Level 6: Metal Backup¶
Protect your Bitcoin with metal backups (worth as much as your house).
What if your signing devices and seed phrases were destroyed in a fire?
For significant Bitcoin holdings, metal backups of your seed phrases ensure your private keys survive extreme conditions like fire, flood, or physical damage. For example, a metal backup created using stainless steel dog tags provides a fireproof, waterproof, and corrosion-resistant copy of your seed phrase.
Why Metal Seed Backup?¶
A metal backup ensures your Bitcoin seed phrase withstands disasters that would destroy paper or digital backups. By stamping your seed phrase into stainless steel or titanium, you create a resilient, long-term artifact.
Which Metal?¶
Not all metals resist environmental damage or are suitable for stamping. The table below compares common metals for hardness (Vickers scale, higher is better for stamping durability) and melting temperature (for fire resistance):
| Metal | Hardness (Vickers) | Melting Temperature | Suitability for Seed Backup |
|---|---|---|---|
| Stainless Steel | ~200–250 HV | ~2,500°F (1,370°C) | Excellent: Affordable, corrosion-resistant, durable, easy to stamp. |
| Titanium | ~300–350 HV | ~3,000°F (1,660°C) | Superior: Lightweight, extremely durable, fire-resistant, slightly harder to stamp. |
| Aluminum | ~25–50 HV | ~1,220°F (660°C) | Poor: Soft, prone to scratches, low fire resistance. |
| Silver | ~25–70 HV | ~1,760°F (960°C) | Poor: Soft, tarnishes, less durable for stamping. |
| Gold | ~20–30 HV | ~1,947°F (1,064°C) | Poor: Very soft, expensive, unsuitable for stamping. |
- Stainless Steel: Affordable and corrosion-resistant. Recommended in grades 304 or 316.
- Titanium: Ultra-durable, lightweight, and fire-resistant but more expensive and slightly harder to stamp. Ideal for high-value backups.
- Tungsten Stamping Kit: Use a tungsten or steel stamping kit with letter stamps. Tungsten stamps are harder, ensuring clear imprints. Available at craft stores or online retailers.
Avoid softer metals like aluminum, silver, or gold, which are less durable and harder to stamp legibly. Verify material quality to ensure durability.
Creating a Metal Seed Backup¶
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Stamp your seed phrase into stainless steel dog tags or fender washers. Generic materials, like dog tags, enhance security by allowing you to control the supply chain and verify materials independently.
While commercial seed backups exist, most are inferior to a DIY solution. The MicroSeed stamp kit, which uses 1" stainless steel fender washers (available at hardware stores), is a good option to simplify the stamping process.
Materials Needed
- Metal Medium: Stainless steel dog tags or fender washers.
- Stamping Kit: Tungsten or steel stamping kit with letter stamps.
- Stamping Surface: A flat, hard surface like a steel anvil or brick for stable stamping. A brick is sufficient but may cause slight imperfections.
For the actual stamping, you'll want to follow the Quarantine Rules discussed previously.
Stamping Process (quarantined)
- Prepare the Seed Phrase:
- Retrieve the seed phrase from your signing device, written on a temporary paper backup during initial setup.
- Work in a secure, private location with no electronic devices nearby.
- Set Up the Stamping Area:
- Place the dog tag or washer on a flat, hard surface (e.g., steel anvil or brick).
- Secure the medium with tape or a holder to prevent movement. Avoid stamping beyond designated areas to prevent damage.
- Stamp the Seed Phrase:
- Use a tungsten stamping kit to imprint each word letter by letter. Apply light, consistent pressure for clear, legible imprints.
- Verify the Backup:
- Test the backup by entering the seed phrase into a temporary, offline wallet (e.g., on a fresh Tails OS session) on a signing device.
- Secure the Backup:
- Store the metal backup in a tamper-evident bag or container.
- Place it in a secure, offline location (e.g., safe deposit box or home safe).
Tamper-Evident Seals¶
For metal backups that don’t require regular access, tamper-evident seals are recommended to ensure no unauthorized access has occurred.
A combination of high-tech and low-tech approaches is advisable. For example, tamper-evident bags are widely available, and an attacker could replace the entire bag, including printed serial numbers, with relative ease. Here are some recommended solutions:
- RFID or NFC Tags: Use specialized tamper-evident tags or cables with RFID or NFC capabilities. Any attempt to break or open the seal disables the RFID or NFC functionality. Replacing the tag requires replicating the exact RFID signal, which is possible but demands a technically sophisticated attacker.
- Tamper-Evident Tape or Bags: These products reveal opening attempts through clear visual indicators (e.g., a “VOID” message after tampering). However, replacing these widely available items is often straightforward, reducing their standalone effectiveness.
- Wax Seals: A classic low-tech solution, wax seals are highly effective but can be replicated by anyone with access to your stamp.
- Unique Markings: Simple yet effective, these can include a signature, hand-etched scratches, or other personalized markings known only to you. These are difficult for an attacker to replicate exactly without detailed knowledge of the design.
In practice, combining multiple techniques -- particularly high-tech and low-tech approaches -- makes unauthorized replacement extremely challenging. For instance, replacing an RFID tag requires a programmable chip, which is difficult but feasible. However, if the attacker must also replicate a custom wax seal and unique hand-drawn markings, the task becomes impractical.
Maintaining Your Metal Backup¶
To ensure your metal backup remains reliable:
- Inspect Regularly: Check annually for corrosion, scratches, or tampering.
- Multiple Backups: Create at least two metal backups, stored in separate, secure locations (e.g., home safe and bank vault).
- Test Periodically: Every few years, verify legibility and test with an offline wallet to confirm accuracy.
- Secure Locations: Store backups in fireproof, waterproof containers in locations accessible only to you and your family (trusted loved ones).
Disclaimer: Metal seed backups require meticulous handling and secure storage. Verify material quality, stamp accurately, and protect backups from theft or exposure. Self-custody carries risks, and you are responsible for protecting your Bitcoin.