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dchest / tweetnacl-js

Port of TweetNaCl cryptographic library to JavaScript
https://tweetnacl.js.org
The Unlicense
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x25519 public key verification #193

Closed morrigan closed 4 years ago

morrigan commented 4 years ago

Hi,

is there way to verify public key in this case? Any suggestions how to do that?

dchest commented 4 years ago

From https://cr.yp.to/ecdh.html:

How do I validate Curve25519 public keys?

Don't. The Curve25519 function was carefully designed to allow all 32-byte strings as Diffie-Hellman public keys. Relevant lower-level facts: the number of points of this elliptic curve over the base field is 8 times the prime 2^252 + 27742317777372353535851937790883648493; the number of points of the twist is 4 times the prime 2^253 - 55484635554744707071703875581767296995. This is discussed in more detail in the curve25519 paper.

There are some unusual non-Diffie-Hellman elliptic-curve protocols that need to ensure ``contributory'' behavior. In those protocols, you should reject the 32-byte strings that, in little-endian form, represent 0, 1, 325606250916557431795983626356110631294008115727848805560023387167927233504 (which has order 8), 39382357235489614581723060781553021112529911719440698176882885853963445705823 (which also has order 8), 2^255 - 19 - 1, 2^255 - 19, 2^255 - 19 + 1, 2^255 - 19 + 325606250916557431795983626356110631294008115727848805560023387167927233504, 2^255 - 19 + 39382357235489614581723060781553021112529911719440698176882885853963445705823, 2(2^255 - 19) - 1, 2(2^255 - 19), and 2(2^255 - 19) + 1. But these exclusions are unnecessary for Diffie-Hellman.

morrigan commented 4 years ago

@dchest thanks! Realized that meanwhile :D