# FBB::DigestBuf(3bobcat)

## Compute Message Digests
(libbobcat-dev_6.11.00)

## 2005-2025

## NAME

FBB::DigestBuf - Computes Message Digests from information inserted into a std::ostream

## SYNOPSIS

#include <bobcat/digestbuf>
Linking option: -lbobcat -lcrypto

## DESCRIPTION

FBB::DigestBuf objects are std::streambuf objects that can be used to initialize std::ostream objects with.

The message digest of all information inserted into such an std::ostream is computed.

All the message digests defined by the OpenSSL library that can be selected by name may be used in combination with DigestBuf objects.

The currently supported message digest algorithms are shown by the openssl list -digest-commands. The names of the digest algorithms can be used to select a digest algorithms for the class's constructor, below.

## NAMESPACE

FBB
All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.

## INHERITS FROM

std::streambuf

## CONSTRUCTORS

- DigestBuf(char const *digestName, size_t bufsize = 1024):
  This constructor initializes the streambuf, setting it up for the message digest algorithm specified with type. The message digest algorithms specified in the DESCRIPTION section may be used here. E.g., to use the sha256 algorithm specify "sha256".

  The bufsize argument specifies the size of the internal buffer used by DigestBuf to store incoming characters temporarily. The provided default argument should be OK in all normal cases.

The copy and move constructors and assignment operators are not available.

## OVERLOADED OPERATOR

- std::ostream &operator<<(std::ostream &out, DigestBuf const &digestbuf):
  The insertion operator is a free function defined in the namespace FBB. It inserts a hash value as a series of hexadecimally displayed values into the provided ostream. See the example below for an illustration.

## MEMBER FUNCTIONS

All members of std::streambuf are available, as FBB::DigestBuf inherits from this class.

- void eoi():
  This member finishes the computation of the message digest computation. It is needed as the DigestBuf object has no external means for deciding whether all information to compute the digest for has yet been received or not. The general approach to follow when computing a message digest is therefore:

          1. create a DigestBuf object
          2. use it to create a std::ostream object
          3. insert information into the ostream object
          4. call the DigestBuf object's eoi() member or insert the
              eoi manipulator into the ostream (see below)
          5. obtain/process the hash value from the DigestBuf object.

- std::string const &hash() const:
  This member returns the hash value computed by the DigestBuf object. Its value is only defined after having called close(). The hash value is returned in a std::string object. This string's length() member contains the number of characters used by the hash value, and its data() member refers to the hash value's characters. Note that a hash value's character value may be 0 (not to be confused with '0').

- void reset():
  This member reinitializes the message digest computation. One a message digest has been computed for, say a stream streamA this member can be called after which the digest for a stream streamB can be computed using the same DigestBuf object.

## MANIPULATOR

- FBB::eoi:
  The eoi manipulator can be inserted into the ostream to complete the digest. It performs the same actions as the eoi member. If inserted into a plain std::ostream nothing happens.

## EXAMPLE

    #include <iostream>
    #include <ostream>
    #include <bobcat/digestbuf>

    using namespace std;
    using namespace FBB;

    int main(int argc, char **argv)
    try
    {
        DigestBuf digestbuf(argc == 1 ? "sha256" : argv[1]);
        ostream out(&digestbuf);

        string hw("hello world\n");

        out << hw << eoi;
        cout << ">" << digestbuf << "<" << endl;

        digestbuf.reset();
        out.write(hw.c_str(), hw.length()) << eoi;
        cout << ">" << digestbuf << "<" << endl;
    }
    catch(exception const &err)
    {
        cout << err.what() << endl;
        return 1;
    }

## FILES

bobcat/digestbuf - defines the class interface

## SEE ALSO

bobcat(7), hmacbuf(3bobcat), std::streambuf

## BUGS

None reported

## BOBCAT PROJECT FILES

- https://fbb-git.gitlab.io/bobcat/: gitlab project page;

Debian Bobcat project files:

- libbobcat6: debian package containing the shared library, changelog and copyright note;
- libbobcat-dev: debian package containing the static library, headers, manual pages, and developer info;

## BOBCAT

Bobcat is an acronym of `Brokken's Own Base Classes And Templates'.

## COPYRIGHT

This is free software, distributed under the terms of the GNU General Public License (GPL).

## AUTHOR

Frank B. Brokken (f.b.brokken@rug.nl).