fivepiece

What's this all about?

Digital cryptography! This is a subject I've been interested in since taking a class with Prof. Fred Schneider back in college. Articles pop up on Hacker News fairly often that pique my interest and this technique is the result of one of them.

Specifically, this is about Lamport signatures. There are many signature algorithms (ECDSA and RSA are the most commonly used) but Lamport signatures are unique because they are formed using a hash function. Many cryptographers believe that this makes them resistant to attacks made possible by quantum computers.

How does a Lamport Signature work?

The algorithm that used to be described here is broken.

A better alternative is described here: https://github.com/sipa/writeups/tree/main/elligator-square-for-bn

Broken Image Repairer

What is the problem?

A long time ago, it was possible to inline images from all kinds of external sources. Since the switch from HTTP to HTTPS, this is no longer possible; only HTTPS sources are allowed. This leads to ugly blurbs like

alt text http://example.com/image.png

instead of a nicely formatted page with images. Sometimes, the links don't even work anymore, even with HTTPS images, which will show like this: ... Luckily, we have the Wayback Machine which is able to rescue some of the lost images. Since a picture often says more than a thousand words, it's important to bring back the post into its original state; important enough to justify the occasional bump of an old post (see below).

Described here is a variant of what has previously been published under the name "P2EP" or Pay-to-endpoint, in which A pays B but B contributes utxos, i.e. it's a coinjoin-payment.

I'm using the term "payjoin" here to refer to using that idea, but not including a URI/endpoint specific to B, and not allowing (as a merchant would) arbitrary payments, which opens up certain problems around snooping attackers (more on this below). So payjoin just means "A pays B but B actively participates and passes across utxos as extra inputs".

I'll defer a more features-focused and non-tech friendly description of what this means to a later blogpost.

  BIP&#58; ??
  Layer&#58; Applications
  Title&#58; SNICKER &#45; Simple Non&#45;Interactive Coinjoin with Keys for Encryption Reused
  Author&#58; Adam Gibson <[email protected]>
  Comments&#45;Summary&#58; No comments yet.
  Comments&#45;URI&#58; &#45;
  Status&#58; Proposed
  Type&#58; Informational
  Created&#58; &#45;

	import hashlib, os, struct, sys, socket, time

	N = 0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2fL
	R = 0xfffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141L
	A = 0L
	B = 7L
	gx = 0x79be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798L
	gy = 0x483ada7726a3c4655da4fbfc0e1108a8fd17b448a68554199c47d08ffb10d4b8L

	b58ab = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"