Digital Security

Abstract

The global network is an open network. Open means unlimited and available to everyone. It was this openness that was responsible for the Internet’s huge popularity in the last century—but it also has its price. There is no central control and this means that it is possible for unauthorized third parties to gain access to the communication sphere of other Internet users. In order to guarantee sufficient protection of privacy and confidentiality it is necessary to use certain cryptology technologies. These enable messages to be encrypted and help to ensure their integrity. Using cryptology methods it is possible to prove the identity of a communication partner and prevent swindlers from wreaking havoc on the Internet under the guise of someone else. After all, Internet communication partners don’t sit face to face or communicate in a way as to be identified physically but are normally far away from each other, often on the other side of the globe. The following chapter provides a brief outline of the methods of cryptography. With such methods, digitalized information as well as the digital communication itself, can be protected against multifarious dangers lurking in our new, net-based world. Examples are presented of the most important of these techniques.

“Certain is that nothing is certain. And not even that.”

—Hans Bötticher, called Ringelnatz,

(1883–1934)

Glossary

Asymmetric encryption (public key encryption)

In the cryptographic method referred to as “public key” each communication partner has a key pair consisting of a so-called public key and a secret, private key. The public key is made available to all participants with whom communication is desired. Participants who wish to communicate with the owner of the public key, encrypt their message with his public key. The owner of the public key can only decode a message encrypted this way with the help of his corresponding, and securely held, private key.

Authentication

Serves to prove the identity of a user or the integrity of a message. Certificates from a trusted body are used to verify identity in authentication. Digital signatures are created and sent with a message to verify its integrity.

Authorization

Access to certain sensitive information resources is often limited to a restricted group of people. When a user requests a secure information resource or its service it is necessary to verify whether he is authorized to use or gain access to the resource or not.

Certificate

Digital certificates are the electronic counterpart of a personal ID. They assign their owner a unique public key and therefore a digital signature, which can only be generated by way of the corresponding private key. Certificates must be issued and signed by a trustworthy third party called a Certificate Authority.

Certificate Authority (CA)

A certificate authority authenticates public keys from registered users based on certificates corresponding to Internet standard RFC 1422. In this way, the user’s identity is verified. The public key of the user is digitally signed along with the user’s name and control information from the CA and issued in this form as a certificate.

Cryptanalysis

Unlike cryptography, in cryptanalysis an attempt is made to break cryptographic procedures, i.e., to determine the contents of the encrypted message without knowing the key. Together cryptanalysis and cryptography are referred to as cryptology.

Cryptography

A branch of computer science and mathematics that involves the design and evaluation of encryption procedures. The aim of cryptography is to protect confidential information from being accessed by unauthorized third parties.

Cryptography procedure

A procedure for the encryption and decryption of data. A distinction is made between weak and strong cryptography procedures, depending on the effort that must be exerted for unauthorized decryption. This effort increases dramatically as the key length grows. Cryptography procedures can be symmetric or asymmetric. Symmetric encryption is based on a secret key used for both encryption and decryption. On the other hand, with asymmetric encryption each participant holds two keys, one is public and other is private and known only to the key holder.

Data integrity

While cryptography cannot prevent data or messages from being altered by unauthorized third parties during transmission, the changes can however be made recognizable through the use of so-called hash functions. These provide a digital fingerprint of the sent data.

Denial of Service (DoS)

This is an Internet attack intended to overload the victim’s system through targeted manipulation so that it can no longer conduct its normal communication tasks or even fails completely. The known weaknesses and errors of Internet services are often used to an advantage in such attacks. To hide the attacker’s identity, the attack is normally carried out from many different, alien computers where the attacker has previously planted his malware. Later at his discretion the actual attack is conducted in a coordinated manner (distributed denial of service). The system administrators of the affected computers are misled concerning the existence of the attack programs that are brought in illegally and stored hidden in the local file system.

Data Encryption Standard (DES)

Symmetric block encryption methods were introduced in 1977 and updated for commercial use in 1993. DES encodes blocks of 64 bits each with an equally long key (56 bits effectively). The DES procedure is composed of 19 rounds altogether, whereby the 16 inner rounds are controlled by the key. The DES procedure presents a 64 bit substitution encryption procedure. This code can be broken today by relatively simple means. A multiple application of DES is carried out with different keys to increase security, for example triple DES (3DES).

Diffie–Hellman procedure

The first publicly known asymmetric encryption procedure, developed in 1976 by W. Diffie, M. Hellman and R. Merkle. Quite similar to the RSA procedure, a mathematical function is used in Diffie-Hellman whose reversal—specifically the problem of the discrete logarithm—is virtually impossible to calculate with a reasonable effort.

Digital signature

Used to authenticate a document, it consists of the digital fingerprint of the document that has been encrypted with the private key of the originator.

DNS poisoning

A targeted, active attack on a DNS server in which the domain names and IP addresses managed there can be manipulated. In this manner, entire domains can be cut off from network traffic, for example, or a targeted re-routing of the data traffic forced.

Identification

The recognition process of a user or a message is called identification. Once identity is established, it can be verified with authentication. Certain authorizations are connected with an identity.

IP spoofing

The targeted manipulation of the IP header of an IP datagram. Most of the time the sender’s IP address is changed by the attacker so he can “spoof” a false identity or further prevent the transmitted datagram from being traced back to him. IP spoofing is the starting point for many further attacks.

Key

A message can be transmitted securely over an insecure medium when its content is concealed from unauthorized third parties by using an encryption procedure (cipher). The original message, the so-called plain text, is transformed into the encrypted message (ciphertext)by way of a transformation function. The transformation function used for encryption can be parameterized with a key. The size of the key space is a measure of the difficulty involved in an unauthorized reversal of the transformation function.

