mechanism that effectively provides its own secure channel, resisting sniffing attacks, man-in-the-middle attacks, and so on that might lead to password compromise, or even just the attacker's somehow masquerading as either the client or server without compromising the password. (This could happen, for example, if the attacker manages to get the server password database.)

On the other hand, you might want to require something that does not build its own secure channel. For example, if you are writing something that will be used only on the console, you will already be assuming a trusted path from the user to your code, so why bother building a secure channel? Similarly, you might already be able to establish an authenticated remote connection to a server through something like SSL, in which case you get a secure channel over which you can do a simpler authentication protocol. (Mutual authentication versus one-sided authentication is therefore another potentially interesting requirement.) Of course, that works only if the server really is authenticated, which people often fail to do properly.

Whether or not you have a secure channel, you will probably want to make sure that you avoid capture replay attacks. In addition, you should consider which possible masquerading scenarios worry you. Obviously, it is bad if an arbitrary person can masquerade as either the client or the server just from watching network traffic. What if an attacker manages to break into a server, however? Should the attacker then be able to masquerade as the user to that server? To other servers where the user has the same credentials (e.g., the same password)?

In addition, when a user shares authentication credentials across multiple servers, should he be able to distinguish those servers? Such a requirement can demand significant trade-offs, because to meet it, you will need either a public key infrastructure or some other secure secret that users need to carry around that authenticates each server. If you are willing to assume that the server is not compromised at account creation time but may be compromised at some later point, you can meet the requirement more easily.

We have already mentioned no susceptibility to password guessing attacks as a possible requirement. When that is too strict, there are other requirements we can impose that are actually reasonable:

• When an attacker steals the authentication database on the server, an offline cracking job should be incredibly difficult—with luck, infeasible, even if the password being attacked is fairly predictable.

• Guessing attacks should be possible only by attempting to authenticate directly with the server, and the login attempt should not reveal any information about the actual password beyond whether or not the guess was correct.

• There should not be large windows of vulnerability where the server has the password. That is, the server should need to see the password only at account initialization time, or not at all. It should always be unacceptable for a server to store the actual password.

No doubt there are other interesting requirements for password systems.

For authentication systems that also do key exchange, there are other interesting requirements you should consider:

Recoverability from randomness problems

You might want to require that the system be able to recover if either the client or the server has a bad source of randomness. That is generally done by using a key agreement protocol, where both sides contribute to the key, instead of a key transport protocol, where one side selects the key and sends it to the other.

Forward secrecy

You might want to require that an attacker who manages to break one key exchange should not be able to decrypt old connections, if he happens to capture the data. Achieving this property often involves some tradeoffs.

Let's look at common technologies in light of these requirements.

8.1.3.1 Traditional UNIX crypt( )

This solution is a single-factor, password-based system. Using it requires a preexisting secure channel (and one that thwarts capture replay attacks). There are big windows of vulnerability because the user's password must be sent to the server every time the user wishes to authenticate. It does not meet any of the desirable security requirements for a password-based system we outlined above (it is susceptible to offline guessing attacks, for example), and the traditional mechanism is not even very strong cryptographically. Using this mechanism on an unencrypted channel would expose the password. Authentication using crypt( ) is covered in Recipe 8.9.

8.1.3.2 MD5 Modular Crypt Format (a.k.a. md5crypt or MD5-MCF)

This function replaces crypt( ) on many operating systems (the API is the same, but it is not backward-compatible). It makes offline cracking attacks a little harder, and it uses stronger cryptography. There are extensions to the basic modular format that use other algorithms and provide better protection against offline guessing; the OpenBSD project's Blowfish-based authentication mechanism is one. Using this mechanism on an unencrypted channel would expose the password. Authentication using MD5-MCF is covered in Recipe 8.10.

8.1.3.3 PBKDF2

You can use PBKDF2 (Password-Based Key Derivation Function 2; see Recipe 4.10) as a password storage mechanism. It meets all the same requirements as the Blowfish variant of MD5-MCF discussed in the previous subsection. Authentication using PBKDF2 is covered in Recipe 8.11.

8.1.3.4 S/KEY and OPIE

S/KEY and OPIE are one-time password systems, meaning that the end user sends a different password over the wire each time. This requires the user and the server to preestablish a secret. As a result, if an attacker somehow gets the secret database (e.g., if he manages to dumpster-dive for an old backup disk), he can masquerade as the client.

