cryptomethods.tex

a very popular packet of cryptography tools,it encloses the most common used algorithm and protocols

subscriber has a personal pair of keys consisting of a {\em secret}
\index{Key!secret} key and a {\em public} key\index{Key!public}. The public
key, as its name implies, is made public, e.g. in a key directory on the
Internet.\par \vskip + 3pt

If Alice\index{Alice}%
\footnote{%
      In order to describe cryptographic protocols participants
      are often named Alice, Bob, \dots (see \cite[p. 23]{Schneier1996cm}). 
      Alice and Bob perform all 
      2-person-protocols. Alice will initiate all protocols and 
      Bob answers. The attackers are named Eve (eavesdropper) and
      Mallory (malicious active attacker).
  } wants to communicate with Bob, then she finds Bob's public key 
in the directory and uses it to encrypt her message to him. She then sends
this cipher text to Bob, who is then able to decrypt it again using his 
secret key. As only Bob knows his secret key, only he can decrypt 
messages addressed to him.
Even Alice who sends the message cannot restore plaintext from the (encrypted)
message she has sent. Of course, you must first ensure that the public key
cannot be used to derive the private key.\par \vskip + 3pt

Such a procedure can be demonstrated using a series of thief-proof letter boxes.
If I have composed a message, I then look for the letter box of the recipient
and post the letter through it. After that, I can no longer read or change the
message myself, because only the legitimate recipient has the key for the
letter box.\par \vskip + 3pt

The advantage of asymmetric procedures is the easy \index{Key management} key management. Let's look again at a network with $n$
subscribers. In order to ensure that each subscriber can establish
an encrypted connection to each other subscriber, each subscriber
must possess a pair of keys. We therefore need $2n$ keys or $n$
pairs of keys. Furthermore, no secure channel is needed before
messages are transmitted, because all the information required in
order to communicate confidentially can be sent openly. In
this case, you simply have to pay attention to the accuracy
(integrity and authenticity) \index{Authenticity} of the public
key. Disadvantage: Pure asymmetric procedures take a lot longer to
perform than symmetric ones.\par \vskip + 3pt

The most well-known asymmetric procedure is the \index{RSA} 
RSA algorithm\index{CrypTool}%
\footnote{%
The RSA algorithm is extensively described in chapter \ref{rsabeweis} and later
within this script. 
The RSA cryptosystem can be executed in many variations with 
CrypTool\index{CrypTool} (using the menu path
{\bf Individual Procedures \textbackslash{} RSA Cryptosystem \textbackslash{}
RSA Demonstration}). 
The topical research results concerning RSA are described 
in chapter \ref{SecurityRSA}.
}%
,
named after its developers Ronald \index{Rivest, Ronald} Rivest, Adi
\index{Shamir, Adi} Shamir and Leonard \index{Adleman, Leonard} Adleman. The RSA algorithm
was published in 1978. The concept of asymmetric encryption was first
introduced by Whitfield Diffie \index{Diffie, Whitfield}  and Martin
\index{Hellman, Martin} Hellman in 1976. Today, the ElGamal \index{ElGamal, Tahir}
procedures also play a decisive role, particularly the \index{Schnorr, C.P.} Schnorr
variant in the \index{DSA} DSA (Digital \index{Signature!digital}Signature
Algorithm).


% --------------------------------------------------------------------------
% \newpage
\subsection[Hybrid procedures]
{Hybrid procedures\footnotemark}
\footnotetext{%
Within CrypTool\index{CrypTool} you can get a visualization of this
technique using the menu path {\bf Crypt \textbackslash{} Hybrid}: 
this dialogue shows the single steps and its dependencies with concrete
numbers. Here the asymmetric algorithm RSA and the symmetric algorithm AES are used.
}\index{Hybrid procedure}

In order to benefit from the advantages of symmetric and asymmetric
techniques together, hybrid procedures \index{Encryption!hybrid} are
usually used (for encryption) in practice. \par \vskip + 3pt

In this case the data is encrypted using symmetric procedures: the key is a
session key\index{Session key} generated by the sender
randomly\footnote{%
An important part of cryptographically secure techniques is to generate 
random numbers. Within CrypTool\index{CrypTool} you can check out
different random number generators using the menu path
{\bf Indiv. Procedures \textbackslash{} Generate Random Numbers}. 
Using the menu path {\bf Analysis \textbackslash{} Analyse Randomness}
you can apply different test methods for random data to binary documents. \\
Up to now CrypTool has concentrated on cryptographically strong 
pseudo random number generators. Only the integrated Secude\index{SECUDE IT Security}
generator involves a "pure" random source. 
}\index{Random}
that is only used for this message.
This session key is then encrypted using the asymmetric procedure and
transmitted to the recipient together with the message. Recipients can determine
the session key using their secret keys and then use the session key to encrypt
the message. In this way, we can benefit from the easy key management
\index{Key management} of asymmetric procedures and encrypt large quantities of data
quickly and efficiently using symmetric procedures.


% --------------------------------------------------------------------------
\subsection{Further details}

Beside the information you can find in the following chapters, many other
books and on a good number of websites the online help of 
CrypTool\index{CrypTool} also offers very many details about the 
symmetric and asymmetric encryption methods.


