NAME¶
FBB::EncryptBuf - Encrypts information using various methods into a std::ostream
SYNOPSIS¶
#include <bobcat/encryptbuf>
Linking option:
-lbobcat
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 will be
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 containg 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 passed over in the clear to the decrypting method. 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 |
|
|
|
|
|
|
|
|
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 |
|
|
|
|
|
|
|
|
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 |
|
|
|
|
|
|
|
|
EBC |
"rc2-ecb" |
|
|
|
CFB |
"rc2-cfb" |
|
|
|
OFB |
"rc2-ofb" |
Key length variable, max. 128 bytes, default length is shown |
|
|
|
|
RC2-40 |
|
|
|
|
|
obsolete: avoid |
|
|
|
|
RC2-64 |
|
|
|
|
|
obsolete: avoid |
|
|
|
|
RC4 |
|
|
|
|
|
Key length is variable, max. 256 bytes. default length is shown |
|
|
|
|
Encrypt again to decrypt. Don’t use DecryptBuf |
|
|
|
|
RC4-40 |
|
|
|
|
|
obsolete: avoid |
|
|
|
|
RC5 |
|
|
|
|
|
|
|
|
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. This may easily be
accomplished using a wrapper class around the output stream using the
facilities offered by
OFilterStreambuf(3bobcat). The
EXAMPLE
section below provides an illustration.
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::Errno 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::Errno 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::Errno 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/errno>
#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 Errno("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(Errno const &err)
{
cout << err.why() << 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_3.01.00-x.dsc: detached signature;
- o
- bobcat_3.01.00-x.tar.gz: source archive;
- o
- bobcat_3.01.00-x_i386.changes: change log;
- o
- libbobcat1_3.01.00-x_*.deb: debian package holding
the libraries;
- o
- libbobcat1-dev_3.01.00-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).