《电子商务安全与保密》 1/48 © 2015-9-19 Guangzhou University PGP 软件简介 PGP( Pretty Good Privacy) 加密软件是美国 PGP Corporation. 出产的免费软件, 作者为

《电子商务安全与保密》 1/48 ©23/4/19 Guangzhou University

PGP 软件简介

PGP( Pretty Good Privacy) 加密软件是美国 PGP Corporation . 出产的免费软件 , 作者为 Philip Zimmermann ( 可自由下载 www.pgp.com) ，可用它对文件、邮件进行加密，在常用的 WINZIP 、 WORD 、 ARJ 、 EXCEL 等软件的加密功能均告可被破解时，选择 PGP 对自己的私人文件、邮件进行加密不失为一个好办法。

除此之外，你还可和同样装有 PGP 软件的朋友互相传递加密文件，安全十分保障。

PGP 中常见的几个概念PGP 的工作原理PGP 常用功能的使用方法


PGP 软件简介—几个常见的概念

• PGP Desktop: A software tool that uses cryptography to protect your data against unauthorized access. Available for Mac OS X and Windows.( 本次课将要介绍是软即是PGP Desktop v 9.0 , 主要用于个人用户 )


• PGP Universal: A tool for enterprises to automatically and transparently secure email messaging for their employees( 一个用于企业环境的加密系统 ).

• PGP Universal environments can include client software, called PGP Universal Satellite, which allows users in a domain protected by a PGP Universal Server to secure email messages all the way to the desktop.

• Version 9.0 and above of PGP Desktop includes the PGP Universal Satellite functionality, thus allowing PGP Desktop users to easily fit into a PGP Universal environment.



• PGP Global Directory: A free, public key server hosted by PGP Corporation( 一个由 PGP 公司维护的免费的公钥服务器） .

• The PGP Global Directory provides quick and easy access to the universe of PGP keys. It uses next-generation key server technology that verifies the email address on a key (so that the key server does not get clogged with unused keys) and lets users manage their own keys. Using the PGP Global Directory significantly enhances your chances of finding the public key of someone with whom you want to send secured messages. PGP Desktop is designed to work closely with the PGP Global Directory.



• PGP Messaging: A feature of PGP Desktop that automatically and transparently supports all of your email clients through policies you control. Also protects many IM clients, such as AIM and iChat (both users must have PGP Messaging enabled).



• PGP Disk: Whole Disk Encryption is a feature of PGP Desktop that protects your entire hard drive or drives, including your boot record, thus protecting all of your files all the time.

• Virtual Disk volumes are another feature of PGP Desktop that let you use part of your hard drive space as an encrypted virtual disk with its own drive letter. If you like, you can use Whole Disk Encryption and Virtual Disk volumes on the same system.



• PGP Zip: A feature of PGP Desktop that lets you put any combination of files and folders into a single encrypted, compressed package for convenient transport or backup. You can encrypt a PGP Zip archive to a PGP key or to a passphrase, allowing you to send the archive to someone who does not even have PGP Desktop on their system.



• PGP Keys: A feature of PGP Desktop that gives you complete control over both your own PGP keys and the keys of those person with whom you are securely exchanging email messages.

• Shredding: A feature of PGP Desktop that lets you completely destroy sensitive data. PGP Shred overwrites files so that even file recovery software cannot recover them.



• Smart cards and tokens: Smart cards and tokens are portable devices on which you can create your PGP keypair or copy your PGP keypair. Creating your PGP keypair on a smart card or token adds security by requiring the smart card or token in order to encrypt, sign, decrypt, or verify.

• So even if an unauthorized person gains access to your computer, your encrypted data is secure because your PGP keypair is with you on your smart card or token. Copying your PGP keypair to a smart card or token is a good way to back it up and an easy way to distribute your public key.



• Conventional cryptography （常规加密技术 , 对称密钥密码体制） : Uses the same passphrase to encrypt and decrypt data. Conventional cryptography is great for data that isn’t going anywhere (because it encrypts and decrypts quickly). However, conventional cryptography is not as well suited for situations where you need to send encrypted data to someone else, especially if you want to send encrypted data to someone you have never met.



• Public-key cryptography( 公开密钥密码体制） : Public-key cryptography uses two keys (called a keypair) for encrypting and decrypting. One of these two keys is your private key; and like the name suggests, you need to keep it private. Very, very private. The other key is your public key, and like its name suggests, you can share it with the general public. In fact, you’re supposed to share.



