IntroductionCustody of the seed is a topic that has always been close to my heart. For the first time, Bitcoin is a system that puts the responsibility of their financial sovereignty on the users' hands. This feature involves a complex balance of opportunities but also risks, given the intrinsic bearer nature of Bitcoin. The ownership of a bitcoin belongs to the person who can digitally sign a transaction with the corresponding private key—no other subject. Person, institution, or bot, whoever controls the private key, controls the bitcoins.
That means that the custody of bitcoin is of fundamental importance regarding the actual possibility of signing transactions and the possibility of adverse events, present or future, of a personal, social, or technological nature that every bitcoiner may encounter within a few years.
Technological, personal, and legal threats must be assessed when considering which storage option to select:
For a comprehensive guide on how to choose the way to backup your seed phrase, you can resort to this new article, again from Jameson:
How to Backup a Seed PhraseOf course, the important thing is that Bitcoin enables the self-custody options ruled out in traditional finance. It is a unique feature that empowers bitcoiners in various adverse scenarios that will be, sadly, more and more common in a few years.
The problem is that self-custody is complex, and the amount of lost bitcoins is a warning for everyone who wants to go down this road: challenges take the most different shapes.
I have been researching this topic for a few years, and one of the first articles to spark my curiosity was:
Gifting satoshis to future generationsRecently, I've been tasked to gift some satoshis to a newborn baby, who is supposed to redeem them when he grows up, about 18 years from now. The challenge was trickier than initially expected, as there are many different ways to store bitcoin, each one with different trade-offs. At the same time, it's hard to predict the state of the Bitcoin industry two decades into the future.
Reading this article, I realised that the custody of a private key through a physical wallet had a few undoubted advantages:
- independence from the technological development in terms of wallets/IT supports
- security concerning the decay of the physical support
- ease of use by the recipient of my gift.
Browsing the internet, I then found a page on Jameson Lopp’s website, a famous Cypherpunk and security expert, where he tested various Metal Seed Storage:
Metal Bitcoin Seed Storage Stress TestIn the first round of testing, one, in particular, caught my attention, as it was the first implementation of the idea of printing the seed on washers by SAFU Ninja:
SAFU Ninja ReviewThe brilliant idea is described in the blog:
Cheap and Easy DIY Metal Bitcoin Seed Storage – Safu Ninja 
The innovation is setting up the storage using pieces easily found in a typical hardware store to print the seed.
The most critical part of the work was stamping the letters on the washers, which turned out to be quite a long, tiring process, yielding not always perfect results when done freehand.
Therefore, a few tinkerers have developed a jig suitable for facilitating the correct orientation, separation, and correct printing of the letters.
The most famous jig is from
Cryptocloacks.
This jig can accurately print the letters with correct spacing and positioning.
The n0nce jig is also available on bitcointalk.org.
n0nce's Steel Washer Backup jig
The advantage of this jig is twofold: first of all, the project is modular. Therefore, it is adaptable to different sizes regarding the washers (the project was born precisely to print on more large size washers) and, above all, adapt to the various dimensions of the punches.
Secondly, n0nce created an improved design of the Blockmit jig, strengthening it in several parts and taking care of the invention with smaller tolerances to reduce the mechanical play between the components.
I will use the n0nce jig in the rest of the guide, but the operation is the same. Finally, in the appendix, we will analyse the differences between the two projects in detail.
As correctly pointed out by Federico Tenga, the BIP39 standard presents "risks of obsolescence" and, above all, risks due to the derivation path used. For this reason, he, in his test, used the WIF format.
Tough I understand perfectly and share his concerns, I decided to use the more usual BIP39 format instead of this project's first iteration. I believe that it presents a better risk/benefit balance on the practical possibility of recovery by the message recipient. Nothing prevents you from hacking the key in that format on the washers: it is a string of 51 characters so you would need only seven washers.
On the internet, the first to have described the whole process was econoalchemist:
Securing a Bitcoin seed phrase in stainless-steel washers (abridged)What can I say? He is an über cool bloke. A complete test with a blowtorch! I don't aspire to that much, and I'll be content to print the washers' resistance to fire corrosion and pressure that I could potentially do. I am afraid it wouldn't differ significantly from the tests made by econoalchemist and Lopp.
