// create 6-digit
public int create6digit() {
    SecureRandom sr = SecureRandom.getInstance("SHA1PRNG");
    int verificationNumber = sr.nextInt(900000) + 100000;
}


@Configuration
public class RetrofitConfig {

    @Bean
    public CoolSMSService coolSMSService(ObjectMapper objectMapper) {
        Retrofit retrofit = new Retrofit.Builder().baseUrl("https://api.coolsms.co.kr/")
                .addConverterFactory(JacksonConverterFactory.create(objectMapper))
                .build();

        return retrofit.create(CoolSMSService.class);
    }
}

@Service
public interface CoolSMSService {

    @POST("sms/2/send")
    Call<CoolSMSResponse> sendSms(@Body CoolSMSRequest coolSMSRequest);
}

public class CoolSMSRequest {
    @JsonProperty("api_key")
    String apiKey;
    String timestamp;
    String salt;
    String signature;
    final String encoding = "base64";

    String to;
    final String from = "<phoneNumber>";
    String text;

    public static CoolSMSRequest of() {
        CoolSMSRequest coolSMSRequest = new CoolSMSRequest();

        try {
            coolSMSRequest.apiKey = "<key>";
            String apiSecret = "<secret>";
            coolSMSRequest.timestamp = String.valueOf(LocalDateTime.now().atZone(ZoneId.systemDefault()).toInstant().toEpochMilli());
            coolSMSRequest.salt = UUID.randomUUID().toString();
            String hmacData = coolSMSRequest.timestamp + coolSMSRequest.salt;

            SecretKeySpec key = new SecretKeySpec(apiSecret.getBytes(), "HmacMD5");
            Mac mac = Mac.getInstance("HmacMD5");
            mac.init(key);
            coolSMSRequest.signature = Base64.encodeBase64String(mac.doFinal(hmacData.getBytes()));
        } catch (Exception ex) {

        }
        return coolSMSRequest;
    }

    public String getApiKey() {
        return this.apiKey;
    }

    public String getTimestamp() {
        return this.timestamp;
    }

    public String getSalt() {
        return this.salt;
    }

    public String getSignature() {
        return this.signature;
    }

    public String getEncoding() {
        return this.encoding;
    }

    public String getTo() {
        return this.to;
    }

    public void setTo(String to) {
        this.to = to;
    }

    public String getFrom() {
        return this.from;
    }

    public String getText() {
        return this.text;
    }

    public void setText(String text) {
        this.text = text;
    }
}

@JsonNaming(PropertyNamingStrategy.SnakeCaseStrategy.class)
public class CoolSMSResponse {
    Boolean successful;
    Integer successCount;
    Integer errorCount;
    String groupId;
    String resultCode;
    String resultMessage;
    LinkedHashMap errorList;

    public Boolean getSuccessful() {
        return this.successful;
    }

    public Integer getSuccessCount() {
        return this.successCount;
    }

    public Integer getErrorCount() {
        return this.errorCount;
    }

    public String getGroupId() {
        return this.groupId;
    }

    public String getResultCode() {
        return this.resultCode;
    }

    public String getResultMessage() {
        return this.resultMessage;
    }

    public LinkedHashMap getErrorList() {
        return this.errorList;
    }
}