Provided by: libbobcat-dev_3.19.01-1ubuntu1_amd64 
NAME
FBB::DecryptBuf - Decrypts information using various methods into a std::ostream
SYNOPSIS
#include <bobcat/decryptbuf>
Linking option: -lbobcat
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:
o 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.
o 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.
o 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.
o 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:
────────────────────────────────────────────────────────────────
method keysize blocksize mode identifier
(bytes) (bytes)
────────────────────────────────────────────────────────────────
AES 16 8 CBC "aes-128-cbc"
EBC "aes-128-ecb"
CFB "aes-128-cfb"
OFB "aes-128-ofb"
24 24 CBC "aes-192-cbc"
EBC "aes-192-ecb"
CFB "aes-192-cfb"
OFB "aes-192-ofb"
32 32 CBC "aes-256-cbc"
EBC "aes-256-ecb"
CFB "aes-256-cfb"
OFB "aes-256-ofb"
────────────────────────────────────────────────────────────────
BLOWFISH 16 8 CBC "bf-cbc"
EBC "bf-ecb"
CFB "bf-cfb"
OFB "bf-ofb"
max key length is 56 bytes, 16 generally used
────────────────────────────────────────────────────────────────
CAMELLIA 16 16 CBC "camellia-128-cbc"
EBC "camellia-128-ecb"
CFB "camellia-128-cfb"
OFB "camellia-128-ofb"
24 CBC "camellia-192-cbc"
EBC "camellia-192-ecb"
CFB "camellia-192-cfb"
OFB "camellia-192-ofb"
32 CBC "camellia-256-cbc"
EBC "camellia-256-ecb"
CFB "camellia-256-cfb"
OFB "camellia-256-ofb"
────────────────────────────────────────────────────────────────
CAST 16 8 CBC "cast-cbc"
EBC "cast-ecb"
CFB "cast-cfb"
OFB "cast-ofb"
min key length is 5 bytes, max is shown
────────────────────────────────────────────────────────────────
DES 8 8 CBC "des-cbc"
EBC "des-ebc"
CFB "des-cfb"
OFB "des-ofb"
────────────────────────────────────────────────────────────────
DESX 8 8 CBC "desx-cbc"
────────────────────────────────────────────────────────────────
3DES 16 8 CBC "des-ede-cbc"
EBC "des-ede"
CFB "des-ede-cfb"
OFB "des-ede-ofb"
────────────────────────────────────────────────────────────────
3DES 24 8 CBC "des-ede3-cbc"
EBC "des-ede3"
CFB "des-ede3-cfb"
OFB "des-ede3-ofb"
Key bytes 9-16 define the 2nd key, bytes 17-24
define the 3rd key
────────────────────────────────────────────────────────────────
RC2 16 8 CBC "rc2-cbc"
EBC "rc2-ecb"
CFB "rc2-cfb"
OFB "rc2-ofb"
Key length variable, max. 128 bytes, default length is shown
────────────────────────────────────────────────────────────────
RC2-40 5 8 "rc2-40-cbc"
obsolete: avoid
────────────────────────────────────────────────────────────────
RC2-64 8 8 "rc2-64-cbc"
obsolete: avoid
────────────────────────────────────────────────────────────────
RC4 16 N.A. "rc4"
Key length is variable, max. 256 bytes. default length is shown
Encrypt again to decrypt. Don’t use DecryptBuf
────────────────────────────────────────────────────────────────
RC4-40 5 N.A. "rc4-40"
obsolete: avoid
────────────────────────────────────────────────────────────────
RC5 16 8 CBC "rc5-cbc"
EBC "rc5-ecb"
CFB "rc5-cfb"
OFB "rc5-ofb"
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.
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
o 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.
o ~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.
o 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.
o 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 intialization 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.
o 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
o 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
o 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/errno>
#include <bobcat/decryptbuf>
using namespace std;
using namespace FBB;
int main(int argc, char **argv)
try
{
if (argc == 1)
throw Errno("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();
// decryptbuf.done(); // optionally
}
catch(Errno const &err)
{
cout << err.why() << endl;
return 1;
}
FILES
bobcat/decryptbuf - defines the class interface
SEE ALSO
bobcat(7), encryptbuf(3bobcat), std::streambuf
BUGS
None reported
DISTRIBUTION FILES
o bobcat_3.19.01-x.dsc: detached signature;
o bobcat_3.19.01-x.tar.gz: source archive;
o bobcat_3.19.01-x_i386.changes: change log;
o libbobcat1_3.19.01-x_*.deb: debian package holding the libraries;
o libbobcat1-dev_3.19.01-x_*.deb: debian package holding the libraries, headers and
manual pages;
o 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).