Pages: 16 (3879 mots) Publié le: 30 mars 2011

Internet accelerates economical flows by online promotion and the selling of material or immaterial products. It allows the creation of new commercial schemes (auctions...) by connecting information. Companies and private individuals exchange through Internet digital contract documents. However in the modern world we cannot rely on the security of our communication networks. It ispractically impossible to physically protect all the links from interference. The use of cryptography to protect communication paths is well established and has undergone significant advances in the last 20 years.

The problem about Internet is that the individuals who communicate do not meet and do not know each other. They do not know if the interlocutor is reliable. The solution to solvethis problem is the Digital Signature. The protocol allows two users to exchange a secret key over an insecure medium without any prior secrets.

A Digital Signature is an electronic signature that can be used to authenticate the identity of the sender of a message, or of the signer of a document. It can also be used to ensure that the original content of the message or document that has beenconveyed is unchanged.

This technology was first defined in the early 1980s with the development of public key cryptography but has received renewed interest as an authentication mechanism on Internet.

This process has been created to full several aims:

➢ Integrity: It allows guaranteeing to the receiver that the document has not been modified.
➢ Confidentiality: Thanks to the DigitalSignature, we cannot reveal the document of someone else.
➢ Authentication: This technology ensures the identity of the sender.
➢ Non-repudiation: The receiver has the proof of the transaction.

In a first chapter, we will explain how the Digital Signature works and will talk about the limits concerning the commercial, technical and law aspects in the sector of e-business in a secondchapter.



The Digital Signature has changed since the apparition of coding technics. In fact, we noticed different systems to code a message, in particular the replacement and the Vigenere’s figure. So we propose to explain both technics prior to explaining how a message is digitally signed today.


The Replacement of aletter by another letter is the oldest technique. We calculate the frequency of one letter’s apparition and we compare with the frequency table of the language to realise the replacement table. It is easier when the text is long enough.

The word STUDENT becomes IUCXDU
The word SCHOOL becomes IVLSSG
The word LISTEN becomes GKIUXD





To translate acode, if we know the chosen algorithm, at first we try to find the most frequent letters. Then we try the most evident replacements. At last, we verify if the decoding message has a meaning. With a long message, it is easy to understand the code.


We change the line for each letter according to a key. For instance, if the key is 2456891, the word STUDENT becomesUXZJMWU

This system is difficult to realise because we need to spread a lot of secret keys.

With software it is easy to create a substitution system of elements, which cannot be decoding by analysing frequencies.

The first system was the DES (Data Encryption Standard), developed in the 60’s, with a key of 56 Bit, with 256 possible solutions. Thus the security is that it is impossible to attackthe key by the frequency method.

So the main problem is the distribution of keys, which has to be secret. The security is ensured thanks to a reliable key distribution. The message can be sent and read after having processed of a key exchange.

We can wonder how to encrypt a message between two different persons who have never met themselves and cannot exchange their keys.

B/ THE...
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