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lydie
Guest
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 Posted: Fri Nov 19, 2004 3:58 pm Post subject: MessageDigest different |
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Hi,
I have an application that get PDF documents from the Web and store
them in a database. I don't want to add the same document many times
so I decided to calculate the digest of these documents.
Here is want I do:
I am getting a document from the WEB as an inputStream using the
document's URL
then I calculate the digest
URL url = new URL("http://www.aoac.org/testkits/030406certrpt.pdf");
DataInputStream bis = new DataInputStream(url.openStream());
bis.mark(Integer.MAX_VALUE);
MessageDigest msg = MessageDigest.getInstance("SHA-1");
byte[] buf = new byte[4096];
long len;
while ( (len = bis.read(buf)) != -1){
msg.update(buf);
}
res1 = msg.digest();
bis.reset();
for(int i=0; i
{
int x = res1[i] &0xFF;
if(x < 0x10) System.out.print("0");
System.out.print(Integer.toHexString(x));
}
If I run this code many times I don't get the same digest.
If I try the same code with a file saved on the computer: it works
perfectly fine.
Here is also the code in case you need it...
File pdf = new File("C:/temp2.pdf");
FileInputStream str = new FileInputStream(pdf);
DataInputStream bis = new DataInputStream(str);
MessageDigest msg = MessageDigest.getInstance("SHA-1");
byte[] buf = new byte[4096];
long len;
while ( (len = bis.read(buf)) != -1){
msg.update(buf);
}
res1 = msg.digest();
for(int i=0; i
{
int x = res1[i] &0xFF;
if(x < 0x10) System.out.print("0");
System.out.print(Integer.toHexString(x));
}
System.out.println();
THanks a lot for your help
Lydie
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Michael Amling
Guest
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| Posted: Sat Nov 20, 2004 2:26 am Post subject: Re: MessageDigest different |
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lydie wrote:
| Quote: | Hi,
I have an application that get PDF documents from the Web and store
them in a database. I don't want to add the same document many times
so I decided to calculate the digest of these documents.
Here is want I do:
I am getting a document from the WEB as an inputStream using the
document's URL
then I calculate the digest
URL url = new URL("http://www.aoac.org/testkits/030406certrpt.pdf");
DataInputStream bis = new DataInputStream(url.openStream());
bis.mark(Integer.MAX_VALUE);
MessageDigest msg = MessageDigest.getInstance("SHA-1");
byte[] buf = new byte[4096];
long len;
while ( (len = bis.read(buf)) != -1){
msg.update(buf);
|
This is only correct if buf is always filled to capacity. What you
want is msg.update(buf, 0, len). When fetching data from the web, you're
apparently getting some partial buffers as your implementation waits for
packets from www.aoac.org.
| Quote: | }
res1 = msg.digest();
bis.reset();
for(int i=0; i
{
int x = res1[i] &0xFF;
if(x < 0x10) System.out.print("0");
System.out.print(Integer.toHexString(x));
}
If I run this code many times I don't get the same digest.
If I try the same code with a file saved on the computer: it works
perfectly fine.
Here is also the code in case you need it...
File pdf = new File("C:/temp2.pdf");
FileInputStream str = new FileInputStream(pdf);
DataInputStream bis = new DataInputStream(str);
|
I recommend getting rid of the DataInputStream, too. The
documentation says "An application uses a data output stream to write
data that can later be read by a data input stream." You're reading data
that was not written by a DataOutputStream.
| Quote: |
MessageDigest msg = MessageDigest.getInstance("SHA-1");
byte[] buf = new byte[4096];
long len;
while ( (len = bis.read(buf)) != -1){
msg.update(buf);
|
This is still wrong (unless you can guarantee the file's length is a
multiple of 4096 bytes), but consistent because there's no delay in
getting data from your local hard drive, so only the last bufferful is
partially filled from the file, and the extra data you're hashing is
leftover from the immediately previous read.
| Quote: | }
res1 = msg.digest();
for(int i=0; i
{
int x = res1[i] &0xFF;
if(x < 0x10) System.out.print("0");
System.out.print(Integer.toHexString(x));
}
System.out.println();
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--Mike Amling
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