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Shopping ListAs mentioned, the company's advantage is to use readily available pieces. This choice certainly helps to have some degree of security by obscurity, as no one will ever really know what you are doing. Indeed, as directly suggested in Safu's guide, if you have something suitable at home, use that, don't even buy it. If you know someone who already owns those punches, borrow them by saying you have to print keys rather than buying them (never mention bitcoin, of course).
So take these models as suggestions:
WashersWashers with an outside diameter of 24 mm, inside diameter of 8 mm, and stainless steel are sufficient. They are found in any hardware store. Twenty-four washers are required for the key and four washers for the additional safety devices. Equip yourself with many extra washers to cope with trials and inevitable errors.
I recommend that they are made of stainless steel. The
most corrosion-resistant steels are 304 and 321.
Bolt We need a 60 mm bolt with a nut, a diameter of 8 mm (the most astute will have noticed that the diameter is the same as that of the washers!).
Also, in this case, the material to choose is stainless steel.
SledgehammerYou need a sledgehammer. A hammer is not enough since the weight must impart enough force to stamp the washer correctly. Again, nothing too sophisticated.
Anvil Who would have thought that your passion for Bitcoin would lead you to buy an anvil!
Given the strength needed to print the washers correctly, you will need sturdy support that won't get damaged by all the strokes. This small 1kg anvil, the smallest I have found, is the perfect fit for you.
Of course, bigger is better, but if you want to save these euros, you can use the smooth side of another club or a metal base. The important thing is that it is a sufficiently large surface to guarantee good stability of the jig and the force applied by the club—a matter of preference. However, I must say that I found this mini anvil very comfortable and suitable for work.
StampsRegarding the stamps set, I have chosen the following 6 mm punch set: it is a set of 36 characters, all letters, digits, and the “&” character. The quality is good, and the dimensioning of the punches is adequate. I found no defects during use, and the print quality did not deteriorate during the whole process.
Safety labelsThese are adhesive labels that, once attached, break, leaving a mark on the surface.
To close the container, you will need safety labels that leave a trace when opened. They must be individually numbered to print the serial number inside the washers.
Hermetic containerWe want to insert the seed in an airtight container for an aesthetic factor and verify its integrity. I found this in a lovely bitcoin orange. Among other things, the internal diameter is 25 mm, therefore ideally suited to accommodate our washers. If you decide to buy a different one, make sure that the diameter and length of the space available inside are sufficient to accommodate your seed.
At this point, the setup is almost complete.
Below is an overview of all the necessary materials.
Other miscellaneous material- A pair of gloves.
- Hot glue gun and stick. To seal the washers.
- Circular labels for security seals.
- The private key in BIP39 format.
- Indelible marker.
The following is an indicative recap of all possible costs (time excluded):
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Stamping Verifying and Sealing the SeedThe procedure is straightforward. Insert the washer into the jig, covering the bottom of the jig with some duct tape. Using that tape immobilises the washer inside the support to not stop after each hammering.
In this way, you will avoid the letters being printed at different distances, but if you are not satisfied with how the letter is printed, you can try to reprint it, completely overlapping the signs. This is the main advantage of using a jig: doing everything freehand, it is easy for a letter to be poorly printed if the punch is hit in a non-optimal manner; with a jig instead, the printing process is rock solid, and if done correctly every letter is printed steadily and firmly into metal. Nevertheless, with a jig, the operation is straightforward and is well represented by the following gif:
The two upper notches are used to identify the correct order of the words, while the eight lower notches are used to print the terms. In the BIP39 standard, words have a maximum length of 8 letters, and the first four letters are sufficient to identify them uniquely. You won't have to worry about data loss if one or two letters aren't perfectly legible.
Place the jig on the anvil and deal a blow.
Proceed like this for all the letters and all the words.
One video, or rather two videos, are worth more than 100 words:
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| Outdoor workstation. Note the discarded washers in the foreground.
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