% --------------------------------------------------------------------------
\newpage
\begin{thebibliography}{99999}
\addcontentsline{toc}{subsection}{Bibliography}

\bibitem[Nichols1996]{Nichols1996} \index{Nichols 1996} 
    Randall K. Nichols, \\
    {\em Classical Cryptography Course, Volume 1 and 2}, \\
    Aegean Park Press 1996;
    or in 12 lessons online at \\
    \href{http://www.fortunecity.com/skyscraper/coding/379/lesson1.htm}
    {\texttt{http://www.fortunecity.com/skyscraper/coding/379/lesson1.htm}}


\bibitem[Schmeh2003]{Schmeh2003}  \index{Schmeh 2003}
        Klaus Schmeh, \\
        {\em Cryptography and Public Key Infrastructures on the Internet},\\ 
        John Wiley \& Sons Ltd., Chichester 2003. \\
        A considerable, up-to-date, easy to read book, which also 
        considers practical problems such as standardisation or
        real existing software.

\bibitem[Schneier1996]{Schneier1996cm} \index{Schneier 1996} 
    Bruce Schneier, \\
    {\em Applied Cryptography, Protocols, Algorithms, and Source Code in C}, \\
    Wiley 1994, 2nd edition 1996.


\bibitem[Coppersmith2002]{Coppersmith2002}  \index{Coppersmith 2002}
        Don Coppersmith, \\
        {\em Re: Impact of Courtois and Pieprzyk results}, \\
        2002-09-19, ``AES Discussion Groups''~ at \\
        \href{http://aes.nist.gov/aes/}
        {\texttt{http://aes.nist.gov/aes/}}

\bibitem[Courtois2002]{Courtois2002}  \index{Courtois 2002}
        Nicolas Courtois, Josef Pieprzyk, \\
        {\em Cryptanalysis of Block Ciphers with Overdefined Systems of Equations}, \\
        received 10 Apr 2002, last revised 9 Nov 2002.\\
        A different version, so called compact version of the first XSL attack,
        was published at Asiacrypt Dec 2002. \\
        \href{http://eprint.iacr.org/2002/044}
        {\texttt{http://eprint.iacr.org/2002/044}}

\bibitem[Ferguson2001]{Ferguson2001}  \index{Ferguson 2001}
        Niels Ferguson, Richard Schroeppel, Doug Whiting, \\
        {\em A simple algebraic representation of Rijndael}, 
        Draft 2001/05/1, \\
        \href{http://www.xs4all.nl/~vorpal/pubs/rdalgeq.html}
        {\texttt{http://www.xs4all.nl/\~{}vorpal/pubs/rdalgeq.html}}

\bibitem[Lucks-DuD2002]{Lucks-DuD2002}  \index{Lucks 2002}
        Stefan Lucks, R\"udiger Weis, \\
        {\em Neue Ergebnisse zur Sicherheit des Verschl\"usselungsstandards AES}, 
        in DuD Dec. 2002.

\bibitem[Robshaw2002a]{Robshaw2002a}  \index{Robshaw 2002}
        S.P. Murphy, M.J.B. Robshaw, \\
        {\em Essential Algebraic Structure within the AES}, 
        June 5, 2002, Crypto 2002,  \\
        \href{http://www.isg.rhul.ac.uk/\~{}mrobshaw/rijndael/rijndael.html}
        {\texttt{http://www.isg.rhul.ac.uk/~mrobshaw/rijndael/rijndael.html}}

\bibitem[Robshaw2002b]{Robshaw2002b}  \index{Robshaw 2002}
        S.P. Murphy, M.J.B. Robshaw, \\
        {\em Comments on the Security of the AES and the XSL Technique}, 
        September 26, 2002, \\
        \href{http://www.isg.rhul.ac.uk/\~{}mrobshaw/rijndael/rijndael.html}
        {\texttt{http://www.isg.rhul.ac.uk/~mrobshaw/rijndael/rijndael.html}}

\bibitem[Wobst-iX2002]{Wobst-iX2002}  \index{Wobst 2002}
        Reinhard Wobst, \\
        {\em Angekratzt - Kryptoanalyse von AES schreitet voran}, 
        in iX Dec. 2002, \\
        plus the reader's remark by Johannes Merkle in iX Feb. 2003.

\end{thebibliography}


% --------------------------------------------------------------------------
\newpage
\section*{Web links}\addcontentsline{toc}{subsection}{Web links}

\begin{enumerate}

\item AES or Rijndael Cryptosystem \\
        \href{http://www.cryptosystem.net/aes}
         {\tt http://www.cryptosystem.net/aes} \\
	\href{http://www.minrank.org/aes/}
         {\tt http://www.minrank.org/aes/}

  \item AES Discussion Groups at NIST \\
	\href{http://aes.nist.gov/aes}{\tt http://aes.nist.gov/aes}

  \item distributed.net: ``RC5-64 has been solved'' \\
        \href{http://distributed.net/pressroom/news-20020926.html}
         {\tt http://distributed.net/pressroom/news-20020926.html}

  \item RSA: ``The RSA Secret Key Challenge'' \\
       \href{http://www.rsasecurity.com/rsalabs/challenges/secretkey/index.html}
        {\tt http://www.rsasecurity.com/rsalabs/challenges/secretkey/index.html}

  \item RSA: ``DES Challenge'' \\
        \href{http://www.rsasecurity.com/rsalabs/challenges/des3/index.html}
         {\tt http://www.rsasecurity.com/rsalabs/challenges/des3/index.html}

  \item Further Links can be found at the CrypTool Homepage \\
        \href{http://www.cryptool.org}
         {\tt http://www.cryptool.org}
	       
\end{enumerate}



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