Digital signature

From Wikiversity
Jump to: navigation, search

The digital signature is a technological tool which allows to ensure the origin of a document or a message and to verify that its content has not been altered. A digitally signed electronic document is as legally valid as a handwriting signed paper document, guaranteeing that it cannot be an object of repudiation.

Electronic signature, asymmetric keys and encryption[edit]

According to Argentinian legislation, signing of documents does not merely mean attribution to the author with certainty, but is also declarative with regard to his agreement with the content of the document. Electronic signature is a concept that attempts to satisfy the characteristics of the handwritten signature using technical systems.

Nowadays, the most accepted signature system in legal orders is the one based on public key asymmetric cryptography. Cryptography allows to transform, through a mathematical formula, a legible document into an unreadable series of signs for those who do not have the secret key to decrypt it.

When the key used to encrypt and decrypt documents is the same, this is a symmetric encryption system. In them, both the message sender and the recipient know and use the same key. The effectiveness of this system is connected to the possibility of keeping the key safely, otherwise any person who knows it will be able to decrypt the message.

In order to resolve this problem, methods that utilize different keys for encryption and decryption of texts have been designed. Those codes are called:

  • Private key, which is has to be well-kept.
  • Public key, which is disclosed to be known by the users.

These are called asymmetric key systems. The first one was created in the 70’s by W. Diffie and M. Hellman and was called RSA system (Rivest, Hamir and Adelaman from MIT).

Keys can be exchanged both for encryption or decryption of documents, provided that later the key which is its pair be used. Digital signatures are a type of electronic signatures. This classification also includes biometric identification systems (e.g. retina or iris scan, fingerprint and palm print recognition, voice recognition), holograph signature verification, etc.

How to digitally sign a document[edit]

In the first place, the signer has to have a pair of keys, which is generated by creating an algorithm. The private key will be saved by the user in the computer hardware or in a smart card, and its owner will be able to access it by using a password. Even though both keys are related, this requirement ensures that it is not feasible to infer or know the private key from the public one.

Moreover, the content of the document which will be signed has to be specified. Then, a program (a hash function) which converts the document to a series of bits called “message digest” is applied to this text. This is a compressed version of the original document. After that, the digest is encrypted by applying the signer’s private key, which results in the digital signature.

The next step is the signature verification by the recipient of the message, who will apply the public key in order to decrypt the text. The verification process consists of the confirmation by the recipient both that the message has been sent by whoever said to have done so (the owner of the private key, authenticity requirement) and that it has not undergone any change since its subscription (integrity requirement). For the issuer of the document to be assured that the message has been received and that it can only be read by the person to whom it was sent, one more step remains to mention, which is “non-repudiation”. In this case, the sender encrypts the message also with the recipient’s public key, so, when it is received, the only way to decrypt it is not only by entering the sender´s public key but also the private key of the recipient. Thus the issuer will ensure that the recipient actually received the message.

Key management and certificate authorities[edit]

In order to achieve an efficient operation of the system, users should be able to get their pair of keys in a safe way , so there must exist a means by which everyone can publish their public key and know the other users’ ones, and specially they need to have confidence in the legitimacy of the foreign keys. Therefore, systems that use certificates issued by certificate authorities have been designed. These institutions are also known as trusted third parties.

A certificate is a digital document which establishes a correspondence between a public key and a person or entity. Whatever the system utilized in generating the key pair, certificate authorities must check by appropriate means the identity and capacity of the person or entity to which it is issued.

Digital signature in Argentina[edit]

In Argentina, Law No. 25.506, sanctioned on November 14, 2001 and enacted on December 11, 2001 determines the legal framework of the digital signature, enabling the use of digital documents for the performing of legal acts and recognizing the usage of the electronic signature and the digital signature. Decree 427/98 grants to the digital signature the same legal value as the handwritten signature for those internal acts of the National Public Administration with no legal effects towards third parties.

Other aspects of the law are:

  • Instituting a national infrastructure in order to provide reliable usage conditions of the digital documents, according to internationally accepted technological standards.
  • Establishing requirements for issuing and managing digital certificates.
  • Regularising the activities of the certificate authorities, determining the scope of their responsibility and the conditions for obtaining the license.
  • Providing mechanisms for recognition of foreign digital certificates.
  • Giving the functions of Enforcement Authority to the Executive Office of the Cabinet of Ministers.

Some applications of the digital signature are: procedures with public entities such as tax returns, court notices, banking operations, distance contracts and foreign trade documents, identification and authentication on the Internet, etc.

Use-Case[edit]

Assume a WHO officer Anna is telling Bert on the phone that she would not recommend to travel to country C because 250 cases of an infectious disease D were identified by the regional public health agency.

Bert knows Anna very well and he knows her phone number, so there is trust between Anna and Bert. But if Bert informs others about the risk via email and they in turn inform their friends, then the chain of trust might not exist anymore. It is possible that someone in the communication network changes the original message from Anna (WHO), e.g. alter the message in way that it is not a problem to travel to country C because there only 2 reported cases.

Write a Message[edit]

At first Anna from WHO writes message as usual.

 ---------------------------------
   MESSAGE from Anna WHO:
   It is not recommended to travel to country C 
   because there are 250 reported 
   cases of disease D.
   best regards,
   Anna from WHO
   ---------------------------------

Sign a Message[edit]

Anna has a private key only Anna has access to this. Anna uses the private key to add a digital signature to the mail.

   ---------------------------------
   MESSAGE from Anna WHO:
   It is not recommended to travel to country C 
   because there are 250 reported 
   cases of disease D.
   best regards,
   Anna from WHO
   ---------------------------------
   DIGITAL SIGNATURE anna@example.com
   SHSUHDWz78293472934SKDL9334SDl....shdaos939423n
  ---------------------------------

Validate Digital Signature[edit]

When Bert or other people receive the mail above, they can check with a public key, that digital signature for the mail is valid. Bert forward the message to Charly and Charly to Diane and so forth. Someone in the chain decides to alter the e-mail into

   ---------------------------------
   MESSAGE from Anna WHO:
   It is still recommended to travel to country C 
   because there are only 2 reported 
   cases of disease D.
   best regards,
   Anna from WHO
   ---------------------------------
   DIGITAL SIGNATURE anna@example.com
   SHSUHDWz78293472934SKDL9334SDl....shdaos939423n
  ---------------------------------

The alteration of the message leads to an invalid digital signature. Because anyone receiving the message can check with the public key of Anna for the mail

anna@example.com

has access to the public key for validation of digital signature of the mail. It is not possible with the current state of the art to create a new valid digital signature without the private key of Anna.

Learning Task[edit]

Bibliography[edit]