• Public key( 公开密钥） : The key you distribute to others so that they can send protected messages to you (messages that can only be decrypted by your private key) and so they can verify your digital signature. Public keys are meant to be widely distributed. Your public and private keys are mathematically related, but there’s no way to figure out someone’s private key if you have their public key.

• Private key （私有密钥） : The key you keep very, very private. Only your private key can decrypt data that was encrypted using your public key. Also, only your private key can create a digital signature that your public key can verify. Do not give your private key, or its passphrase, to anyone!



• Encrypting( 加密） : The process of scrambling data so that if an unauthorized person gets access to it, they cannot do anything with it (it’s so scrambled, it’s meaningless).

• Signing( 签名） : The process of applying a digital signature to data using your private key. Because data signed by your private key can be verified only by your public key, the ability to verify signed data with your public key proves that your private key signed the data and thus proves the data is from you.

• Decrypting( 解密） : The process of taking encrypted (scrambled) data and making it meaningful again. When you receive data that has been encrypted using your public key, you use your private key to decrypt the data.



• Key server: A repository for keys. Some companies host key servers for the public keys of their employees, so other employees can find their public keys and send them protected messages.



PGP- 软件简介 -PGP 的工作原理

• Conventional cryptography

• In conventional cryptography, also called secret-key or symmetric-key encryption, one key is used both for encryption and decryption. The Data Encryption Standard (DES) is an example of a conventional cryptosystem that is widely employed by the Federal Government. Figure 1-2 is an illustration of the conventional encryption process.


• Key management and conventional encryption

• Conventional encryption has benefits. It is very fast( 最主要优点是速度快） . It is especially useful for encrypting data that is not going anywhere. However, conventional encryption alone as a means for transmitting secure data can be quite expensive simply due to the difficulty of secure key distribution （最大的困难是密钥分配问题） . Recall a character from your favorite spy movie: the person with a locked briefcase handcuffed to his or her wrist. What is in the briefcase, anyway? It's probably not the missile launch code/ biotoxin formula/ invasion plan itself. It's the key that will decrypt the secret data.



• For a sender and recipient to communicate securely using conventional encryption, they must agree upon a key and keep it secret between themselves. If they are in different physical locations, they must trust a courier, the Bat Phone, or some other secure communication medium to prevent the disclosure of the secret key during transmission. Anyone who overhears or intercepts the key in transit can later read, modify, and forge all information encrypted or authenticated with that key. From DES to Captain Midnight's Secret Decoder Ring, the persistent problem with conventional encryption is key distribution: how do you get the key to the recipient without someone intercepting it?



• Public key cryptography• The problems of key distribution are solved by

public key cryptography, the concept of which was introduced by Whitfield Diffie and Martin Hellman in 1975. (There is now evidence that the British Secret Service invented it a few years before Diffie and Hellman, but kept it a military secret — and did nothing with it. [J H Ellis:



• The Possibility of Secure Non-Secret Digital Encryption, CESG Report, January 1970]) Public key cryptography is an asymmetric scheme that uses a pair of keys for encryption: a public key, which encrypts data, and a corresponding private, or secret key for decryption. You publish your public key to the world while keeping your private key secret. Anyone with a copy of your public key can then encrypt information that only you can read. Even people you have never met.



• It is computationally infeasible to deduce the private key from the public key( 由于采用单向陷门函数，反算私有密钥是非常困难的） . Anyone who has a public key can encrypt information but cannot decrypt it. Only the person who has the corresponding private key can decrypt the information.



• The primary benefit of public key cryptography is that it allows people who have no preexisting security arrangement to exchange messages securely( 无需事先安排，即可实现安全通信） . The need for sender and receiver to share secret keys via some secure channel is eliminated （不再需要安全的信道） ; all communications involve only public keys, and no private key is ever transmitted or shared.



• Some examples of public-key cryptosystems are Elgamal (named for its inventor, Taher Elgamal), RSA (named for its inventors, Ron Rivest, Adi Shamir, and Leonard Adleman), Diffie-Hellman (named, you guessed it, for its inventors), and DSA, the Digital Signature Algorithm (invented by David Kravitz).


