Public key cryptography brings major security technologies to the desktop in the Windows 2000 environment. The network now is provided with the ability to allow users to safely:
• Transmit over insecure channels
• Store sensitive information on any commonly used media
• Verify a person’s identity for authentication
• Prove that a message was generated by a particular person
• Prove that the received message was not tampered with in transit
Algorithms based on public keys can be used for all these purposes. The most popular public key algorithm is the standard RSA, which is named after its three inventors: Rivest, Shamir, and Adleman. The RSA algorithm is based on two prime numbers with more than 200 digits each. A hacker would have to take the ciphertext and the public key and factor the product of the two primes. As computer processing time increases, the RSA remains secure by increasing the key length, unlike the DES algorithm, which has a fixed key length.
Public key algorithms provide privacy, authentication, and easy key management, but they encrypt and decrypt data slowly because of the intensive computation required. RSA has been evaluated to be from 10 to 10,000 times slower than DES in some environments, which is a good reason not to use public key algorithms for bulk encryption.
Document letterhead can be easily created on a computer, so forgery is a security issue. When information is sent electronically, no human contact is involved. The receiver wants to know that the person listed as the sender is really the sender and that the information received has not been modified in any way during transit. A hash algorithm is implemented to guarantee the Windows 2000 user that the data is authentic. A hash value encrypted with a private key is called a digital signature. Anyone with access to the corresponding public key can verify the authenticity of a digital signature. Only a person having a private key can generate digital signatures. Any modification makes a digital signature invalid.
The purpose of a digital signature is to prevent changes within a document from going unnoticed and also to claim the person to be the original author. The document itself is not encrypted. The digital signature is just data sent along with the data guaranteed to be untampered with. A change of any size invalidates the digital signature.
When King Henry II had to send a message to his troops in a remote location, the letter would be sealed with wax, and while the wax was still soft the king would use his ring to make an impression in it. No modification occurred to the original message if the seal was never broken during transit. There was no doubt that King Henry II had initiated the message, because he was the only person possessing a ring that matched the waxed imprint. Digital signatures work in a similar fashion in that only the sender’s public key can authenticate both the original sender and the content of the document.
The digital signature is generated by a message digest, which is a number generated by taking the message and using a hash algorithm. A message digest is regarded as a fingerprint and can range from a 128-bit number to a 256-bit number. A hash function takes variable-length input and produces a fixed-length output. The message is first processed with a hash function to produce a message digest. This value is then signed by the sender’s private key, which produces the actual digital signature. The digital signature is then added to the end of the document and sent to the receiver along with the document.
Since the mere presence of a digital signature proves nothing, verification must be mathematically proven. In the verification process, the first step is to use the corresponding public key to decrypt the digital signature. The result will produce a 128-bit number. The original message will be processed with the same hash function used earlier and will result in a message digest. The two resulting 128-bit numbers will then be compared, and if they are equal, you will receive notification of a good signature.
If a single character has been altered, the two 128-bit numbers will be different, indicating that a change has been made to the document, which was never scrambled.
Public key cryptography can provide authentication instead of privacy. In Windows 2000, a challenge is sent by the receiver of the information. The challenge can be implemented one of two ways. The information is authenticated because only the corresponding private key could have encrypted the information that the public key is successfully decrypting.
In the first authentication method, a challenge to authenticate involves sending an encrypted challenge to the sender. The challenge is encrypted by the receiver, using the sender’s public key. Only the corresponding private key can successfully decode the challenge. When the challenge is decoded, the sender sends the plaintext back to the receiver. This is the proof for the receiver that the sender is truly the sender. For example, when Alice receives a document from Bob, she wants to authenticate that the sender is really Bob. She sends an encrypted challenge to Bob, using his public key. When he receives the challenge, Bob uses his private key to decrypt the information.
The decrypted challenge is then sent back to Alice. When Alice receives the decrypted challenge, she is convinced that the document she received is truly from Bob. The second authentication method uses a challenge that is sent in plaintext. The receiver, after receiving the document, sends a challenge in plaintext to the sender. The sender receives the plaintext challenge and adds some information before adding a digital signature.
The challenge and digital signature now head back to the sender. The digital signature is generated by using a hash function and then encrypting the result with a private key, so the receiver must use the sender’s public key to verify the digital signature. If the signature is good, the original document and sender have at this point been verified mathematically.
