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So I have a compressed private key. Is it possible to get the uncompressed version of it? If yes, how? Examples in Javascript would be much appreciated.

vinhboy
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2 Answers2

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Ok I answered my own question. From: https://github.com/pointbiz/bitaddress.org/blob/master/src/bitcoinjs-lib.eckey.js#L186-L222

bytes = Bitcoin.Base58.decode(compressed_or_uncompressed_private_key);

After some processing, this bytes-array can then be re-encoded into a compressed or uncompressed private key.

vinhboy
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I know this is an old post. But according to documentation (https://en.bitcoin.it/wiki/Wallet_import_format)

Add a 0x01 byte at the end if the private key will correspond to a compressed public key a compressed private key should have a 01 flag at the end... If it corresponded to a compressed public key, the WIF string will have started with K or L instead of 5 (or c instead of 9 on testnet).

Taking the key from the example on the site

5HueCGU8rMjxEXxiPuD5BDku4MkFqeZyd4dZ1jvhTVqvbTLvyTJ

The key should be derived by decoding the key using Base58, removing the last 4 bytes (8 characters) corresponding to the checksum.

800c28fca386c7a227600b2fe50b7cae11ec86d3bf1fbe471be89827e19d72aa1d507a5b8d

For the compressed version add a 01 byte at the end

800c28fca386c7a227600b2fe50b7cae11ec86d3bf1fbe471be89827e19d72aa1d01

And re derive the double SHA256 hash for the checksum. Using Python (http://www.herongyang.com/Bitcoin/Block-Data-Calculate-Double-SHA256-with-Python.html) or (https://learnmeabitcoin.com/guide/checksum) result is:

a62019d20340a1de1b5f254f07f2f6c96ad5165218459ab4f3c8f5a7c0e12183

Take the first 4 bytes from the hash (a62019d2) and add it to the key after the 01 byte.

800c28fca386c7a227600b2fe50b7cae11ec86d3bf1fbe471be89827e19d72aa1d507a5b8d01a62019d2

Encode the result in Base58 and you should have the compressed key.

KwdMAjGmerYanjeui5SHS7JkmpZvVipYvB2LJGU1ZxJwYvP98617

Notice the key begins with K. I was unable to confirm with any other info published on the internet.

MRGiacalone2005
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