[Crypto in CTF] Classical Cipher

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Classical Cipher by oalieno

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凱薩密碼 (Caesar Cipher) 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 C D E F G H I J K L M N O P Q R S T U V W X Y Z A B 明⽂密⽂ Hello World Jgnnq Yqtnf Shift 2 格

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凱薩密碼 (Caesar Cipher) 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 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 C D E F G H I J K L M N O P Q R S T U V W X Y Z A B 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 0 1 明⽂密⽂ x + 2 mod 26 加密 y − 2 mod 26 解密密鑰 2

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凱薩密碼 (Caesar Cipher) • 最簡單的破解⽅法就是窮舉 26 種可能 • ROT13 就是密鑰是 13 的凱薩密碼

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A ffi ne Cipher 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 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 I N S X C H M R W B G L Q V A F K P U Z E J O T Y D 8 13 18 23 2 7 12 17 22 1 6 11 16 21 0 5 10 15 20 25 4 9 14 19 24 3 明⽂密⽂ 5x + 8 mod 26 加密 21(x − 8) mod 26 解密密鑰 (5,8) 21 是 5 的模逆元

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A ffi ne Cipher 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 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 I N S X C H M R W B G L Q V A F K P U Z E J O T Y D 8 13 18 23 2 7 12 17 22 1 6 11 16 21 0 5 10 15 20 25 4 9 14 19 24 3 明⽂密⽂ 21(x − 8) mod 26 解密明⽂ R y = 17 21(17 − 8) mod 26 = 7 密⽂ H

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Substitution Cipher • 都是把字元互換，可以推廣為更⼀般的 Substitution Cipher • 也就是把整張字⺟替換表當作密鑰 • 可以⽤頻率分析破解 • ⼯具：https://quipqiup.com/

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Frequency Analysis • 統計英⽂字⺟出現的頻率 • 對照出原本的明⽂

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Vigenère Cipher H E L L O W O R L D 7 4 11 11 14 22 14 17 11 3 W O R D W O R D W O 22 14 17 3 22 14 17 3 22 14 D S C O K K F U H R 3 18 2 14 10 10 5 20 7 17 明⽂密鑰 + 密⽂密鑰 : WORD

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Vigenère Cipher 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 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 A C D E F G H I J K L M N O P Q R S T U V W X Y Z A B D E F G H I J K L M N O P Q R S T U V W X Y Z A B C E F G H I J K L M N O P Q R S T U V W X Y Z A B C D F G H I J K L M N O P Q R S T U V W X Y Z A B C D E G H I J K L M N O P Q R S T U V W X Y Z A B C D E F H I J K L M N O P Q R S T U V W X Y Z A B C D E F G I J K L M N O P Q R S T U V W X Y Z A B C D E F G H J K L M N O P Q R S T U V W X Y Z A B C D E F G H I K L M N O P Q R S T U V W X Y Z A B C D E F G H I J L M N O P Q R S T U V W X Y Z A B C D E F G H I J K M N O P Q R S T U V W X Y Z A B C D E F G H I J K L N O P Q R S T U V W X Y Z A B C D E F G H I J K L M O P Q R S T U V W X Y Z A B C D E F G H I J K L M N P Q R S T U V W X Y Z A B C D E F G H I J K L M N O Q R S T U V W X Y Z A B C D E F G H I J K L M N O P R S T U V W X Y Z A B C D E F G H I J K L M N O P Q S T U V W X Y Z A B C D E F G H I J K L M N O P Q R T U V W X Y Z A B C D E F G H I J K L M N O P Q R S U V W X Y Z A B C D E F G H I J K L M N O P Q R S T V W X Y Z A B C D E F G H I J K L M N O P Q R S T U W X Y Z A B C D E F G H I J K L M N O P Q R S T U V X Y Z A B C D E F G H I J K L M N O P Q R S T U V W Y Z A B C D E F G H I J K L M N O P Q R S T U V W X Z 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

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Bacon Cipher

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Bacon Cipher HELLO AAbbb AAbAA AbAbb AbAbb AbbbA To encode a message each letter of the plaintext… TO encODe A MEsSagE eAch LettEr of the plaintext… 隨便選⼀段句⼦按照 A 或 b 套⽤⼤⼩寫查表轉換

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豬圈密碼 https://en.wikipedia.org/wiki/Pigpen_cipher

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豬圈密碼 https://en.wikipedia.org/wiki/Pigpen_cipher

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柵欄密碼 HELLOWORLD HLOOL ELWRD HLOOLELWRD

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柵欄密碼 HELLOWORLD HLOOL ELWRD HLOOLELWRD

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⼯具 • https://cryptii.com/ • https://www.dcode.fr/ • https://gchq.github.io/CyberChef/ • https://quipqiup.com/

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總結 • 古典密碼學的種類數不勝數 • 每個⼈都可以創造⾃⼰的密碼 • 在 CTF 中就是通靈題的存在 • 看誰比較會通靈、比較會 Google