What do I do to secure my SEEDPLATE?
Most people now understand that metal seed backups are paramount, as paper burns. The challenge is how to store clear-text secrets? Anyone with physical access could use it and gain complete control of your funds! Encrypted digital backups are great, but they are not discrete and you could be compelled to produce the passphrase.
Enter Seed XOR, a plausibly deniable means of storing secrets in two or more parts that look and behave just like the original secret. One 12 or 24-word seed phrase becomes two or more parts that are also BIP-39 compatible seeds phrases. These should be backed up in your preferred method, metal or otherwise. These parts can be individually loaded with honeypot funds as each one has same word length, with the last being the checksum and will work as such in any normal BIP-39 compatible wallet.
This one more solution for your game-theory arsenal.
- Q: I'm lazy, can I do this to my Existing Seed?
- A: Yes. You can split the words you have already in your Coldcard, making 2, 3 or 4 new SEEDPLATES. You could also any number of existing SEEDPLATES you have, and combine them to make a new random wallet that is the XOR of their values. Effectively that makes a new random wallet.
Seed XOR works by taking any number of seed phrases in BIP-39 style, and simply XOR-ing them together, bit-by-bit into a new phrase. All seed phrases have to be of the same length.
The last word contains checksum. For the "parts" (sometimes called "shares") this checksum is calculated as normal for BIP-39, but those final bits are not used in the XOR process. But the checksums still protects the integrity of the individual parts. In 24-words XOR last 8 bits are checksum and in 12-words XOR last 4 bits are checksum.
Useful properties of this approach:
- Every "part" looks and operates as a valid BIP-39 wallet.
- All the parts can be combined in any order and you arrive at the same result.
- You must have all parts, because any combination of less than all parts is a valid Seed XOR wallet too.
- Each "part" can be recorded on a SEEDPLATE like normal and no new recording tools are needed. No information about you original seed is leaked by finding up to N-1 of the parts.
- You can store funds on the seeds of any part, and any subset of parts, which opens even more duress options.
We recommend storing the checksum word of the original wallet along with your N parts. This allows you to be sure you've gotten all the parts and assembled them correctly. This does reveal 3 bits of your real wallet however, and also reveals that a working and correct subset of parts has been assembled.
It is not hard to calculate a Seed XOR on paper (or to verify or reconstruct a seed split by Coldcard). Below is a complete example, and a lookup table that allows you to XOR together hex digits. You can do the XOR at the bit level, but we recommend looking up each word and finding it's 3-digit hex value (0x000 to 0x7FF), and going hex-digit by hex-digit (4 bits).
How Parts are Generated
Create new parts on your Coldcard:
Advanced > Danger Zone > Seed Functions > Seed XOR > Split Existing
You can choose between 2, 3 or 4 parts. You can also choose (next screen) to generate them deterministically or using the TRNG. The advantage of the deterministic approach is you'll always get the same answers, so you can check that you've recorded the correct words right the next day.
When the parts are made deterministically, we take a double-SHA256 over
a fixed string (
Batshitoshi), your master secret, and the text
1 of 4 parts which changes for each part.
In random mode, we simply pick 32 random bytes (and then double-SHA256 them) from the Coldcard's True Random Number Generator (TRNG)..
This is done to make all but the one part. The final part is the value needed to get back to your secret, so it's the XOR of the other N-1 parts. It contains just as much entropy as the other parts.
So many possible duress games are possible once you've split your seed up, and you are able to "give up" all of the seed phrases, except one, and the attackers will still get nothing. You can load various possible combinations of your Seed XOR's with various amounts, so none are obviously empty and so on.
Any two or more SEEDPLATES you have already encoded can be used together to make a new wallet based on their XOR. No changes to their existing values are needed... just import the set into a new Coldcard and effectively a new random seed is in play at that point.
One downside of the deterministic approach is that it allows attackers to verify they have a seed that was split by Coldcard. They can import the N parts into a Coldcard, and then split them again on that Coldcard, and should arrive at the same values. If they don't then either you used the TRNG, or they have some subset of all the parts.
You can pick your XOR parts randomly, and the result when XOR'ed together, is a random wallet. However, it would be best to get the last word checksum recorded correctly, so please use a tool such as the Coldcard to lookup the 24th word and save that (for each part). For example, you might take a fresh Coldcard (no secret) and draw 23 words from a hat. After providing the 23rd word, the Coldcard will show 8 possible final words. You can pick randomly from that list, or simply use the first one, and then cancel the seed import process on the Coldcard. Record that final word along with the others on a SEEDPLATE.
