bionic (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
with.
All information inserted into such a std::ostream is encrypted and written into a std::ostream that is
given as argument to EncryptBuf’s constructor.
All 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. Unless provided at construction time or explicitly set
thereafter an initialization vector containing randomly selected values will be used. The initialization
vector that is actually used can be obtained from the EncryptBuf object. This is important, as the
matching decrypting object needs to know the initialization vector that was used when encrypting the
data. 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:
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
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 message encrypt algorithm
specified with type. The encryption algorithms that can be used are listed in the table found in
the DESCRIPTION section. 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 at the time encryption starts it will be filled by the EncryptBuf object with
random data. When the key and/or the iv is too small for the requested method they will be
expanded by adding the required number of zero valued bytes.
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 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 to
store incoming characters temporarily. The provided default argument should be OK in all normal
cases. 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::EncryptBuf inherits from this class. Some of the
std::streambuf’s member are overridden or are hidden by EncryptBuf. In normal situations these inherited
members will not be used by programs using EncryptBuf objects.
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 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 encrypted. When called after encryption has started an FBB::Exception
exception will be thrown.
o void setKey(std::string const &key, size_t numberOfBytes = 0):
This member can be used to specify the key and its length after construction time but before any
data has been encrypted. When called after encryption has started an FBB::Exception exception will
be thrown. The size of the key is assumed to be the number of bytes in the key’s data. If another
key length is required the member function’s second parameter can be used to specify the length of
the key in bytes.
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 <fstream>
#include <bobcat/exception>
#include <bobcat/encryptbuf>
#include <bobcat/ohexstreambuf>
#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)";
string key(argv[2]);
string iv;
if (argc > 3)
iv = argv[3];
EncryptBuf encryptbuf(cout, argv[1], key, iv);
ostream out(&encryptbuf);
cerr << "Block length: " << encryptbuf.blockLength() << ’\n’ <<
"Key length: " << encryptbuf.keyLength() << ’\n’ <<
"Max Key length: " << EVP_MAX_KEY_LENGTH << ’\n’ <<
"IV length: " << encryptbuf.ivLength() << endl;
cerr << encryptbuf.iv().length() << ’ ’;
OHexStreambuf ohsb(cerr);
ostream ohs(&ohsb);
ohs.write(encryptbuf.iv().data(), encryptbuf.iv().length()) << flush;
cerr << endl;
out << cin.rdbuf();
}
catch(exception const &err)
{
cout << 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/ofilterstreambuf>
class Skip256: public FBB::OFilterStreambuf
{
size_t d_count;
public:
Skip256(std::ostream &os)
:
OFilterStreambuf(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), ofilterstreambuf(3bobcat), std::streambuf
BUGS
None reported
DISTRIBUTION FILES
o bobcat_4.08.02-x.dsc: detached signature;
o bobcat_4.08.02-x.tar.gz: source archive;
o bobcat_4.08.02-x_i386.changes: change log;
o libbobcat1_4.08.02-x_*.deb: debian package holding the libraries;
o libbobcat1-dev_4.08.02-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).