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FBB::DecryptBuf(3bobcat) Decrypt information FBB::DecryptBuf(3bobcat)


FBB::DecryptBuf - Decrypts information using various methods into a std::ostream


#include <bobcat/decryptbuf>
Linking option: -lbobcat -lcrypto


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:

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.
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.
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.
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. unless the initial 256 bytes produced by the cipher are discarded.


All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.


FBB::CryptBuf, in turn inheriting from std::streambuf


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.
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.


All members of std::streambuf are available, as FBB::DecryptBuf inherits from this class.

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).
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).


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.


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.


#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)

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; }


bobcat/decryptbuf - defines the class interface


bobcat(7), encryptbuf(3bobcat), std::streambuf


None reported

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bobcat_5.11.01-x.tar.gz: source archive;
bobcat_5.11.01-x_i386.changes: change log;
libbobcat1_5.11.01-x_*.deb: debian package containing the libraries;
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Bobcat is an acronym of `Brokken’s Own Base Classes And Templates’.


This is free software, distributed under the terms of the GNU General Public License (GPL).


Frank B. Brokken (

2005-2022 libbobcat-dev_5.11.01