Version 5.0 The LEDA User Manual

10.5 Authentication ( OMACCoder )
1. Definition
The class OMACCoder can be used for authentication. It implements the One-Key CBC
MAC algorithm by Iwata and Kurosawa [49]. A MAC (= message authentication code)
is a kind of checksum that is generated for a message m. In contrast to a usual checksum
a MAC does not only depend on m but also on a secret key k. A MAC can be used if a
sender wants to transfer m through an insecure channel to a receiver. If the sender also
transmits the MAC then the receiver can verify that m has not been altered and that it
originates from someone who knows k.
Important: OMACCoder does not encipher or decipher any data, it only computes and
verifies MACs!
The behaviour of this coder depends on the MAC in stream flag. If it is false then encoding
and decoding are equivalent: Both methods copy the source stream to the target stream
and compute a MAC. If the flag is true then in encoding mode the source stream is copied
to the target stream, a MAC is computed and appended to the target stream. In decoding
mode the MAC at the end of source stream is removed and the original data is copied to
the target stream, a MAC is computed and compared with the MAC found in the source
stream. If the two MACs differ an error is signaled. The method MAC is valid may be
used to check whether the source stream is authentic.
The class OMACCoder< BlkCipher > is parameterized with a block-cipher BlkCipher,
which can be one of the following: Rijndael (default), Blowfish, Twofish (see Section 10.3
for more information). OMACCoder also supports fast seek operations (see Section 9.22).
#include < LEDA/coding/authentication.h >
2. Creation
OMACCoder< BlkCipher > C(streambuf ∗ src stream = 0,
streambuf ∗ tgt stream = 0, bool own streams = false);
creates an instance C which uses the given source and target
streams. If own streams is set, then C is responsible for the destruction
of the streams, otherwise the pointers src stream and tgt stream
must be valid during the life-time of C.
OMACCoder< BlkCipher > C(const char ∗ src file name, const char ∗ tgt file name);
creates an instance C which uses file-streams for input and output.