Base64 encoding in SQL Server 2005 T-SQL

Here is the code for the functions that will do the work

-- To Base64 string
CREATE FUNCTION [dbo].[fn_str_TO_BASE64]
(
    @STRING NVARCHAR(MAX)
)
RETURNS NVARCHAR(MAX)
AS
BEGIN
    RETURN (
        SELECT
            CAST(N'' AS XML).value(
                  'xs:base64Binary(xs:hexBinary(sql:column("bin")))'
                , 'NVARCHAR(MAX)'
            )   Base64Encoding
        FROM (
            SELECT CAST(@STRING AS VARBINARY(MAX)) AS bin
        ) AS bin_sql_server_temp
    )
END
GO

-- From Base64 string
CREATE FUNCTION [dbo].[fn_str_FROM_BASE64]
(
    @BASE64_STRING NVARCHAR(MAX)
)
RETURNS NVARCHAR(MAX)
AS
BEGIN
    RETURN (
        SELECT 
            CAST(
                CAST(N'' AS XML).value('xs:base64Binary(sql:variable("@BASE64_STRING"))', 'VARBINARY(MAX)') 
            AS NVARCHAR(MAX)
            )   UTF8Encoding
    )
END

Example of usage:

DECLARE @CHAR NVARCHAR(256) = N'e.g., سلام جیران or В России'
SELECT [dbo].[fn_str_FROM_BASE64]([dbo].[fn_str_TO_BASE64](@CHAR)) as converted

Here's a modification to mercurial's answer that uses the subquery on the decode as well, allowing the use of variables in both instances.

DECLARE
    @EncodeIn VARCHAR(100) = 'Test String In',
    @EncodeOut VARCHAR(500),
    @DecodeOut VARCHAR(200)    

SELECT @EncodeOut = 
    CAST(N'' AS XML).value(
          'xs:base64Binary(xs:hexBinary(sql:column("bin")))'
        , 'VARCHAR(MAX)'
    )
FROM (
    SELECT CAST(@EncodeIn AS VARBINARY(MAX)) AS bin
) AS bin_sql_server_temp;

PRINT @EncodeOut

SELECT @DecodeOut = 
CAST(
    CAST(N'' AS XML).value(
        'xs:base64Binary(sql:column("bin"))'
      , 'VARBINARY(MAX)'
    ) 
    AS VARCHAR(MAX)
) 
FROM (
    SELECT CAST(@EncodeOut AS VARCHAR(MAX)) AS bin
) AS bin_sql_server_temp;

PRINT @DecodeOut

The simplest and shortest way I could find for SQL Server 2012 and above is BINARY BASE64 :

SELECT CAST('string' as varbinary(max)) FOR XML PATH(''), BINARY BASE64

For Base64 to string

SELECT CAST( CAST( 'c3RyaW5n' as XML ).value('.','varbinary(max)') AS varchar(max) )

( or nvarchar(max) for Unicode strings )

I know this has already been answered, but I just spent more time than I care to admit coming up with single-line SQL statements to accomplish this, so I'll share them here in case anyone else needs to do the same:

-- Encode the string "TestData" in Base64 to get "VGVzdERhdGE="
SELECT
    CAST(N'' AS XML).value(
          'xs:base64Binary(xs:hexBinary(sql:column("bin")))'
        , 'VARCHAR(MAX)'
    )   Base64Encoding
FROM (
    SELECT CAST('TestData' AS VARBINARY(MAX)) AS bin
) AS bin_sql_server_temp;

-- Decode the Base64-encoded string "VGVzdERhdGE=" to get back "TestData"
SELECT 
    CAST(
        CAST(N'' AS XML).value(
            'xs:base64Binary("VGVzdERhdGE=")'
          , 'VARBINARY(MAX)'
        ) 
        AS VARCHAR(MAX)
    )   ASCIIEncoding
;

I had to use a subquery-generated table in the first (encoding) query because I couldn't find any way to convert the original value ("TestData") to its hex string representation ("5465737444617461") to include as the argument to xs:hexBinary() in the XQuery statement.

I hope this helps someone!