XOR Lookup Table
- XOR = EOR = ⊕ = Exclusive OR Wikipedia
- values in table are: x ⊕ y in hex
- go sideways for first digit, then look down for second digit
- in fact, doesn't matter if you do row or column first
- example: 2 XOR 6 => 4 same as 6 XOR 2 => 4
- any values XOR itself is zero (diagonal on this table)
- alternative view: (x) XOR (y) = flip bits of (x) that are set in (y)
- XOR with zero does nothing (flips no bits)
- XOR with 0xF flips all four bits
- XOR with self flips all set bits, so gives zero
- to XOR three values together, do (a⊕b)=X then (X⊕c)=answer
- right to A, down to B ... take that number, and go to that column
- down to C, that is answer: a ⊕ b ⊕ c
24 Words XOR Seed Example Using 3 Parts
Seed A (1 of 3)
1=romance [5DC], 2=wink [7DE], 3=lottery , 4=autumn [07D], 5=shop , 6=bring [0E1], 7=dawn [1BF], 8=tongue , 9=range [58E], 10=crater , 11=truth [74E], 12=ability , 13=miss [46E], 14=spice [68C], 15=fitness [2BF], 16=easy [22E], 17=legal [3FB], 18=release [5A9], 19=recall [59B], 20=obey [4BF], 21=exchange , 22=recycle [59F], 23=dragon , 24=room [5DF] A = 5DC 7DE 420 07D 635 0E1 1BF 723 58E 194 74E 001 46E 68C 2BF 22E 3FB 5A9 59B 4BF 275 59F 210 5DF
Seed B (2 of 3)
1=lion , 2=misery [46D], 3=divide [1FF], 4=hurry [37D], 5=latin [3EB], 6=fluid [2CD], 7=camp , 8=advance [01F], 9=illegal , 10=lab [3E0], 11=pyramid , 12=unaware , 13=eager , 14=fringe [2E8], 15=sick [63E], 16=camera , 17=series , 18=noodle [4B0], 19=toy , 20=crowd [1A2], 21=jeans [3BD], 22=select [61A], 23=depth [1D9], 24=lounge  B = 411 46D 1FF 37D 3EB 2CD 106 01F 388 3E0 578 763 227 2E8 63E 105 620 4B0 733 1A2 3BD 61A 1D9 422
Seed C (3 of 3)
1=vault [78E], 2=nominee [4AF], 3=cradle [18F], 4=silk , 5=own [4F0], 6=frown [2EC], 7=throw [70A], 8=leg [3FA], 9=cactus , 10=recall [59B], 11=talent [6EB], 12=worry [7EE], 13=gadget [2F5], 14=surface [6D1], 15=shy [63C], 16=planet [52F], 17=purpose , 18=coffee , 19=drip , 20=few [2AC], 21=seven , 22=term [6FB], 23=squeeze [69C], 24=educate  C = 78E 4AF 18F 644 4F0 2EC 70A 3FA 100 59B 6EB 7EE 2F5 6D1 63C 52F 573 169 219 2AC 625 6FB 69C 234
Calculation (XOR each hex digit)
A = 5DC 7DE 420 07D 635 0E1 1BF 723 58E 194 74E 001 46E 68C 2BF 22E 3FB 5A9 59B 4BF 275 59F 210 5DF B = 411 46D 1FF 37D 3EB 2CD 106 01F 388 3E0 578 763 227 2E8 63E 105 620 4B0 733 1A2 3BD 61A 1D9 422 C = 78E 4AF 18F 644 4F0 2EC 70A 3FA 100 59B 6EB 7EE 2F5 6D1 63C 52F 573 169 219 2AC 625 6FB 69C 234 | | | | | | | XOR = 643 71C 450 544 12E 0C0 7B3 4C6 706 7EF 4DD 08C 4BC 2B5 2BD 604 0A8 070 0B1 7B1 7ED 57E 555 3xx
Resulting Seed Phrase
1=silent , 2=toe [71C], 3=meat , 4=possible , 5=chair [12E], 6=blossom [0C0], 7=wait [7B3], 8=occur [4C6], 9=this , 10=worth [7EF], 11=option [4DD], 12=bag [08C], 13=nurse [4BC], 14=find [2B5], 15=fish [2BD], 16=scene , 17=bench [0A8], 18=asthma , 19=bike [0B1], 20=wage [7B1], 21=world [7ED], 22=quit [57E], 23=primary  final word between: gas  - lend [3FF] correct final word: indoor 
12 Words XOR Seed Example Using 3 Parts
Seed A (1 of 3)
1=romance [5DC], 2=wink [7DE], 3=lottery , 4=autumn [07D], 5=shop , 6=bring [0E1], 7=dawn [1BF], 8=tongue , 9=range [58E], 10=crater , 11=truth [74E], 12=ability  A = 5DC 7DE 420 07D 635 0E1 1BF 723 58E 194 74E 001
Seed B (2 of 3)
1=boat [0C6], 2=unfair , 3=shell [62B], 4=violin [7A2], 5=tree [73F], 6=robust [5DA], 7=open [4D9], 8=ride [5CB], 9=visual [7A7], 10=forest [2D9], 11=vintage [7A1], 12=approve  B = 0C6 768 62B 7A2 73F 5DA 4D9 5CB 7A7 2D9 7A1 056
Seed C (3 of 3)
1=lion , 2=misery [46D], 3=divide [1FF], 4=hurry [37D], 5=latin [3EB], 6=fluid [2CD], 7=camp , 8=advance [01F], 9=illegal , 10=lab [3E0], 11=pyramid , 12=unhappy [76A] C = 411 46D 1FF 37D 3EB 2CD 106 01F 388 3E0 578 76A
Calculation (XOR each hex digit)
A = 5DC 7DE 420 07D 635 0E1 1BF 723 58E 194 74E 001 B = 0C6 768 62B 7A2 73F 5DA 4D9 5CB 7A7 2D9 7A1 056 C = 411 46D 1FF 37D 3EB 2CD 106 01F 388 3E0 578 76A | | | XOR = 10B 4DB 3F4 4A2 2E1 7F6 460 2F7 1A1 0AD 597 73x
Resulting Seed Phrase
1=cannon [10B], 2=opinion [4DB], 3=leader [3F4], 4=nephew [4A2], 5=found [2E1], 6=yard [7F6], 7=metal , 8=galaxy [2F7], 9=crouch [1A1], 10=between [0AD], 11=real  final word between: toward  - tree [73F] correct final word: trade 
- It's not possible to calculate the checksum of the final seed phrase on paper (needs SHA256).
- But it must start with the indicated digit(s). If using 24 words XOR, there will be only one suitable choice offered by the Coldcard in that range (x00 to xFF), once you have entered the other 23 words.
- The checksum of each of the XOR-parts protects the final result, assuming your XOR math is correct.