pack25519 function
Implementation
void pack25519(final Uint8List o, final List<int> n) {
int i, j, b;
final List<int> m = gf(), t = gf(n);
car25519(t);
car25519(t);
car25519(t);
for (j = 0; j < 2; ++j) {
m[0] = t[0] - 0xffed;
for (i = 1; i < 15; ++i) {
m[i] = t[i] - 0xffff - ((m[i - 1] >> 16) & 1);
m[i - 1] &= 0xffff;
}
m[15] = t[15] - 0x7fff - ((m[14] >> 16) & 1);
b = (m[15] >> 16) & 1;
m[14] &= 0xffff;
sel25519(t, m, 1 - b);
}
for (i = 0; i < 16; ++i) {
o[2 * i] = t[i] & 0xff;
o[2 * i + 1] = t[i] >> 8;
}
}