Survival Guide - Encryption, Authentication

This is a survival guide covering the mind-numbing topics of Cryptography, Encryption, Authorization and Authentication. For the mathematically challenged (and who is not) the maths involved in cryptography are gruesome in the extreme and are covered, if at all, at the level of 'stuff happens'. The guide concentrates on system functionality and consists almost exclusively of descriptions and explantations rather than specific commands or implementations. Much of this stuff is background for SSL/TLS and X.509 certificates.

Contents:

• Teminology
• Cryptographic Overview
• Symmetric Encryption
• Asymmetric Encryption
• Digests and Hashes
• Message Authentication code (MAC)
• Digital Signatures

Terminology

A non-exhaustive list of terms used in security. Many of these terms are explained further in the text others are left dangling either to create a sense of tension in the reader or, more likely, because we have no idea what they mean.

Authentication	The process or procedures used to verify that data or information asserting that it originates from a source can only have come from that source. Techniques used include digital signatures, MACs.
Authorization	When a user has Authenticated themselves they are typically Authorized, based on their login credentials or account properties to access, or not access, certain systems resources such as files, applications or network resources. The term privilege is sometimes used synonymously for Authorization, thus a user may have enough privileges to access resource X but not resource Y. They are Authorized to access X but not Authorized to access Y.
cipher	A cipher a.k.a. an encryption algorithm. To cipher a.k.a. the process of encryption.
Clear Text	A.k.a. plain text. A block of data to which no encryption process has been applied or a block of data that results from a decrpytion or deciphering operation.
decipher	Apply an decrpytion algorithm to an encrypted block of text and, given the appropriate keys turn it into plain text. Most, but not all, ciphers use the same algorithm to encrypt and decrpyt.
Encryption	The process of transforming data using a cipher (or encrpytion algorithm). Techniques used may be symmetric or asymmetric.
Plain Text	A.k.a. clear text. A block of data to which no encription has been applied or a block of data that results from a de-crpytion or deciphering operation.

Cryptographic Overview

Cryptography accoding to Webster is "the enciphering and deciphering of messages in secret code or cipher; also : the computerized encoding and decoding of information".

It is the process of transforming (enciphering or encrypting) data (called clear or plain text) using some process (an cipher or encryption algorithm) into some gobbledegook that can only be transformed back (deciphered or de-crypted) to plain text if the recipient has some some secret knowledge such as a key or a set or keys.

Historically the ciphers or algorithms used formed the secret. For example shift every character one position left that we all used as kids when sending secret messages to our friends. The weakness here is that if the method or algorithm is discovered all communication sent using that algorithm (or cipher) can be broken. A new algorithm has to be created and propagated to all parties.

Modern crytography assumes that the bad guys will discover the cryptographic algorithm, indeed, for reason we will see later, the algorithms themselves are widely published. Instead the secret lies with a unique key or keys which are used by the algorithm to transform the data. If the key is exposed or compromised (discovered by a bad guy) then by simply replacing the key, but keeping the algorithm the same, we can recommence confidential communication. The bad guy has to start all over again to discover the key with no greater knowledge than before but with, hopefully, more careful key maintainancee procedures in place.

Cryptographic algorithms are not provably secure. Instead they are widely published and exposed to repeated attack by dedicated researchers and specialists (black hat testers) who love this kind of stuff. Only having resisted repeated and sustained attacks are the algorithms used operationally. Since research into the cyptographic algorithms is ongoing it can occasionally mean that apparently robust, mature algorithms need to be replaced when weaknesses are discovered. A recent example here relates to theoretical weaknesses being discovered in the MD5 digest algoritm. While it is possible to use a brute force attack to find a key, cryptographic systems use a concept known as computationally infeasable which simply means that it would take cost too much or take too long to mount such a brute force attack. Computationally infeasable is based on todays's technology and is therefore relative not absolute and does change over time.

Crytography may be used for three purposes:

1. Confidentiality: Only the parties to the communication can understand the messages or data sent between the parties.

2. Authentication: The data could only have come from a known source.

3. Data Integrity: The data received by one party was the data sent by the other party and was not manipulated or compromised.

One or more of the above may be provided by a single algorithm or may be provided by a combination of algorithms and methods.

First the basic techniques. Modern cryptographic techniques are either symmetric or asymmetric.

Symmetric Cryptography

Symmetric encryption algorithms, also called single-key, shared-secret, or even, confusingly, private-key systems, use a single key (or set of keys) to encrypt and decrypt the data. This single key - the shared secret - must be securely exchanged between the parties that will use it prior to the actual secure communication. The limitations of shared-secret systems are twofold. First, the key must be distributed securely in a process called key management, which itself is not trivial. Second, the method of securing the key once distributed lies with all the parties to the communication: "I trust myself but do I trust all the other parties to keep the key secret?". If shared-secret is compromised at any of the parties then it is compromised for all parties that use it. Symmetric algorithms use significantly less computational resources than their asymmetric counterparts.

