Lost Half of Private Key - Are Bitcoins Safe?

[TrailingComet]

Absolutely yes. Virtually zero chance of working out the rest of the string

[Harley997]

Thanks for answers.

Reason I asked is that I prefer to give half a key to Alice and Bob and the other half to Charlie and Dennis. I consider this a backup strategy in case something happens to me (so my family can inherit) or I lose my notes (e.g. house burns down). To add safety I could use multisig, but the risks to worry about should be elsewhere; may these conspire and steal my coins, did a screen capture see the private key, did I write them down without any typos?

You have really not exposed only 1/2 of your private key to someone. You have exposed your entire key to two parties, only that each party only has 1/2 of the key.

If you think that you will die in the medium future (within several years) and you are sure that you will not need to spend X amount of bitcoin prior to your death, it would be advisable to simply give whoever you wish to receive your bitcoin upon your death now. This eliminates the issue of having to trust multiple people to carry out your wishes.

If you don't think that you will die in the medium future and/or you may need some of this bitcoin, then you could give each party 1/2 of your blockchain.info password and keep the identifer in a safe deposit box that they would be able to access upon your death. An additional security measure would be to have a backup of your blockchain wallet that can be decrypted with the password that you give the two parties, and then change your blockchain password.

[Berghoff]

It makes it 340,282,366,920,938,463,463,374,607,431,768,211,456 times easier to brute force.

So if you could guess 10^9 per second it'd only take you 10^24 years instead of 10^63 years.

That needed digit grouping.

[Chemistry1988]

Writing down your key is not a good idea as even one "typo" would cause your entire "key" (it wouldn't actually be a key anymore) worthless

Actually, a private key is just a big number (up to 2^256, or more correctly 0xFFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFE BAAE DCE6 AF48 A03B BFD2 5E8C D036 4141).

So with a typo, you would get another valid key, which statistically speaking should be almost sure to be associated with an empty address.

[ranochigo]

It would still be secure, just that it can be easier, but still close to impossible, no one can brute force so many letters and words in such a short time. The best would be transferring your bitcoins to a new address.

[InwardContour]

Writing down your key is not a good idea as even one "typo" would cause your entire "key" (it wouldn't actually be a key anymore) worthless

Actually, a private key is just a big number (up to 2^256, or more correctly 0xFFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFE BAAE DCE6 AF48 A03B BFD2 5E8C D036 4141).

So with a typo, you would get another valid key, which statistically speaking should be almost sure to be associated with an empty address.

depending on what "language" the key is written in it may not translate back over to HEX format within the possible ranges of private keys

Also as you said, the new private key would almost certainly be worthless as it would (almost) certainly be associated with an address with no (spent or unspent) inputs