I wanted to gift some bitcoin to a friend, and came up with a fun idea of writing them a poem with words making up an BIP-39 mnemonic word list. Then they can easily type in the 12 words in Electrum and take control of their wallet (and maybe transfer the amount to a wallet I don't have access to :).

BIP-39 Fundamentals

Basic idea of BIP 39 is that there is a wordlist of 2048 words, so each word choice encodes 11 bits (2^11 = 2048) of entropy. With 12 words, you have 12*11=132 bits, enough for 128 bits of of true entropy and a 4 bit checksum. You can read all about it in the BIP-39 itself.

Now only problem is, that the last word is not random, but must match the top 4 bits of SHA256 checksum of the preceding 128 bits. So essentially you can choose the 11 first words, and then try to see which choices of 12th word end up with a valid word list mnemonic.

One could manually type stuff into Electrum word list box, but trying 2048 options sounds pretty frustrating (on average, every 16th try will work). So let's do it in Python!

Validating a BIP-39 word list in Python

First, grab the English wordlist — and yes BIP-39 is not the best way as it depends on the word list, but it is standard enough. Then we read it in with Python:

import hashlib, binascii, sys

nums = {}
wordlist = []
with open('english.txt') as fin:
    i = 0
    for word in fin:
        nums[word.strip()] = i
        wordlist.append(word.strip())
        i += 1

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Just a quick note after yesterday's S2 Pico OLED tutorial. I encountered some hiccups getting Serial.println() to work on Arduino with this board. Essentially nothing would come out of Serial Monitor after reprogramming.

I think the core of the issue is that the ESP32-S2 has native USB. ESP8266 and older ESP32 boards used a USB serial converter chip, so programming over serial vs. printing to serial happened without any glitches to the USB. Now with native USB I think here's what happens:

1. You press Button 0, cycle Reset and release B0
2. ESP32-S2 boots into "programming mode" and initializes native USB as COM port
3. You hear the USB connection sound as COM port is assigned
4. Arduino reprograms the flash
5. You manually press reset
6. USB COM port actually drops at this point
7. When you have Serial.begin(); in your code, it now initializes native USB as COM port again
8. You hear the "USB chime" again from your computer, and COM port is assigned

Now if you're used to having Arduino Serial monitor open all the time, the same COM13 that was there during programming on my PC is now a "new" COM13. It seems the serial monitor doesn't notice the change. Solution is simple:

1. Reprogram your chip.
2. Reset, wait for the "chime"
3. Only now open the serial monitor

The irksome thing is, that I'll now need a delay in setup() to see what's going on. Maybe I have an old version of Arduino or something. If you know another solution, you're welcome to drop me a line (me at codeandlife.com)

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Just received the Wemos S2 pico board from AliExpress, and thought I'd write a simple tutorial on how to use it with Arduino, as Wemos' Getting started guide was a bit outdated on Arduino config and did not have an OLED example.

Quick Background

I've been planning to make a DIY hardware Bitcoin wallet just for fun. To make it even remotely secure — once you assume attackers have your internet connected devices pwned it pretty much varying degrees of tinfoil — it's essential that you have an external display and a button to print out your secret key or which address you're signing your coins to go.

My ESP8266 supply was running low (have been using ), and not sure if it has enough memory, I looked what Wemos might have nowadays, since I've used their nice D1 Mini in several projects, such as the ATX power control. I was very happy to discover they had this Wemos S2 Pico available at a reasonable 8 € price point from LoLin AliExpress store , having an SSD-1306 compatible OLED display and even a button. Perfect!

Note: there are clones for Wemos products for cheaper, but I like to show my support even if it costs a dollar or two more!

Setting up Arduino for ESP32-S2 Support

Following Wemos' Getting Started tutorial, I realized the Boards list did not include any ESP32-S2 modules. I checked that I had the "latest" 1.0.6 version installed. After some googling lead me to this Adafruit page, I realised that I needed 2.0.x version that is served from a different location (latest ESP32 branch now lives in Github).

After following the installation instructions — essentially replacing the old Espressif "Additional Boards Manager URL" in Arduino Preferences with the new one — I updated the ESP32 package to 2.0.1 and voilà: There is now the "ESP32S2 Dev Module" available in the ESP32 Boards section. Since Wemos' instructions, the USB CDC setting had changed a bit, this is how I set it up (changes made highlighted):

Note that the S2 Pico requires you to hold Button 0 down, press Reset button and release the Button 0 to enter flashing mode. This will change the COM port! Thankfully, it seems to stay in that mode so you should not be in a rush to flash.

