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I am using BouncyCastle to generate .NET core packet signatures. I want to be able to get the v value for the Ethereum signature.

To these ends, and with my understanding of the meaning of the v value, my aim is to take the source for ECDSASigner, modify it to a custom local version, and return the correct v value on the basis of the discovered X coordinate in that code.

What I don't know though is which of the values 27..30 (27..28 in Ethereum yellow paper), correspond to which of the possible outputs. Is eg: 27==positive Y for given X?

Also, btw where is this stuff documented? YellowPaper.io refers to "The Literature" but where?

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I think I found it. Basically this code from the bitcoin core libraries define the v value

*recid = (overflow ? 2 : 0) | (secp256k1_fe_is_odd(&r.y) ? 1 : 0);

That gets added to 27 for Ethereum, while 29 and 30 are invalid according to YellowPaper.io

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  • Please note that if an implementation supports EIP-155, instead of using 27 or 28, it uses CHAIN_ID* 2 + 35. So the value of "v" can also be 37 or 38
    – Linmao Song
    Commented Jan 15, 2018 at 12:05
  • Thanks. Also, to add, it seems the v value definition comes from the sec specification for compressing points to octet strings secg.org/sec1-v2.pdf (section 2.3.3)
    – Sentinel
    Commented Jan 15, 2018 at 13:30

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