FBB::ISymCryptStreambuf(3) Input Filtering stream buffer doing symmetric encryption


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


The information made available by ISymCryptStreambuf objects has been subject to symmetric encryption or decryption. The information to be encrypted or decrypted is made available to ISymCryptStreambuf object via std::istream objects.

The class ISymCryptStreambuf is a class template, using a FBB::CryptType template non-type parameter. Objects of the class FBB::ISymCryptStreambuf<FBB::ENCRYPT> encrypt the information they receive, objects of the class FBB::ISymCryptStreambuf<FBB::DECRYPT> decrypt the information they receive. See also section ENUMERATION below.

All symmetric encryption methods defined by the OpenSSL library that can be selected by name may be used in combination with EncryptBuf objects. To select a particular encryption method an identifier is passed to the constructor. E.g., "aes-128-cbc" indicating the AES (Rijndael) method, using 128 bit sized keys and blocks using `cbc' mode (see below for an explanation).

When providing shorter keys than expected by the method the provided key is extended by adding the required number of 0-bytes. (zero valued bytes, not '0' characters).

Most modes use an initialization vector. The initialization vector must be provided at construction time. The matching decrypting object needs to know the initialization vector that was used when encrypting the data: the application must ensure that the matching decryption object receives the same initialization vector as the one that was provided to the encryption object. Initialization vectors are not security sensitive in the sense that they can be sent in the clear to the decryption object. What is important, though, is that they contain random data when used `for real'. When an initialization vector is specified that is shorter than expected by the method it will be extended with the required number of 0-bytes.

Block ciphers use one of the following four encryption modes:

  • 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.
  • 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.
  • 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.
  • 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:

max key length is 56 bytes, 16 generally used
min key length is 5 bytes, max is shown
Key bytes 9-16 define the 2nd key, bytes 17-24
define the 3rd key
Key length variable, max. 128 bytes, default length is shown
obsolete: avoid
obsolete: avoid
Key length is variable, max. 256 bytes. default length is shown
Encrypt again to decrypt. Don't use DecryptBuf
obsolete: avoid
Key length variable, max. 256 bytes, rounds 8, 12 or 16,
default # rounds is 12

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.


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




All members of FBB::IFilterStreambuf are available, as ISymCryptStreambuf inherits from this class.

Overloaded move and/or copy assignment operators are not available.


ISymCryptStreambuf objects either encrypt or decrypt information. ISymCryptStreambuf objects of the class FBB::ISymCryptStreambuf<FBB::ENCRYPT> encrypt the data they receive, ISymCryptStreambuf objects of the class FBB::ISymCryptStreambuf<FBB::DECRYPT> decrypt the data they receive.

The values ENCRYPT and DECRYPT are defined in the enum CryptType, which is defined in the FBB namespace.


  • ISymCryptStreambuf<CryptType>( std::istream &in, char const *type, std::string const &key, std::string const &iv, size_t bufSize = 100, size_t filterBufSize = 1000, ENGINE *engine = 0):
    This constructor initializes the streambuf. - ISymCryptStreambuf<ENCRYPT> objects perform encryption;
    ISymCryptStreambuf<DECRYPT> objects perform decryption;
    - ISymCryptStreambuf<CryptType> objects obtain the bytes to encrypt or decrypt from std::istream &in;
    - The encryption method to use is specified by the type parameter. E.g., "bf-cbc" selects the Blowfish Cipher Block Chaining method;
    - The symmetric key to use is specified by the key parameter;
    - The initialization vector is specified by the iv parameter;
    - The FBB::ISymCryptStreambuf internally used buffer will contain bufSize characters. The default value is the smallest value that is used. When a smaller bufSize value is specified, the default value is used;
    - FBB::ISymCryptStreambuf's IFilterStreambuf base class is initialized with a buffer of size filterBufSize, using a lower bound of 100;
    - The parameter ENGINE can be used to specify a hardware acceleration engine, as supported by the used encryption/decryption method. Its default argument value indicates that no hardware acceleration is available. Copy- and move constructors are not available.


The example shows the construction of an ISymCryptStreambuf<ENCRYPT> object ebuf which is used to initialize a std::istream object. The information read from this istream is encrypted using the Blowfish CBC method. A ISymCryptStreambuf<DECRYPT> object (dbuf reads the information from that stream and decrypts it again). The std::istream din object is initialized with the ISymCryptStreambuf<DECRYPT> object, and its contents is sent to std::cout. The information that is presented at std::cin and that appears at std::cout should be identical.

#include <iostream>
#include <bobcat/isymcryptstreambuf>
using namespace std;
using namespace FBB;
int main()
    ISymCryptStreambuf<ENCRYPT> ebuf(cin, "bf-cbc",
                                    "1234567890", "1234567890");
    istream ein(&ebuf);
    ISymCryptStreambuf<DECRYPT> dbuf(ein, "bf-cbc",
                                    "1234567890", "1234567890");
    istream din(&dbuf);
    cout << din.rdbuf();


bobcat/isymcryptstreambuf - defines the class interface


Sep/Oct 2013: due to a change in library handling by the linker (cf. http://fedoraproject.org/wiki/UnderstandingDSOLinkChange and https://wiki.debian.org/ToolChain/DSOLinking) libraries that are indirectly required are no longer automatically linked to your program. With BigInt this is libcrypto, which requires programs to link to both bobcat and crypto.


  • bobcat_4.02.00-x.dsc: detached signature;
  • bobcat_4.02.00-x.tar.gz: source archive;
  • bobcat_4.02.00-x_i386.changes: change log;
  • libbobcat1_4.02.00-x_*.deb: debian package holding the libraries;
  • libbobcat1-dev_4.02.00-x_*.deb: debian package holding the libraries, headers and manual pages;
  • http://sourceforge.net/projects/bobcat: public archive location;


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


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


Frank B. Brokken ([email protected]).