公钥基础设施设计与实施详解

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《公钥基础设施实施与设计》是一本深入讲解公钥基础设施（Public-Key Infrastructure, PKI）的实用指南。本书主要关注于PKI的核心概念、架构设计以及关键组件的功能和管理。以下是对各章节内容的详细解析： 1. **前言**：书中首先介绍书目的目标，适合哪些读者群体以及书籍的组织结构。作者强调了使用的一般性约定，为后续章节的学习奠定基础。 2. **第1章：密码学基础** - 本章涵盖了密码学的基本原理，如加密、解密、哈希函数等。重点讨论了加密技术在信息安全中的应用，特别是数字签名，它是PKI中的核心组成部分，用于验证数据的完整性和来源的真实性。 3. **第2章：公钥基础设施基础** - 这里定义了什么是PKI，它包括硬件、软件、人员、策略和规程等要素。讲解了PKI的主要组件，如证书权威机构（CA）、证书生命周期过程，以及如何与PKI系统协作。 4. **第3章：PKI架构** - 章节介绍了PKI架构的不同类型，包括单一CA架构（适用于小型或特定应用场景）、企业级PKI架构（涉及多层次管理和安全性要求）和混合架构（结合了不同类型的PKI元素）。作者还讨论了选择合适架构的重要性，并提供了决策建议。 5. **第4章：CA功能** - 作为PKI的核心部分，CA负责证书的签发、撤销和管理。本章详细解释了CA的功能，如证书的签发流程、证书撤销策略，以及制定证书政策和认证实践声明（CPS）的重要性，通过示例展示了Solv, Inc.公司的CPS。 6. **第5章：证书管理** - 本章聚焦于证书的注册、请求、审批和撤销过程，以及确保安全的认证和授权方法。这部分内容对于理解PKI的有效运作至关重要。综上，本书围绕PKI的核心要素展开，从基础概念到实践操作，帮助读者全面理解和设计实施一个安全、可靠的公钥基础设施。无论是技术人员、安全专家还是IT管理员，都能从中受益匪浅，掌握如何有效地构建、管理和维护PKI环境。

Cryptography has now become an industry standard for providing information security, trust, controlling

access to resources, and electronic transactions. Its use is no longer limited to just securing sensitive military

information. In fact, cryptography is now recognized as one of the major components of the security policy of

an organization.

Before moving further with cryptography, let us first look at a few terms that are commonly associated with

cryptography:

Plaintext: Is the message that has to be transmitted to the recipient. It is also commonly referred to as

cleartext.

•

Encryption: Is the process of changing the content of a message in a manner such that it hides the

actual message.

•

Ciphertext: Is the output that is generated after encrypting the plain text.•

Decryption: Is the reverse of encryption and is the process of retrieving the original message from its

encrypted form. This process converts ciphertext to plaintext.

•

Hash algorithm: Is an algorithm that converts text string into a string of fixed length.•

Key: Is a word, number, or phrase that is used to encrypt the cleartext. In computerbased

cryptography, any text, key word, or phrase is converted to a very large number by applying a hash

algorithm on it. The large number, referred to as a key, is then used for encryption and decryption.

•

Cipher: Is a hash algorithm that translates plaintext into an intermediate form called ciphertext, in

which the original message is in an unreadable form.

•

Cryptanalysis: Is the science of breaking codes and ciphers.•

Before looking at the details of various cryptographic techniques, let us now look at the steps involved in the

conventional encryption model:

A sender wants to send a Hello message to a recipient.1.

The original message, also called plaintext, is converted to random bits known as ciphertext by using

a key and an algorithm. The algorithm being used can produce a different output each time it is used,

based on the value of the key.

The ciphertext is transmitted over the transmission medium.3.

At the recipient end, the ciphertext is converted back to the original text using the same algorithm and

key that were used to encrypt the message.

This process is also shown in Figure 1−5.

Figure 1−5: Conventional encryption model

Having looked at an overview of cryptography, let us now look at the various cryptography techniques

available. For the purpose of classification, the techniques are categorized on the basis of the number of keys

that are used. The two main cryptography techniques are

Single key cryptography: This cryptography technique is based on a single key. It is also known as

symmetric key or private key or secret key encryption.

•

Public key cryptography: This cryptography technique is based on a combination of two keyssecret

key and public key. It is also known as asymmetric encryption.

•

The Basics of Cryptography

Let us look at each of these methods in detail.

Single Key Cryptography

The process of encryption and decryption of information by using a single key is known as secret key

cryptography or symmetric key cryptography. In symmetric key cryptography, the same key is used to encrypt

as well as decrypt the data. The main problem with symmetric key algorithms is that the sender and the

receiver have to agree on a common key. A secure channel is also required between the sender and the

receiver to exchange the secret key.

