What are the qualities of a good block cipher? Beginner, Intermediate, Advanced, Expert [closed]

Question

Notice: I'm doing this to learn. I know not to use a homemade cipher for anything that requires security. I also have heard before I'm not supposed to be trying this, but I learn best through trying to make things from scratch, even if I fail.

Ciphers are incredibly complex, to the point most people probably don't appreciate them, myself included. Trying to make one from scratch might help with this, so I'm looking for a list of qualities/achievables that can be worked towards for years to come.

This is the list so far, for a good block cipher at different learning levels:

Beginner :

1. Encrypted output can not be broken by hand. The level of encryption during WW1 or so.

Intermediate :

1. Changing a single bit in the input should have a 50% average chance of changing each bit of output.

2. Each byte should have a 1/256 chance of not changing (basically #1 ^ 8)

3. The number of bytes found in similar by search should be very few (I need to write down the math, but my estimate is around 5%)

Advanced:

1. Not sure yet?

Expert :

1. Immune to Differential cryptanalysis?

---

This list is very incomplete, but gives an example of what I'm looking for.

What is your list of achievable qualities for block cipher at different skill levels?

Answer 1

I don't think we can really separate things into skill levels as such, but there plenty of targets to aim for in terms of security goals. You'll want resistance to:

• Differential cryptanalysis
  • Truncated differential cryptanalysis
  • Integral cryptanalysis
  • Impossible differential cryptanalysis
  • Higher order differential cryptnalysis
  • Boomerang attacks
• Linear cryptanalysis
  • Differential-Linear attack
• Rotational cryptanalysis
• Slide attacks

• Meet in the middle attacks

  • Biclique_attacks

• Brute force search

• Side Channel attacks
  • Timing attacks
• Algebraic attacks
• Partitioning cryptanalysis
• Attacks that don't exist yet (at least, publicly)

Some resistance can be achieved via:

• Large enough keysize
  • Mandatory for simple brute force search
• Large enough blocksize
  • A large enough size can preclude the conditions for a slide attack to be applicable
• Avoiding using secret information in ways that leak timing information
  • No lookup tables
  • No shuffling of arrays according to secret information
  • No secret dependent rotations
• High levels of diffusion that cause the avalanche effect quickly

Some other desirable traits:

• Software efficiency on a wide variety of platforms (i.e. 8 bit, 32 bit, 64 bit)
• Key agility
  • Ideally the key schedule is invertible
  • Ideally the actual keys used each round are extracted from the invertible key schedule in a weakly one way manner (first preimage resistant)

Don't forget to obey Kerckhoffs Principle

After you've done all that, you might like to make your construction tweakable

Some of the mathematical concepts related to cipher design are:

• Information theory
  • Information Entropy
• Finite Fields
• MDS matrix
  • Matrix
  • Coding theory
• Boolean functions
  • S-box (meaning NxM mapping, not a lookup table)
  • Bent functions