NAME

FBB::DecryptBuf - Decrypts information using various methods into a std::ostream

SYNOPSIS

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

DESCRIPTION

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

All information inserted into such a std::ostream is decrypted and written into a std::ostream that is given as argument to DecryptBuf’s constructor.

All encryption methods defined by the OpenSSL library that can be selected by name may be used in combination with DecryptBuf objects. Most likely the information will have been encrypted using an EncryptBuf object, selecting a particular encryption method. The encryption method used when encrypting information should also be specified when constructing a DecryptBuf object. Likewise, the constructor expects a key and initialization vector. The key and initialization vector that was passed to the EncryptBuf object must be passed to DecryptBuf’s constructor as well.

Block ciphers use one of the following four encryption modes:

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CBC (Cipher Block Chaining):
The first block is XOR-ed by the initialization vector and then encrypted using the specified method. Subsequent blocks are XOR-ed by the encrypted version of the preceding block. Due to the initialization vector dictionary attacks are infeasible, as long as the initialization vector is truly random.

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ECB (Electronic Code Book):
Each block is encrypted by itself, using the specified encryption method. Although an initialization vector may be specified, it is not used. This method is susceptible to dictionary attacks and should therefore be avoided, unless you know what you’re doing.

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CFB (Cipher Feednack):
This method allows a block cipher to be used as a stream cipher. It uses an initialization vector, which should be unique and random for each new stream of data that is encrypted using the method. Encryption can only start after the first data block has been received.

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OFB (Output Feednack):
This is an alternative way to use a block cipher as a stream cipher. It is somewhat more susceptible to traditional data manipulation attacks, which can usually be thwarted when a message authentication code is added to the information as well. Like CFB it uses an initialization vector, which should again be unique and random for each new stream of data that is encrypted.

The following table presents an overview of methods that are currently available. Methods for which the block size is specified as N.A. are stream ciphers; other methods are block ciphers:

The RC4 stream cipher is subject to a well-known attack (cf. http://www.wisdom.weizmann.ac.il/~itsik/RC4/Papers/Mantin1.zip) unless the initial 256 bytes produced by the cipher are discarded.

NAMESPACE

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

INHERITS FROM

std::streambuf

CONSTRUCTOR/DESTRUCTOR

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DecryptBuf(std::ostream &outStream, char const *type, std::string const &key, std::string const &iv, size_t bufsize = 1024):
This constructor initializes the DecryptBuf object preparing it for the message decrypt algorithm specified with type. The decryption algorithms that can be used are listed in the table found in the EncryptBuf(3bobcat) manual page. As an example: to use the AES method on 192 bit keys and blocks in CBC mode specify "aes-192-cbc". The key parameter refers to the key to be used, the iv parameter refers to the initialization vector to use. The iv’s length cannot be zero. When using ECB modes no initialization vector is used. In those cases any non-empty initialization vector may be provided.

The constructor throws an FBB::Exception exception if an unknown encryption method was specified.

The constructor’s first parameter refers to the std::ostream to receive the decrypted information.

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

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~DecryptBuf():
The destructor calls the done() member (see below), prevending any exception that function might throw from leaving the destructor. In this case only a non thread-safe way to determine whether the decryption was successfully completed is available through the static member function lastOK() (see below). There is no copy constructor, nor move constructor (as std::streambuf doesn’t support either).

MEMBER FUNCTIONS

All members of std::streambuf are available, as FBB::DecryptBuf inherits from this class. Some of the std::streambuf’s member are overridden or are hidden by DecryptBuf. In normal situations these inherited members will not be used by programs using DecryptBuf objects.

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void done():
This member can be called to indicate that all information to be decrypted has been received. It throws an FBB::Exception exception if decryption fails (resulting from providing the DecryptBuf object with incorrect (usually improperly padded) input). If not explicitly called it is called by DecryptBuf’s destructor, preventing its exception from leaving the destructor.

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void setIv(std::string const &iv):
This member can be used to specify the initialization vector to use after construction time but before any data has been decrypted. When called after decryption has started or when specifying an empty initialization vector an FBB::Exception exception will be thrown. When using ECB modes no initialization vector is used. In those cases any non-empty initialization vector may be provided.

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bool setRounds(size_t nRounds):
This member can only be used with the RC5 decryption method to set the number of rounds of the algorithm to 8, 12 or 16. When the number of rounds were updated successfully the member returns true. It returns false in other cases (e.g., called for other decryption methods than RC5 or the requested number of rounds differ from 8, 12 or 16).

STATIC MEMBER

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bool lastOK():
This member is a non thread-safe way to determine whether the decryption has succeeded when the DecryptBuf object’s done member has not been called and the object has been destroyed. In that case the object’s destructor will call done to complete the decryption. The member lastOK returns true if the DecryptBuf object destroyed last could complete its decryption successfully and returns false otherwise.

PROTECTED MEMBER

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EVP_CIPHER_CTX *cipherCtx():
Classes derived from DecryptBuf may use this member to gain direct access to the EVP_CIPHER_CTX pointer used by the DecryptBuf object. This pointer is a pointer to an opaque structure used by many OpenSSL functions to set or query parameters of an decryption method.

EXAMPLE

#include <iostream>
#include <fstream>
#include <bobcat/exception>
#include <bobcat/decryptbuf>
using namespace std;
using namespace FBB;
int main(int argc, char **argv)
try
{


    if (argc == 1)


        throw Exception(1) << 


                    "1st arg: method, 2nd arg: key, 3rd arg: file to "


                    "decrypt (to stdout), 4th arg: iv";


    cerr << "Key: `" << argv[2] << "’\n"


            "IV:  `" << argv[4] << "’\n";


    DecryptBuf decryptbuf(cout, argv[1], argv[2], argv[4]);


    ostream out(&decryptbuf);


    ifstream in(argv[3]);


    out << in.rdbuf();
}
catch(exception const &err)
{


    cout << err.what() << endl;


    return 1;
}

FILES

bobcat/decryptbuf - defines the class interface

SEE ALSO

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

BUGS

None reported

DISTRIBUTION FILES

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bobcat_4.08.06-x.dsc: detached signature;

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bobcat_4.08.06-x.tar.gz: source archive;

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bobcat_4.08.06-x_i386.changes: change log;

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libbobcat1_4.08.06-x_*.deb: debian package holding the libraries;

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libbobcat1-dev_4.08.06-x_*.deb: debian package holding the libraries, headers and manual pages;

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http://sourceforge.net/projects/bobcat: public archive location;

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).