Secret Key Agreement via Public Key
The PKI of Windows 2000 permits two parties to agreed on a secret key while they use nonsecure communication channels. Each party generates half the shared secret key by generating a random number, which is sent to the other party after being encrypted with the other party’s public key. Each receiving side then decrypts the ciphertext using a private key, which will result in the missing half of the secret key. By adding both random numbers together, each party will have an agreed-upon shared secret key, which can then be used for secure communication even though the secret key was first obtained through a nonsecure communication channel.
Bulk Data Encryption without Prior Shared Secrets
The final major feature of public key technology is that it can encrypt bulk data without generating a shared secret key first. The biggest disadvantage of using asymmetric algorithms for encryption is the slowness of the overall process, which results from the necessary intense computations; the largest disadvantage of using symmetric algorithms for encryption of bulk data is the need for a secure communication channel for exchanging the secret key. The Windows 2000 operating system combines symmetric and asymmetric algorithms to get the best of both worlds at just the right moment.
For a large document that must be kept secret, because secret key encryption is the quickest method to use for bulk data, a session key is used to scramble the document.
To protect the session key, which is the secret key needed to decrypt the protected data; the sender encrypts this small item quickly by using the receiver’s public key. This encryption of the session key is handled by asymmetric algorithms, which use intense computation but do not require much time, due to the small size of the session key. The document, along with the encrypted session key, is then sent to the receiver. Only the intended receiver will possess the correct private key to decode the session key, which is needed to decode the actual document. When the session key is in plaintext, it can be applied to the ciphertext of the bulk data and then transform the bulk data back to plaintext.
The Windows Server 2008 PKI does many things behind the scenes. Thanks in part to auto enrollment and certificate stores (places where certificates are kept after their creation), some PKI-enabled features such as EFS work with no user intervention at all. Others, such as IPSec, require significantly less work than would be required without an advanced operating system. Even though a majority of the PKI is handled by Server, it is still instructive to have an overview of how certificate services work.
1. First, a system or user generates a public/private key pair and then a certificate request.
2. The certificate request, which contains the public key and other identifying information such as user name, is forwarded on to a CA.
3. The CA verifies the validity of the public key. If it is verified, the CA issues the certificate.
4. Once issued, the certificate is ready for use and is kept in the certificate store, which can reside in Active Directory. Applications that require a certificate use this central repository when necessary.
In practice, it isn’t terribly difficult to implement certificate services, as the following sidebar shows. Configuring the CA requires a bit more effort, as does planning the structure and hierarchy of the PKI—especially if you are designing an enterprise-wide solution.
In our previous discussion of public and private key pairs, two users wanted to exchange confidential information and did so by having one user encrypt the data with the other user’s public key. We then discussed digital signatures, where the sending user “signs” the data by using his or her private key. Did you notice the security vulnerability in these methods?
Of the three general types of certificates found in a Windows PKI, the user certificate is perhaps the most common. User certificates are certificates that enable the user to do something that would not be otherwise allowed. The Enrollment Agent certificate is one example. Without it, even an administrator is not able to enroll smart cards and configure them properly at an enrollment station. Under Windows Server 2008, required user certificates can be requested automatically by the client and subsequently issued by a certification authority (discussed below) with no user intervention necessary.
Also known as computer certificates, machine certificates (as the name implies) give the system—instead of the user—the ability to do something out of the ordinary. The main purpose for machine certificates is authentication, both client-side and server-side. As stated earlier, certificates are the main vehicle by which public keys are exchanged in a PKI. Machine certificates are mainly involved with these behind-the-scenes exchanges, and are normally overseen by the operating system. Machine certificates have been able to take advantage of Windows’ autoenrollment feature since 2000 Server was introduced.
The term application certificate refers to any certificate that is used with a specific PKI-enabled application. Examples include IPSec and S/MIME encryption for e-mail. Applications that need certificates are generally configured to automatically request them, and are then placed in a waiting status until the required certificate arrives. Depending upon the application, the network administrator or even the user might have the ability to change or even delete certificate requests issued by the application.
Source of Information : Syngress The Best Damn Windows Server 2008 Book Period 2nd Edition
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