Fix curv25519 coordinates #162
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Dec 29, 2021
qalisander
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Jul 1, 2023
* Add test vectors for curve25519 coordinates * Make (q-1)/4 actually (q-1)/4 and not (q-3)/4 * Fix curve25519 coordinates by clearing the parity bit (cherry picked from commit bc626bb)
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The current code had the following bugs:
ycoordinate it took the bytes serialization of the point and interpreted it as theycoordinate, ignoring the fact that the serialization encodes the parity bit of thexcoordinate in the most significant bit (which is the reason djb made the field elements are 255 bits)xcoordinate there was a bug inxrecoverusing(q-3)/4instead of(q-1)/4.xcoordinate it did not flip it to the right parity using the parity bit.I generated test vectors using https://github.com/dalek-cryptography/curve25519-dalek and fixed the issues making the test pass.
references:
https://ed25519.cr.yp.to/python/ed25519.py
https://datatracker.ietf.org/doc/html/rfc8032#section-5.1.3
https://github.com/dalek-cryptography/curve25519-dalek/blob/076cf34/src/edwards.rs#L193
https://github.com/dalek-cryptography/curve25519-dalek/blob/076cf34/src/edwards.rs#L518
https://github.com/dalek-cryptography/curve25519-dalek/blob/076cf34/src/field.rs#L229