In addition, the user will need to keep some kind of physical token, like a sheet of one-time passwords (which will occasionally need to be refreshed) or a calculator to compute correct passwords. To avoid exposing the password if the server database is compromised, the user will also need to reinitialize the server from time to time (and update her calculator).

These mechanisms do not provide their own secure channel. S/KEY, as specified, relies on MD4, which is now known to be cryptographically broken. If it's used on an unencrypted channel, no information about the password is revealed, but an attacker can potentially hijack a connection.

8.1.3.5 CRAM

CRAM (Challenge-Response Authentication Mechanism) is a password-based protocol that avoids sending the password out over the wire by using a challenge-response protocol, meaning that the two ends each prove to the other that they have the secret, without someone actually sending the secret. Therefore, CRAM (which does not itself provide a secure channel) can be used over an insecure channel. However, it is still subject to a number of password attacks on the server, particularly because the server must store the actual password. Therefore, you should not use CRAM in new systems.

8.1.3.6 Digest-Auth (RFC 2617)

Digest-Auth is one of the authentication mechanisms specified for HTTP/1.1 and later (the other is quite weak). It does not provide a secure channel, and it provides only moderate protections against attacks on passwords (much of it through an optional nonce that is rarely used).

8.1.3.7 SRP

All of the mechanisms we've looked at so far have been password-based. None of them create their own secure channel, nor do they provide mutual authentication. SRP (Secure Remote Password) is a password-based mechanism that does all of the above, and it has a host of other benefits:

Client-server authentication

SRP not only allows a server to authenticate clients, but it also allows clients to know that they're talking to the right server—as long as the authentication database isn't stolen.

Protection against information leakage

SRP also prevents all but a minimal amount of information leakage. That is, an attacker can try one password at a time by contacting the server, but that is the only way he can get any information at all about the password's value. Throttling the number of allowed login attempts to a few dozen a day should reasonably thwart most attacks, though it opens up a denial of service risk. You might consider slightly more sophisticated throttling, such as a limit of 12 times a day per IP address. (Of course, even that is not perfect). A far less restrictive method of throttling failed authentication attempts is discussed in Recipe 8.8.

Protection against compromise

SRP protects against most server-compromise attacks (but not a multiserver masquerading attack, which we do not think is worth worrying about anyway). It even prevents an attacker who compromises the server from logging into other machines using information in the database.

Key exchange

Another big benefit is that SRP exchanges a key as a side effect of authentication. SRP uses public key cryptography, which can be a denial-of-service issue.

The big problem with SRP is that patents cover it. As a result, we do not explore SRP in depth. Another potential issue is that this algorithm does not provide forward secrecy, although you could easily introduce forward secrecy on top of it.

8.1.3.8 Basic public key exchange

There are plenty of strong authentication systems based on public key cryptography. These systems can meet most of the general requirements we've discussed, depending on how they're implemented.

Generally, the public key is protected by a password, but the password-protected key must be transported to any client machine the user might wish to use. This is a major reason why people often implement password-based protocols instead of using public key-based protocols. We discuss a basic protocol using public key cryptography in Recipe 8.16.

8.1.3.9 SAX

SAX (Symmetric Authenticated eXchange) is a protocol that offers most of the same benefits of SRP, but it is not covered by patents. Unlike SRP, it does not use public key encryption, which means that it minimizes computational overhead. There is a masquerading attack in the case of server compromise, but it effectively requires compromise of two servers and does not buy the attacker any new capabilities, so it is not very interesting in practice.

SAX has two modes of use:

• You can avoid leaking any information about the password if the user is willing to carry around or memorize a secret provided by the server at account creation time (that secret needs to be entered into any single client only once, though).

• Otherwise, SAX can be used in an SRP-like manner, where the user need not carry around anything other than the password, but information about the password can be learned, but primarily through guessing attacks. Someone can mount an offline dictionary attack on the server side, but the cost of such an attack can be made prohibitive.

If an attacker somehow gets the secret database (e.g., if he manages to dumpster-dive for an old backup disk), he can masquerade as the client. PAX is a similar protocol that fixes this problem.

8.1.3.10 PAX

PAX (Public key Authenticated eXchange) is a basic two-way authenticating key exchange using public key encryption that uses passwords to generate the keys. The server needs to know the password once at initialization time, and never again.

