Compressing BIP-39 Seeds

[SirArthur2]

I'd an idea to compress BIP-39 seeds, make them to become a tiny string suitable for QR or RFID storage.

The process:

• We convert the wordlist into line numbers.
• We base58 encode the corresponding line number to the given word, so that each word will be turn into exactly 2 characters (zero padded)
• We can add a + sign to the end of the generated output or not, indicating if the seed is expanded with a unknown word or not

Eg:

Let's assume the following seed (freshly generated using BIP-39 tool):

Code:

cannon garment estate enforce remind attract about that east retreat uncle route method practice chunk

As QR this will generate a massive code that low definition cameras struggle to read. Representing a 102 characters long string.

Using the base58 word-to-line-number compressor, we get

Code:

5dEFBhBFS6345XuAdSQZeT2LMQP6d

, a 30 chars long string, rendering a much smaller and easy readable QR code.

Sample Python code for this (requires tkinter and the wordlist, named as english.txt in the same folder as the .py script - the wordlist can be found at BIP-39's Github [ https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt ]):

Code:

#!/usr/bin/python3
import os
import base64
import tkinter as tk
import tkinter.messagebox as messagebox
from typing import Mapping, Union

BITCOIN_ALPHABET = \
    '123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz'
dir_path = os.path.dirname(__file__)
with open(f"{dir_path}/english.txt", "r") as f:
    words = [line.strip() for line in f if line.strip()]

win = [Suspicious link removed]()
win.title("BIP-39 Compressor/Decompressor")
frame1 = tk.Frame(master=win)
frame1.grid(row=0, column=0, padx=5, pady=5)

label1 = tk.Label(master=frame1, text="Decompressed Seed")
label1.grid(row=0, column=0)
label2 = tk.Label(master=frame1, text="Your words BIP-39 seed")
label2.grid(row=0, column=2)
decomp = tk.Text(master=frame1, height=10)
decomp.grid(row=1, column=0, columnspan=3)

btn = tk.Button(master=frame1, text="Compress", command=lambda: compressSeed())
btn.grid(row=2, column=2)

frame2 = tk.Frame(master=win)
frame2.grid(row=1, column=0, padx=5, pady=5)

label3 = tk.Label(master=frame2, text="Compressed Seed")
label3.grid(row=0, column=0)
label4 = tk.Label(master=frame2, text="Your compressed seed")
label4.grid(row=0, column=2)
comp = tk.Text(master=frame2, height=1)
comp.grid(row=1, column=0, columnspan=3)

btn2 = tk.Button(master=frame2, text="Decompress", command=lambda: decompressSeed())
btn2.grid(row=2, column=2)
expanded_var = tk.BooleanVar()
check_btn = tk.Checkbutton(
    master=win,
    text="Mark as Expanded (adds '+' to output)",
    variable=expanded_var,
)
check_btn.grid(row=3, column=0, sticky='w', pady=10) 

def b58encode(i: int, default_one: bool = True, alphabet: bytes = BITCOIN_ALPHABET) -> str:
    if not i and default_one:
        return alphabet[0:1]
    string = ""
    base = len(alphabet)
    while i:
        i, idx = divmod(i, base)
        string = alphabet[idx:idx+1] + string
    return string

def b58decode(v: Union[str, bytes], alphabet: bytes = BITCOIN_ALPHABET,
              *, autofix: bool = False) -> int:
    map = _get_base58_decode_map(alphabet, autofix=autofix)
    decimal = 0
    base = len(alphabet)
    for char in v:
        decimal = decimal * base + map[char]
    return decimal

def _get_base58_decode_map(alphabet: bytes,
                           autofix: bool) -> Mapping[int, int]:
    invmap = {char: index for index, char in enumerate(alphabet)}
    groups = [b'0Oo', b'Il1']
    for group in groups:
        pivots = [c for c in group if c in invmap]
        if len(pivots) == 1:
            for alternative in group:
                invmap[alternative] = invmap[pivots[0]]
    return invmap

def compressSeed():
    bip_words = decomp.get("1.0", "end-1c").strip().split()
    out = []
    for word in bip_words:
        if word in words:
            pos = words.index(word)
            out.append(b58encode(pos + 1))
        else:
                    messagebox.showinfo("Seed Error", "The word " + word + " wasn't found in the dictionary!")
    compressed_seed = "".join(out)
    
    # Append '+' if expanded option is selected
    if expanded_var.get():
        compressed_seed += '+'
    
    comp.delete("1.0", "end-1c")
    comp.insert("1.0", compressed_seed)

def decompressSeed():
    compressed_seed = comp.get("1.0", "end-1c").strip()
    
