Test whether a and b are coprime (gcd == 1), possibly mutating a.
1 if gcd(|a|,|b|) == 1, 0 if not coprime, or a negative value on error.
| Name | Description |
|---|---|
| a | First value; may be overwritten as a scratch during the gcd. |
| b | Second value (not modified). |
| ctx | BN_CTX scratch space. |