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Suppose I'm stuck in the middle of nowhere with no internet access and I want to make a payment to a certain Bitcoin address.

Is it possible to do a transfer via postal mail, and if so what information would I need to send?

Assume the following:

  1. I have both the recipient's Bitcoin address and my (the sender's) own private key(s) memorized or written down on a piece of paper.
  2. The person who receives the mail (my agent) has access to the Bitcoin network and will act on my behalf.
  3. The mail is delivered securely (i.e. no chance of theft)

What information would I need to send in the mail?

Consider the following possibilities:

  • A) Trust

    1. I trust my agent to do the transfer to the given address (e.g. the agent is the recipient), but I can't trust my agent with my private key(s)
    2. I trust my agent with all my information (private keys and all)
    3. I do not trust my agent at all

  • B) Access to a computer

    1. I have access to a computer, so I can run any program/scripts that may be required for generating any signatures
    2. I don't have access to a computer (and I'm not Rain Man)
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  • "I have access to a computer" "I don't have access to a computer" Huh? – Nick ODell Mar 28 '13 at 0:44
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    @NickODell, Manish is asking us to consider both those possibilities – Highly Irregular Mar 28 '13 at 1:13
  • @NickODell That's right, what are my options if (1) I have access to a computer, and what if (2) I don't. – Manish Mar 28 '13 at 1:19
  • If no one else answers this by the time I have time tomorrow, I'm all over this. – Colin Dean Mar 28 '13 at 3:26
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  • A1B1: Assuming you've prepared in advance, there are some nice offline wallet options.

  • A1B2: This one's tricky. I'm going to assume that you have access to a computer before running off into the woods, or what have you. I'm also going to assume that you know exactly the amount and address in advance.

    1. Generate the transaction and sign it, but don't broadcast it (can be done with Bitcoin Armory)

    2. Print out the transaction

    3. When you want to make the transaction, mail the transaction to your agent.
  • A2B?: Easiest one. Just write down your private key and describe the transaction.
  • A3B?: Well, you're screwed, because the agent could just never make the transaction.
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  • Thanks! About A3, so they couldn't just send the funds to another address (not the intended recipient) if I sent them the transaction as in A1B2? And I'm assuming that's because of the signature. – Manish Mar 28 '13 at 19:00
  • I suppose for A3 one option would be to build redundancy by having multiple agents, and multiple payments could be prevented by having only the exact amount available via that particular sender address (so only one of the transactions would go through). – Manish Mar 28 '13 at 19:07
  • @Manish "couldn't just send the funds to another address" "because of the signature" Yep. "multiple payments could be prevented" As long as you hand the exact same TX to every agent, you don't need to. – Nick ODell Mar 28 '13 at 21:18
  • Great answer, you beat me to it! A1B2 is probably the most secure option of these. – Colin Dean Mar 29 '13 at 0:24
  • @NickODell "As long as you hand the exact same TX to every agent ..." Hmm, how does that work? A transaction says "Send N bitcoins from X address to Y address (signed by X)" so what keeps it from going through multiple times as long as there's funds in X? – Manish Mar 29 '13 at 12:04

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