Is there a way to sign a transaction with BTC or any Altcoin without a computer using only pen and paper?

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Was wondering if this is possible to do. Lets say Satoshi Nakamoto ended up prison in a foreign country and he was only able to communicate with the outside world by writing letters using only a pen and paper and being able to send it anywhere in the world. How can he sign a Bitcoin/Altcoin transaction using only pen and paper when he has memorized the following:

  1. A brain wallet address.
  2. Private Key

Now he would have to create a transaction and then sign it. And for him to broadcast the transaction, he would send it to someone while keeping his coins safe aswell. How can this be done using only pencil and paper?

Patoshi パトシ

Posted 2015-12-02T01:13:29.077

Reputation: 8 911

not the same. nothing to do with mining. – Patoshi パトシ – 2015-12-02T07:22:30.463

Answers

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He cannot for a bunch of reasons:

  1. He has no idea what the unspent transaction hashes are (not memorized).
  2. He does not have a good way to know where he's sending the coins.
  3. In order to actually sign the transaction, Satoshi would need to do ECC math on the SECP256K1 curve, which he hasn't memorized.

Supposing that he's given an unsigned transaction to sign and he somehow has memorized the parameters of the SECP256K1 curve, it would be possible, but it's likely to take him a very long time to sign a single transaction. Point doubling alone is 14 operations of 256 bit numbers using Jacobian coordinates. For signing anything, you usually need to double the generator point 256 times.

Basically, he can if given all these numbers, but it's likely not (A) secure since he'll leave have the paper around which has evidence of his private key which he needs in order to sign and (B) efficient in any sense of the word since it's very likely he'll make mistakes and a single mistake will render the signature invalid.

Jimmy Song

Posted 2015-12-02T01:13:29.077

Reputation: 7 067

i wonder what would make it possible to do it by hand? if not btc, maybe some other coin or algo? – Patoshi パトシ – 2015-12-02T07:23:15.140

you can do it with a check. he can certainly sign a physical check =). seriously, though, most alt coins use ECC math and require you to memorize the generator point, for example, which is a 256 bit x 256 bit coordinate over a prime field which you also have to memorize. not easy to do by hand even if you had all the numbers available. – Jimmy Song – 2015-12-02T17:23:57.613

reason why im asking is because i've been pondering how would someone utilize a memorized brain wallet to sign a transaction without revealing the brain wallet or private key itself. this would be an interesting use case for many things i'd imagine. – Patoshi パトシ – 2015-12-02T20:07:47.630

depends on what you mean by reveal, but this can be done fairly easily with software. no one's written it because brain wallets are not secure and it's highly recommended that you don't use one. – Jimmy Song – 2015-12-02T22:26:29.730

what if you had a 200 character brain wallet and its not something you find in a book or any public media? and it has every type of alphanumeric character. – Patoshi パトシ – 2015-12-02T23:06:09.100

at that point, you might as well memorize a private key. regardless, it's generally not a good idea. that said, nothing is stopping you from creating your own tool to do this. it's actually not even that hard to make. – Jimmy Song – 2015-12-03T02:34:15.357

Asked: 2015-12-02T01:13:29.077

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Active: 2015-12-02T04:32:53.430