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For a well-designed algorithm, enciphering the same plaintext but with a different key should produce a totally different ciphertext. Similarly, decrypting ciphertext with the wrong key should produce random-looking gibberish. If the decryption key is lost, encrypted data should not in practice be recoverable — at least for high quality encryption algorithms and large enough key sizes.
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' law — "only secrecy of the key provides security", or "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. 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, social engineering or inspection of a Post-it note stuck to the side of a terminal) 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 algorithmA symmetric-key algorithm is an algorithm for cryptography that uses the same cryptographic key to encrypt and decrypt the message. Actually, it is sufficient for it to be easy to compute the decryption key from the encryption key and vice versa. Other tes. A newer class of "public key" cryptographic algorithms was discovered in the 1970sMillennia: 1st millennium 2nd millennium 3rd millennium Centuries: 19th century 20th century 21st century Decades: 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s 2010s 2020s Years: 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 Events and trends which use a pair of keys, one to encrypt and one to decrypt. These asymmetric key algorithmIn cryptography, an asymmetric key algorithm uses a pair of different, though related, cryptographic keys to encrypt and decrypt. The two keys are related mathematically; a message encrypted by the algorithm using one key can be decrypted by the same algos allows 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 difficult or impossible, 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.
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 length 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. Recently, a message encrypted using a 109-bit key elliptic curve algorithm was broken by brute force. 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 2004) been proven mathematically, so a theoretical breakthrough could make everything you've encrypted an open book. This is another reason to err on the side of choosing longer keys.
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