OpenSSL实战指南：常用特性和命令解析

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"openssl cookbook" 《OpenSSL Cookbook》是由Ivan Ristić编写的，第二版出版，该书主要关注OpenSSL最常用的功能和命令。它与《Bulletproof SSL and TLS》一书紧密关联，后者是关于理解并部署SSL/TLS和PKI（公钥基础设施）以保护服务器和网络应用安全的专业著作，受到了业界专家的高度评价。 OpenSSL是一个强大的安全套接字层密码库，包含了各种主要的密码算法、常用的密钥和证书封装管理功能以及SSL协议，并提供丰富的应用程序供测试或其他目的使用。这本书面向系统管理员、开发者和IT安全专业人员，旨在教会读者如何利用OpenSSL来防止窃听和冒充攻击，确保网络安全。《OpenSSL Cookbook》的内容可能涵盖了以下关键知识点： 1. **OpenSSL基础**：介绍OpenSSL库的基本概念，包括它的结构、组件和工作原理。 2. **证书管理**：详细解释如何创建、签发、验证和管理X.509数字证书，这是SSL/TLS安全通信的基础。 3. **密钥生成**：涵盖不同类型的密钥（如RSA、DSA、ECDSA等）的生成方法，以及密钥的安全存储和备份。 4. **SSL/TLS协议**：深入解析SSL/TLS协议的工作流程，包括握手过程、加密算法的选择和会话恢复机制。 5. **安全性最佳实践**：提供关于如何配置服务器以实现最高安全性的建议，如避免使用弱密码、禁用不安全的协议版本和加密套件等。 6. **OpenSSL命令行工具**：详细介绍如何使用OpenSSL命令行工具进行各种操作，如`openssl s_client`、`openssl s_server`、`openssl req`等。 7. **故障排查**：指导读者解决在部署和使用OpenSSL过程中可能遇到的问题，如证书链问题、证书过期警告等。 8. **安全策略**：讨论如何制定和实施有效的SSL/TLS策略，包括证书信任链的构建和证书吊销列表（CRL）的管理。 9. **现代加密标准**：涵盖最新的加密标准，如TLS 1.3，以及对前向安全性和完美前向保密（PFS）的理解。 10. **PKI架构**：介绍公共密钥基础设施的组成和运作，包括证书权威机构（CA）、注册机构（RA）的角色和交互。通过《OpenSSL Cookbook》，读者将能够熟练掌握OpenSSL的使用，提升网络安全防护能力，确保数据在传输过程中的安全。无论是在服务器配置、应用程序开发还是网络安全审计中，这本书都是一份宝贵的参考资料。

6 Chapter 1: OpenSSL

Cipher commands (see the `enc' command for more details)

aes-128-cbc aes-128-ecb aes-192-cbc aes-192-ecb

aes-256-cbc aes-256-ecb base64 bf

bf-cbc bf-cfb bf-ecb bf-ofb

camellia-128-cbc camellia-128-ecb camellia-192-cbc camellia-192-ecb

camellia-256-cbc camellia-256-ecb cast cast-cbc

cast5-cbc cast5-cfb cast5-ecb cast5-ofb

des des-cbc des-cfb des-ecb

des-ede des-ede-cbc des-ede-cfb des-ede-ofb

des-ede3 des-ede3-cbc des-ede3-cfb des-ede3-ofb

des-ofb des3 desx rc2

rc2-40-cbc rc2-64-cbc rc2-cbc rc2-cfb

rc2-ecb rc2-ofb rc4 rc4-40

seed seed-cbc seed-cfb seed-ecb

seed-ofb zlib

Building a Trust Store

OpenSSL does not come with any trusted root certiﬁcates (also known as a trust store), so if

you’re installing from scratch you’ll have to ﬁnd them somewhere else. One possibility is to

use the trust store built into your operating system. This choice is usually ﬁne, but default

trust stores may not always be up to date. A better choice—but one that involves more work

—is to turn to Mozilla, which is putting a lot of effort into maintaining a robust trust store.

For example, this is what I did for my assessment tool on SSL Labs.

Because it’s open source, Mozilla keeps the trust store in the source code repository:

https://hg.mozilla.org/mozilla-central/raw-file/tip/security/nss/lib/ckfw/builtins…

/certdata.txt

Unfortunately, their certiﬁcate collection is in a proprietary format, which is not of much

use to others as is. If you don’t mind getting the collection via a third party, the Curl project

provides a regularly-updated conversion in Privacy-Enhanced Mail (PEM) format, which you

can use directly:

http://curl.haxx.se/docs/caextract.html

But you don’t have to write a conversion script if you’d rather download directly from Mozilla.

