Null cipher
Encyclopedia
A null cipher is an ancient form of encryption
where the plaintext
is mixed with a large amount of non-cipher material. It would today be regarded as a simple form of steganography
. Null ciphers can also be used to hide ciphertext, as part of a more complex system.
In classical cryptography a null is intended to confuse the cryptanalyst. Typically, a null will be a character which decrypts to obvious nonsense at the end of an otherwise intelligible phrase. In a null cipher, most of the characters may be nulls.
An example follows (Kipper 9):
Taking the first letter in each word successively yields the real message: "Newt is upset because he thinks he is President."
.
Examples of this are the "eNull", "aNull", "Null" and "aDH" cipher suites in OpenSSL and the "NULL Encryption Algorithm" in IPSec.
slow and expensive. Whenever a cypher needs to be sent to a human for
semantic processing, this substantially increases the cost of decryption.
A decoy cypher can take the form of noise - sending copious messages of
encrypted garbage plaintext. This decreases the signal-to-noise ratio
for humans trying to interpret decrypted "plaintext" messages.
A decoy cypher can also take the form of misleading information - for example,
in an onion cypher
, most of the layers may contain information that
when decrypted will produce a message that directly misleads the person
reading it - often resulting in them taking actions against their interest -
such as signalling that they are evesdropping by responding to a specific
false signal, false flag attacks, or causing them to suspect the wrong
parties. The actual message can still be contained at some level of
the onion - but preferably not the lowest level - which may include
an innocuous message so that if all layers are decrypted the core seems innocent.
(see noise decoy cypher).
Encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
where the plaintext
Plaintext
In cryptography, plaintext is information a sender wishes to transmit to a receiver. Cleartext is often used as a synonym. Before the computer era, plaintext most commonly meant message text in the language of the communicating parties....
is mixed with a large amount of non-cipher material. It would today be regarded as a simple form of steganography
Steganography
Steganography is the art and science of writing hidden messages in such a way that no one, apart from the sender and intended recipient, suspects the existence of the message, a form of security through obscurity...
. Null ciphers can also be used to hide ciphertext, as part of a more complex system.
In classical cryptography a null is intended to confuse the cryptanalyst. Typically, a null will be a character which decrypts to obvious nonsense at the end of an otherwise intelligible phrase. In a null cipher, most of the characters may be nulls.
An example follows (Kipper 9):
- News Eight Weather: Tonight increasing snow. Unexpected precipitation smothers eastern towns. Be extremely cautious and use snowtires especially heading east. The [highway is not] knowingly slippery. Highway evacuation is suspected. Police report emergency situations in downtown ending near Tuesday.
Taking the first letter in each word successively yields the real message: "Newt is upset because he thinks he is President."
Identity function encryption
In modern cryptology, null cipher (or NONE cipher) is also defined as choosing not to use encryption in a system where various encryption options are offered, such as for testing/debugging, or authentication-only communication. Thus the text is the same before and after encryption. In mathematics such a function is known as the identity functionIdentity function
In mathematics, an identity function, also called identity map or identity transformation, is a function that always returns the same value that was used as its argument...
.
Examples of this are the "eNull", "aNull", "Null" and "aDH" cipher suites in OpenSSL and the "NULL Encryption Algorithm" in IPSec.
Decoy Cypher
The weak link in decryption is the human in the loop. Human computation isslow and expensive. Whenever a cypher needs to be sent to a human for
semantic processing, this substantially increases the cost of decryption.
A decoy cypher can take the form of noise - sending copious messages of
encrypted garbage plaintext. This decreases the signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
for humans trying to interpret decrypted "plaintext" messages.
A decoy cypher can also take the form of misleading information - for example,
in an onion cypher
Onion routing
Onion routing is a technique for anonymous communication over a computer network. Messages are repeatedly encrypted and then sent through several network nodes called onion routers. Like someone unpeeling an onion, each onion router removes a layer of encryption to uncover routing instructions, and...
, most of the layers may contain information that
when decrypted will produce a message that directly misleads the person
reading it - often resulting in them taking actions against their interest -
such as signalling that they are evesdropping by responding to a specific
false signal, false flag attacks, or causing them to suspect the wrong
parties. The actual message can still be contained at some level of
the onion - but preferably not the lowest level - which may include
an innocuous message so that if all layers are decrypted the core seems innocent.
(see noise decoy cypher).