Email systems that were previously used by isolated organizations in relatively secure conditions are now being connected to public networks that cannot be considered sufficiently secure.
Organizations and government agencies are vulnerable to hackers, unscrupulous system employees, and skilled eavesdroppers.
Cryptographic technology can provide secure communications and effective email security.
Public key cryptography is ideal for secure electronic data exchange without the problems associated with the distribution of encryption keys that arise in symmetric or single-key cryptography.
Digital signatures can provide authentication of subscribers equivalent to that achievable when certifying documents with a handwritten signature.
The adoption of international and industrial standards now makes it possible to use these technologies in e-mail systems for the secure transmission of any digital information between any subscribers via any networks.
In data transmission systems, security management is carried out by various services.
The following security services are traditional: ensuring the confidentiality of information, maintaining integrity, authentication, eliminating failures when transmitting or receiving messages, access control.
Any e-mail security system must ensure that information is kept confidential to prevent unauthorized disclosure of classified information.
The security of documents transmitted by e-mail must be ensured both when stored in the computer system and when transmitted between systems or correspondents.
Due to the high vulnerability of information transmission networks to interception, any unprotected message transmitted over the network cannot be considered undisclosed.
Maintaining the integrity of information is required to prevent accidental or intentional destruction of data or delay in its transmission.
The authentication service is the most important and critical in the security system, since without an accurate verification of the identity of the correspondent, all other security measures are meaningless. To prevent an illegal user from disguising himself as a legitimate one, passwords and personal identification numbers (PIN) are often used.
However, simple passwords and PINs are not effective enough to protect sensitive information because they can be guessed, intercepted, or compromised.
Preventing non-repudiation of messages is important in two situations.
If the sender of an electronic document can later deny his involvement, the recipient will potentially suffer a loss and the sender will benefit.
Similarly, failure to acknowledge receipt of a document opens the door to disputes about that receipt (and/or the time of delivery).
Access control aims to ensure that access to system elements is restricted to authorized users, and protects the hardware, communications environment, application software, and data.
However, access control does not protect against fraudulent actions by authorized persons.
The use of cryptography to protect message content has a centuries-old history.
Until recently, all cryptosystems required that communication partners use a single secret key to encrypt and decrypt messages.
Of these so-called symmetric cryptosystems, the most well-known are systems using the DES data encryption standard (USA) and the more cryptographically secure international IDEA data encryption algorithm.
However, these systems become impractical when there are a large number of network users due to the complexity of distributing encryption keys.
Thus, with 500 users, 125,000 different secret keys would be required for secure communication, each with each other.
If many users use the same secret encryption key, there is a risk of its disclosure or forgery.
Public key cryptosystems mainly use the RSA algorithm.
They solve the problems of key distribution and are free from the shortcomings inherent in symmetric cryptosystems.
In the near future, public key cryptosystems will become the basis for electronic data interchange (EDI) systems and e-mail in the government and commercial sectors.
In the future, they will become an integral part of all electronic media.
These systems make it possible to establish secure communications between network subscribers without prior exchange of secret information and to ensure reliable authentication using digital signatures.
The RSA cryptosystem has been subjected to over 15 years of scrutiny and analysis, and has stood the test.
Other public-key systems have been proposed, but most have proven to be less secure than the RSA system.
The RSA cryptographic algorithm has been adopted by a number of standards committees around the world. Where it has not yet become an official standard, it is a de facto standard.
Each public key of the RSA cryptosystem has a user secret key that is matched to it. Secret keys are generated by the user's hardware (computer, smart card, or smart disk).
Public keys are published in a directory or are located in a reference file, and secret keys should be accessible only to users who own them. It is impossible to determine the (paired) secret key matched to a known public key.
The US government recently adopted the Skipjack cryptographic algorithm, implemented on the Clipper and Capstone cryptographic chips, as its standard.
This cryptographic algorithm, developed under the direction of the US National Security Agency (NSA), contains a post-access for law enforcement, which is an obstacle to international adoption of this standard.
Recently discovered weaknesses in the Skipjack cryptographic algorithm allow a skilled operator to scramble messages so that they cannot be decrypted by authorized law enforcement. These weaknesses, and serious criticism of the algorithm by computer experts, could significantly limit its use.
A digital signature, like a traditional handwritten signature, contains information about the sender of a message.
But a digital signature can also verify the integrity of a message and can irrefutably prove to a third party that only the sender of the message could have created the digital signature.
If the message needs to be kept secret, then the entire message, together with the attached digital signature, is encrypted.
The DES or IDEA cryptographic algorithm is used for this.
The keys of these algorithms are encrypted using the public key of the message recipient, who decrypts the key of the symmetric cryptographic algorithm, decrypts the text of the message using it, and then decrypts the digital signature contained in the message using the sender's public key.
Encryption using the DES or IDEA algorithm is much faster than using the public key algorithm.
