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My question is about the difference between compressed keys and addresses, and i know it was addressed in another question but my question is less about the theory and more about how the keys are used practically in Bitcoin.

So heres what I did: I used dumpprivkey for an address (183Tw2TqXKkbk5ZeocTYwmxg8x46ADXb6c) in the .7.2 client, and got L5DTi7rgqsPg7Vq1vTU2dP2HWMEsNM5w5JCqEDXXXXXXXXXXXX. When i pasted this into bitaddress.org's wallet details script it returned several things. First, it showed a different address (1FKswUuFsjWKyr4ZPSe3vEpp1iCXXChBZm), but it also showed the address above and said it was the compressed version. I sent like 0.0001 to each of the 2 addresses and my satoshi client only shows a transaction recieved from the 183 (compressed) address.

Can someone please explain in practical terms what the difference is and why they both even exist in Bitcoin? Any help is appreciated.

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^^ bitaddress.org* –  leximus prime Jan 2 '13 at 7:03

1 Answer 1

The reason they exist both, is because Satoshi didn't know about compressed public keys, and it was only recently discovered that they would be possible to use without compatibility problems (support only exists as of version 0.6 of the reference client).

The advantage is clear: their public keys are smaller, resulting in smaller transactions on the network, saving block chain size for everyone.

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thanks for the answer, but what i really dont understand is, in terms of usability, how come when i sent to the compressed, the satoshi client recieved the transaction, and when i sent the uncompressed, it did not. also, why do sites such as blockchain.info see them as 2 different addresses. also, is it possible to determine the uncompressed from the compressed, and vice versa? –  leximus prime Jan 2 '13 at 7:02
    
The address is just the hash (in base58 encoding) of the serialized public key. Compressed public keys are a more efficient serialization, so they correspond to a different address. Given a public key, you can determine the "compressed address" and the "uncompressed address", but not from just an address. Clients can only receive coins on compressed addresses if they support compressed public keys in the first place. –  Pieter Wuille Jan 2 '13 at 12:15
    
Still, shouldn't a post-0.6.0 client recognize both the send to the compressed address, and the send to the uncompressed address, because it has the private key sufficient to spend either? –  Quizzical Nov 27 '13 at 2:16
    
What's the point? No peer can have the address for the wrong one in case you only give out one. No need to double the number of addresses checked if we know only one is ever shown. –  Pieter Wuille Nov 27 '13 at 22:01
    
Does this mean if custom software gives out the base58 address for an uncompressed address it is unspendable or that it is spendable through RPC ? –  makerofthings7 Feb 14 at 13:46

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