We employ cryptography to achieve four main goals: confidentiality, integrity, authentication, and non-repudiation.
Confidentiality is the assurance that only the intended recipient can read the contents of the message. Integrity ensures that the message cannot be changed between the source and the destination. Authentication validates the identity of the sender of any information. Non-repudiation is closely tied to authentication and integrity, in that it establishes that the authenticated person is the only person that could have sent the message and that the message is, in fact, what the sender originally sent.
Confidentiality, integrity, and authentication can be implemented individually, though generally, confidentiality strongly implies integrity. Non-repudiation is the establishment of integrity and authentication together. This section will cover the following topics:
- Symmetric versus asymmetric
- Fundamental differences and encryption methods
- Transport encryption
- Non-repudiation
- Hashing
- Key escrow
- Steganography
- Digital signatures
- Use of proven technologies
- Elliptic curve and quantum cryptography
Symmetric versus Asymmetric
The two general types of encryption in use in computing today are symmetric (or secret key) encryption and asymmetric (or public key) encryption.
In symmetric encryption, the same key is used to both encode and decode a message. This key must be kept secret, so the encryption must be between two, and only two, parties.
In asymmetric encryption, there are two keys in use. One, the public key, is known to everybody. This key can be used to encode information, but it cannot decode information. Anyone can encode information using the public key to encode information, but only the holder of the second key can decode that information. The second key in asymmetric encryption is the private key. Each public key has a corresponding private key. The private key can decrypt data that has been encrypted with the public key. This allows any public user to send data that can only be read by one person, the holder of the private key. The private key must be kept secret.
Fundamental Differences and Encryption Methods
In symmetric encryption, there are two basic classes of ciphers, or encrypting methods. One is to take a block of data of predetermined size and apply the cipher to that data. This is known as a block cipher. The other method is to apply the encryption to the data one digit at a time, known as a stream cipher.
Stream ciphers tend to encrypt and decrypt more quickly, but they also tend to be more vulnerable to unauthorized decryption. For instance, the WEP vulnerability discussed in the wireless section in Chapter 3 is due to the reuse of initialization vectors in the implementation of the RC4 stream cipher.
Block ciphering, on the other hand, tends to be computationally expensive. With a stream cipher, the encrypting host just has to add or subtract a certain number of bits or bytes depending on the cipher position. With a block cipher, the mass of data of a specific size is put through a series of complex mathematical processes to output ciphertext that is the same size as the input, but not easily related to the input. As the output of a block cipher is not easily relatable to the input of a block cipher, it does not risk being broken by reuse.
Transport Encryption
Transport encryption is an application of cryptography to protect network communications. In transport encryption, the entirety of the content between two end points is encrypted for the duration of a connection. One common application of transport encryption is a VPN. The traffic between both points is encrypted before it is sent. When transport encryption is in use, the connection that is made is referred to as an encrypted connection.
Non-repudiation
As mentioned earlier, non-repudiation exists when a message is guaranteed to be from a certain source, and guaranteed to be unchanged from the message the sender originally sent. With a guarantee of non-repudiation, it is impossible for the original sender to deny being the true sender.
One method of guaranteeing the source and integrity of a file is through the use of a digital signature. This is the addition of a cryptographically generated file created with the private key of a certain user. This signature file can only be created by the owner of the private key, and the signature will only verify successfully against a completely unchanged file. This firmly establishes both authentication of the sender and integrity of the sent information.
Hashing
Hashing is the application of a one-way cryptographic function to a block of data to create a short string of text, or a hash, of a defined length. This hash will not be strictly unique, as the size of the hash will not be as large as all possible inputs. Though a collision (matching hash from differing inputs) is possible, it is highly unlikely.
Because any individual hash does not correspond to only one input, it is impossible to determine the original input used to create the hash. Because the chance of the hash of two different blocks of data corresponding by chance is so low, a hash can be used to verify the integrity of a file.