Man-in-the-middle attack

An attack at a secured connection between two communication partners, whereby the attacker interacts with both partners (man-in-the-middle attack). The communication is intercepted or forged without it being noticed.

MD5 (Message Digest 5)

A one-way hash function used in many cryptographic algorithms that generates a digital fingerprint of the input data. It was developed by Ron Rivest to succeed the MD4 hash function. MD5 uses a key length of 128 bits and is implemented in e.g., PGP and SMTP.

Message digest

A short digital fingerprint of a message that is generated by using a hash function on the message to be transmitted. If the message digest is encrypted with the sender’s private key via an asymmetric encryption procedure, one receives a digital signature of the original message. This can be used to help check the sender’s identity and the authenticity of the message.

Nonce (aka salt)

Randomly selected unique value that can be used in a secured communication to prevent so-called playback attacks. In such attacks an unauthorized third party attempts to exert influence by using a previously recorded and later duplicated message. A nonce value may be used only once in the communication.

One-way function

A simply calculated mathematical function whose inverse function is virtually impossible to calculate with a reasonable effort. The one-way function is used in cryptography as an encryption procedure or for the generation of so-called digital fingerprints, via the use of one-way hash functions. A one-way hash function is also referred to as a message authentication code (MAC), data authentication code (DAC), digital signature or message digest (MD).

Packet filter

Special software or dedicated hardware that filters the data traffic in a network or between an internal LAN and the global Internet. IP datagrams are analyzed and a determination made as to whether the datagram is forwarded or blocked depending on the given source or target address, packet type or other parameters. Packet filters are used as access control systems, e.g., in firewalls.

Packet sniffer

A network application or dedicated hardware with the task of monitoring data traffic on the network layer of a LAN. In a diffusion network in which all computers use a shared communication medium, a packet sniffer can read and analyze every single packet. The actual purpose of packet sniffers is to monitor and analyze the network data traffic, yet they can also be misused for the purpose of break-ins and spying.

Playback attack

Simple attack on a computing system. By monitoring data communication the encrypted passwords are recorded and then used in a later attack to break into the system being monitored.

Pretty Good Privacy (PGP)

A system for the secure handling of email traffic developed in 1991 by Phil Zimmermann. PGP is freely available for most hardware platforms and operating systems. It offers email encryption with symmetric encryption procedures (Triple DES, IDEA, CST), securing the symmetric key via an asymmetric encryption procedure (RSA) as well as securing the integrity of emails (MD5 message digest) and preserving the authenticity of the communication partner (digital signatures). PGP is the most widely used system for the safe transport of emails.

Privacy

Only the sender and the receiver are allowed access to the contents of a confidential message. If an unauthorized third party spies on communication (eavesdropping), confidentiality can no longer be guaranteed and results in a loss of privacy.

Public Key Infrastructure (PKI)

In the application of asymmetric public key encryption, each participant needs a key pair. It consists of a public key, which is available to everyone, and a secret (private key), which is only accessible to the holder. To prevent abuse, the assignment of the participant’s public key must be verified by a trusted third party—the certificate authority (CA)—by means of a certificate. The rules as to how this certificate is created (security policy) must be established in a way that is publicly accessible. A PKI includes all organizational and technical measures that are necessary for the secure use of an asymmetric encryption procedure for encryption or for digital signatures.

Request for Comments (RFC)

New Internet technologies that develop in the course of expert discussion are retained in so-called RFCs (Request for Comments). As part of the Internet standardization process a sequentially numbered group of documents has arisen from this, in which technologies, standards and other items pertaining to the Internet are documented and standardized.

RSA procedure

The most well known asymmetric encryption procedure, it is named after its developers Rivest, Shamir and Adleman. Just as Diffie-Hellman encryption, the RSA procedure works with two keys. One is accessible to everyone—the public key—and the other is a private key that must be kept secret. RSA is based on number theory and the prime factorization problem. Encryption with a reasonable effort is not possible without knowledge of the private key.

Secret key encryption

The oldest family of encryption procedures in which both the sender and the receiver have an identical, private key for the encryption and decryption of a message. A distinction is made between block ciphers, in which the message to be encrypted is broken down into blocks of a fixed length before encryption, and stream ciphers. In the latter case, the to-be-encrypted message is treated as a text stream for which a one-time key of identical length is generated. Encryption of the message is carried out character by character with this key. One problem with secret key encryption involves the exchange of the key, which must be kept secret from third parties.

Steganograph

A special type of encryption whereby the message to be encrypted is concealed inside another message with information irrelevant to the hidden message. The primary aim is to hide the fact that encrypted information is being transmitted.

Strong cryptography

Designation for encryption procedures with the highest level of security, for which no known practical methods for breaking the encryption exist. The necessary calculation for breaking a key typically depends on the length of the key used. Thus a method with a specific key length is referred to as a strong cryptographic procedure. This limit however shifts constantly with the ever-growing power of computing systems.

Transport Layer Security (TLS)

TSL is waiting in the wings to become the potential successor to SSL. This protocol, at the transport layer of the TCP/IP communication model, promises even more security in Internet communication. The TLS specification has been raised by the IETF to Internet standard RFC 2246.

Trapdoor function

This is a one-way function whose inverse can be calculated relatively easily with knowledge of additional—so-called trapdoor—information. Without knowing the trapdoor information this however proves virtually impossible.

User authentication

Proving a user’s identity is done via special authentication methods, e.g., a password mechanism or a biometric authentication procedure such as fingerprinting.