import java.security.PrivateKey;
import java.security.Signature;
import java.nio.charset.StandardCharsets;
import java.util.Base64;
import java.security.KeyFactory;
import java.security.spec.PKCS8EncodedKeySpec;
import java.util.UUID;

public class DYSign {
    public static void main(String []args) {
                // 请求时间戳
                long timestamp = System.currentTimeMillis()/1000L;
                // 开发者填入自己的小程序app_id
                String appId = "testAppId";
                // 随机字符串
                String nonceStr = UUID.randomUUID().toString(); 
                // 应用公钥版本,每次重新上传公钥后需要更新,可通过「开发管理-开发设置-密钥设置」处获取
                String keyVersion = "1";                
                // 应用私钥,用于加签 重要：1.测试时请修改为开发者自行生成的私钥;2.请勿将示例密钥用于生产环境;3.建议开发者不要将私钥文本写在代码中
                String privateKeyStr = "MIIEvgIBADANBgkqhkiG9w0BAQEFAASCBKgwggSkAgEAAoIBAQCZSHNcFfthd/bV\nYexEJWOBVEjjDcXjfr1fYevuraNFfMmLPKV836BbvCiUSWHzJYEpkJ934e/j28NB\nEcEbPDLiGlLTd6AVwR22TkUwpLr41oQprz0HKFwhVPZ0HQCGIv0pVMA53TFSitIq\niqbNLmgm5yzSNqNy1t/0X/RfqEtA6Eoxw9u/Sx57i+pBFuLlZYanlm57+b7t1khg\n9JHvF0ulo7DScyJ4qgrD7oQf0RIQB0rqCFIeYuYO1cfvnxb9x4DPodEyVoAM4i9Y\ndFop9ZHt73W/icuLku/P8/G1+arzB5b7S1S3ky5/KdS8AEA9Ww5czZcdf9Jgm2S6\nRymjFGjzAgMBAAECggEBAIryGNgdePyWcSJmHHR9a+CdFWD0aDBa/7CJpAN8VKc1\ngcB8Xgp+7+6X9jTM/EQa+CVEWrmiDgF/gVPnkyNsAzff4rqcEnoFzzglZSS9/lp4\nod7jYa+uTy1LxgflDkeJSfEASStqrT4EZpR3kNInQfQZ1BBNxQXhb6smm+9mL6kK\nQJjAqBgEqtUAmNv0GnH89ZPPgZuIZeeL4cb4BhMEoa5MBnI+HDf07cN1nECQXRJl\nHU/iyhAPfP7RpO8O9KGDEDE36qebu0Cu4yUjWANXiqECFv93sQzONotkl3VPealv\nXM+jzGT7YdgHo/t3QKE8flMBo/XUzGTqi8j5AOXiaBkCgYEAylKVtjQMYgg4qMwd\n9Je+KZ9qL6QVHCsB2NPUt8N99oj70efsG4aGaEAadr8meNhIJ5lpoK+FXqSBIbbD\nS/xeOVI3XoMx/EdKLw/ZNi87G/EHYK9z7Fr3W7q8DFXe2hZ/ojFXC/aaBanjVVBK\n/6RfPzXfnx+vGX/t1FhcLC+yQD8CgYEAwfMtrXfH+3dW77dxXT/CTFJVs/o1K2qb\nepnQ6A33KMHPLBtPZZ6z5rzIO7OMSNItOTXTEoRXHmOKc5FtXGtbCvGSRByb6FgD\nWG3p2Bp2sZLuJBzXmLbSnEbHTNHM6uTgxNgWAh8pYpjPY8xF7BqYz2rGT47OPBmc\ntRzDjnzjak0CgYAqkM1mk/S2+zvQZ4E14GblouBYPZEjZ/jvgUGTl9F8eL1iIAUQ\nlXDZpgLrULPrYLVtf101rTfF/Z4dVbIo3mOEc8OqYre1d9onpJHyUGWDL2Z59O/S\nniDEb7j4b2h/QZSArxi9L5if8GofnNDqj85qIg92Dthr6PpEXoKl2TMLSQKBgFzQ\nBHHYukiqSV4ZyRQ4qMBhPkYMXFlUgObgqMoDtN06MewHfa1BjxHCEYgQWfeXLLEO\nAt3/mrkeJWk8lLr/XOgVxkr17d34EFHG93rE3zwG9hMuAjZAdvT2IfWvCIL32GAa\nkB2fz+ww+D3nySY9bBcGH7R+wE6eaxF4nFSZizKZAoGBAJzuaWCnVK0djfgvUsjm\nGUtyDvgyREcpAsXvES1pB2NLVeEUxm0uRtj6k4DhCv3rJfUwfMr0+sa9NUnXuaSR\nVqLYvAD8bNPKXwn7ymzQ7WioCqmZuUhLnQRppkjhfQGKLH0MnMw9Xh9FwJ9kzGNE\nUnTEhaaHsoaHMlLlRET32gyG".replace("\n","");
                // 生成好的data
                String data = "{\"skuList\":[{\"skuId\":\"1\",\"price\":9999,\"quantity\":1,\"title\":\"标题\",\"imageList\":[\"https://dummyimage.com/234x60\"],\"type\":401,\"tagGroupId\":\"idxxx\"}],\"outOrderNo\":\"1213\",\"totalAmount\":9999,\"limitPayWayList\":[],\"payExpireSeconds\":3000,\"orderEntrySchema\":{\"path\":\"page/index/index\",\"params\":\"{\\\"poi\\\":\\\"6601248937917548558\\\",\\\"aweme_useTemplate\\\":1}\"}}";        
                        
