focal (3) encryptbuf.3bobcat.gz
NAME
FBB::EncryptBuf - Encrypts information using various methods into a std::ostream
SYNOPSIS
#include <bobcat/encryptbuf>
Linking option: -lbobcat -lcrypto
DESCRIPTION
FBB::EncryptBuf objects are std::streambuf objects that can be used to initialize std::ostream objects.
All information inserted into such a std::ostream is encrypted and written into an std::ostream that is
given as argument to EncryptBuf’s constructor.
All encryption methods supported by the OpenSSL library that can be selected by name may be used by
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).
Most modes use initialization vectors. Unless provided at construction time EncryptBuf objects create
random initialization vectors. The initialization vector that is actually used can be obtained from the
EncryptBuf object. This is important, as the matching decryption object needs to know the initialization
vector that was used when encrypting the data. Initialization vectors are not considered confidential and
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 encryption method it is automatically extended with 0-bytes to the required length.
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 EncryptBuf(std::ostream &outStream, char const *type, std::string const &key, std::string const
&iv, size_t bufsize = 1024):
This constructor initializes the EncryptBuf object preparing it for the encryption algorithm
specified with type. The encryption algorithms that can be used are listed in the abovementioned
table. E.g., to use the AES method on 24 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. Both key and iv may contain non-displayable characters. When iv.length() is zero it
will be filled by the EncryptBuf object with random data. When the iv is too small for the
requested method it is expanded by adding the required number of zero valued bytes.
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 to receive the encrypted information.
Be aware of the fact that the encrypted information most likely contains non-displayable
characters.
The bufsize argument specifies the size in bytes of the internal buffer used by EncryptBuf
temporarily storing incoming characters. The provided default argument can most likely be kept
as-is.
o ~EncryptBuf():
Normally, once all information has been inserted into the encryption stream the end manipulator
(see below) is inserted to complete the encryption process. Alternatively, the encryption process
ends once the EncryptBuf’s destructor is called. E.g., if encStream is the std::ostream to receive
the information to encrypt and inStream is the std::istream containing the information to encrypt
then
endStream << inStream.rdbuf();
completes the decryption once EncryptBuf’s destructor is called. Alternatively,
encStream << inStream.rdbuf() << end;
can be used to immediately complete the encryption process.
Copy and move constructors (and assignment operators) are not available.
MEMBER FUNCTIONS
All members of std::streambuf are available, as FBB::EncryptBuf inherits from this class.
o size_t blockLength() const:
This member returns the block size (in bytes) that are used by the specified method.
o size_t ivLength() const:
This member returns the size (in bytes) of the initialization vector that is used by the specified
method.
o std::string iv() const:
This member returns a reference to the initialization vector that is used by the specified method.
Be advised that the initialization vector may contain non-displayable characters.
o size_t keyLength() const:
This member returns the size of the key (in bytes) that are used by the specified method.
o size_t rounds() const:
This member can only be used with the RC5 encryption method to query the number of rounds of the
algorithm. It returns the currently used number of rounds or 0 if the member is called for another
encryption method than RC5.
o bool setRounds(size_t nRounds):
This member can only be used with the RC5 encryption 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 encryption 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 EncryptBuf may use this member to gain direct access to the EVP_CIPHER_CTX
pointer used by the EncryptBuf object. This pointer is a pointer to an opaque structure used by
many OpenSSL functions to set or query parameters of an encryption method.
EXAMPLE
#include <iostream>
#include <iomanip>
#include <fstream>
#include <bobcat/exception>
#include <bobcat/ohexbuf>
#include "../encryptbuf"
#include <openssl/evp.h>
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: (opt): iv, "
"stdin: file to encrypt (to stdout)\n"
"e.g., driver aes-128-cbc somekey < driver.cc > /tmp/enc\n";
string key(argv[2]);
string iv;
if (argc > 3)
iv = argv[3];
EncryptBuf encryptbuf(cout, argv[1], key, iv, 50);
ostream out(&encryptbuf);
size_t ivLength = encryptbuf.iv().length();
cerr << "Block length: " << encryptbuf.blockLength() << "\n"
"Key length: " << encryptbuf.keyLength() << "\n"
"Max Key length: " << EVP_MAX_KEY_LENGTH << "\n"
"actual IV length: " << ivLength << "\n"
"IV =\n";
OHexBuf ohb{ cerr, ivLength << 1 };
ostream outHex(&ohb);
outHex << encryptbuf.iv();
cerr << ’\n’ << dec;
out << cin.rdbuf() << eoi;
}
catch(exception const &err)
{
cerr << err.what() << endl;
return 1;
}
To ignore the initial 256 bytes generated by the RC4 algorithm a simple wrapper class around the eventual
output stream can be used. Here is an illustration:
#include <ostream>
#include <bobcat/ofilterbuf>
class Skip256: public FBB::OFilterBuf
{
size_t d_count;
public:
Skip256(std::ostream &os)
:
OFilterBuf(os),
d_count(0)
{}
private:
virtual int overflow(int c)
{
if (d_count == 256)
out().put(c);
else
++d_count;
return c;
}
};
Next, an Skip256 object is used to define an intermediate std::ostream that is then passed to the
EncryptBuf object. E.g., only showing the essential steps defining the EncryptBuf object:
Skip256 skip256(std::cout);
std::ostream out(&skip256);
EncryptBuf encryptbuf(out, "rc4", key, "");
FILES
bobcat/encryptbuf - defines the class interface
SEE ALSO
bobcat(7), decryptbuf(3bobcat), ofilterbuf(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).