Since it is practically impossible to find two documents that would have the same hash value, a document's hash, often referred to as a "thumbprint", can be used to uniquely identify this document. If you are to develop a document repository, such as a database of image files, and want to prevent the same document to appear in your database twice, you can store the document's thumbprint together with the document itself. This way every time a new document is to be added. you can easily check whether it already exists in your database by computing the new file's hash value and looking for a match in your database.

Most database management systems are capable of storing arbitrary files in database tables as blobs. MS Access can stores blobs in fields of the type OLE Object. The corresponding data type in SQL Server is called IMAGE.

AspUpload can save uploaded files in the database in just one line of code. The following code snippet captures files uploaded with a browser and saves them in the field Blob of the table Images:

<% Set Upload = Server.CreateObject("Persits.Upload") Upload.Save "c:\upload" ' temp location For Each File in Upload.Files    File.ToDatabase "DSN=AspEncrypt;", _       "INSERT INTO IMAGES(BLOB) VALUES(?)" Next %>

The method File.ToDatabase accepts two parameters: an ODBC connection string and an SQL INSERT or UPDATE statement where the actual blob is denoted by a question mark (?).

  Adding Hash-Computing Code

The following ASP code (also found in the file Samples/File_ID/UploadAndID.asp of the installation) captures files uploaded with a browser, computes their hash values, checks whether the values are already in in the database and if not saves the files to the database. The code uses the Images table of the AspEncrypt.mdb sample database shipped with the product.

<% Set Upload = Server.CreateObject("Persits.Upload") Upload.Save "c:\upload" Set CM = Server.CreateObject("Persits.CryptoManager") Set Context = CM.OpenContext("", True) Set Hash = Context.CreateHash ' create SHA Hash object ' Save files in the database as blobs along with their thumbprints For Each File in Upload.Files    Hash.Reset ' clear hash    Hash.AddFile File.Path ' compute file hash    HashValue = Hash.Value.Hex    ' Check if thumbprint already exists in database    set rs = Server.CreateObject("adodb.recordset")    SQL = "select * from Images where Thumbprint = '" & HashValue & "'"    rs.Open SQL, "DSN=AspEncrypt;UID=;PWD=;"    If rs.EOF Then       ' Build SQL INSERT statement       SQL = "insert into Images( Blob, Thumbprint) values(?, '" & HashValue & "')"       ' Save file in the database       File.ToDatabase "DSN=AspEncrypt;UID=;PWD=;", SQL    Else       Response.Write "File " & File.Path & " is already in the database"    End If Next %>