// the given io.Reader. If random is nil, crypto/rand.Reader is used.
// If length <= 0, the access key length is chosen automatically.
//
// GenerateAccessKey returns an error if length is too small for a valid
// access key.
func GenerateAccessKey(length int, random io.Reader) (string, error) {
	if random == nil {
		random = rand.Reader
	}
	if length <= 0 {
		length = accessKeyMaxLen
	}