Using The OLED on Wemos S2 Pico

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This post outlines the basics of creating a project that combines Go (or "golang" as googling for "go" is a pain — why didn't the guys at Google think of this?) native backend serving a web UI / GUI running on SvelteKit.

In a nutshell, this involves creating a new go project, creating a simple web server program that supports serving files from a static folder, and finally creating a SvelteKit project and configuring it to produce static content into that folder. But let's do a short detour on why this might be useful!

Combining native executable with Web UI

Native graphical user interfaces are not easy on any platform, and after looking at Qt, WxWidgets, Electron etc. I decided all had either major shortcomings, huge learning curves or resulted in way too large packages.

Doing a native web server, on the other hand, is quite easy using Go. I also investigated C and C++, but at least on Windows you very quickly run into MinGW vs. Visual Studio issues, runtimes, build systems and all that chaos, whereas Go pretty much produces executables with minimum fuss.

Once you have a web server, you can just serve a web UI and the user can run the executable and open the UI in their browser.

Simple web server with Go

Once you are comfortable creating a "Hello world" level app in Go, making a simple app for web server is not too hard:

$ mkdir project
$ cd project
project$ go mod init example/project

Here's a simple web server you can paste into main.go

package main

import (
    "encoding/json"
    "log"
    "mime"
    "net/http"
)

func databases(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "application/json")
    w.Header().Set("Access-Control-Allow-Origin", "*") // for CORS
    w.WriteHeader(http.StatusOK)
    test := []string{}
    test = append(test, "Hello")
    test = append(test, "World")
    json.NewEncoder(w).Encode(test)
}

func main() {
    // Windows may be missing this
    mime.AddExtensionType(".js", "application/javascript")

    http.Handle("/test", http.HandlerFunc(databases))
    http.Handle("/", http.FileServer(http.Dir("static")))
    log.Fatal(http.ListenAndServe(":8080", nil))
}

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Today as I was finishing my Go+SvelteKit article, I ran into frustrating Chrome error message:

Failed to load module script: Expected a JavaScript module script but the server responded with a MIME type of "text/plain". Strict MIME type checking is enforced for module scripts per HTML spec.

Don't you just love Chrome? It knows what it needs to do (load a JavaScript module), but utterly refuses to do that because of a wrong MIME type. This happened with a client-side SvelteKit application, when it tried to open some part of the .js code.

At the time of writing, it seemed I could not find the answer easily to this one, but there actually seems to be a StackOverflow solution discussing this. But to help others hitting the same issue:

The problem on my Windows install was likely that Windows 10 registry did not contain a MIME type definition for .js files. Informing user how to tweak registry to get your program working is not ideal, but thankfully you can augment the mime types:

import "mime"

func main() {
	// Windows may be missing this
	mime.AddExtensionType(".js", "application/javascript")

    // And then you create the FileServer like you normally would
	http.Handle("/", http.FileServer(http.Dir("static")))
}

After adding the mime fix, remember to force reload Chrome page (hold Control key down while you press refresh), otherwise the problem persists as Chrome does not really bother reloading the offending files.

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Continuing from my Project Euler Problem 1, here are the next two Euler problems and some reflection!

Project Euler Problem 2

The second problem asks to find the sum of even Fibonacci numbers numbers below four million. For procedural languages, this is quite walk in the park, but for me a concise method to produce the series in Clojure proved a bit of a brain nugget!

Python 3

For Python, I decided to make a simple generator that yields the Fibonacci numbers below a given treshold N. Rest is just list comprehension:

def fib(N):
    i, j = 0, 1
    while j < N:
        yield(j)
        i, j = j, i+j

print(sum(i for i in fib(4e6) if i%2==0))

C++

In C++ it's natural to just loop with a conditional addition. This time I decided to drop using namespace std; to save a few characters. Without i, j = j, i+j syntax, the temporary variable ij is a bit ugly.

#include <iostream>

int main() {
    int i=0, j=1, s=0;

    while(j<4000000) {
        if(j%2==0) s += j;
        int ij = i + j; i = j; j = ij;
    }
    std::cout << s << std::endl;
}
}

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