Conversion scripts are available in Perl or Go. I describe both in the following sections.

Note

If you do end up working on your own conversion script, note that Mozilla’s root

certiﬁcate ﬁle actually contains two types of certiﬁcates: those that are trusted and

8 Chapter 1: OpenSSL

2012/06/04 09:52:29 Failed to parse certificate starting on line 23068: negative …

serial number

In my case, there was one invalid certiﬁcate that the Go X.509 library couldn’t handle, but

otherwise the conversion worked as expected.

Key and Certiﬁcate Management

Most users turn to OpenSSL because they wish to conﬁgure and run a web server that sup-

ports SSL. That process consists of three steps: (1) generate a strong private key, (2) cre-

ate a Certiﬁcate Signing Request (CSR) and send it to a CA, and (3) install the CA-provided

certiﬁcate in your web server. These steps (and a few others) are covered in this section.

Key Generation

The ﬁrst step in preparing for the use of public encryption is to generate a private key. Before

you begin, you must make several decisions:

Key algorithm

OpenSSL supports RSA, DSA, and ECDSA keys, but not all types are practical for use in

all scenarios. For example, for web server keys everyone uses RSA, because DSA keys are

effectively limited to 1,024 bits (Internet Explorer doesn’t support anything stronger)

and ECDSA keys are yet to be widely supported by CAs. For SSH, DSA and RSA are

widely used, whereas ECDSA might not be supported by all clients.

Key size

The default key sizes might not be secure, which is why you should always explicitly

conﬁgure key size. For example, the default for RSA keys is only 512 bits, which is simply

insecure. If you used a 512-bit key on your server today, an intruder could take your

certiﬁcate and use brute force to recover your private key, after which he or she could

impersonate your web site. Today, 2,048-bit RSA keys are considered secure, and that’s

what you should use. Aim also to use 2,048 bits for DSA keys and at least 256 bits for

ECDSA.

Passphrase

Using a passphrase with a key is optional, but strongly recommended. Protected keys

can be safely stored, transported, and backed up. On the other hand, such keys are

inconvenient, because they can’t be used without their passphrases. For example, you

might be asked to enter the passphrase every time you wish to restart your web server.

Key Generation 9

For most, this is either too inconvenient or has unacceptable availability implications.

In addition, using protected keys in production does not actually increase the security

much, if at all. This is because, once activated, private keys are kept unprotected in

program memory; an attacker who can get to the server can get the keys from there

with just a little more effort. Thus, passphrases should be viewed only as a mechanism

for protecting private keys when they are not installed on production systems. In other

words, it’s all right to keep passphrases on production systems, next to the keys. If you

need better security in production, you should invest in a hardware solution.

To generate an RSA key, use the genrsa command:

$ openssl genrsa -aes128 -out fd.key 2048

Generating RSA private key, 2048 bit long modulus

....+++

...................................................................................…

+++

e is 65537 (0x10001)

Enter pass phrase for fd.key: ****************

Verifying - Enter pass phrase for fd.key: ****************

Here, I speciﬁed that the key be protected with AES-128. You can also use AES-192 or AES-256

(switches -aes192 and -aes256, respectively), but it’s best to stay away from the other algo-

rithms (DES, 3DES, and SEED).

Warning

The e value that you see in the output refers to the public exponent, which is set to

65,537 by default. This is what’s known as a short public exponent, and it signiﬁcantly

improves the performance of RSA veriﬁcation. Using the -3 switch, you can choose 3

as your public exponent and make veriﬁcation even faster. However, there are some

unpleasant historical weaknesses associated with the use of 3 as a public exponent,

which is why generally everyone recommends that you stick with 65,537. The latter

choice provides a safety margin that’s been proven effective in the past.

Private keys are stored in the so-called PEM format, which is just text:

$ cat fd.key

-----BEGIN RSA PRIVATE KEY-----

A small number of organizations will have very strict security requirements that require the private keys to be protected at any cost. For them,

the solution is to invest in a Hardware Security Module (HSM), which is a type of product speciﬁcally designed to make key extraction impossi-

ble, even with physical access to the server. To make this work, HSMs not only generate and store keys, but also perform all necessary operations

(e.g., signature generation). HSMs are typically very expensive.

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