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In the SEC#1 elliptic curve cryptography standard, the encoding of the public key involve a leading octet:

  • 00h: The public key is the point at infinity.
  • 02h, 03h: The public key is the compressed point.
  • 04h: The public key contain both x and y coordinates.

What is (or was) the value 01h for? Had there been other values defined for ECC?

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I don't know for sure, I can only bring additional intel, and a theory.

This convention already existed in ANS X9.62-1998, section 4.3.6 Point-to-Octet-String Conversion, which also assigns $06$ and $07$ to indicate another 'hybrid' encoding method. Just like in compressed form, the parity of the format indicator encodes a quantity. Quoting:

The octet string representation of the point at infinity $\mathcal O$ shall be a single zero octet $PC = 00$. (…)
If the compressed form is used (…) Assign the value $02$ to the single octet $PC$ if $\tilde y_p$ is 0, or the value $03$ if $\tilde y_p$ is 1. (…)
If the uncompressed form is used (…) Assign the value $04$ to the single octet $PC$. (…)
If the hybrid form is used (…) Assign the value $06$ to the single octet¹ if $\tilde y_p$ is 0, or the value $07$ if $\tilde y_p$ is 1. (…)

So my theory is that bit 0 of the identifier $PC$ is reserved for an additional bit in formats where that makes sense, with the format itself encoded in the other bits of PC. Thus $01$ and $05$ are unassigned, because there is no additional bit to be coded for the point at infinity or when using uncompressed form.

Notice that the point at infinity is not a valid public key in ECDSA, and would be a lousy one.


¹ sic: the mention $PC$ is missing.

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  • $\begingroup$ Coincidentally, today's XKCD is about the razor. $\endgroup$
    – fgrieu
    Nov 13 at 13:18
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    $\begingroup$ Although by 2002 rfc3279 (for point(s) in a PKIX certificate) allowed only compressed and uncompressed, not hybrid. (It did retain the X9.62 explicit parameters for a custom (Weierstrass) curve group, which rfc5480 in 2009 removed.) $\endgroup$ Nov 13 at 22:44

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