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It is possible to get it from here: https://blockchain.info/de/q/pubkeyaddr https://blockchain.info/q
Is it possible with the standard client? How?
I am aware that the blockchain only contains the pubkey after a tx from that address.
Is it possible to get the public key of a bitcoin address I do not have the private keys for with the standard client?
Answers
18
It is impossible to compute the public key of an address, as the address is computed from the hash of the public key. You can retrieve the public key from address with the reference client using the validateaddress RPC call (or in the debug window of Bitcoin-Qt), but that simply fetches it from the wallet, and only works if the address is your.
What do you need the public key for, if it's not your address?
9
It is impossible. Given an ECDSA (compressed 65Bytes or not 33Bytes) public-key K, a Bitcoin address is generated using the cryptographic hash functions SHA-256 and RIPEMD-160. The public-key is hashed twice: HASH160 = RIPEMD-160(SHA-256(K)). The Bitcoin address is computed directly from this HASH160 value as
base58(0x00 || HASH160 || bSHA-256(SHA-256(0x00 || HASH160))/2224c). "||=concatenation"
so it is impossible to reverse the hash to get the public key.
@PieterWuille : but payment transactions are performed to a pubkey… Right ? – user2284570 – 2019-03-28T01:21:11.383
it depends on the transaction type – Badr Bellaj – 2019-03-28T11:40:57.053
I wrote this in the sense this is a requirement. – user2284570 – 2019-03-28T11:45:30.817
the pub key is shared with the network so no need to reverse an address to get the pubkey – Badr Bellaj – 2019-03-28T15:23:56.833
@user2284570 What is a "payment transaction"? – Pieter Wuille – 2019-03-28T19:32:03.100
@PieterWuille a transaction which bear value with a receipient which is not a miner. – user2284570 – 2019-03-28T21:32:42.697
In that case, no; there are almost no pay-to-pubkey outputs at all. – Pieter Wuille – 2019-03-28T21:41:38.307
9
To be honest, it IS possible, but you need a signature made by that address. From that point, you can get the public key.
See this piece of code: https://github.com/bitcoinjs/bitcoinjs-message/blob/master/index.js#L57
1
I would like to encrypt messages by abusing bitcoin/namecoin signing keys as encryption keys. see: https://bitcointalk.org/index.php?topic=145098 I am aware there might be peripheral risks to using the same keys for signing and encryption.
– kermit – 2013-06-13T20:07:02.6001To elaborate, a public key is 33 bytes long, whereas an address is a 20-byte hash of the public key. Because the information is smaller, therefore it is impossible to compute the correct public key from an address. – Nayuki – 2016-01-11T02:30:16.517
don't i need the full pub key for an address i do NOT own when creating a new multisig address with "addmultisigaddress"? – Albert s – 2016-01-23T21:27:24.323
It IS possible to recover the public key from the signature though (so you can recover the public key of an address already seen on the blockchain), see the answer below. – None – 2017-06-28T14:09:03.540
Wait, shouldn't it be possible to lookup int he blockchain and see if that person spent from that address and then get the public key used to sign the transaction? – Guerlando OCs – 2017-07-26T08:30:28.440
@PieterWuille : but payment transactions are performed to a pubkey… Right ? – user2284570 – 2019-03-28T01:21:07.093
What are "payment transactions"? – Pieter Wuille – 2019-03-28T01:48:46.310