sql server base64解密函数

USE [LVP]
GO
/****** Object:  UserDefinedFunction [dbo].[base64_decode]    Script Date: 2016/5/26 10:47:27 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
Create FUNCTION [dbo].[base64_decode]
(
@encoded_text varchar(max)
)
RETURNS varbinary(max)
AS
BEGIN
DECLARE
@output varbinary(max),
@block_start int,
@encoded_length int,
@decoded_length int,
@mapr binary(122)
IF LEN(@encoded_text) & 3 > 0
OR @encoded_text LIKE '%[^ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=]%' COLLATE Latin1_General_Bin
RETURN NULL
SET @output = 0x
-- The nth byte of @mapr contains the base64 value of the character with an ASCII value of n.
-- eg. 65th byte = 0x00 = 0 = value of 'A'
SET @mapr =
0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF -- 1-33
+ 0xFFFFFFFFFFFFFFFFFFFF3EFFFFFF3F3435363738393A3B3C3DFFFFFF00FFFFFF -- 33-64
+ 0x000102030405060708090A0B0C0D0E0F10111213141516171819FFFFFFFFFFFF -- 65-96
+ 0x1A1B1C1D1E1F202122232425262728292A2B2C2D2E2F30313233 -- 97-122
--get the number of blocks to be decoded
SET @encoded_length = LEN(@encoded_text)
SET @decoded_length = @encoded_length / 4 * 3
--for each block
SET @block_start = 1
WHILE @block_start < @encoded_length
BEGIN
--decode the block and add to output
--BINARY values between 1 and 4 bytes can be implicitly cast to INT
SET @output = @output +
CAST(CAST(
SUBSTRING(@mapr, ASCII(SUBSTRING(@encoded_text, @block_start    , 1)), 1) * 262144
+ SUBSTRING(@mapr, ASCII(SUBSTRING(@encoded_text, @block_start + 1, 1)), 1) * 4096
+ SUBSTRING(@mapr, ASCII(SUBSTRING(@encoded_text, @block_start + 2, 1)), 1) * 64
+ SUBSTRING(@mapr, ASCII(SUBSTRING(@encoded_text, @block_start + 3, 1)), 1)
AS int) AS binary(3))
SET @block_start = @block_start + 4
END
IF RIGHT(@encoded_text, 2) = '=='
SET @decoded_length = @decoded_length - 2
ELSE IF RIGHT(@encoded_text, 1) = '='
SET @decoded_length = @decoded_length - 1
RETURN SUBSTRING(@output, 1, @decoded_length)
END