Heres an example that illustrates the process of single key cryptography. Alice wants to send a For Your Eyes

message to Bob and wants to ensure that only Bob is able to read the message. To secure the transmission,

Alice generates a secret key, encrypts the message with this key, and sends the message to Bob.

Figure 1−6 represents the process of secret key cryptography.

Figure 1−6: Secret key cryptography

Now, to read the encrypted message, Bob would need the secret key that has been generated by Alice. Alice

can give the secret key to Bob in person or send the key to Bob by any other means available. If Alice sends

the key to Bob in person, it could be time−consuming depending on the physical distance between the two of

them or other circumstances such as Bobs availability. After Bob receives the secret key, he can decrypt the

message to retrieve the original message.

Many secret key algorithms were developed on the basis of the concept of secret key cryptography. The most

widely used secret key algorithms include

Data Encryption Standard (DES)•

Triple−DES (3DES)•

International Data Encryption Algorithm (IDEA)•

RC4•

CAST−128•

Advanced Encryption Standard (AES)•

Let us consider these algorithms in detail in the following sections.

Data Encryption Standard (DES)

DES, which is an acronym for the Data Encryption Standard, is the common name for the Federal Information

Processing Standard (FIPS) 46−3. It describes the Data Encryption Algorithm (DEA). DEA is also defined in

the ANSI standard X3.92. The DES algorithm is one of the most widely used encryption algorithms in the

world. The Data Encryption Standard (DES) algorithm was developed by the IBM team in the 1970s and was

adopted by National Institute of Standards and Technology (NIST) for commercial applications.

The Basics of Cryptography

Note Refer to RFCs 1827 and 2144 for more information on DES.

DES is still surrounded by controversy. This controversy was originally fueled by the following facts:

The key length used by this algorithm was reduced to 56 bits by the U.S. government, although the

original design called for a key length of 128 bits, leading to a compromise on security. Although the

algorithm for DES was published, the rationale for the design was never published.

•

DES became widely available to the U.S. public and to approved users in other countries. However,

DES was excluded by the U.S. government from protection of any of its own classified information.

•

The major weaknesses and attacks that are faced by DES are described below.

Brute Force Attack The simplest attack to decipher a DES key is the brute force attack. The brute force

attack on the DES algorithm is feasible because of the relatively small key length (56 bit) and ever−increasing

computational power of the computers. Until the mid−1990s, brute force attacks were beyond the capabilities

of hackers because the cost of computers that were capable of hacking was extremely high and unaffordable.

With the tremendous advancement in the field of computing, high−performance computers are relatively

cheaper and, therefore, affordable. In fact, general purpose PCs today can be successfully used for brute force

attacks. Many hackers today are using more powerful techniques, such as Field Programmable Gate Array

(FPGA) and Application−Specific Integrated Circuits (ASIC) technology that provide faster and cheaper

means of hacking.

You can break through any cipher by trying all keys that possibly exist. However, in brute force attacks, the

time taken to break a cipher is directly proportional to the length of the key. In a brute force attack, keys are

randomly generated and applied to the ciphertext until the legitimate key is generated. This key decrypts the

data into its original form. Therefore, the encryption key length is a major factor that needs to be considered

while choosing a key. The longer the encryption keys, the stronger the security. For example, in case of a

32−bit long key, the number of steps required to break the cipher are about 2

or 10

. Similarly, a 40−bit key

requires about 2

steps. This is something which can be achieved in one week by anyone sitting on his

personal computer. A 56−bit key is known to have been broken by professionals and governments by using

special hardware in a few months time. Today, 128−bit encryption is considered to be the safest and most

reliable means of encrypting messages.

Note On January 19, 1999, a group of computer enthusiasts from all over the world formed a coalition to

decipher a DES encrypted ciphertext and as a result recovered the key in a record−breaking time of 22

hours and 15 minutes. This coalition was known as Distributed.Net. Its members worked with DES

Cracker and a worldwide network of nearly 100,000 PCs on the Internet to recover the key. The DES

Cracker machine was specially designed for this purpose.

Note For more information on brute force attacks, refer to RFCs 2228 and 2557.

Differential Cryptanalysis Attack The differential cryptanalysis attack looks specifically at pairs of

ciphertexts whose plaintext have some specific differences. It analyzes these differences as the plaintext

propagates through the various rounds of DES when they are encrypted with the same key.

This technique chooses pairs of plaintext with a fixed difference. Two plaintexts can be chosen at random, as

long as they satisfy specific difference conditions. Then, using the differences in the resulting ciphertexts,

different probabilities can be assigned to different keys. As more and more ciphertext pairs are analyzed, one

key emerges, as the most probable candidate key.

Note For more information on differential cryptanalysis attack, refer to RFC 2144.

The Basics of Cryptography

Linear Cryptanalysis Attack Linear Cryptanalysis attack was invented by Mitsuru Matsui in 1993. This

method is based on the concept that if you XOR some of the plaintext bits together, XOR some ciphertext bits

together, and then XOR the results, you will get a single bit that is the XOR of some of the key bits. A large

number of such plaintexts/ciphertexts pairs are then used to guess the values of the key bits. The greater the

volume of the base data, the more reliable is the guess.

