declare @iDoc int, @Name varchar(100), @EncodedText nvarchar(max), @DecodedArcText nvarchar(max)
declare @xml varchar(8000) = ' <encoded text from file> '
EXEC sp_xml_preparedocument @idoc OUTPUT, @xml

SELECT TOP 1 @Name=Name, @EncodedText=Value
FROM  OPENXML (@idoc, '/CryptoConvert/Body/Messages/Message',2)
            WITH (ID  int '@id',
		  KeyId varchar(10) '@keyId',
                  Name varchar(100) '@name',
                  Value nvarchar(max) 'text()'
                  )

SELECT @Name, @EncodedText

select cast(N'' as xml).value('xs:base64Binary(sql:variable("@EncodedText"))', 'varbinary(max)')  

select @DecodedArcText = cast(cast(N'' as xml).value('xs:base64Binary(sql:variable("@EncodedText"))', 'varbinary(max)')  as nvarchar(max)) --  as [decoded_value]; -
select @DecodedArcText, LEN(@EncodedText), LEN(@DecodedArcText)

EXEC sp_xml_removedocument @idoc

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declare @x xml = ...

select
 t.n.value('@name', 'varchar(200)'),
 t.n.value('.', 'varbinary(max)')
from
 @x.nodes('/CryptoConvert/Body/Messages/Message') t(n);