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cryptography comes from the greek words for "secret writing". professionals make a distenction between ciphres and codes.
A cipher is a character for charecter or bit for bit transformation, without regard to the linguistic straucture of the massege .
A code replace one word with another word or symbol. codes are not used anymore, although they have a glorius history. the most successiful code ever devised was used by the u.s armer forces during world war 2 in the pacific.
They simple had navajo indians talking to each other using specific navajo words for molatary
terms.
Encryption methods have historically been divided into two categories.
1.substitution ciphers
2.transposition ciphers
SUBSTITUTION CIPHERS:
substitution ciphers each letter or group of letters is replaced by another letter or group of letters to disguise it.
one of the oldest known ciphers is the caesar cipher.in this method
a becomes D, b becomes E ,c becomes F..........and z becomes C
EX: attack becomes DWWDFN
the above example plain text will be given in lower case letters,and cipher text in upper case letters
the next improvement is to have each of the symbol in the plain text say,the 26 letters for simplicity ,map onto some other letter.
ex: 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
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
the general system of symbol-for-symbol substitution is called a monoalphabetic substitution,with the key
being the 26 letterstring corresponding to the alphabet
ex: plaintext is attack would be transformed into the cipher text QZZQEA
TRANSPOSITION CIPHERS:
We use this method reorder the letters but do not disguish them.
Ex:the common transposition cipher, is columnar transposition . the cipher is a key word by a word or phase not containing any repeated letters.
Ex:MEGABUCK is the key
the purpose of the key is the number the columns, column i being under the key letter closet to the alphabet and so on.
The plain text is written horizontally, in rows, padded to fgl the matain if need be. the cipher text is read out by columns, starting with the column whose key letter is lowest.
M E G A B U C A
7 4 5 1 2 8 3 6
p l e a s e t r
a n s t e r o n
e m i l l i o n
d o l l a r s t
o m y s w i s s
b a n k a c c o
u n t s i x t w
o t w o a b c d
PLAIN TEXT:
Please transfer one million dollars to my swiss bank acount six two two
CIPHER TEXT:
AFLLSKSOSELAWALATOOSSCTCLNMOMANTESILYNTWRNNTSOWDIAEDOBEOERIRICXB
A cipher is a character for charecter or bit for bit transformation, without regard to the linguistic straucture of the massege .
A code replace one word with another word or symbol. codes are not used anymore, although they have a glorius history. the most successiful code ever devised was used by the u.s armer forces during world war 2 in the pacific.
They simple had navajo indians talking to each other using specific navajo words for molatary
terms.
Encryption methods have historically been divided into two categories.
1.substitution ciphers
2.transposition ciphers
SUBSTITUTION CIPHERS:
substitution ciphers each letter or group of letters is replaced by another letter or group of letters to disguise it.
one of the oldest known ciphers is the caesar cipher.in this method
a becomes D, b becomes E ,c becomes F..........and z becomes C
EX: attack becomes DWWDFN
the above example plain text will be given in lower case letters,and cipher text in upper case letters
the next improvement is to have each of the symbol in the plain text say,the 26 letters for simplicity ,map onto some other letter.
ex: 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
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
the general system of symbol-for-symbol substitution is called a monoalphabetic substitution,with the key
being the 26 letterstring corresponding to the alphabet
ex: plaintext is attack would be transformed into the cipher text QZZQEA
TRANSPOSITION CIPHERS:
We use this method reorder the letters but do not disguish them.
Ex:the common transposition cipher, is columnar transposition . the cipher is a key word by a word or phase not containing any repeated letters.
Ex:MEGABUCK is the key
the purpose of the key is the number the columns, column i being under the key letter closet to the alphabet and so on.
The plain text is written horizontally, in rows, padded to fgl the matain if need be. the cipher text is read out by columns, starting with the column whose key letter is lowest.
M E G A B U C A
7 4 5 1 2 8 3 6
p l e a s e t r
a n s t e r o n
e m i l l i o n
d o l l a r s t
o m y s w i s s
b a n k a c c o
u n t s i x t w
o t w o a b c d
PLAIN TEXT:
Please transfer one million dollars to my swiss bank acount six two two
CIPHER TEXT:
AFLLSKSOSELAWALATOOSSCTCLNMOMANTESILYNTWRNNTSOWDIAEDOBEOERIRICXB