This protocol is similar to SAX, but has some minor advantages because it uses public key cryptography. For example, you can back away from using passwords (for example, you might take the key and put the client's private key onto a smart card, obviating the need to type in a password on the client end). Additionally, if an attacker does get the authentication database, he nonetheless cannot masquerade as the client.

PAX can be used in one of two modes:

• You can get all the advantages of a full public-key based system if the user is willing to carry around or memorize a secret provided by the server at account creation time (that secret needs to be entered into any single client only once, though).

• Otherwise, PAX can be used in an SRP-like manner, where the user need not carry around anything other than the password; information about the password can be learned, but only through guessing attacks.

As with SRP, you can easily layer forward secrecy on top of PAX (by adding another layer of cryptography; see Recipe 8.21).

Unlike SRP, PAX is not believed to be covered by patents.

8.1.3.11 Kerberos

Kerberos is a password-based authentication mechanism that requires a central authentication server. It does not use any public key cryptography whatsoever, instead relying on symmetric cryptography for encryption and authentication (typically DES or Triple-DES in CBC mode with MD5 or SHA1 for authentication).

Although Kerberos never transmits passwords in the clear, it does make the assumption that users will not use weak passwords, which is a poor assumption to make, because users will invariably use passwords that they find easy to remember. That typically also makes these passwords easy for an attacker to guess or to discover by way of a dictionary attack.

Kerberos does assume that the environment in which it operates is insecure. It can overcome a compromised system or network; however, if the system on which its central database resides is compromised, the security afforded by Kerberos is seriously compromised.

We cover authentication with Kerberos in Recipe 8.13. Because of the complexity of the SSPI API in Windows, we do not cover Kerberos on Windows in this book. Instead, recipes are available on our web site.

8.1.3.12 Windows NT LAN Manager (NTLM)

Windows NT LAN Manager is a password-based protocol that avoids sending the password out over the wire by using a challenge-response protocol, meaning that the two ends each prove to the other that they have the secret, without someone actually sending the secret. Therefore, NTLM (which does not itself provide a secure channel) can be used over an insecure channel. However, it is still subject to a number of password attacks on the server, particularly because the server must store the actual password.

Windows uses NTLM for network authentication and for interactive authentication on standalone systems. Beginning with Windows 2000, Kerberos is the preferred network authentication method on Windows, but NTLM can still be used in the absence of a Kerberos infrastructure.

Because of the complexity of the SSPI API in Windows, we do not cover authentication with NTLM in this book. Instead, recipes are available on our web site.

8.1.3.13 SSL certificate-based checking

Secure Sockets Layer (SSL) and its successor, Transport Layer Security (TLS), use certificates to allow entities to identify entities in a system. Certificates are verified using a PKI where a mutually trusted third party vouches for the identity of a certificate holder. See Recipe 10.1 for an introduction to certificates and PKI.

Certificates are obtained from a trusted third party known as a certification authority (CA), which digitally signs the certificate with its own private key. If the CA is trusted, and its signature on the certificate is valid, the certificate can be trusted. Certificates typically also contain other important pieces of information that must also be verified—for example, validity dates and the name of the entity that will present the certificate.

To be effective, certificates require the mutually trusted third party. One of the primary problems with certificates and PKI is one of revocation. If the private key for a certificate is compromised, how is everyone supposed to know that the certificate should no longer be trusted? CAs periodically publish lists known as certificate revocation lists (CRLs) that identify all of the certificates that have been revoked and should no longer be trusted, but it is the responsibility of the party verifying a certificate to seek out these lists and use them properly. In addition, there is often a significant window of time between when a CA revokes a certificate and when a new CRL is published.

SSL is widely deployed and works sufficiently well for many applications; however, because it is difficult to use properly, it is often deployed insecurely. We discuss certificate verification in Recipe 10.4 through Recipe 10.7.

8.1.4 See Also

• Thinking Putty article on defeating biometric fingerprint scanners: http://www.puttyworld.com/thinputdeffi.html

• RFC 1510: The Kerberos Network Authentication Service (V5)

• RFC 2617: HTTP Authentication: Basic and Digest Access Authentication

• Recipe 4.10, Recipe 8.8, Recipe 8.9, Recipe 8.10, Recipe 8.11, Recipe 8.12, Recipe 8.13, Recipe 8.16, Recipe 8.21, Recipe 10.1, Recipe 10.4, Recipe 10.5, Recipe 10.6, Recipe 10.7