Key (cryptography)


Key (cryptography)

In cryptography, a key is a piece of information (a parameter) that determines the functional output of a cryptographic algorithm or cipher. Without a key, the algorithm would produce no useful result. In encryption, a key specifies the particular transformation of plaintext into ciphertext, or vice versa during decryption. Keys are also used in other cryptographic algorithms, such as digital signature schemes and message authentication codes.

Contents

Need for secrecy

In designing security systems, it is wise to assume that the details of the cryptographic algorithm are already available to the attacker. This principle is known as Kerckhoffs' principle — "only secrecy of the key provides security", or, reformulated as Shannon's maxim, "the enemy knows the system". The history of cryptography provides evidence that it can be difficult to keep the details of a widely-used algorithm secret (see security through obscurity). A key is often easier to protect (it's typically a small piece of information) than an encryption algorithm, and easier to change if compromised. Thus, the security of an encryption system in most cases relies on some key being kept secret.

Keeping keys secret is one of the most difficult problems in practical cryptography; see key management. An attacker who obtains the key (by, for example, theft, extortion, dumpster diving or social engineering) can recover the original message from the encrypted data.

Encryption algorithms which use the same key for both encryption and decryption are known as symmetric key algorithms. A newer class of "public key" cryptographic algorithms was invented in the 1970s which uses a pair of keys, one to encrypt and one to decrypt. These asymmetric key algorithms allow one key to be made public while retaining the private key in only one location. They are designed so that finding out the private key is extremely difficult, even if the corresponding public key is known. A user of public key technology can publish their public key, while keeping their private key secret, allowing anyone to send them an encrypted message.

Key sizes

For the one-time pad system the key must be at least as long as the message. In encryption systems that use a cipher algorithm, messages can be much longer than the key. The key must, however, be long enough so that an attacker cannot try all possible combinations.

A key length of 80 bits is generally considered the minimum for strong security with symmetric encryption algorithms. 128-bit keys are commonly used and considered very strong. See the key size article for a fuller discussion.

The keys used in public key cryptography have some mathematical structure. For example, public keys used in the RSA system are the product of two prime numbers. Thus public key systems require longer key lengths than symmetric systems for an equivalent level of security. 3072 bits is the suggested key length for systems based on factoring and integer discrete logarithms which aim to have security equivalent to a 128 bit symmetric cipher. Elliptic curve cryptography may allow smaller-size keys for equivalent security, but these algorithms have only been known for a relatively short time and current estimates of the difficulty of searching for their keys may not survive. As of 2004, a message encrypted using a 109-bit key elliptic curve algorithm had been broken by brute force. [1]The current rule of thumb is to use an ECC key twice as long as the symmetric key security level desired. Except for the random one-time pad, the security of these systems has not (as of 2008) been proven mathematically, so a theoretical breakthrough could make everything one has encrypted an open book. This is another reason to err on the side of choosing longer keys.

Key choice

To prevent a key from being guessed, keys need to be generated truly randomly and contain sufficient entropy. The problem of how to safely generate truly random keys is difficult, and has been addressed in many ways by various cryptographic systems. There is a RFC on generating randomness (RFC 1750, Randomness Recommendations for Security). Some operating systems include tools for "collecting" entropy from the timing of unpredictable operations such as disk drive head movements. For the production of small amounts of keying material, ordinary dice provide a good source of high quality randomness.

When a password (or passphrase) is used as an encryption key, well-designed cryptosystems first run it through a key derivation function which adds a salt and compresses or expands it to the key length desired, for example by compressing a long phrase into a 128-bit value suitable for use in a block cipher.

See also

References

  1. ^ The Internet Encyclopedia, by Hossein Bidgoli, John Wiley, 2004, ISBN 0-471-22201-1, p. 567 [1]

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Public-key cryptography — In an asymmetric key encryption scheme, anyone can encrypt messages using the public key, but only the holder of the paired private key can decrypt. Security depends on the secrecy of that private key …   Wikipedia

  • Transient-key cryptography — is a form of public key cryptography wherein keypairs are generated and assigned to brief intervals of time instead of to individuals or organizations. In a transient key system, private keys are used briefly and then destroyed, which is why it… …   Wikipedia

  • private key cryptography — UK US noun [uncountable] business, computing a way of keeping Internet messages secret in which a single key (=letter or number) changes the message into code and back again Thesaurus: codes and codificationhyponym communicating by email or text… …   Useful english dictionary

  • public key cryptography — kriptografija viešuoju raktu statusas T sritis informatika apibrėžtis Metodų ir standartų rinkinys, leidžiantis subjektui (žmogui, organizacijai, aparatinei arba programinei įrangai) elektroniškai paliudyti savo tapatumą arba pasirašyti ir… …   Enciklopedinis kompiuterijos žodynas

  • public-key cryptography — noun A form of cryptography which allows users to communicate securely without previously agreeing on a shared secret key (abbreviation PKC) Syn: asymmetric cryptography, public key encryption …   Wiktionary

  • public key cryptography — noun The subfield of cryptography in which all information used to encode a message is assumed to be publicly available …   Wiktionary

  • private key cryptography — UK / US noun [uncountable] business, computing a way of keeping Internet messages secret in which a single key (= letter or number) changes the message into code and back again …   English dictionary

  • public key cryptography — noun a method of encryption of electronic data sent from one person to another which relies on two authentication keys, one public and one private, the two keys being connected by an algorithm which makes the combination unique, neither key being …   Australian English dictionary

  • Криптография с открытым ключом/PUBLIC KEY CRYPTOGRAPHY — разработана Уайтфильдом Диффи (Whitfielf Diffi). Использует пару ключей, причем каждая пара обладает следующими свойствами: что либо зашифрованное одним из них может быть расшифровано с помощью другого; имея один ключ из пары, называемый открытым …   Толковый словарь по информационному обществу и новой экономике

  • Key disclosure law — Key disclosure laws, also known as mandatory key disclosure, is legislation that require individuals to surrender cryptographic keys to law enforcement. The purpose is to allow access to material for confiscation or digital forensics purposes and …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.