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/* Added to ref10 for Tor. We place this in the public domain. Alternatively,
* you may have it under the Creative Commons 0 "CC0" license. */
#include "fe.h"
#include "ed25519_ref10.h"
int ed25519_ref10_pubkey_from_curve25519_pubkey(unsigned char *out,
const unsigned char *inp,
int signbit)
{
fe u;
fe one;
fe y;
fe uplus1;
fe uminus1;
fe inv_uplus1;
/* From prop228:
Given a curve25519 x-coordinate (u), we can get the y coordinate
of the ed25519 key using
y = (u-1)/(u+1)
*/
fe_frombytes(u, inp);
fe_1(one);
fe_sub(uminus1, u, one);
fe_add(uplus1, u, one);
fe_invert(inv_uplus1, uplus1);
fe_mul(y, uminus1, inv_uplus1);
fe_tobytes(out, y);
/* propagate sign. */
out[31] |= (!!signbit) << 7;
return 0;
}