# Учебно-методическое пособие Для студентов, аспирантов Таганрог 2008

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## Cryptographic Algorithms and Protocols

Cryptography is the science of devising methods that allow information to be sent in a secure form in such a way that the only person able to retrieve this information is the intended recipient.

The basic principle is this: A message being sent is known as plaintext. The message is then coded using a cryptographic algorithm. This process is called encryption. An encrypted message is known as ciphertext, and is turned back into plaintext by the process of decryption.

It must be assumed that any eavesdropper has access to all communications between the sender and the recipient. A method of encryption is only secure if even with this complete access, the eavesdropper is still unable to recover the original plaintext from the ciphertext.

In the last few decades cryptographic algorithms, being mathematical by nature, have become sufficiently advanced that they can only be handled by computers. This in effect means that plaintext is binary in form, and can therefore be anything; a picture, a voice, an e-mail or even a video - it makes no difference, a string of binary can represent any of these.

Where possible, use cryptographic techniques to authenticate information and keep the information private (but don't assume that simple encryption automatically authenticates as well). Generally you'll need to use a suite of available tools to secure your application.

Cryptographic protocols and algorithms are difficult to get right, so do not create your own. Instead, where you can, use protocols and algorithms that are widely-used, heavily analyzed, and accepted as secure. When you must create anything, give the approach wide public review and make sure that professional security analysts examine it for problems. In particular, do not create your own encryption algorithms unless you are an expert in cryptology, know what you're doing, and plan to spend years in professional review of the algorithm. Creating encryption algorithms (that are any good) is a task for experts only.

A number of algorithms are patented; even if the owners permit ``free use'' at the moment, without a signed contract they can always change their minds later, putting you at extreme risk then. In general, avoid all patented algorithms - in most cases there's an unpatented approach that is at least as good or better technically, and by doing so you avoid a large number of legal problems.

Often, your software should provide a way to reject ``too small'' keys, and let the user set what ``too small'' is. For RSA keys, 512 bits is too small for use. There is increasing evidence that 1024 bits for RSA keys is not enough either; Bernstein has suggested techniques that simplify brute-forcing RSA, and other work based on it (such as Shamir and Tromer's "Factoring Large Numbers with the TWIRL device") now suggests that 1024 bit keys can be broken in a year by a \$10 Million device. You may want to make 2048 bits the minimum for RSA if you really want a secure system, and you should certainly do so if you plan to use those keys after 2015.

When you need a security protocol, try to use standard-conforming protocols such as IPSec, SSL (soon to be TLS), SSH, S/MIME, OpenPGP/GnuPG/PGP, and Kerberos. Each has advantages and disadvantages; many of them overlap somewhat in functionality, but each tends to be used in different areas:

• Internet Protocol Security (IPSec). IPSec provides encryption and/or authentication at the IP packet level. However, IPSec is often used in a way that only guarantees authenticity of two communicating hosts, not of the users. As a practical matter, IPSec usually requires low-level support from the operating system (which not all implement) and an additional keyring server that must be configured. Since IPSec can be used as a "tunnel" to secure packets belonging to multiple users and multiple hosts, it is especially useful for building a Virtual Private Network (VPN) and connecting a remote machine. As of this time, it is much less often used to secure communication from individual clients to servers. The new version of the Internet Protocol, IPv6, comes with IPSec ``built in,'' but IPSec also works with the more common IPv4 protocol. Note that if you use IPSec, don't use the encryption mode without the authentication, because the authentication also acts as integrity protection.

• Secure Socket Layer (SSL) / TLS. SSL/TLS works over TCP and tunnels other protocols using TCP, adding encryption, authentication of the server, and optional authentication of the client (but authenticating clients using SSL/TLS requires that clients have configured X.509 client certificates, something rarely done). SSL version 3 is widely used; TLS is a later adjustment to SSL that strengthens its security and improves its flexibility. Currently there is a slow transition going on from SSLv3 to TLS, aided because implementations can easily try to use TLS and then back off to SSLv3 without user intervention.

SSL/TLS is the primary method for protecting http (web) transactions. SSL is relatively easy to use in programs, because most library implementations allow programmers to use operations similar to the operations on standard sockets like SSL_connect(), SSL_write(), SSL_read(), etc. A widely used OSS/FS implementation of SSL (as well as other capabilities) is OpenSSL, available at http://www.openssl.org/.

• OpenPGP and S/MIME. There are two competing, essentially incompatible standards for securing email: OpenPGP and S/MIME. OpenPHP is based on the PGP application; an OSS/FS implementation is GNU Privacy Guard from http://www.gnupg.org/. Currently, their certificates are often not interchangeable.

• SSH. SSH is the primary method of securing ``remote terminals'' over an internet, and it also includes methods for tunelling X Windows sessions. However, it's been extended to support single sign-on and general secure tunelling for TCP streams, so it's often used for securing other data streams too (such as CVS accesses). The most popular implementation of SSH is OpenSSH http://www.openssh.com/, which is OSS/FS. Typical uses of SSH allows the client to authenticate that the server is truly the server, and then the user enters a password to authenticate the user (the password is encrypted and sent to the other system for verification). Current versions of SSH can store private keys, allowing users to not enter the password each time. To prevent man-in-the-middle attacks, SSH records keying information about servers it talks to; that means that typical use of SSH is vulnerable to a man-in-the-middle attack during the very first connection, but it can detect problems afterwards. In contrast, SSL generally uses a certificate authority, which eliminates the first connection problem but requires special setup (and payment!) to the certificate authority.

• Kerberos. Kerberos is a protocol for single sign-on and authenticating users against a central authentication and key distribution server. Kerberos works by giving authenticated users "tickets", granting them access to various services on the network. When clients then contact servers, the servers can verify the tickets. Kerberos is a primary method for securing and supporting authentication on a LAN, and for establishing shared secrets (thus, it needs to be used with other algorithms for the actual protection of communication). Note that to use Kerberos, both the client and server have to include code to use it.

Many of these protocols allow you to select a number of different algorithms, so you'll still need to pick reasonable defaults for algorithms (e.g., for encryption).

What can be a plaintext?

What should a specialist take into account when he wants to create his own or use some developed products?

Choose a security protocol and give its characteristic.

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