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    Week 5 - Cryptography

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    Tahir Bashir
    The document discusses various polyalphabetic and transposition ciphers used throughout history to encrypt messages including the Vigenère cipher, autokey cipher, one-time pad, rail fence cipher, and row transposition ciphers. It also introduces the concept of using multiple ciphers sequentially, known as product ciphers, to increase security over a single cipher. Finally, it briefly discusses steganography as an alternative approach to hiding information.

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    Week 5 - Cryptography

    Uploaded by

    Tahir Bashir
    25 views13 pages
    The document discusses various polyalphabetic and transposition ciphers used throughout history to encrypt messages including the Vigenère cipher, autokey cipher, one-time pad, rail fence cipher, and row transposition ciphers. It also introduces the concept of using multiple ciphers sequentially, known as product ciphers, to increase security over a single cipher. Finally, it briefly discusses steganography as an alternative approach to hiding information.

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    Week 5- Cryptography

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    The document discusses various polyalphabetic and transposition ciphers used throughout history to encrypt messages including the Vigenère cipher, autokey cipher, one-time pad, rail fence cipher, and row transposition ciphers. It also introduces the concept of using multiple ciphers sequentially, known as product ciphers, to increase security over a single cipher. Finally, it briefly discusses steganography as an alternative approach to hiding information.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    25 views13 pages

    Week 5 - Cryptography

    Uploaded by

    Tahir Bashir
    The document discusses various polyalphabetic and transposition ciphers used throughout history to encrypt messages including the Vigenère cipher, autokey cipher, one-time pad, rail fence cipher, and row transposition ciphers. It also introduces the concept of using multiple ciphers sequentially, known as product ciphers, to increase security over a single cipher. Finally, it briefly discusses steganography as an alternative approach to hiding information.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    of 13

    Cryptography

    Week 5
    Polyalphabetic Ciphers
    • another approach to improving security is to use
    multiple cipher alphabets
    • called polyalphabetic substitution ciphers
    • makes cryptanalysis harder with more alphabets to
    guess and flatter frequency distribution
    • use a key to select which alphabet is used for each
    letter of the message
    • use each alphabet in turn
    • repeat from start after end of key is reached
    Vigenère Cipher
    • simplest polyalphabetic substitution cipher is
    the Vigenère Cipher
    • effectively multiple caesar ciphers
    • key is multiple letters long K = k1 k2 ... kd
    • ith letter specifies ith alphabet to use
    • use each alphabet in turn
    • repeat from start after d letters in message
    • decryption simply works in reverse
    Example
    • write the plaintext out
    • write the keyword repeated above it
    • use each key letter as a caesar cipher key
    • encrypt the corresponding plaintext letter
    • eg using keyword deceptive
    key: deceptivedeceptivedeceptive
    plaintext: wearediscoveredsaveyourself
    ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ
    Security of Vigenère Ciphers
    • have multiple ciphertext letters for each
    plaintext letter hence letter frequencies are
    obscured but not totally lost
    Autokey Cipher
    • ideally want a key as long as the message
    • Vigenère proposed the autokey cipher
    • with keyword is prefixed to message as key
    • knowing keyword can recover the first few letters
    • use these in turn on the rest of the message
    • but still have frequency characteristics to attack
    • eg. given key deceptive
    key: deceptivewearediscoveredsav
    plaintext: wearediscoveredsaveyourself
    ciphertext:ZICVTWQNGKZEIIGASXSTSLVVWLA
    One-Time Pad
    • if a truly random key as long as the message is
    used, the cipher will be secure
    • called a One-Time pad
    • is unbreakable since ciphertext bears no
    statistical relationship to the plaintext
    • since for any plaintext & any ciphertext there
    exists a key mapping one to other
    • can only use the key once though
    • have problem of safe distribution of key
    Transposition Ciphers
    • now consider classical transposition or
    permutation ciphers
    • these hide the message by rearranging the
    letter order
    • without altering the actual letters used
    • can recognise these since have the same
    frequency distribution as the original text
    Rail Fence cipher
    • write message letters out diagonally over a
    number of rows
    • then read off cipher row by row
    • eg. write message out as with key=2:
    m e m a t r h t g p r y
    e t e f e t e o a a t
    • giving ciphertext
    MEMATRHTGPRYETEFETEOAAT
    …
    To decipher a rail fence cipher, we divide
    the ciphertext in half and reverse the
    order of the steps of encipherment, that
    is, write the ciphertext in two rows and
    read off the plaintext in a zig-zag fashion.
    (Note that if there are an odd number of letters,
    we include the “middle” letter on the top
    line.)
    Row Transposition Ciphers
    • a more complex scheme
    • write letters of message out in rows over a
    specified number of columns
    • then reorder the columns according to some
    key before reading off the rows
    Key: 3 4 2 1 5 6 7
    Plaintext: a t t a c k p
    o s t p o n e
    d u n t i l t
    w o a m x y z
    Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ
    Product Ciphers
    • ciphers using substitutions or transpositions are not
    secure because of language characteristics
    • hence consider using several ciphers in succession to
    make harder, but:
    – two substitutions make a more complex substitution
    – two transpositions make more complex transposition
    – but a substitution followed by a transposition makes a new
    much harder cipher
    • this is bridge from classical to modern ciphers
    Steganography
    • an alternative to encryption
    • hides existence of message
    – using only a subset of letters/words in a longer
    message marked in some way
    – using invisible ink
    – Pin punctures
    • has drawbacks
    – high overhead to hide relatively few info bits

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