IANA "COSE Key Type Parameters" includes the following:
| Key Type | Name | Label | CBOR Type |
|---|---|---|---|
| 2 (EC2) | crv | -1 | int/tstr |
| 2 (EC2) | x | -2 | bstr |
| 2 (EC2) | y | -3 | bstr/bool |
| 2 (EC2) | d | -4 | bstr |
The information implies that there are cases where the type of y-coordinate for ECDSA can be a boolean value (not an integer value). Does anyone know the reason? I'd like to know how to treat the boolean value when I actually encounter the case.
The relevant specification is RFC 9053: CBOR Object Signing and Encryption (COSE): Initial Algorithms, Section 2.1. ECDSA.
It seems that I have found the answer in "2.3.3 Elliptic-Curve-Point-to-Octet-String Conversion" of "SEC 1: Elliptic Curve Cryptography". The document says that a certain case "allows the y-coordinate to be represented compactly using a single bit".