recover key from signature method about elliptic HOT 5 CLOSED

Nice! Do you feel like you might be interested in contributing this?

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@indutny if you don't mind me being slow and bugging you. :P

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from section 4.1.6 Public Key Recovery Operation

1. Let x = r + jn.
2. Convert the integer x to an octet string X of length mlen using the conversion routine
   specified in Section 2.3.7, where mlen = d(log2 p)/8e or mlen = dm/8e.
3. Convert the octet string 0216kX to an elliptic curve point R using the conversion routine
   specified in Section 2.3.4. If this conversion routine outputs “invalid”, then do another
   iteration of Step 1.
4. If nR 6= O, then do another iteration of Step 1.
5. Compute e from M using Steps 2 and 3 of ECDSA signature verification.
6. For k from 1 to 2 do the following.
   1. Compute a candidate public key as:
      Q = r−1(sR − eG).
   2. Verify that Q is the authentic public key. (For example, verify the signature of a
      certification authority in a certificate which has been truncated by the omission of
      Q from the certificate.) If Q is authenticated, stop and output Q.
   3. Change R to −R.

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WIP RP #37

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landed in 3.1.0!

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