3 - 2 - The Data Encryption Standard (22 min)new

it's interesting. In fact Lucifer had a
number of variations but one of the later
但是其中一种新变种有128位的密钥长度
variations in fact had a key length that
was 128 bits and a block length that's
function, and I'm not gonna talk much
about key expansion. But I just wanted to
但我想提一下，事实上，密钥扩张也是描述
分组密码的重要部分
mention that, in fact, key expansion is
expansion mechanism, and one needs to
需要指定回合函数。本节我关注回合函数
specify the round function. In the segment
here, I'm gonna focus on the round
我不会讨论太多密钥扩张的细节
search. And basically this 1997 experiment
1997年的实验预示了DES的末路，
它意味着DES本身不再安全了
kinda spelled the doom of DES. It meant
that DES is itself, is no longer secure.
in the next segment. And then I also
我之前提过，分组密码由迭代构造。特别地
mentioned before that block ciphers are
often built by iteration. In particular,
我们看的分组密码里的迭代
also a big part of describing how block
好，那么我们来看看DES的历史
cipher works. Okay, so let's talk about
the history of DES. Essentially, in the
1970年代初，IBM发现他们的消费者要求
于是国家标准研究所对外发出请求
And as a result, the National Institute of
Standards, as it became known, issued a
征求下一代分组密码方案
request for proposals. For our next
generation's block cipher standard and in
very popular up until recently, as the
曾是网络的主要加密机制
当然，现在它已被AES和其他密码替代
main encryption mechanism for the web. Of
standardization process and then finally
最后于1976年由国家安全局采用为联邦标准
in 1976, the national bureau standard
adopted DES as a federal standard. And, in
事实上DES很有意思，它的密钥长度比Lucifer要少
later on. But in this segment, I wanna
本节我想描述DES的工作原理
DES是个(历史上)异常成功的密码
describe how DES works. Now, DES is a
cipher, it's an amazingly successful
它被用于银行业界。事实上
we're gonna look at block ciphers that are
built by a form of iteration where a key K
开始时密钥K被扩张成一组回合密钥，
然后回合函数应用到输入信息上
is first expanded into a bunch of round
was a source of many complaints over its
特别地，早在1997年，DES就被穷举密钥攻击
life. In particular, already back in 1997,
DES was broken by exhaustive search
破解了，意思是让计算机遍历所有
使用这个格式。我想更清楚点
事实上，指定一个这种类型的分组密码
format. I just wanna be clear that, in
fact, to specify a block cipher of this
需要指定密钥扩张的机制
type, one ห้องสมุดไป่ตู้eeds to specify the key
its suppliers to actually have a good grip
可以在卖给政府的产品中有用
to algorithm that they could use in
products sold to the government. So in
1973年国家安全局（当时的称谓）
本节我们讲DES
canonical examples, triple DES and AES. In
this segment, we're gonna talk about DES,
然后在下一节讨论3DES
and we'll talk about triple DES, actually,
事实上IBM提交了Lucifer的一个变种
standard. And in fact IBM submitted a
variant of Lucifer. That variant actually
这个变种经过一些标准化的修改之后
went through some modification during the
现在我们理解了什么是分组密码
So now that we understand what block
ciphers are, let's look at a classic
我们看一个经典的例子叫数据加密标准
DES。简单提醒一下大家
example called the Data Encryption
1973 the National Bureau of Standards as
it was called at the time put out a
放出了一个征求分组密码的项目
选中的会成为联邦标准
request for proposals for a block cipher
that is going to become a federal
分组大小也是128位。好，1973年
政府发现它买了很多商业的
also 128 bits. Okay, in 1973 the governor
realized that it's buying many commercial
已下架的计算机，所有它希望这些计算机
提供商能有个好的算法
off-the-shelf computers and so it wanted
银行使用这个网络来处理支票
DES被用来保护这些事务的完整性
checks with one another. And DES is used
for integrity in those transactions. It's
DES也被商用。事实上它不久前很流行
also used in commerce. In fact, it was
cipher. It's been used in the banking
industry. In fact, there's a classic
有一个经典的网络叫做电子打扫房间
network called the Electronic
Clearing House, which banks use to clear
group, and the head of that group, was
在70年代初，他设计了Lucifer密码
Horst Feistel, who, in the early 70s,
designed a cipher called Lucifer. Now,
有趣的是，事实上Lucifer有很多变种
事实上这些决定，特别是减少密钥长度
fact, these decisions, especially the
decision to reduce the key length, is
是导致很多人怀疑DES寿命的原因
going to be a key length yield of DES and
round functions are applied, we obtain the
resulting cipher text, okay? And again,
我们要看看数据加密标准DES如何工作
what we're gonna look at, how DES, the