Classical Crypto

By: Luong-Sorin VA, IMIT

Dith Nimol, IMIT

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!

Double Transposition Cipher

Transpose 2 times Thwart learning from statistical information

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

OTP with same key

OTP with same key

Codebook Cipher

A kind of substitution cipher By words or phrases Used by Germany during WW1

Codebook Cipher

Codebook Cipher

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