                String byteAuthorization = getByteAuthorization(privateKeyStr, data, appId, nonceStr, timestamp, keyVersion);
                System.out.println(byteAuthorization);
    }
        
        public static String getByteAuthorization(String privateKeyStr, String data, String appId, String nonceStr, long timestamp, String keyVersion) {
                String byteAuthorization = "";
                try {
                        // 生成签名
                        String signature = getSignature(privateKeyStr, "POST", "/createSignOrder", timestamp, nonceStr, data);                        
                        // 构造byteAuthorization
                        StringBuilder sb = new StringBuilder();
                        sb.append("SHA256-RSA2048 ").
                                append("appid=").append(appId).append(",").
                                append("nonce_str=").append(nonceStr).append(",").
                                append("timestamp=").append(timestamp).append(",").
                                append("key_version=").append(keyVersion).append(",").
                                append("signature=").append(signature);
                        byteAuthorization = sb.toString();        
                } catch (Exception ex) {
                        ex.printStackTrace();
                        return "";
            }                
                return byteAuthorization;
         }
 
         public static String getSignature(String privateKeyStr, String method, String uri, long timestamp, String nonce, String data) throws Exception {
            String rawStr = method + "\n" +
                    uri + "\n" +
                    timestamp + "\n" +
                    nonce + "\n" +
                    data + "\n";
            Signature sign = Signature.getInstance("SHA256withRSA");
            sign.initSign(string2PrivateKey(privateKeyStr));
            sign.update(rawStr.getBytes(StandardCharsets.UTF_8));
            return Base64.getEncoder().encodeToString(sign.sign());
        }
    
        public static PrivateKey string2PrivateKey(String privateKeyStr) {
            PrivateKey prvKey = null;
            try {
                byte[] privateBytes = Base64.getDecoder().decode(privateKeyStr);
                PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(privateBytes);
                KeyFactory keyFactory = KeyFactory.getInstance("RSA");
                prvKey = keyFactory.generatePrivate(keySpec);
            } catch (Exception ex) {
                ex.printStackTrace();
            }
            return prvKey;
        }
}

openssl
genrsa -out private_key.pem 2048
rsa -in private_key.pem -pubout -out public_key.pem
pkcs8 -topk8 -inform PEM -in private_key.pem -outform PEM -nocrypt -out rsa_private_key.pem
rsa -in rsa_private_key.pem -pubout -out rsa_public_key.pem
exit
 
// 注：生成出来的私钥文件为rsa_private_key.pem, 公钥文件为rsa_public_key.pem 
// PKCS8格式,私钥以-----BEGIN PRIVATE KEY-----开头，-----END PRIVATE KEY-----结尾