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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


FBB::DecryptBuf objects are std::streambuf objects that can be used to initialize std::ostream objects with.
All information inserted into such a std::ostream is decrypted and written into a std::ostream that is given as argument to DecryptBuf’s constructor.
All encryption methods defined by the OpenSSL library that can be selected by name may be used in combination with DecryptBuf objects. Most likely the information will have been encrypted using an EncryptBuf object, selecting a particular encryption method. The encryption method used when encrypting information should also be specified when constructing a DecryptBuf object. 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.
Information about the various encryption methods and cipher modes as well as information about how keys and initialization vectors are handled is found in the EncryptBuf(3bobcat) manual page and need not be repeated here.


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




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 message decrypt algorithm specified with type. The decryption algorithms that can be used are listed in the table found in the EncryptBuf(3bobcat) manual page. 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 to use. The iv’s length cannot be zero. 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::Errno exception if an unknown encryption method was specified.
The constructor’s first parameter refers to the std::ostream to receive the decrypted information.
The bufsize argument specifies the size in bytes of the internal buffer used by DecryptBuf to store incoming characters temporarily. The provided default argument should be OK in all normal cases.
The destructor calls the done() member (see below), prevending any exception that function might throw from leaving the destructor. In this case only a non thread-safe way to determine whether the decryption was successfully completed is available through the static member function lastOK() (see below). There is no copy constructor, nor move constructor (as std::streambuf doesn’t support either).


All members of std::streambuf are available, as FBB::DecryptBuf inherits from this class. Some of the std::streambuf’s member are overridden or are hidden by DecryptBuf. In normal situations these inherited members will not be used by programs using DecryptBuf objects.
void done():
This member can be called to indicate that all information to be decrypted has been received. It throws an FBB::Errno exception if decryption fails (resulting from providing the DecryptBuf object with incorrect (usually improperly padded) input). If not explicitly called it is called by DecryptBuf’s destructor, preventing its exception from leaving the destructor.
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 decrypted. When called after decryption has started or when specifying an empty intialization vector an FBB::Errno exception will be thrown. When using ECB modes no initialization vector is used. In those cases any non-empty initialization vector may be provided.
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).


bool lastOK():
This member is a non thread-safe way to determine whether the decryption has succeeded when the DecryptBuf object’s done member has not been called and the object has been destroyed. In that case the object’s destructor will call done to complete the decryption. The member lastOK returns true if the DecryptBuf object destroyed last could complete its decryption successfully and returns false otherwise.


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.


#include <iostream>
#include <fstream>
#include <bobcat/errno>
#include <bobcat/decryptbuf>
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: file to " "decrypt (to stdout), 4th arg: iv");
cerr << "Key: `" << argv[2] << "’\n" "IV: `" << argv[4] << "’\n";
DecryptBuf decryptbuf(cout, argv[1], argv[2], argv[4]); ostream out(&decryptbuf); ifstream in(argv[3]);
out << in.rdbuf(); // decryptbuf.done(); // optionally } catch(Errno const &err) { cout << err.why() << endl; return 1; }


bobcat/decryptbuf - defines the class interface


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


None reported


bobcat_3.01.00-x.dsc: detached signature;
bobcat_3.01.00-x.tar.gz: source archive;
bobcat_3.01.00-x_i386.changes: change log;
libbobcat1_3.01.00-x_*.deb: debian package holding the libraries;
libbobcat1-dev_3.01.00-x_*.deb: debian package holding the libraries, headers and manual pages;
o public archive location;


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 (
2005-2012 libbobcat1-dev_3.01.00-x.tar.gz