BIP-0340: Remove last remaining mention of Jacobi symbol

Jacobi symbol can be confusing because it may suggest that the modulus is
composite.

Thanks to Alan Szepieniec for pointing out this issue.

Author: jonasnick

bip-0340.mediawiki

@@ -85,7 +85,7 @@ In the case of ''R'' the third option is slower at signing time but a bit faster
 for elliptic curve operations). The two other options require a possibly
 expensive conversion to affine coordinates first. This would even be the case if the sign or oddness were explicitly coded (option 2 in the list above). We therefore choose option 3.
 
-For ''P'', using the third option comes at the cost of computing a Jacobi symbol (test for squaredness) at key generation or signing time, without avoiding a conversion to affine coordinates (as public keys will use affine coordinates anyway). We choose the second option, making public keys implicitly have an even Y coordinate, to maximize compatibility with existing key generation algorithms and infrastructure.
+For ''P'', using the third option comes at the cost of computing whether the Y coordinate is square at key generation or signing time, without avoiding a conversion to affine coordinates (as public keys will use affine coordinates anyway). We choose the second option, making public keys implicitly have an even Y coordinate, to maximize compatibility with existing key generation algorithms and infrastructure.
 
 Implicit Y coordinates are not a reduction in security when expressed as the number of elliptic curve operations an attacker is expected to perform to compute the secret key. An attacker can normalize any given public key to a point whose Y coordinate is even by negating the point if necessary. This is just a subtraction of field elements and not an elliptic curve operation<ref>This can be formalized by a simple reduction that reduces an attack on Schnorr signatures with implicit Y coordinates to an attack to Schnorr signatures with explicit Y coordinates. The reduction works by reencoding public keys and negating the result of the hash function, which is modeled as random oracle, whenever the challenge public key has an explicit Y coordinate that is odd. A proof sketch can be found [https://medium.com/blockstream/reducing-bitcoin-transaction-sizes-with-x-only-pubkeys-f86476af05d7 here].</ref>.