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Applied Cryptography Notes

Applied Crytography Notes#

Technology trumps politics

This is about stopping major governments from reading your files

Types of encryption:

  • file
  • email
  • digital signatures
  • digital certified mail
  • secure election protocols
  • digital cash

Natural low of cryptography - much easier to user than it is to break

Encryption and decryption grows linearly with keylength but cryptoanalysis grows exponentially

What we have learnt is the NSA circumvents and does not break cryptography. It hacks the computers doing the encryption and decryption.

You have to be targeted - bulk collection is not feasible.

The technical difficulties of implementing cryptography are far more difficult than the mathematical challenges of making crypto secure.


  • plaintext - message
  • encryption - disguising a message
  • ciphertext - encrypted message
  • decryption - turning ciphertext into a message
  • cryptography - art and science of keeping messages secure
  • Cryptanalysis - art and science of breaking ciphertext

Plaintext (M) undergoes encryption (E) to achieve ciphertext (C). Decryption (C) goes the other way.

  • Authentication - receiver know the origin / sender is who they say they are
  • Integrity - ensure the message has not been modified in transit
  • Nonrepudiation - sender should no be able to falsely deny that they sent a message

Cryptographic algorithm or cipher - function used for encryption / decryption.

A restricted algorithm works by keeping the algorithm secret - the problem is that when a person leaves the group you have to change the algorithm. No standardization or quality control.

Modern cryptography solves teh resticted algoritm problem with a key. The range of possible values is called the keyspace

Encryption and decryption operations use a key.

Some systems have a separate encryption key from the decryption key

The security is based on the key - meaning that the algorithm can be published and analysed. If an eavesdropper knows your algorithm - she still can’t read the message without the key.

Types of key-based algorithms:

  • Symmetric - the encryption key can be calculated from the decryption key. Usually a single key - that both sender and received agree on. The security rests on the key - the key must remain secret.
    • Stream ciphers - work on a single bit at a time
    • block ciphers - work on groups of bits
  • Public-key algorithms - Designed so enryption key is different from the decryption key. Decryption key cannot be calculated from the encryption key. The encryption key can be made public. A complete stranger can encrypt a message with the encryption key. But only the person with the corresponding decryption key can decrypt the message. The encryption key is the public key. The decrpytion key is the private key or secret key.

Sometimes messages are encrypted with the private key and decrpyed with the public key - used in digital signatures.

The point of cryptography is to keep the plaintext message secret.

Crpytanalysis is retrieving the plaintext message without access to the key.

Types of attacks:

  • ciphertext-only - non plaintext
  • known-plaintext - has access to ciphertext and messages
  • chosen-plaintext - chooses what the message is that gets encrypted - can choose ones that yield more info about the key
  • adaptive chosen plaintext - can modify choice about chosen attack
  • chosen-ciphertext - can choose ciphertext to be decrypted and has access to decrypted message
  • chosen-key attack - knows relationship between keys
  • rubber-hose cryptanalysis - the best way - blackmail or torture

Security of Algorithms#

If the cost required to break an algorithm is greater than the value of the encrypted data then you’re probably safe

If the amont of data encrypted with a single key is less than the amount of data necessary to break the algorithm your probably safe

value of data decreases over time


  • total break - key is found
  • global deduction - finds an alternate algorithm without finding K
  • instance deduction - finds the plaintext of intercepted ciphers
  • information deduction - gains informatino about the key or plaintext

algorithm is unconditionally secure if no matter how much ciphertext - there is not enough info to recover the plaintext

Every other cryptosystem is breakable in a cipher-only attack - by trying every possible key and seeing if the message is meaningful. The brute-force attack.

We are more interested in computationally secure - if it cannot be broken with available resources.

Complexity measurement:

  • Data complexity - amount of data input
  • processing complexity - time to perform the attack (work factor)
  • storage requirements - amount of memory needed


Hiding secret messages in other messages - like invisible ink, pin puntures. More recently messages are hidden in graphic images. Replacing the least significant bit of each byte with bits of a message. You can store a 64 kilobyte message in a 1024 x 1024 grayscale image.

Substitution and Transposition Ciphers#

Before computers - character based algorithms were used. Substituting or transposing (changing the order) characters. These days the philosophy remains the same just that work is done on bits instead of characters - a change in the alphabet size from 26 to 2.

Types of Substitution Ciphers:

  • simple substitution (monoalphabetic) - each character is replaced with a corresponding ciphertext character
  • homophonic substitution - single character can map to one of several ciphertext characters
  • polygram substitution - blocks of characters are encrypted in groups
  • polyalphabetic substitution - multiple simple substitution ciphers

Caesar cipher - each plaintext character is substituted with that 3 to the right - modulo 26.

In transposition ciphers the plaintext remains the same but the order of characters is shuffled around.

Simple columnar transposition is writing the horizontal plaintext as ciphertext vertically.

Transposition is troublesome as it uses lots of memory and requires messages to be shorter than a certain length.

Rotor Machines#

Mechanical automation of encryption. A 4-rotor machine takes substitution from the output of one rotar as input to the next. The combination of rotors and gearing makes it secure.

Best example is the German Enigma from World War 2. The German Enigma had three rotors, chosen from a set of five, a plugboard that slightly permuted the plaintext, and a reflecting rotor that caused each rotor to operĀ­ate on each plaintext letter twice

Simple XOR#

Exclusive OR. A standard operation on bits.

…a lot of good info in this book…just it goes a bit too deep on certain subjects and fundamentals are not concisely explained. It is a tad long winded for someone not wanting to study cryptography.


  • Applied Cryptography Protocols, Algorithms and Source Code in C - Bruce Schneier