Question

md5 加密为什么要update2次？

加密的时候为什么要update()两次呢？
byte[] salt = new byte[12];

random.nextBytes(salt);
MessageDigest md5 = MessageDigest.getInstance("MD5");
md.update(salt);
md.update(password.getBytes("UTF-8"));

byte[] digest = md.digest();

Answer 1

/* * MD5加密 */ private String getMD5Str(String str) { MessageDigest messageDigest = null; try { messageDigest = MessageDigest.getInstance("MD5"); messageDigest.reset(); messageDigest.update(str.getBytes("UTF-8")); } catch (NoSuchAlgorithmException e) { System.out.println("NoSuchAlgorithmException caught!"); System.exit(-1); } catch (UnsupportedEncodingException e) { e.printStackTrace(); } byte[] byteArray = messageDigest.digest(); StringBuffer md5StrBuff = new StringBuffer(); for (int i = 0; i < byteArray.length; i++) { if (Integer.toHexString(0xFF & byteArray[i]).length() == 1) md5StrBuff.append("0").append(Integer.toHexString(0xFF & byteArray[i])); else md5StrBuff.append(Integer.toHexString(0xFF & byteArray[i])); } //16位加密，从第9位到25位 return md5StrBuff.substring(8, 24).toString().toUpperCase(); }

Answer 2

md5.update 会将每次字符串拼接
上述最终算的是 salt+password.getBytes("UTF-8") 的md5