MegaLab.it

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* A Message From Philip Zimmermann
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There is no advantage for using the keys larger than about 3000 bits. The 128-bit session keys have the same work factor to break as a 3000 bit RSA or DH key. Therefore, the larger keys contribute nothing to security, and, in my opinion, spread superstition and ignorance about cryptography. They also slow everything down and burden the key servers and everyone's keyrings, as well as cause interoperability problems with present and future releases of PGP. Perhaps even more importantly, they also undermine other people's faith in their own keys that are of appropriate size. While it may have been well-intentioned, this massive expansion of key size is a disservice to the PGP community.

Also, larger DSA keys don't contribute anything unless the hash grows bigger with it. That requires selecting a good well-designed bigger hash that has been specifically designed to have the full work factor for breaking it. Using two SHA1 hashes in that manner has not been adequately shown to achieve this result.

Anyone with a sophisticated understanding of cryptography would not make the keys bigger this way.

Experimental code that we put into PGP during its development should not be used. It was protected with conditional compilation flags and should never have been revealed to uninformed users who decide to perform a "public service" by enabling the code and releasing it. This is part of the reason why we ask people not to release code changes on their own, but to send them to us, so that we may incorporate some of them (if they seem like good ideas) into our next product release. That is how PGP enhancements from the user community have always been managed since PGP source code was released in 1991.

-Philip Zimmermann

Per dire che nemmeno i ricercatori sono in grado di mettersi d'accordo tra di loro, per via della natura altamente speculativa della materia. Quindi stabilire cosa è "migliore" rispetto a cosa, mi pare un po' un terno al lotto....

Riguardo i migliori, oltre alla crittografia asimmetrica di PGP (la mia preferita di sempre, visto l'alto livello di sicurezza garantito dalla doppia chiave pubblica/privata), c'è l'algoritmo di cifratura AES che è stato adottato dalla NSA. Ed è la prima volta, nella storia della crittografia, che un'istituzione di sicurezza considera talmente sicuro un algoritmo da permettere che sia di pubblico dominio, continuando nel contempo ad utilizzarlo per proteggere le informazioni classificate.

Come si decrittano? Il concetto alla base è la fattorizzazione, che ce la insegnano alle scuole medie.

Fattorizzare un numero di due cifre, è facile.
Fattorizzare un numero con tante cifre, è difficile.
Tutto qui.

Queste informazioni vengono dal corso di sicurezza che ho seguito ormai l'anno scorso, se vi servono dati più precisi mi costringete a cercare tra i bit dei miei archivi

In 2002, a theoretical attack, termed the "XSL attack", was announced by Nicolas Courtois and Josef Pieprzyk, showing a potential weakness in the AES algorithm. Several cryptography experts have found problems in the underlying mathematics of the proposed attack, suggesting that the authors may have made a mistake in their estimates. Whether this line of attack can be made to work against AES remains an open question. For the moment, the XSL attack against AES appears speculative; it is unlikely that anyone could carry out the current attack in practice.

Side channel attacks

Side channel attacks do not attack the underlying cipher, but attack implementations of the cipher on systems which inadvertently leak data.
In April 2005, D.J. Bernstein announced a cache timing attack that he used to break a custom server that used OpenSSL's AES encryption. The custom server was designed to give out as much timing information as possible, and the attack required over 200 million chosen plaintexts. Some say the attack is not practical over the internet with a distance of one or more hops [5]; Bruce Schneier called the research a "nice timing attack." [6]
In October 2005, Adi Shamir and two other researchers presented a paper demonstrating several cache timing attacks(PDF file) against AES. One attack was able to obtain an entire AES key after only 800 writes, in 65 milliseconds. This attack requires the attacker to be able to run programs on the same system that is performing AES encryptions.

thomas ha scritto:Home page del corso del Prof. Ozalp Babaoglu (C.d.L. Informatica, Università di Bologna)

thomas ha scritto:Fattorizzare un numero con tante cifre, è difficile.

Bondo ha scritto:

thomas ha scritto:Home page del corso del Prof. Ozalp Babaoglu (C.d.L. Informatica, Università di Bologna)

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thomas ha scritto:

Bondo ha scritto:

thomas ha scritto:Home page del corso del Prof. Ozalp Babaoglu (C.d.L. Informatica, Università di Bologna)

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