Hashing can provide a great deal of confidence that any data received has not been changed in transmission. For example, when a user wishes to download a file and verify its authenticity, the software company can post the file for download, along with a hash generated from the file. The user can download the file and use well-known processes to create a hash for the file. The user can then compare the hash he or she generated to the hash the software company posted. Even if the user acquired the file from a source other than the software company, the hash can assure the user that the file remains unchanged.
Hashing can also be used to store passwords securely and to reduce the risk of a password database being compromised. When authenticating a password against the database, the authentication service will make a hash of the submitted password and compare the password to the hash stored in the database.
The generation of a hash relies only on well-known processes, not any secret cryptographic information. While hashing verifies integrity, it does not authenticate the identity of the sender. MD5 and SHA-1 are common cryptographic hash functions, though MD5 is now considered insecure.
Key Escrow
In asymmetric encryption, what happens when the digital copy of a secret key is lost? If there is only one copy of the secret key, any files that have been encrypted with the public key become permanently irretrievable.
One method that can be employed to protect against the loss of data is the use of a key escrow service. The key escrow service must be a trusted third party. If an unauthorized user gains access to the private key, he or she will be able to read data encrypted with the public key.
For symmetric encryption, the same principles apply. Because symmetric encryption uses the same key to both encode and decode the information, access to the key is required to read any data protected by symmetric encryption. No matter which method is used to encrypt your data, access to the secret key will allow the reading of the data.
Retrieving a key from key escrow will grant access to encrypted data. It is vital that any process to retrieve a key from escrow meet strict security requirements, such as multifactor authentication and multiple person authorization.
Steganography
Steganography is a form of cryptography that does not necessarily indicate that there is a message at all. Information is often embedded in existing files, or injected into otherwise normal streams of data.
Steganography allows a user to hide a message in plain sight. Information can be hidden in images, sound files, or even imprinted on physical objects. Though steganography does not necessarily rely on encryption, it can allow communication without detection.
Digital Signatures
The addition of a digital signature to a file functions similarly to a hash of that file. A digital signature verifies the integrity of a file, and that nothing has been changed since the file was signed. A digital signature also includes an identity authorization component.
Unlike a simple hash function, digital signature creation requires a private key as an input. This creates a special digital signature that can be read, if accompanied by the signed file. The digital signature verification process requires comparing the contents of the digital signature file to the signed file. If the signature and file match, both the origin and the integrity of the file can be assured.
Use of Proven Technologies
Many specific technologies exist to implement the different types of cryptography discussed so far. For symmetric cryptography, we might use 3DES, AES, or Blowfish. For public key cryptography we might use PGP/GPG with RSA.
When implementing cryptography, it is important to use cryptography that has been open to public scrutiny and is still considered secure. New technologies may represent a theoretical improvement in security, but a proven technology with no known practical attacks is a safe bet for strong protection in at least the short term.
When commonly used technologies are found to be insecure, they should be phased out. WEP, SSH, and DES have well-known vulnerabilities that, though they are safe against casual prying eyes, will not prevent a determined or sophisticated attacker from gaining access to your data.
Elliptic Curve and Quantum Cryptography
Advances in cryptography are constantly being proposed, tested, and refined. Currently, asymmetric encryption takes a large amount of processing power. This is because current public key encryption performs a number of mathematical functions on the product of two large prime numbers.
Elliptic curve cryptography uses curves that are defined mathematically to generate key pairs. This process is less computationally expensive and may prove an effective encoding method.
Another innovation is the use of quantum cryptography. Quantum Key Distribution is the most commonly known method of using the properties of quantum mechanics to communicate securely. Eavesdropping on quantum communications introduces changes to the communication itself. Quantum Key Distribution relies on this property of quantum mechanics, rather than on the mathematical difficulty of factoring the products of large numbers, to ensure secure key distribution.
These methods provide theoretical advantages in cost of computation and communications security. Given time, they may well grow to become standards in our cryptographic toolkit.