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Agenda of Classical Crypto
Terms related to Crypto Simple Substitution Cipher Double Transposition Cipher One-Time Pad Codebook Cipher Conclusion
Terms related to Crypto
Cryptology: art and science of making and breaking “secret codes.”
Cryptography: making of “secret codes.” Cryptanalysis: breaking of “secret codes.” Crypto is a synonym for any or all of the
above (and more).
Simple Substitution Cipher
Ciphertext: substitute the letter of the alphabet (n) places ahead of the Plaintext.
Plaintext: substitute the letter of the alphabet (n) places backward of the Ciphertext.
n {0, 1, 2,…….,25} (n) acts as Key Ex: if (n=3) then
Simple Substitution Cipher
Attack– Exhaustive key search?– A Long keys, Max = 26! ≈ 288 possible keys– It would take 8900 millennia to exhaust
Key Size (bits)
Number of Alternative Keys
Time required at 106 Decryption/µs
32 232 = 4.3 x 109 2.15 milliseconds
56 256 = 7.2 x 1016 10 hours
128 2128 = 3.4 x 1038 5.4 x 1018 years
168 2168 = 3.7 x 1050 5.9 x 1030 years
Another possible attack
Compare with English letters frequency counts
Ex: “E” is the most common Same, try and guess, but more clever!
One Time Pad (OTP)
OTP strengths:– not easy to break if key is genuinely random– the cipher text is likewise completely random and so cannot be broken– not only function with 26 alphabets, but also 256 ASCII characters or
other set.– also use XOR function
OTP weakness:– Key need to have same length as Plaintext so data to send is double
making the traffic of network become slow– Key can be intercepted when sending– Difficulty of creating volumes of genuinely random numbers
One Time Pad (OTP)
Spy named Alice want to encrypt the plaintext message
Spy Bob receives Alice’s message, he decrypts it using the same key.
OTP with same key
In case messages are in depth Attacker recognizes pattern of ciphertext Try to guess Plaintext (Say P1) ! Then figure out key (Say K) Then use K to decrypt other chipertext
Conclusion
Weak :– Simple substitute cipher: attacker can guess in ciphertext message– Codebook cipher
Strong :– One time pad: is provably secure cryptosystem
Attacker can’t guess or break code Random key never reused
– Double Transposition Cipher Employed by modern block cipher.
Which cipher to choose?– Secure– Large enough key space