focal (3) decryptbuf.3bobcat.gz
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.
All information inserted into such an std::ostream is decrypted and written to an std::ostream passed as
argument to DecryptBuf’s constructor.
All encryption methods supported by the OpenSSL library that can be selected by name may be used by
DecryptBuf objects. In practice the information has previously been encrypted by an EncryptBuf object,
using the same encryption method. 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
FBB::CryptBuf, in turn inheriting 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 decryption algorithm
specified by type. The decryption algorithms that are available are listed in the abovementioned
table. 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 that was used when encrypting the original information. The IV is not
considered confidential. 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 encryption method is specified that is
not supported by OpenSSL.
The constructor’s first parameter refers to the std::ostream receiving the decrypted information.
The bufsize argument specifies the size in bytes of the internal buffer used by DecryptBuf
temporarily storing incoming characters. The provided default argument can most likely be kept
as-is.
o ~DecryptBuf():
Normally, once all encrypted information has been inserted into the decryption stream the eoi
manipulator (see below) is inserted to complete the decryption process. Alternatively, the
decryption process ends once the DecryptBuf’s destructor is called. E.g., if decStream is the
std::ostream to receive the encrypted information and encStream is the std::istream containing the
encrypted information then
decStream << encStream.rdbuf();
completes the decryption once DecryptBuf’s destructor is called. Alternatively,
desStream << encStream.rdbuf() << eoi;
can be used to immediately complete the decryption process.
Copy and move constructors (and assignment operators) are not available.
MEMBER FUNCTIONS
All members of std::streambuf are available, as FBB::DecryptBuf inherits from this class.
o void eoi():
This member can be called instead of using the manipulator to end the decryption process. It
throws an std::exception if decryption fails (which may happen if the DecryptBuf object is
provided with incorrect (usually improperly padded) input).
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).
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.
MANIPULATOR
o FBB::eoi:
The eoi manipulator can be inserted into the decryption stream to complete the decryption process.
If it is inserted into an std::ostream that is not initialized with either a DecryptBuf or an
EncryptBuf nothing happens.
EXAMPLE
#include <iostream>
#include <fstream>
#include <bobcat/exception>
#include <bobcat/ohexbuf>
#include <openssl/evp.h>
#include <openssl/err.h>
#include "../decryptbuf"
#include "../../encryptbuf/encryptbuf"
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: iv, "
"4th arg: file to decrypt (to stdout)";
// e.g., driver aes-128-cbc somekey iv-from-encryptbuf-driver
// /tmp/enc > /tmp/driver.dec
cerr << "Key: `" << argv[2] << "’\n"
"IV: `" << argv[3] << "’\n";
DecryptBuf decryptbuf(cout, argv[1], argv[2], argv[3]);
ostream out(&decryptbuf);
ifstream in(argv[4]);
if (not in)
throw Exception{} << "can’t read `" << argv[4] << ’\n’;
out << in.rdbuf() << eoi;
}
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
o bobcat_5.02.00-x.dsc: detached signature;
o bobcat_5.02.00-x.tar.gz: source archive;
o bobcat_5.02.00-x_i386.changes: change log;
o libbobcat1_5.02.00-x_*.deb: debian package holding the libraries;
o libbobcat1-dev_5.02.00-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).