密码学-关键词加密法.ppt

经典单码加密法,,Keyword Cipher,1. select a keyword - if any letters are repeated drop the second and all other occurrences from the keyword 1.选择一个关键词，如果这个关键词中有重复的字母，则去除第一次出现之外的所有相同字母2. write the keyword below the alphabet, fill in the rest of the space with the remaining letters in the alphabet in their standard order 2.将关键词写在字母表的下方，并用字母表中除了关键词之外的其他字母按标准的顺序填写余下的空间Example,The keyword is MAGICNET,plaintext,ciphertext,这样的话,明文字母A就由密文字母M来替换 ，B则由A来替换，. . .,明文：HELP I AM LOST 密文：TCHO B MJ HLRS,Exercise,The keyword is COUNT,plaintext,ciphertext,Starting Position,The keyword does not have to start at the beginning of the plaintext alphabet（关键词不从第一字母开始） it could start at any letter for example, “count” could start at “k”,ciphertext,plaintext,Example,If the keyword is “visit” (note, the second “i” in visit is dropped below) starting at “a” and the plaintext is “next”, the application is:,plaintext,ciphertext,N E X T,K,,,,,A,,,X,,,Q,Keyword in CAP,运行CAP软件，输入一些明文，然后从Cipher菜单选取Keyword菜单项，将弹出一个Keyword Cipher窗口。

关键词加密法的分析法,关键词加密法当然比简单移位加密法功能更强，因为要尝试所有的关键词及其位置，显然比所有25种移位加密密钥要困难得多 这种攻击法的原理就是计算机用字典中的每个词进行尝试 ，直至找到一个能破解该加密法的为止 用穷举查找攻击法（即尝试所有可能的关键词）在今天也是可行的，但不是可取的Breaking a Keyword,The keyword cipher is not secure, in fact it is easy to break One reason why it is useful to study such a cipher is that in order to break this cipher you must use some of the most fundamental tools of cryptanalysis（破译关键词密码需要一些密码分析基本工具）,Challenge,Consider a simple monoalphabetic keyword substitution cipher such as the following:,GJXXN GGOTZ NUCOT WMOHY JTKTA MTXOB YNFGO GINUG JFNZV QHYNG NEAJF HYOTW GOTHY NAFZN FTUIN ZBNFG NLNFU TXNXU FNEJC INHYA ZGAEU TUCQG OGOTH JOHOA TCJXK HYNUV OCOHQ UHCNU GHHAF NUZHY NCUTW JUWNA EHYNA FOWOT UCHNP HOGLN FQZNG OFUVC NZJHT AHNGG NTHOU CGJXY OGHYN ABNTO TWGNT HNTXN AEBUF KNFYO HHGIU TJUCE AFHYN GACJH OATAE IOCOH UFQXO BYNFG,Basic Cryptanalysis,The most basic observation of cryptanalysis is that every letter of a language has a personality of its own if every plaintext t is changed to a ciphertext m, then in the ciphertext, m assumes the personality of t to the trained observer, the personality of a letter gives away its identity 对训练有素的观察者来说，字母的特性揭示了其真实面目。

Some of these personality characteristics are: frequency of occurrence contact with other letters position within words 字母的特性主要有：出现的频率、与其他字母的联系、在单词中的位置Letter Frequency,What is the most frequent letter in English? Actually the frequency depends on the type of text. A widely used frequency table of 400 letters of standard English（英文字母的频率）,Letter: A B C D E F G H I J K L M N O P QCount: 32 6 12 16 42 8 6 24 26 2 2 14 12 28 32 8 1 Letter: R S T U V W X Y ZCount: 26 24 36 12 4 6 2 8 1In Order: ETAONIRSH DLUCMPFYWGB VJKQXZ,Letter Frequency,The frequency count for the challenge text is:,A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 17 4 13 0 7 17 23 26 5 12 3 2 2 36 25 1 5 0 0 23 20 3 6 9 13 8,We could compare this with the expected frequency:,Standard: ETAONIRSHDLUCMPFYWGBVJKQXZCipher: NHOGTUAFCYJXZEWIQBKVLMPDRS （密文）,Result: OLUUE OOANC EIHAN PJATD . . .,However, while relative frequencies may shift slightly,(i may be more frequent than a), they do not stray far from their area in the frequency table,Letter Frequency,当然，这只是个概数，不会与每篇文章的字母出现频率完全一样，通常越短的信息的字母频率与上面的相比就越不相符，越短的信息也就越难破解了。

如： John,a jazzist,will go to Japan in June to join a jazz party. 其中，J的百分比是12.8，出现6次，而E的百分比是0，对其进行频率分析就毫无意义 又如：UOCT XH.若 明码表 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 密码表 Q W E R T Y U I O P A S D F G H J K L Z X C V B N M 则它被翻译为GIVE UP. 但若 明码表 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 密码表 Q W U E T R Y I X P A S C H O M D F G J K L Z V B N 则它被翻译为COME IN. 究竟哪个是真的，不得而知但当它放进一篇文章里，就可从前后推知它应怎样 翻译了Frequency Groups,High Frequency Group E T A O N I R S H Medium Frequency Group D L U C M Low Frequency Group P F Y W G B V Rare Group J K Q X Z,字母在分组中的位置可能会 发生变化，但很少会出现在 其他的分组中去。

Single Frequency Reasoning,Things to look for in a frequency report If there are hills and valleys similar to standard English then the cipher is most likely a substitution, so（密文字母频率的高峰低谷部分） Find the break between high frequency and medium frequency (look for a 2% drop between two letters) The most frequent letter is probably “e” or at least “t” or “a”,WARNING: this is only useful if you have enough text to maintainthe “average” picture of frequency distributions,Frequency Report,Again consider the frequency count for the challenge:,N H O G T U A F C Y J X Z E W I Q B K V L M P D R S 36 26 25 23 23 20 17 17 13 13 12 9 8 7 6 5 5 4 3 3 2 2 1 0 0 0,Where is the break - that is, which set of characters are in the high frequency group?,Out of the possible high frequency group which is e?,Challenge Digrams,This chart lists the digrams formed by the most frequent letters in the ciphertext:,First task - identify (or confirm) e,N is a good possibility by frequency counts,。