Component bdlde_crc32 [Package bdlde]

Provide a mechanism for computing the CRC-32 checksum of a dataset. More...

Namespaces
namespace	bdlde

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Detailed Description

Outline

• Purpose
• Classes
• Description
  • Usage

Purpose:

Provide a mechanism for computing the CRC-32 checksum of a dataset.

Classes:

bdlde::Crc32

stores and updates a CRC-32 checksum

See also:

Description:

This component implements a mechanism for computing, updating, and streaming a CRC-32 checksum (a cyclic redundancy check comprised of 32 bits). This checksum is a strong and fast technique for determining whether or not a message was received without errors. Note that a CRC-32 checksum does not aid in error correction and is not naively useful in any sort of cryptographic application. Compared to other methods such as MD5 and SHA-256, it is relatively easy to find alternate texts with identical checksum.

Usage:

The following snippets of code illustrate a typical use of the bdlde::Crc32 class. Each function would typically execute in separate processes or potentially on separate machines. The senderExample function below demonstrates how a message sender can write a message and its CRC-32 checksum to a bdex output stream. Note that Out may be a typedef of any class that implements the bslx::OutStream protocol:

  void senderExample(Out& output)
      // Write a message and its CRC-32 checksum to the specified 'output'
      // stream.
  {
      // prepare a message
      bsl::string message = "This is a test message.";

      // generate a checksum for 'message'
      bdlde::Crc32 crc(message.data(), message.length());

      // write the message to 'output'
      output << message;

      // write the checksum to 'output'
      const int VERSION = 1;
      crc.bdexStreamOut(output, VERSION);
  }

The receiverExample function below illustrates how a message receiver can read a message and its CRC-32 checksum from a bdex input stream, then perform a local CRC-32 computation to verify that the message was received intact. Note that In may be a typedef of any class that implements the bslx::InStream protocol:

  void receiverExample(In& input)
      // Read a message and its CRC-32 checksum from the specified 'input'
      // stream, and verify the integrity of the message.
  {
      // read the message from 'input'
      bsl::string message;
      input >> message;

      // read the checksum from 'input'
      bdlde::Crc32 crc;
      const int VERSION = 1;
      crc.bdexStreamIn(input, VERSION);

      // locally compute the checksum of the received 'message'
      bdlde::Crc32 crcLocal;
      crcLocal.update(message.data(), message.length());

      // verify that the received and locally-computed checksums match
      assert(crcLocal == crc);
  }