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MD5 How-to: How To Get the MD5 Hash of a Stream

Applies to: ~>1.0

There are two overloads of the TPJMD5.Calculate and TPJMD5.Process methods that can be used to create MD5 hashes of streams. One version takes a single TStream parameter and gets the hash of the data in the stream after the current position while the other version hashes a given number of bytes from the current stream position.

Code examples in this how-to all use TPJMD5.Calculate because it lends itself to more concise code, but TPJMD5.Process can be used for the same purpose. For details of the differences see Understand the Calculate and Process methods.

The most common requirement is to take the MD5 hash of a whole stream. This is done like this:

function GetMD5OfWholeStream(const Stm: TStream): TPJMD5Digest;
  Stm.Position := 0;
  Result := TPJMD5.Calculate(Stm);

The main point to note is that we ensure the stream position is at the start so that TPJMD5.Calculate processes the whole stream.

It is much less likely that you will need to process parts of a stream, but it can be done. There are three main cases:

  1. Process N bytes at the start of a stream
  2. Process N bytes at the end of a stream
  3. Process N bytes from a given offset in the stream

Assume you have a stream of length greater that 1Kb, then the following code fragments get the MD5 of the first, last and middle 1Kb of data:

  OneKb: Int64 = 1024;
  DFirst, DMiddle, DLast: TPJMD5Digest;
  Stm: TStream;
  // ...
  // create Stm and ensure it contains at least 1Kb of data
  // ...
  Assert(Stm.Size >= OneKb);

  // get MD5 of 1st 1Kb of data
  Stm.Seek(0, soFromBeginning);
  DFirst := TPJMD5.Calculate(Stm, OneKb);

  // get MD5 of last 1Kb of data
  Stm.Seek(OneKb, soFromEnd);
  DLast := TPJMD5.Calculate(Stm);  // reads to end

  // get MD5 of middle 1Kb of data
  Stm.Seek((Stm.Size - OneKb) div 2, soFromBeginning);
  DMiddle := TPJMD5.Calculate(Stm, OneKb);

  // ...
  // do something with digests
  // ...

See Also