An introductory presentation on cryptography. From ancient ciphers to modern public key encryption, it follows the evolution of a science and how it affects society.

1. THE SCIENCE OF SECRECY
INTRODUCTION TO
CRYPTOGRAPHY
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A presentation by:
Somaditya Basak

2. What is Cryptography?
 The term cryptography comes from the Greek term kryptos
which means "hidden, secret"; and gráphō, which means
"writing".
 Cryptography is the practice and study of hiding information,
i.e. means of converting information from its normal,
comprehensible form into an incomprehensible format,
rendering it unreadable without secret knowledge.
 Modern cryptography intersects the disciplines of
mathematics, computer science, and engineering.
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3. Terminology
 Plaintext: The information a sender wishes to transmit to a
receiver. It is the input fed into an encryption algorithm.
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4. Terminology (contd.)
 Cipher: An algorithm for performing encryption or
decryption — a series of well-defined steps that can be
followed as a procedure.
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5. Terminology (contd.)
 Cipher text: The unintelligible gibberish obtained as a
result after applying a cipher on a plaintext.
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6. Terminology (contd.)
Encryption: The process of transforming information
(plaintext) using an algorithm (cipher) to make it unreadable to
anyone except those possessing special knowledge, usually
referred to as a key. The result of the process is encrypted
information (cipher text).
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7. Terminology (contd.)
Decryption: The reverse of encryption, to make the encrypted
information readable again (i.e. to make it unencrypted).
Key: A piece of information which is generated at the time
of encryption and is required to decrypt the encrypted data.
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8. Applications of Cryptography
Defense forces: To ensure secrecy of
communication.
Intelligence Agencies: To decrypt intercepted
communication among terrorist outfits and other
countries.
E-Commerce (online shopping, net banking):
To ensure secrecy of confidential information like
credit card numbers during transactions.
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9. Classification of Ciphers
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10. Classical Ciphers
 Substitution: A substitution cipher is a method of
encryption by which units of plaintext are replaced with
cipher text according to a sequential order.
 Caesar’s shift cipher
 Atbash
 ROT13
 Affine
 Transposition: The units of the plaintext are rearranged
in a different and usually quite complex order.
 Caesar’s Box Cipher
 Columnar transposition
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11. Substitution: Caesar’s Shift Cipher
 Caesar’s shift cipher is one of the simplest substitution cipher.
 It is a type of substitution cipher in which each letter in the plaintext is replaced by a
letter some fixed number of positions down the alphabet.
 For example, with a shift of 3, A would be replaced by D, B would become E, and so on.
Plain: 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
Cipher: 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
(the shift parameter is +3)
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12. Substitution: Atbash Cipher
 Atbash is another simple substitution cipher .
 It consists in substituting the first letter for the last, the second for
the one before last, and so on, therefore reversing the alphabet.
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: ZYXWVUTSRQPONMLKJIHGFEDCBA
First 13 letters: A|B|C|D|E|F|G|H|I|J|K|L|M
Last 13 Letters: Z|Y|X|W|V|U|T|S|R|Q|P|O|N
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13. Substitution: ROT13 Cipher
 ROT13: Applying ROT13 to a piece of text merely requires
examining its alphabetic characters and replacing each one by the
letter 13 places further along in the alphabet, wrapping back to the
beginning if necessary.
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14. Substitution: ROT13 Cipher
 ROT13 is its own inverse; that is, to undo ROT13, the
same algorithm is applied, so the same action can be used
for encoding and decoding.
 Only those letters which occur in the English alphabet are
affected; numbers, symbols, whitespace, and all other
characters are left unchanged.
 Because there are 26 letters in the English alphabet and
26 = 2 × 13, the ROT13 function is its own inverse:
 ROT13(ROT13(x)) = ROT26(x) = x for any text x.
 In other words, two successive applications of ROT13
restore the original text (in mathematics, this is sometimes
called an involution; in cryptography, a reciprocal cipher).
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15. Substitution: Affine Cipher
 In Affine cipher, each letter in an alphabet is mapped to
its numeric equivalent and then encrypted using a
simple mathematical function.
 The encryption function for this example will be
 y = E(x) = (5x + 8) mod 26
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16. Transposition: Caesar’s Box Cipher
 This cipher requires the encoder to omit any spaces and then
rewrite the plaintext in a square column-wise and read row-wise to
get the cipher text.
 So for example to encode the phrase 'What an unusual box', first
omit the spaces to get 'WHATANUNUSUALBOX' and then write
them in a box as follows:
 To write this in code, you would then print
'WAULHNSBAUUOTNAX'
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17. Columnar Transposition Cipher
 In a columnar transposition, the message is written
out in rows of a fixed length, and then read out
column by column, and the columns are chosen in
some scrambled order.
