Glossary / Addresses & keys
Uncompressed public key
- Definition
- An uncompressed public key writes out both coordinates of a curve point behind a 0x04 prefix, taking 65 bytes, and survives today mainly in keys generated before 2012.
Bitcoin's earliest software recorded the full x and y values, which is why coins from 2009 and 2010 sit behind addresses derived from 65 byte keys. SegWit then closed the door on the format, since BIP-143 forbids uncompressed keys in version 0 witness programs and no bc1q address can use one. If you are recovering an old paper wallet, this is the detail that decides whether your funds appear.
How it works
An uncompressed public key has a layout as literal as its name. One prefix byte of 0x04 announces the format, then 32 bytes give the x coordinate, then 32 bytes give the y coordinate, for 65 bytes altogether. Nothing has to be computed to read it: both halves of the point are already there, which is exactly why the original implementation chose it on the hardware of the time.
Validity rules have never singled the format out at the base layer. A node checking a legacy pay-to-public-key-hash or pay-to-script-hash spend accepts an uncompressed key without complaint, and coins locked to one in 2010 remain spendable with no deadline. What changed is everything built on top: BIP-32 wallets derive compressed keys only, so a seed phrase can never produce an uncompressed key, and SegWit made them invalid inside witness version 0 spends.
The cost of the extra 32 bytes is paid at spending time. A legacy input carrying a compressed key runs about 148 vbytes; the same input with an uncompressed key runs about 180 vbytes, roughly 22 percent more for an identical spend. Consolidating a dozen such inputs during a fee spike is where that difference stops being trivia.
Where you see it
Three artifacts still hand people uncompressed keys, and all three date from the same era.
Paper wallets printed by the generator sites of 2011 to 2013 usually produced an address beginning with 1 and a Wallet Import Format string beginning with 5, the marker for the uncompressed side of the format. Physical coins in the Casascius series, minted between 2011 and 2013, hid the same kind of string under a tamper-evident hologram. And private keys exported from very early wallet software predate the switch entirely.
Recovering an old key
The trap is not the encoding, it is that one private key produces two different addresses. Hashing the 65-byte key gives one legacy address; hashing the 33-byte compressed version of the same key gives a completely different one. Import the key into modern software that assumes compression and the balance can look like zero while the coins sit untouched at the other address.
Wallets that handle old material well, Electrum among them, check both derivations when you sweep a key rather than import it. Sweeping is also the safer operation: it moves the coins into an address controlled by your current seed phrase, so a printout that has been sitting in a drawer, photographed by a phone or seen by a previous owner, stops being able to spend anything. Check both address forms on a block explorer before deciding a recovery failed.