Note For more information on linear cryptanalysis attacks, refer to RFC 2144.

Triple Data Encryption Standard (3DES)

Triple−DES is a minor variation of DES. Although, three times slower than DES, it can be much more secure,

if used properly. In todays scenario, Triple−DES is implemented more widely than DES. This is because DES

is easy to break with the help of advanced technology that is widely available today. On the other hand, 3DES

has proved to be an extremely reliable solution because of the longer key length that it uses. This extended

length of key plays an important role in eliminating many of the shortcut attacks that can be used to reduce the

amount of time it takes to break DES.

You can increase the effective key length of your cryptographic system by using the Triple Pass DES through

the process known as EDE (Encrypt, Decrypt, and Encrypt). When you use triple pass DES, it first encrypts

the plaintext data with a 56−bit key. The ciphertext so obtained is then decrypted by using a different key.

When you decrypt ciphertext with some different key it gives some garbage. Finally, you encrypt the garbage

with the first key. This process of using triple pass DES for encryption, decryption, and again encryption is

commonly referred to as EDE.

Figure 1−7 explains the process of 3DES. This is why this encryption method is referred to as Triple−DES.

Figure 1−7: Process of 3DES

Note Triple−DES has been adopted by ANSI as the standard X9.52 and has been proposed as a revision to

FIPS 46, known as draft FIPS 46−3.

Note Refer to RFCs 1828 and 2420 for more information on Triple−DES.

International Data Encryption Algorithm (IDEA)

The International Data Encryption Algorithm (IDEA) is a symmetric block cipher developed by Xuejia Lai

and James Massey of the Swiss Federal Institute of technology. It uses a 128−bit key to encrypt data in blocks

of 64 bits. This is why it is referred to as a block cipher method. IDEA is designed to facilitate both software

and hardware implementation.

The major factors that make IDEA a strong algorithm are:

The key length is long enough to prevent comprehensive key searches. IDEA uses a key length of 128

bits, which makes it very secure.

•

The ciphertext is not easily decipherable from the plaintext and the key. IDEA effectively masks the

statistics of how the ciphertext depends on the statistics of the plaintext.

•

The Basics of Cryptography

IDEA was developed to provide a high level of security with ease of implementation. Due to its strength and

reliability IDEA is now used worldwide in many banking and industry applications.

Note You can find more information about the use of the IDEA Encryption Algorithm in a Certificate

Management System in RFC 3058.RC2

RC2 or Rons Code 2 is a 64−bit block cipher that was designed by Ron Rivest. It uses variable−sized keys.

This algorithm was designed to replace DES. The code for this algorithm was not made public. However,

many companies have licensed RC2 for use in their products. RC2 is being used in a number of software

packages, such as Lotus Notes, Microsoft Windows, Internet Explorer, and Netscape Communications

Navigator and Communicator. In addition, RC2 forms an integral component of S/MIME as it provides

privacy and interpretability between the export versions and domestic versions of products that use S/MIME.

Note You can find more information about RC2 in RFC 2268.

RC4

RC4 is a cipher that was also designed by Ron Rivest, who was the co−inventor of the RSA cipher. It is used

in a number of commercial systems like Lotus Notes and Secure Netscape.

XRef For more information on RSA, refer to the RSA section in this

chapter.

It is a cipher with a key size of up to 2048 bits (256 bytes). It is listed in the category of relatively fast and

strong cipher methods. It is a stream cipher that creates a stream of random bytes and XORs these bytes with

the text. Using RC4 with the same key on two different messages makes it very weak. It is thus useful in

situations, in which a new key can be chosen for each message.

Note You can find more information about RC4 in RFC 2246.

RC5

RC5 is yet another block cipher designed by Ron Rivest for RSA Security in 1994. Along with a variable key

size, and a variable number of rounds, the size of RC5 data blocks is variable. The block size can range from

32 bits, 64 bits, to 128 bits. Similarly, the number of rounds can range from 0 to 255, while the key can range

from 0 bits to 2040 bits in size.

Note You can find more information about RC4 in RFC 2040.

CAST−128

Carlisle Adams developed CAST−128 in May 1997. This algorithm uses a variable key length and uses block

sizes of 64 bits.

The key lengths supported by CAST−128 vary from 40 bits to 128 bits, in increments of 8 bits. For key sizes

that range up to 80 bits, the data block undergoes 12 rounds of encryption, while for key sizes of more than 80

bits, the algorithm has 16 rounds. For the keys whose sizes are less than 128 bits, zeroes are added to the

rightmost (or the least significant) bits until the total length of the key result is 128 bits. This is done because

the algorithm must have an input key of 128 bits in length.

The Basics of Cryptography

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