Examples of common symmetric key algorithms are DES, AES, IDEA, and RC4, and typical key sizes are 64, 128, or 192 bits. Figure 1 shows the operational use of a shared secret for classic confidential communications. Note: The term shared secret, which describes a single key (or set of keys) used, or shared, by both ends of the communication should not be confused with secret sharing, which describes a process whereby the shared, or single, secret key is broken up into parts and shared between multiple persons to make it more secure.

Figure 1 - Symmetric Cryptography

Asymmetric Cryptography

Asymmetric encryption algorithms use a pair of keys and are generally referred to as public-key cryptographic systems or sometimes as nonsecret encryption (a slight oxymoron). In these systems, data (called plain-text in the jargon) that is encrypted with one key can only be decrypted with the paired key. Given one key, it is computationally infeasible to derive the paired key. The system works by making one key, called the public key, widely available, while maintaining the other key, surprisingly called the private key, a secret. This process has an interesting side effect. If a message is encrypted with a private key and can be decrypted with its paired public key, then only the owner of the private key could have done it. This property is used in digital signatures and is described later. Aysymmetric algorithms use significant computational resources in comparison with their symmetric counterparts and ytherefore are generally not used to encrypt streams of data.

The most widely used public-key encryption systems are RSA (after the inventors Rivest, Shamir, and Adelman) and epileptic curves. Typical key sizes for public-key systems are 512 bits, 1,024 bits, or higher. The public keys of a private/public key pair can be safely stored in a public service such as DNS, while the private key must be maintained securely in a private location. Figure 2 illustrates the use of public-key cryptography for classic confidential communications.

Figure 2 - Asymmetric Cryptography

Public-key systems have one significant limitation, in that they rely on knowing, or trusting, that the public key which will be used in communications with a person or organization really is the public key of the person or organization and has not been spoofed by a malicious third party.. The method by which this is usually accomplished is sometimes called a Public Key Infrastructure (PKI), in which a trusted third party securely manages, or attests to the authenticity of, public keys. If the third party is requested to provide the public key of X, they are trusted to provide the correct key. The third party is trusted to have satisfied themselves by some process - attestation, notarization, and so on-that X is the one and only, or globally unique, X. The most common method for making available public keys that have been verified by a third pary is to embed them in an X.509 certificate.

Message Digests (Hashes)

To provide data integrity, any message could be simply encrypted. Thus, only the possessor of the single key (in symmetric systems) or the public key (in asymmetric systems) could decrypt it. However, encryption systems use complex mathematical functions, and are therefore big users of CPU resources. To encrypt all messages may incur unacceptably high overheads. Fortunately, other techniques can be used to reduce this load. The most common is a lightweight procedure called a one-way hash, simply a hash, or more commonly a message digest. The hash or digest algorithm creates a unique and relatively small fixed-size block of data (irrespective of the original message length) that cannot be reversed. The messages being sent typically include both the plain text (unencrypted) and a digest of the message. The hash algorithm is applied to the received plain text and if the result matches the message digest, this means the received data was not altered. The message digest is in some senses similar in concept to a checksum but has significantly different mathematical properties.

The most common forms of message digest are MD5 and SHA-1 (part of the SHA family). Figure 3 shows the message digest in action.

Figure 3 - Message Digest or One-Way Hash

Message Authentication Code (MAC)

In the symmetric or shared-secret world, the process of authentication and data integrity uses what is called a Message Authentication Code (MAC). The MAC combines the message digest with the shared key. The key part authenticates the sender, and the hash (or digest) part ensures data integrity.

The most common forms of MACs are HMAC-MD5 and HMAC-SHA-1. Figure 4 shows how the MAC is used. Note: The MD5 hash algorithm, and by implication any algorithm that uses it, such as RSA-MD5, has been moved to a "not recommended" status in most IETF documents, due to some theoretical weaknesses published in early 2005. These weaknesses do not invalidate the use of the algorithm.

Figure 4 - Message Authentication Code (MAC)

Digital Signatures

In the asymmetric or public-key world, the process of authentication and data integrity uses what is called a digital signature. The message being sent is again hashed to create a message digest using, say, MD5 or SHA-1 to ensure data integrity. The resulting message digest is then encrypted using the private key of the sender. Both the plain-text message and the encrypted digest are sent to the other party. The receiver decrypts the message digest using the public key of the sender, applies the hash algorithm to the plain-text data, and if the results match, then both the authenticity of the sender and the integrity of the data are assured.

Typical key sizes for digital signature systems are 512 bits, 1,024 bits, or higher. The most common digital signature algorithms are RSA-MD5, RSA-SHA-1, and Digital Signature Architecture (DSA; a US Government standard). Figure 5 shows how the digital signature is used. Note: The MD5 hash algorithm, and by implication any algorithm that uses it, such as RSA-MD5, has been moved to a "not recommended" status in most IETF documents, due to some theoretical weaknesses published in early 2005. These weaknesses do not invalidate the use of the algorithm.

Figure 5 - Digital Signature

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