Introduction to Cryptography – C839 Course Lecture Notes and Practice Material

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The swapping of blocks of ciphertext.



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1 Cryptography Transposition



3 Cipher 4 Diffusion


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Terms in this set (158)



Cryptography "The science of altering communication so that it cannot be
understood without having the key."


Mono-Alphabet Substitution Substituting one character of ciphertext for each character of
plaintext.




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Affine Cipher Formula This is the formula for what cipher:


ax + b (mod M)


Homophonic Substitution One of the earlier attempts to make substitution ciphers more robust
by masking letter frequencies


Diffusion Changes to one character in plaintext affecting multiple characters
in ciphertext. This is done with transposition and scrambling things
up.


Confusion Attempts to make the relationship between statistical frequencies of
ciphertext & the actual key as complex as possible. This is done using
substitution operations.


Symmetric Encryption Formula C = E (k,p)


Ciphertext = Encryption (key, plaintext)


Symmetric Decryption Formula P = E (k,c)


Plaintext = Encryption (key, ciphertext)




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Substitution Changing some part of the plaintext for some matching part of
ciphertext. It is essentially XORing the plaintext with the key.


Transposition The swapping of blocks of ciphertext.


Round Function A function performed with each iteration of the Feistel Cipher.


Keyspace The number of possible keys. A large set of values the algorithm
chooses from when it needs to create a key. This is all the values that
SHOULD be used.


Key schedule The generation of sub keys from a single key. It is an algorithm that,
given the key, calculates the sub keys for rounds.


Cipher The algorithm(s) needed to encrypt & decrypt a message.


Key Random bits used in encrypting a message.


Plaintext Information which is transferred or stored WITHOUT cryptographic
protection.


Key Clustering An instance when two different keys generate THE SAME ciphertext.




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Feistel Function / Feistel Cipher Forms the basis for most block ciphers. Splits the block of plaintext
into two parts (L0 and R0), then applies round function to one of the
halves. The output of each round function is then XORed with the
other half.


Unbalanced Feistel Cipher Uses a modified structure where L0 and R0 (both halves of the
plaintext) are NOT EQUAL lengths.


Electronic Codebook (ECB) The most basic encryption mode where each block is encrypted
INDEPENDENTLY, but identical plaintext blocks are encrypted into
identical ciphertext blocks. You can encrypt the same thing and get
THE SAME result.


Cipher-Block Chaining (CBC) Uses INPUT from the first round (previous ciphertext) as INPUT for
the next round. Each block of plaintext is XORed with the previous
ciphertext before encryption.


Propagating Cipher-Block Chaining (PCBC) Designed to cause small changes in ciphertext to PROPAGATE
indefinitely when decrypting & encrypting. Each block of plaintext is
XORed with the XOR of the previous plaintext block AND previous
ciphertext block.




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