1 / 10

# 第四讲 古典替换密码 - PowerPoint PPT Presentation

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about ' 第四讲 古典替换密码' - gavin-valentine

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
1。多字母表替换密码 (Polyalphabetic ciphers)
• 增强密码安全性的方法是使用多字母替换密码
• hence the name polyalphabetic ciphers
• 使得密码分析更困难，因为有较多的密码表去猜测
• 打乱了字母出现的频率
• 用一个密钥决定对每个字母使用什么字母变换表
• 密钥的第I个字母指定第I个字母表ith letter of key specifies ith alphabet to use
• 依次使用每个字母表use each alphabet in turn
• repeat from start after end of key is reached
2. Vigenère Cipher
• 最简单的多字母替换密码（Vigenère Cipher）
• ——多重 caesar ciphers
• 密钥是多字母长度 K = k1 k2 ... kd
• 数学表示：
• Encryption is done using
• Eki(a): a -> a + ki (mod 26)
• Decryption is done using
• Dki(a): a -> a - ki (mod 26)
3. Vigenère Example
• write the plaintext out
• under it write the keyword repeated
• then using each key letter in turn as a caesar cipher key
• encrypt the corresponding plaintext letter
• Plaintext THISPROCESSCANALSOBEEXPRESSED
• Keyword CIPHERCIPHERCIPHERCIPHERCIPHE
• Plaintext VPXZTIQKTZWTCVPSWFDMTETIGAHLH
3. Vigenère Example
• C -> CDEFGHIJKLMNOPQRSTUVWXYZAB
• I -> IJKLMNOPQRSTUVWXYZABCDEFGH
• P -> PQRSTUVWXYZABCDEFGHIJKLMNO
• H -> HIJKLMNOPQRSTUVWXYZABCDEFG
• E -> EFGHIJKLMNOPQRSTUVWXYZABCD
• R -> RSTUVWXYZABCDEFGHIJKLMNOPQ
• ABCDEFGHIJKLMNOPQRSTUVWXYZ
• to map the above plaintext letters.
• \'T\' uses key \'C\' maps to \'V\'
• \'H\' uses key \'I\' maps to \'P\'
• \'I\' uses key \'P\' maps to \'X\'
• etc
4. Beauford Cipher
• 类似 Vigenère密码，但顺序相反
• 数学描述:
• Encryption is done using
• Eki(p): p -> ki - p (mod 26)
• Decryption is done using
• Dki(c): c -> ki - c (mod 26)
• eg. if using a single key letter of \'d\' have translation alphabet
• Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ
• Cipher: DCBAZYXWVUTSRQPONMLKJIHGFE
5. Variant-Beauford Cipher
• 类似 Vigenère密码，但是移位向左
• 实际上是Vigenère 逆密码 (decrypts it)
• 数学表示：
• Encryption is done using
• Eki(a): a -> a - ki (mod 26)
• Decryption is done using
• Dki(a): a -> a + ki (mod 26)
• eg. Variant-Beauford key \'c\' (shift left 2 places)
• same as Vigenère key \'y\' (shift right 24 places)
6. Autokey Cipher
• 更多的字母替换表可以提高密码的安全性
• Vigenère 提出了自动密钥密码（ autokey cipher ）
• 密钥字作为消息的前缀组成密钥
• knowing keyword can recover the first few letters
• then use these in turn on the rest of the message
• eg. given key "DECEPTIVE" and message "WE ARE DISCOVERED SAVE YOURSELF"
• key: DECEPTIVEWEAREDISCOVEREDSAV
• plaintext: WEAREDISCOVEREDSAVEYOURSELF
• ciphertext: ZICVTWQNGKZEIIGASXSTSLVVWLA
Problems with autokey ciphers
• Autorkey cipher 看起来好象比较安全
• 实际上仍然有语言特征characteristics are used by the key as the message
• ie. a key of \'E\' will be used more often than a \'T\' etc
• hence an \'E\' encrypted with a key of \'E\' occurs with probability (0.1275)2 = 0.01663, about twice as often as a \'T\' encrypted with a key of \'T\'
• given sufficient ciphertext this can be broken
• if a truly random key as long as the message is used, the cipher will be secure
• called a Vernam Cipher or One-Time pad, we meet this later