The cryptographic function of an undisclosed SQL Server password _mssql

If the users of the MSSQL information is interested, may find master.dbo.sysxlogins inside the user's password, but, password field if not NULL is a bunch of binary can not understand, this password is how to encrypt it?
In fact, as long as a careful look at the master.dbo.sp_addlogin will know, MSSQL SP can see the code, is really good.
Let's take a look at how it is done, note that this line of select @passwd = Pwdencrypt (@passwd), this time after the @passwd is encrypted, let us also try
DECLARE @ClearPWD varchar (255)
DECLARE @EncryptedPWD varbinary (255)
SELECT @ClearPWD = ' Test '
SELECT @EncryptedPWD = CONVERT (varbinary (255), Pwdencrypt (@ClearPWD))
SELECT @EncryptedPWD
It looks good, it's really encrypted, but how can I restore it?
　　
Password encryption is one-way, with encrypted ciphertext to compare it.
Continue to look at other users related to the SP, you can find Master.dbo.sp_password inside the contents of the password comparison.
Pwdcompare (@old, password, (case when xstatus&2048 = 2048 THEN 1 ELSE 0))
Do not have to pay attention to xstatus, this is a state mask, generally we use when the direct use of 0 can be
DECLARE @ClearPWD varchar (255)
DECLARE @EncryptedPWD varbinary (255)
SELECT @ClearPWD = ' Test '
SELECT @EncryptedPWD = CONVERT (varbinary (255), Pwdencrypt (@ClearPWD))
SELECT Pwdcompare (@ClearPWD, @EncryptedPWD, 0)
SELECT pwdcompare (' Errorpassword ', @EncryptedPWD, 0)
So we can use these two functions to encrypt our passwords.