LESSON 9 E-MAIL SECURITY part VII

9.3.2 Digital Signatures
A digital signature is generated by your e-mail software and your private key to assure the
authenticity of your e-mail. The purpose of the signature is twofold. The first is to certify it
came from you. This is called non-repudiation. The second is to ensure the contents have not
been altered. This is called data integrity. The way an e-mail program accomplishes this is by
running the contents of your message through a one way hash function. This produces a fixed
size output of your e-mail called a message digest. This is a unique value, and if the
mathematical algorithm that produces it is strong, the message digest has the following
attributes.
  The original message can’t be reproduced from the digest.
  Each digest is unique.
After the digest is created, it is encrypted with your private key. The encrypted digest is
attached to the original message along with your public key. The recipient then opens the
message, and the digest is decrypted with your public key. The digest is compared to an
identical digest generated by the recipients’ mail program. If they match, then you’re done.
If not, your mail client will let you know the message has been altered. There are 2 types of
signing / encryption functions, S/MIME and PGP. S/MIME is considered to be the corporate
and government choice, possibly because it uses the less labor intensive certificate authority
model for authentication, and because it is more easily implemented through Microsoft's
Outlook Express e-mail program. PGP is more often the choice of the computer user
community, because it is based on a non-centralized web of trust for authentication, where a
user's trustworthiness is validated through the 'friend of a friend' system, where you agree that,
if you trust me, then you can also trust those people who I trust, and because members of the
computer user community don't really care if it takes them four hours to figure out how to

make PGP work with Thunderbird – they consider these types of challenges to be a form of
recreation.

9.3.2  数字签名

数字签名是由电子邮件软件和私人密码产生的，主要用来确保电子邮件的真实性。签名的目的是双重的。第一点是验证发邮件人是不是你。这称作不可否认，第二点是确保内容没有被篡改。这称作数据完整性。电子邮件应用程序通过将你的邮件信息进行一种哈希算法运算来完成数字签名。这样你的邮件会产生一个固定大小的输出，邮件摘要。这很特别，如果这个数学算法很强大，产生的摘要有下面两个属性：

      原始邮件不能通过摘要复原

      每个摘要都是唯一的

当摘要产生后，会通过你的私人密码加密。加密后的摘要作为原始邮件的附件。接收者打开邮件，摘要是经过你的私人密码加密的。接收者的电子邮件程序会产生一个完全相同的摘要，用来和寄过来的摘要比对。如果相同，你成功发送邮件，如果不同，电子邮件客户端会通知你你的电子邮件被篡改了。有两种签名/加密函数，S/MIME 和 PGP。S/MIME一般是企业和政府的选择，可能因为验证模式比较简单，在Microsoft's Outlook Express邮件软件比较好用。PGP是基于非集中性网络信任度鉴定，用户的可信度通过“朋友的朋友”系统验证，如果你相信我，那么你也可以相信我所信任的人，因为通产网络电脑用户不会在意是否还要花费他们4个小时来弄明白怎样让PGP搭配Thunderbird工作---他们将这种挑战当做在创造。（英语水平是在太低了，自己都不知道这句话到底是什么意思）

 

9.3.3 Getting a certificate
If you are interested in getting a digital certificate or digital ID, you need to contact a
Certificate Authority (Verisign and thawte are the most well known, although a web search
may find others.) Both require you to provide identification to prove to them that you are who
you are. You can get a free certificate from thawte, but they require a significant amount of
personal information, including a government identification number (such as a passport, tax
id or driver's license). Verisign charges a fee for its certificate and requires that you pay this fee
with a credit card, but asks for less personal information. (Presumably, Verisign is relying on the
credit card company to validate your personal information.) These requests for information
may seem intrusive, but remember, you are asking these companies to vouch for your
trustworthiness. And – as always – check with your parents or guardians before you give out
any personal information (or run up large balances on their credit cards).
The biggest disadvantage to using a certificate authority is that your private key is available
to someone else – the certificate authority. If the certificate authority is compromised, then
your digital ID is also compromised.

 

9.3.3 获得证书

拂过你对获得数码证书或者数码ID感兴趣，你需要联系认证机构（Verisign和thawte是最有名的，能上网搜到他们），这两个认证机构都需要你提供身份证明你的身份。你可以从Thawte得到免费证明，但是他们需要分量很重的个人信息，包括一个政府鉴定号（像护照，纳税人号或者驾驶执照）。Verisign会收费，并需要你用信用卡交费，但需要你给的个人信息相对少些。（大概，Verisign通过信用卡公司来确定你的个人信息）这些对个人信息的要求看起来有点侵犯隐私，但记住，你需要这些公司证明你的可信度。和往常一样，在你给出任何个人信息之前，与你的父母或监护人商量一下（或者减少对你信用卡的使用，使用他们的信用卡吧（我是这么理解的，不知道对不对））

通过认证机构的一大缺点是总有某个人知道你的密码---这个人就是认证机构。如果认证机构被破解了，那么你的数字号码也被破解了。