+ /*
+ * Decode the point ('u' coordinate). This should be reduced
+ * modulo p, but we prefer to avoid the dependency on
+ * br_i31_decode_reduce(). Instead, we use br_i31_decode_mod()
+ * with a synthetic modulus of value 2^255 (this must work
+ * since G was truncated to 255 bits), then use a conditional
+ * subtraction. We use br_i31_decode_mod() and not
+ * br_i31_decode(), because the ec_prime_i31 implementation uses
+ * the former but not the latter.
+ * br_i31_decode_reduce(a, G, 32, C255_P);
+ */
+ br_i31_zero(b, 0x108);
+ b[9] = 0x0100;
+ br_i31_decode_mod(a, G, 32, b);
+ a[0] = 0x107;
+ br_i31_sub(a, C255_P, NOT(br_i31_sub(a, C255_P, 0)));
+