# FBB::HMacBuf(3bobcat)

## Compute HMAC Message Digests
(libbobcat-dev_5.07.03)

## 2005-2021

## NAME

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

## SYNOPSIS

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

## DESCRIPTION

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

All information inserted into such a std::ostream is used to compute a message HMAC from.

All the message digest algorithms defined by the OpenSSL library that can be selected by name may be used in combination with HMacBuf objects.

The following message hmac algorithms are currently supported: mull, md2, md5, sha, sha1, sha224, sha256, sha384, sha512, dss, dss1, ecdsa, mdc2, ripemd160. These very names are the ones to use to select the particular digest algorithm for the class's constructor, below. It is quite possible that future releases of the openssl library will support additional message digest algorithms. The header file openssl/evp.h lists all available hmac algorithms (in that file look for EVP_MD *EVP_: a message digest algorithm immediately follows the 2nd underscore. E.g., const EVP_MD *EVP_md4(void) which refers to the md4 message digest algorithm).

## NAMESPACE

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

## INHERITS FROM

std::streambuf

## CONSTRUCTORS

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

  The constructor's first argument defines the key to be used when computing the HMAC message digest.

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

Copy and move constructors (and assignment operators) are not available.

## OVERLOADED OPERATOR

- std::ostream &operator<<(std::ostream &out, HMacBuf const &hmacbuf):
  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::HMacBuf inherits from this class.

- void eoi():
  This member completes the message digest computation. It is needed as the HMacBuf object has no external means for deciding whether all information to compute the digest for has yet been received or not. Alternatively, the eoi manipulator can be inserted into the stream computing the message digest (see below) The general approach for computing a message hmac is therefore:

          create a HMacBuf object
          use it to create a std::ostream object
          insert information into the ostream object
          call the HMacBuf object's eoi() member or insert eoi into the ostream
          object
          obtain/process the hash value from the HMacBuf object.

- std::string const &hash() const:
  This member returns the hash value computed by the HMacBuf 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 hmac computation. One a message hmac has been computed for, say a stream streamA this member can be called after which the hmac for a stream streamB can be computed using the same HMacBuf object.

- void eoi():
  This member can be called to complete the message digest computation. Instead of calling this member the eoi manipulator (see below) can be used.

## MANIPULATOR

- FBB::eoi:
  The eoi manipulator can be inserted into the ostream to complete the digest computation. If it is inserted into a plain std::ostream nothing happens.

## EXAMPLE

    #include <iostream>
    #include <ostream>
    #include <cstring>
    #include <iomanip>
    #include <bobcat/exception>
    #include "../hmacbuf"

    using namespace std;
    using namespace FBB;

    int main(int argc, char **argv)
    try
    {
        if (argc < 3)
            throw Exception{} << "Arg1: key, arg2: digest method required";

        string key(argv[1]);

        HMacBuf hmacbuf{ key, argv[2] };
        ostream out(&hmacbuf);

        string hw{ "hello world\n" };

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

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

## FILES

bobcat/hmacbuf - defines the class interface

## SEE ALSO

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

## BUGS

None reported

## BOBCAT PROJECT FILES

- https://fbb-git.gitlab.io/bobcat/: gitlab project page;
- bobcat_5.07.03-x.dsc: detached signature;
- bobcat_5.07.03-x.tar.gz: source archive;
- bobcat_5.07.03-x_i386.changes: change log;
- libbobcat1_5.07.03-x_*.deb: debian package containing the libraries;
- libbobcat1-dev_5.07.03-x_*.deb: debian package containing the libraries, headers and manual pages;

## 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).