If a message is intended for several users, it only needs to be encrypted once.
The keys of symmetric cryptographic algorithms are encrypted separately for each recipient using their public key.
All this significantly increases the efficiency of the process. A digital signature is a measure of protection against counterfeiting or loss of data, which cannot be provided by a regular handwritten signature. If an encrypted message is changed before it reaches the recipient, this will lead to a change in the digital signature, which will serve as a signal to the recipient about the change in the message.
The main standards related to digital signatures can be international, governmental, national or even industrial.
They provide a basic level of trust and enable the development of interoperable products.
For software developers and vendors, the recommendations of the International Organization for Standardization IS 9796 (digital signatures) and ISO X.500 are important if they are interested in selling their products on the global market.
The US National Institute of Standards and Technology (NIST) has approved the federal information processing standard FIPS186, which is a standard for the digital signature DDS (Digital Signature Standard). This standard may be a competitor to the digital signature algorithm in the RSA cryptosystem.
The DSS standard technology in combination with Capstone cryptochips may be of interest to companies and organizations that do business with, or intend to do business with, U.S. government agencies and institutions.
The American National Standards Institute (ANSI) Recommendations X9.30 and X9.31 define the processes for generating digital signatures using the DSS standard and the RSA algorithm, respectively. They are consistent with the ISO X.500 and IS9796 recommendations.
Before standards organizations took concrete steps to implement the DSS standard or the RSA algorithm, a number of influential organizations and developers working with RSA Data Security created a working standard to facilitate the development and production of interoperable hardware and software.
This standard, called PKCS (Public Key Cryptography Standard), has become a de facto industry standard and has been adopted by leading software vendors.
Public Key Certification
To verify digital signatures, the recipient of messages needs to know the public key of the sender and be sure that this key belongs to the sender.
To do this, public key cryptography systems must have some kind of authentication structure.
Public key certification is a process in which a trusted party uses its digital signature to verify some digital record that associates a particular user's name or identifier with a corresponding public key.
When there are a large number of users or correspondents with whom a particular user may exchange messages, it is inefficient to reliably verify the public keys of all correspondents.
However, public key cryptography contains ways to reduce such checks. These include the certification method, which allows the user to act as a trusted third party.
To do this, the user uses his secret key and digitally signs a small digital record (certificate).
Each such certificate contains the name of the authentication authority, the certified public key, the name of the owner of this key, the secret number assigned by the authentication authority to this owner, and the start and end dates of the certificate's validity period.
These certificates do not require secret storage, since their certification by a digital signature eliminates the possibility of forgery.
Secret Key Protection
The secrecy of digital signatures depends on maintaining the secrecy of users' secret keys.
Since secret keys contain tens and even hundreds of digits, they must be stored in electronic memory.
The best storage method is provided by Smart Disks. These disks look like a 3.5 floppy disk but contain microprocessors and a magnetic interface with the read/write heads of a floppy disk drive.
Smart Disks plug directly into the floppy disk port of a personal computer and therefore do not require additional hardware, unlike smart cards.
Each smart disk generates its own key pair (secret/public). The secret key is never output in clear text from the disk, which reduces the likelihood of compromising the secret key. The public key is freely transferred for its certification and distribution.
If smart disks are not accessible to the user, the secret key can be stored on his workstation.
In this case, the secret key is encrypted with a key generated from the password entered by the user.
Users usually do not choose strong passwords, so this method of storing the secret key is susceptible to cryptanalysis using a «dictionary».
The X.435 standard in email systems.
CCITT has put into effect a standard for the transmission (in electronic information interchange systems) of messages in the standard US X.12 format or the international EDIFACT format.
This X.435 standard defines the type of messages in EDI systems and the services used within the framework of the global X.400 MHS standard (message delivery or non-delivery reports, message storage capacity, message delivery priority, etc.).
The X.435 standard has attracted great interest from U.S. government agencies using EDI systems.
This is due to the inclusion of the X.400 and X435 standards in the Government Open System Interconnect Profile (GOSIP).
Many EDI software developers have either already included X.435 in their products or have announced their intention to do so.
The use of the X.435 standard allows for the cost of delivering EDI messages to be reduced by combining them with e-mail messages for delivery over any network that uses the X.400 standard.
E-mail is evolving from text messaging systems within individual organizations with relatively reliable security to EDI messaging systems, as well as audio and video information, over various networks to subscribers located in different parts of the world.
The security of these systems is becoming critical. Encryption and digital signatures now provide security for individual users of such systems, and organizations and government agencies can rely on the privacy of their important communications systems to be protected.
The introduction of public-key cryptography and the standardization of its applications will effectively solve many previously considered insoluble security and law enforcement problems.
This can be considered the transition to global secure messaging.
Defense Electronics.- 1994 .-26, №8.-P. 24-26.