ElGamal 是 Diffie-Hellman 的变种，在 PGP 中， Diffie-Hellman/DSS(

Digital Signature Standard) 中的密钥就是 ElGamal 密钥．


• Because conventional cryptography was once the only available means for relaying secret information, the expense of secure channels and key distribution relegated its use only to those who could afford it, such as governments and large banks (or small children with secret decoder rings). ( 在公钥密码体制之前，对称密钥系统是唯一可行的加密技术，只有拥有安全信息传递私密密钥的机构才可能使用，因此只有政府或大银行才能进行加密数据传输）．



• Public key encryption is the technological revolution that provides strong cryptography to the adult masses. Remember the courier with the locked briefcase handcuffed to his wrist? Public-key encryption puts him out of business (probably to his relief). ( 公钥密码体制是一个革命性的变革，它使普通大众也可以加密技术带来的便利 - 　例如通过 PGP 实现文件、邮件、即时消息、磁盘或虚拟磁盘的加密质。 )



• How PGP works

– PGP combines some of the best features of both conventional and public key cryptography. PGP is a hybrid cryptosystem. (PGP 集中了对称密钥系统和公开密钥系统的优势，可以说明 PGP 是一个混和系统）。



• When a user encrypts plaintext with PGP, PGP first compresses the plaintext. Data compression saves modem transmission time and disk space and, more importantly, strengthens cryptographic security. Most cryptanalysis techniques exploit patterns found in the plaintext to crack the cipher. Compression reduces these patterns in the plaintext, thereby greatly enhancing resistance to cryptanalysis. (Files that are too short to compress or which don't compress well aren't compressed.)



• PGP then creates a session key( 会话密钥） , which is a one-time-only secret key( 一次一密密钥） . This key is a random number( 一次一密采用的是一个随机数） generated from the random movements of your mouse and the keystrokes you type.

• This session key works with a very secure, fast conventional encryption algorithm to encrypt the plaintext; the result is ciphertext( 采用快速，安全性高的加密算法） .

• Once the data is encrypted, the session key is then encrypted to the recipient‘s public key. This public key-encrypted session key is transmitted along with the ciphertext to the recipient.( 而会话密钥则被公钥加密）

• 上述过程见下图





• Decryption works in the reverse. The recipient's copy of PGP uses his or her private key to recover the temporary session key, which PGP then uses to decrypt the conventionally-encrypted ciphertext.

•



• The combination of the two encryption methods combines the convenience of public key encryption with the speed of conventional encryption.

• Conventional encryption is about 1, 000 times faster than public key encryption( 对称密钥的计算速度是公钥的1000 位） .

• Public key encryption in turn provides a solution to key distribution and data transmission issues. Used together, performance and key distribution are improved without any sacrifice in security （而公钥则很好地解决了会话密钥的分发问题） .



对称加密体制 :AES IDEA TripleDES 及其它

公钥密码体制 :RSA Diffie-Hellman/DSS



• Keys• A key is a value that works with a cryptographic algorithm to produce

a specific ciphertext. Keys are basically really, really, really big numbers.

• Key size is measured in bits; the number representing a 1024-bit key is darn huge. In public key cryptography, the bigger the key, the more secure the ciphertext. However, public key size and conventional cryptography's secret key size are totally unrelated.

• A conventional 80-bit key has the equivalent strength of a 1024-bit public key. A conventional 128-bit key is equivalent to a 3000-bit public key. Again, the bigger the key, the more secure, but the algorithms used for each type of cryptography are very different and thus comparison is like that of apples to oranges. ( 公钥体制中 1024位的密钥强度与对称密钥体制中的 80 位相当，对称密钥中的 128位密钥相当于公钥的 3000 位左右）。



• While the public and private keys are mathematically related, it's very difficult to derive the private key given only the public key; however, deriving the private key is always possible given enough time and computing power. This makes it very important to pick keys of the right size; large enough to be secure, but small enough to be applied fairly quickly. Additionally, you need to consider who might be trying to read your files, how determined they are, how much time they have, and what their resources might be.



• Larger keys will be cryptographically secure for a longer period of time. If what you want to encrypt needs to be hidden for many years, you might want to use a very large key. Of course, who knows how long it will take to determine your key using tomorrow's faster, more efficient computers? There was a time when a 56-bit symmetric key was considered extremely safe.



