Describes the One Time pad cipher. A classical encryption technique inspired by Vernam.

1. One Time Pad
(Vernam Cipher)
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• Patrick
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• Rufus
• Mark K.
• Mark C.
• Adams
• Kelvin M.
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2. One Time Pad
(Vernam Cipher)
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3. OTP History
• First described by Frank Miller in 1882 the one-time pad was
re-invented in 1917 and patented a couple of years later. It is
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derived from the Vernam cipher, named after Gilbert Vernam,
one of its inventors.
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• Used by spies of the former USSR (Russia) during the 1960s
• Used a stack of small very thin pages, each with a series
of random numbers on them. After use, a page would be
destroyed immediately
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4. OTP-Working Mechanism
• The encryption-key has at least the same length as the
plaintext and consists of truely random numbers
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• Each letter of the plaintext is 'mixed' with one element
from the OTP
• This results in a ciphertext that has no relation with the
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plaintext when the key is unknown. At the receiving end,
the same OTP is used to retrieve the original plaintext
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5. OTP Rules
• The OTP should consist of truely random numbers
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• Precisely two copies of the OTP should exist.
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• The OTP should only be used once.
• Both copies of the OTP are destroyed immediately after
use.
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6. Why OTP is Unbreakable
• The key is atleast as long as the message
• The key is truly random (not auto-generated)
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• Key and plain text calculated modulo 10/26/2
• Each key should only be used once & destroyed by
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sender and receiver
• There should only be 2 copies of the key (1 for sender
and 1 for receiver)
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7. OTP Criticism
• Distribution of the key was a challenge
• Adding numbers to the plaintext manually, is a time-
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consuming task. It is therefore sometimes thought that
OTPs are no longer considered practical
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• Counter: Improved computing power now
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8. OTP Example
• Problem: Alice wishes to send the message "HELLO" to Bob
• ROE: Assume two pads of paper containing identical random
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sequences of letters were somehow previously produced and
securely issued to both
• Additional Info:Alice chooses the appropriate unused page
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from the pad.
• How: The way to do this is normally arranged for in advance,
as for instance 'use the 12th sheet on 1 May', or 'use the next
available sheet for the next message'. The material on the
selected sheet is the key for this message. Each letter from the
pad will be combined in a predetermined way with one letter of
the message. It is common, but not required, to assign each
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letter a numerical value: e.g. "A" is 0, "B" is 1, and so on.

9. OTP Example (Modulo26)
• Here, we combine the key and the message using
modular addition.
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• The numerical values of corresponding message and key
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letters are added together, modulo 26.
• If key material begins with "XMCKL" and the message is
"HELLO", then the coding would be……..
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10. OTP Encryption
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11. OTP Encryption [contd.]
• If a number is larger than 25, then the remainder after
subtraction of 26 is taken in modular arithmetic fashion.
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• This simply means that if your computations "go past" Z,
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you start again at A.
• The ciphertext to be sent to Bob is thus "EQNVZ". Bob
uses the matching key page and the same process, but
in reverse, to obtain the plaintext.
• Here the key is subtracted from the ciphertext, again
using modular arithmetic…… 11

12. OTP Decryption
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• NB: If a number is negative then 26 is added to make the
number positive 12

13. OTP Cryptanalysis
• Suppose Eve intercepts Alice's ciphertext: "EQNVZ".
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• If Eve had infinite computing power, she would quickly
find that the key "XMCKL" would produce the plaintext
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"HELLO", but she would also find that the key "TQURI"
would produce the plaintext "LATER"
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14. OTP Cryptanalysis [contd]
• It is possible to "decrypt" out of the ciphertext any
message whatsoever with the same number of
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characters, simply by using a different key, and there is
no information in the ciphertext which will allow Eve to
choose among the various possible readings of the
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ciphertext
• Thus, OTP coined, the “Perfect Cipher”
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15. Try This One:
• Message: COME
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• Key: BABY
• References:
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http://www.cs.miami.edu/~burt/learning/Csc609.051/notes/02.html
http://www.cryptomuseum.com/crypto/otp.htm
http://practicalcryptography.com/ciphers/
http://en.wikipedia.org/wiki/One-time_pad 15