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July 30, 2018
Hash Function Attacks Illustrated

Here are some illustrated explanations of the main ways in which cryptographic hash functions can be attacked, and be resistant to those attacks.

Zooko Wilcox's blog post Lessons From The History Of Attacks On Secure Hash Functions gives us a nice overview of these and I've quoted his concise explanations below. Check out his great post for more detail and history on this topic.

A cryptographic hash function is an important building block in the cryptographic systems that keep us safe in our communications on the internet.

A hash function takes some input data and generates a hopefully unique string of bits for each different input. The same input always generates the same result.

Diagram of an ideal hash function

The input to a secure hash function is called the pre-image and the output is called the image.

I use the following key below:

A red square

Red for inputs which can be varied by an attacker.

A green square

Green for inputs which can't be varied under the attack model.

To attack a hash function the variable inputs are generally iterated on in a random or semi-random brute-force manner.

Collision attack

Diagram of a collision attack on a hash function

A hash function collision is two different inputs (pre-images) which result in the same output. A hash function is collision-resistant if an adversary can't find any collision.

Pre-image attack

Diagram of a pre-image attack on a hash function

A hash function is pre-image resistant if, given an output (image), an adversary can't find any input (pre-image) which results in that output.

Second-pre-image attack

Diagram of a pre-image attack on a hash function

A hash function is second-pre-image resistant if, given one pre-image, an adversary can't find any other pre-image which results in the same image.

Hopefully these diagrams help to clarify how these attacks work!

Read more of my posts on the subject of cryptography.

June 13, 2018
Browser Blockchain in ClojureScript

I built a little blockchain-in-a-browser in ClojureScript to help understand the underlying algorithms.

You can simulate a network of peers by opening multiple browser tabs. Each peer can mine blocks and make transactions independently and the resulting blockchain will resolve conflicts correctly across all tabs.

A blockchain works by laying down a chain of blocks of transaction data.

Bitcoin whitepaper SPV

Each block in the chain contains a cryptographic hash with two important properties:

The proof-of-work is accomplished by iteratively updating a nonce until a low-probability hash is discovered.

These two properties mean a blockchain is digital amber.

Insect embedded in amber

If somebody wants to modify a transaction deep inside the amber it would be very difficult because they would have to re-create every layer of the blockchain by doing as much work as the original process required.

In my browser blockchain the hashing is implemented like this:

(hash-object [timestamp transactions previous-hash nonce])

As you can see the previous block's hash is included in the current block.

The hashing is performed iteratively in a loop until a hash with at least one byte of leading zeroes is found:

(loop [c 0]
  (let [candidate-block (make-block (now) transactions previous-hash new-index (make-nonce))]
    (if (not= (aget (candidate-block :hash) 0) 0)
      (recur (inc c))
      candidate-block)))

Jan. 27, 2014
3d Printable Sixteen Sided Hexadecimal Die

Hexadecimal Die

This is a Hexadecimal die that you can 3d print. Download the STL file or get the source code on GitHub. Hopefully useful when generating private keys and the like.