• 在 PGP 中密钥是以加密的形式存储的 . PGP 存储密钥磁盘中的两上文件中，一个为公钥，另一个为私有密钥。

• These files are called keyrings(PGP 中存储密钥的文件称为 Keyrings). As you use PGP, you will typically add the public keys of your recipients to your public keyring.

• Your private keys are stored on your private keyring. If you lose your private keyring, you will be unable to decrypt any information encrypted to keys on that ring.( 在 PGP 中私是非常重的，一但私钥丢失，则你将无法解密你的任何资料 ). 而你的密钥则通过你输入的password 或 passphrase 来加密的 .



• A passphrase should be hard for you to forget and difficult for others to guess. It should be something already firmly embedded in your long-term memory, rather than something you make up from scratch( 你选择的passphrase 必须不易忘记 , 且外人容易猜测 .)

Why? Because if you forget your passphrase, you are out of luck. Your private key is totally and absolutely useless without your passphrase and nothing can be done about it.

• PGP is cryptography that will keep major governments out of your files. It will certainly keep you out of your files, too



生成密钥对 ( 公开密钥和私有密钥 )

• 使用 PGP 之前，首先需要生成一对密钥，这一对密钥其实是同时生成的，其中的一个我们称为公钥，意思是公共的密钥，你可以把它分发给你的朋友们，让他们用这个密钥来加密文件，另一个我们称为私钥，这个密钥由你保存，你是用这个密钥来解开加密文件的。打开“开始”中“ PGP” 的“ PGP KEYS” ，可看到以下的画面。点击图标或者用菜单 key>new key 开始生成密钥。 PGP 有一个很好的密钥生成向导，只要跟着它一步一步做下去就可以生成密钥 ! 　　

PGP- 软件简介 - 常用功能使用


输入 Name 和 E-mail



设置密钥类型



输入 Passphrass : 应该尽长且字母数字混杂



生成密钥， master key( 用于签名）　 subkey 用于加密解密



如果系统与 Internet 有连接的话，你可选择是否将公钥发到 Global Directory 中



加密和解密• PGP Zip archives can be any of three types:

– PGP Zip archives encrypted to a public key. If you are sending the PGP Zip archive to one or more persons whose public keys you have, you should encrypt the archive to their public keys; thus, only the intended recipients can open the archive. The recipients must have PGP Desktop installed.( 若要发给另一个 , 则用其公钥加密 , 公钥用于加密会话密钥 ).



• PGP Zip archives encrypted to a passphrase. If you prefer to encrypt to a passphrase or you’re sending the archive to multiple recipients, some of whom you don’t have their public key, you can specify conventional encryption and encrypt the archive using a passphrase (you will need to communicate the passphrase to the recipients). The recipients must have PGP Desktop installed.( 若文件的接收者没有 PGP, 则你只能将文件用 passphrase 加密 , 然后你要设法通知文件的接收者 , 此时文件的接收者要安装有 PGP, 此时你输入的 passphrase 用于加密会话密钥 ).



• PGP Zip archives that are self decrypting. If the person to whom you are sending an archive does not have PGP Desktop, then you need to create a self-decrypting PGP Zip archive, called a PGP Zip SDA. A PGP Zip SDA will be slightly larger (the executable increases the file size somewhat) than a regular PGP Zip archive, but it can be opened on a Windows system that does not have PGP Desktop installed. A PGP Zip SDA created on a Windows system can only be opened on another Windows system.( 如果文件的接收者没有安装 PGP, 则你还可以将文件加密成自我释放型 , PGP zip SDA(self-decrypting PGP zip Archive).




修改加密算法


PGP Messaging

When secure email messaging is enabled, PGP Desktop monitors the email traffic between your email client and your mail server. Depending on the circumstances, PGP Desktop will intercede on your behalf to encrypt, sign, decrypt, or verify messages( 当启用消息监控功能后， PGP 会邮件 Client 和邮件 Server 之间的数据传输，并进行相应的加密、解密、签名等处理） .



Shredding

If you want to completely destroy sensitive files without leaving fragments of their data behind, use the PGP Shred utility. When you delete a file using Shred, the file is immediately overwritten (even on systems with virtual memory) and all traces of the file are removed so that it cannot be retrieved even by using disk recovery software.


Documents

《电子商务安全与保密》 1/48 © 2015-9-19 Guangzhou University PGP 软件简介 PGP( Pretty Good Privacy) 加密软件是美国 PGP Corporation. 出产的免费软件, 作者为