 Both the width of the rows and the permutation of
the columns are usually defined by a keyword.
 For example, the word ZEBRAS is of length 6 (so
the rows are of length 6), and the permutation is
defined by the alphabetical order of the letters in the
keyword.
 In this case, the order would be "6 3 2 4 1 5".
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18. Columnar Transposition Cipher
 For example, suppose we use the keyword ZEBRAS and the
message WE ARE DISCOVERED. FLEE AT ONCE.
 In a columnar transposition, we write this into the grid as:
Z E B R A S
6 3 2 4 1 5
W E A R E D
I S C O V E
R E D F L E
E A T O N C
E
 This results in the following ciphertext:
EVLN ACDT ESEA ROFO DEEC WIREE
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19. Rotor Machine
 A rotor machine is an electro-mechanical device used for
encrypting and decrypting secret messages.
 The primary component is a set of rotors which are rotating disks
with an array of electrical contacts on either side. The wiring
between the contacts implements a fixed substitution of letters,
scrambling them in some complex fashion.
 The most widely known rotor cipher device is the German Enigma
machine used during World War II. It was broken by Alan Turing.
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20. Modern Ciphers
 Private Key ciphers are a class of algorithms for
cryptography that use identical cryptographic keys
for both decryption and encryption.
 A single secret key shared by sender and receiver
(which must also be kept private)
 The sender and receiver must securely share a key
in advance.
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21. Private Key Ciphers
 Stream Ciphers: A stream cipher is a private key
cipher where plaintext bits are combined with a
pseudorandom cipher bit stream (key stream),
typically by an exclusive-or (XOR) operation.
 E.g. A5/1 is a stream cipher used to provide over-the-air
communication (mainly voice) privacy in the GSM cellular
telephone standard
 Turing is a stream cipher developed at Qualcomm for CDMA.
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22. Private Key Ciphers
 Block Ciphers: A block cipher is a private key cipher operating on
fixed-length groups of bits, called blocks. A block cipher
encryption algorithm might take (for example) a 128-bit block of
plaintext as input, and output a corresponding 128-bit block of
cipher text. The exact transformation is controlled using a second
input — the secret key.
 E.g. DES, AES
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23. Public Key Encryption
 The distinguishing technique used in public key cryptography is
the use of asymmetric key algorithms, where the key used to
encrypt a message is not the same as the key used to decrypt it.
 The asymmetric key algorithms are used to create a
mathematically related key pair: a secret private key and a
published public key.
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24. Public Key Encryption
 Unlike symmetric key algorithms, it does not require
a secure initial exchange of one or more secret keys
to both sender and receiver.
 Each user has a pair of cryptographic keys — a
public key and a private key.
 The private key is kept secret, whilst the public key
may be widely distributed.
 The keys are related mathematically, but the private
key cannot be feasibly (i.e. in actual or projected
practice) derived from the public key.
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25. Public Key Encryption
 Messages are encrypted with the recipient's public
key and can only be decrypted only with the
corresponding private key.
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26. Public Key Encryption
 It was the discovery of such algorithms which
revolutionized the practice of cryptography beginning
in the middle 1970s.
 E.g. RSA (which stands for Rivest, Shamir and
Adleman, who first publicly described it), PGP (short
for Pretty Good Privacy)
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27. Decryption Techniques
 Brute force attack is a strategy used to break the
encryption of data which involves traversing all
possible keys until the correct key is found.
 The selection of an appropriate key length depends
on the practical feasibility of performing a brute force
attack.
 The resources required for a brute force attack scale
exponentially with increasing key size, not linearly.
 As a result, doubling the key size for an algorithm
does not simply double the required number of
operations, but rather squares them.
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28. Decryption Techniques
 Dictionary attack is a technique for defeating a cipher
or authentication mechanism by trying to determine its
decryption key or passphrase by searching likely
possibilities.
 Dictionary attack uses a brute-force technique of
successively trying all the words in an exhaustive list
called a dictionary (from a pre-arranged list of values).
 In contrast with a normal brute force attack, where a
large proportion key space is searched systematically, a
dictionary attack tries only those possibilities which are
most likely to succeed, typically derived from a list of
words like a dictionary (hence the phase dictionary
attack), or easily-predicted variations on words, such as
appending a digit.
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29. Conclusion
 Cryptography is a very important field of study in
today’s world whether in our daily lives or in national
security.
 It is evolving at a rapid pace.
 Theoretically, no cipher can provide absolute
secrecy. Given enough time and computational
power, any encrypted data can be deciphered.
 Only practical limitations (time and computing
resources) makes an encryption technique
sufficiently secure.
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30. THANK YOU
Questions???
Twitter: @dityabasak
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