2

Peter Wuille recently explained how ECDSA pubkey recovery is done, in response to my question.

So it's basically that for a given base64 signature, the value v in v,r,s provides the specific coordinates, hence the recid (recovery ID).

Example from BitcoinCore test vectors:

> sig =
> "H8PgOb/liZzt3QQHJn9kLBqH7E/i+SC6JTwYGtdNdOjnXzFqXnHMZqP7oZ1wb1QiQ3H/kF8xC9Yx7pK9ddlx8TA"
> addr = "1K5Z1nxN4mjUgCLpSXMRkeZxuAMpbn2CQB" wif =
> "KwfJTiKdcjNMjBu4ksgGd21EZXz6JomoZNbirP3nfd3K9ZMXMEUi"
> 
> v,r,s = vrs = (31,
> 88597177789312009809148107221292570613390338668815747761545214128303675599079L,
> 43057030252916568867525408201971649068117337291455262356277580652864892694832L)

The value of v is 27 + recid for uncompressed keys and 31 + recid for compressed keys.

Ive run a Python loop using pybitcointools which signs a message using a random key, and I've yet to ever see v=29 or v=30. Why is this? Is it by design, or is it just a very low probability event?

3

For random signatures, it is an extremely low probability event. Around 1 in 2128, so it will likely never ever actually happen.

However, you can easily construct a valid signature that has one of those, and the recovery algorithm will give you an actual public key for it. It just won't be a public key anyone actually knows the secret key for.

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