SQL Server Magazine

A recent Windows 2000 Magazine Web site Instant Poll found that 75 percent of respondents running Windows NT 4.0 were running the high-encryption (i.e., 128-bit) version of the OS. If your business falls into that category, you might be running the high-encryption system on faith, unaware of the protection that the 128-bit version provides. To answer the question, "What type of protection does the high-encryption version of NT 4.0 provide?" I spent hours digging through documents, security standards, and publications, and polling my colleagues for answers to the following 10 questions.

1. What's encryption?
To understand how NT implements encryption and how encryption protects a system, you first need to know a few basic cryptographic concepts. Encryption is the process of transforming a message (often called plaintext) into ciphertext, a string that appears random and meaningless. Only someone who knows how to unscramble (i.e., decrypt) the ciphertext can read an encrypted message. RSA Laboratories defines cryptography as the "art and science of using mathematics to secure information." Mathematics is the key to a good cryptographic system. Mathematical algorithms that perform encryption are called ciphers.

Cryptography comprises two types of mathematical functions: hashing algorithms and functions that employ a key to transform data. When you use a hashing algorithm to transform data, reversing the algorithm and converting the hash value back into the original message is mathematically impossible. Hashes obscure the length of the original message. When you encrypt data with a function that employs a key, you can decipher the message only when you have the key or keys that the function employs. For more information about encryption, see "Related Articles in Previous Issues."

2. Where do hash algorithms fit in?
Hash algorithms perform a one-way transform on data, which means that you can't invert or reverse a good hash function to reproduce the original data, as you can when you use key-based ciphers. Cryptographic systems commonly use hash functions that produce a fixed-length output, regardless of the size of the input. Therefore, a hash value can represent any piece of data—from a simple password to a 1000-page document—in as few as 16 bytes. When the OS uses a hash value in this way, the value is called a Message Digest (MD).

When you apply a hash function to a document, the fixed-length output uniquely identifies the original document. When the hashed value of the received document is identical to that of the sent document, the receiver knows that no one has modified the document in transit. If the hash value isn't identical, the document is suspect.

Hash functions also help keep passwords secure. Instead of storing a clear-text password, most OSs store a function of a password's hashed value, also called a password derivative. When a user attempts to authenticate, the OS compares the stored password derivative with similarly transformed logon credentials from the client. When the derivatives match, the OS authenticates the user.

3. What's the difference between secret-key ciphers and public-key ciphers?
Key-based ciphers have two primary variants: secret key and public key. Generally speaking, secret-key ciphers are fast to calculate but are difficult to implement securely, whereas public-key ciphers are slower to compute but are much more secure.

Secret-key ciphers. Secret-key ciphers use one key to encrypt and decrypt data. The oldest and most widely used secret-key algorithm is the Data Encryption Standard (DES), which uses a 56-bit key. After several companies demonstrated the inherent weaknesses of 56-bit DES, the US government introduced an improved version called Triple DES (3DES), an extended algorithm that uses one, two, or even three keys and three calculation steps. RSA Laboratories' RC2 and RC4 secret-key ciphers employ algorithms that are more difficult to break than 56-bit DES.

The disadvantage of the secret-key method is that the sender and the receiver must agree on and exchange the secret key that they will use to secure their communication. Developing a technique by which to securely exchange a secret key is difficult. For this reason, key-management protocols exist to securely exchange keys between two systems that an unsecured network connects.

Public-key ciphers. Public-key ciphers use two keys. The most common implementation uses a public key to encrypt data and a private key that is mathematically related to the public key to decrypt data. The private key is the only number the algorithm can use to decrypt the message. Also, unless security has been compromised, only the intended recipient of the message knows the private key. The primary disadvantage of public-key encryption is that it's computationally intensive. RSA, which is the most common public-key algorithm, comes in two flavors: RSA Key Exchange and RSA Signature.

The fastest and most secure cryptographic implementations combine public-key and secret-key algorithms to achieve the best of both worlds. Computationally intensive public-key cryptosystems securely exchange keys and verify the identity of the sender, whereas the much faster secret-key algorithms encrypt data and authenticate both the sender and the receiver (because they exchange and employ the same secret key). According to RSA Laboratories' FAQ, DES computes roughly 100 times faster in software and computes between 1000 and 10,000 times faster in hardware than RSA's public-key algorithm.

4. Why are key lengths important?
If you're concerned enough about security to encrypt your data, you're also concerned that someone might compromise your encrypted data while it's in transit. People who steal and decrypt data for fun or profit are probably well versed in cryptanalysis, which RSA Laboratories defines as "the art and science of breaking encryption or any form of cryptography." The ease with which an intruder can decipher or break in to an intercepted message depends on the length of the encryption key.

The longer a key is in bits, the more time an intruder must devote to trying number combinations to discover the key that will decrypt a message. A 56-bit key presents a large but definite limit to the number of combinations such an intruder would need to check. A malicious user with access to a large array of computers working in parallel can break a 56-bit key encrypted message in a matter of minutes. When a key is 128 bits long, a brute-force attack is 4.7 sextillion (4.7 x 10²¹) times more difficult.

Even when someone successfully breaks a key, the key can expose only messages encrypted with that key. To limit the amount of data that a stolen or cracked key can compromise, most cryptographic implementations support configurable key lifetimes. In an extremely secure scenario, a key might be good for only one message exchange; in a less demanding environment, a key might expire 1 hour after the final message exchange.

5.What type of encryption does NT 4.0 support?
NT is available in two encryption strengths: standard and high. Encryption strength refers specifically to the length of the keys that NT employs for its RC2 and RC4 secret-key algorithms. Microsoft released the OS with a standard encryption strength of 40 bits but upgraded the standard strength to 56 bits in Service Pack 6a (SP6a). The standard version of NT installs the base Cryptographic Service Provider (CSP), which uses either 40-bit or 56-bit keys, depending on the service pack level.

High-encryption (i.e., 128-bit) updates have been available as separate downloads since SP3. The high-encryption version installs the enhanced provider, which extends the key length of RC2 and RC4 secret-key ciphers to 128 bits, doubles the key length for RSA public-key ciphers, and includes DES or 3DES. Because key length is the most important factor in determining how easily a malicious user can crack an intercepted message, the high-encryption version of NT delivers much stronger protection.

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