    # Remove trailing '+' if present (expanded seed indicator)
    if compressed_seed.endswith('+'):
        compressed_seed = compressed_seed[:-1]
        messagebox.showinfo("Expanded Seed Alert", "The seed appears to be expanded with an unknown secret extra word!")

    decompressed_words = []
    i = 0
    while i < len(compressed_seed):
        chunk = compressed_seed[i:i+2]
        if not chunk:
            break
        
        pos = b58decode(chunk)
        decompressed_words.append(words[pos - 1])
        i += 2

    decomp.delete("1.0", "end-1c")
    decomp.insert("1.0", " ".join(decompressed_words))

win.mainloop()

Footnote: sorry for posting from a new account, but I don't know where's my TOTP code (and don't quite remember to had create one) for my old account.

[odolvlobo]

I'd an idea to compress BIP-39 seeds, make them to become a tiny string suitable for QR or RFID storage.

A BIP-39 phrase is an encoding of a binary value. If you want to "compress" the phrase, you can use the binary value instead. It can't be compressed any more than that.

If you want text, I think base-64 is a better choice, because it is more compact and more widely used.

[BattleDog]

This is not really compression, BIP39 words are already an encoding of entropy plus checksum. Each word is basically an 11-bit index into a 2048-word list, so replacing the words with line numbers and then base58'ing those numbers is not squeezing magic juice out of it. You are just moving from a human-readable standard format into a custom format that future-you and/or tired-you will have to remember how to decode.

For QR use, sure, representing the raw index/entropy data directly can make the QR smaller. That part is reasonable. But I would not casually store this on RFID/NFC unless the threat model is "I hope nobody waves a reader near my backup." Seeds are not coupons. Anything electronic that can be read conveniently can usually be stolen conveniently too.

The bigger danger is making a clever backup that only one Python script understands. Wallets understand BIP39 words. Humans can check BIP39 words. Recovery tools understand BIP39 words. Your custom base58 seed-string with a "+ means unknown expanded word" flag is the sort of thing that feels elegant now and becomes archaeological pain in 2031 when you are trying to recover coins from an old drawer and a half-dead laptop. If doing this for fun, fine. If doing this for actual funds, keep the real BIP39 phrase backed up in a boring, standard, preferably non-electronic way. Boring survives. Clever often needs tech support.

[SirArthur2]

That's some valid criticism, let me address it.

A BIP-39 phrase is an encoding of a binary value. If you want to "compress" the phrase, you can use the binary value instead. It can't be compressed any more than that.

We are talking about compress to human-usable format, not binary formats. The computed seed for that textual representation is

Code:

eac38d38b91495d46f12e28790c78cf7c5da2937f3b1ff7332ad68007f06316e8156be8520dc880db3946fac8e0e896f6ce27a367e204ac3feb50676a56ab10c

...or decimal 7377687107399363427919506940445526085943085060767798863389917008452289517492375 9375047662272726680500679881548796212384941982210304075203137347288298516924490 7766500354732069958083853735531831032887267586702456712895290533588458780

This has no use for humans, it's the computer technical part.

If you want text, I think base-64 is a better choice, because it is more compact and more widely used.

Even base58 is too much, base46 would be the ideal base size [sqrt(2048)]. Base64 wouldn't give any advantage, the whole idea is to compact a number in maximum of 2 chars.

The bigger danger is making a clever backup that only one Python script understands.

You can even do it by hand with pen and paper, sorry for use GTK in the sample Pyhton, all you need is to split it by pairs, convert it from b58 to decimal and check what line number it corresponds to in BIP-39 wordlist, even Windows Notepad can show line numbers, and if you open in Github instead of downloading the file, Github UI already shows you to the line number.

Code:

Using: https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt
5d = decimal 268; line 268 cannon
EF = 768 = garment
Bh = 620 = estate
BF = 594 = enforce
(...)

As for the "+" it's to remind you that is not the whole seed, the whole purpose of add a custom word to the derivation path is so that if someone gets your seed backup won't be able to get your funds, but you can write it like:

Code:

5dEFBhBFS6345XuAdSQZeT2LMQP6d+<your custom word>

[nc50lc]

We are talking about compress to human-usable format, not binary formats. The computed seed for that textual representation is

They are talking about the entropy that the words represent, not the binary seed that's computed from it.

In your example, those 15 words represent a considerably short 160-bit value (excluding the checksum).
In Hex:

Code:

216bfd35a51b5c1d801eff45b703b25e48c352ca

in Base64:

Code:

IWv9NaUbXB2AHv9FtwOyXkjDUso=

Since you mentioned BIP39 tool, you can find it by toggling "Show entropy details" after typing/generating the seed phrase.