go/src/crypto/rand/util.go

// Prime returns a number of the given bit length that is prime with high probability.
// Prime will return error for any error returned by rand.Read or if bits < 2.
func Prime(rand io.Reader, bits int) (*big.Int, error) { … }

// Int returns a uniform random value in [0, max). It panics if max <= 0, and
// returns an error if rand.Read returns one.
func Int(rand io.Reader, max *big.Int) (n *big.Int, err error) { … }