In order to transfer information from Alice to

Bob and for Bob to learn new information through that transfer,

both Alice and Bob need to agree on how to do the information transfer.

This how to, can include coding,

which represents information by quotes or characters.

For example, which language to use when two people talk to each other.

And, the algorithm. For example,

the cipher which converts the plaintext to ciphertext.

And, the protocol, or the interaction routine between Alice and Bob.

Kerckhoff's principle states that Eve knows the system

that Alice and Bob use for information transfer including the coding scheme,

the algorithm, the protocol, and so on.

Kerckhoff's principle is also called Open Design or Shannon's Maxim,

which states that the enemy knows the system.

Cryptography requires information asymmetry

and there's a part of what Alice and Bob

know that is designed to be secret against the attacker Eve.

This is often called, the key,

and significant effort is expended to make the key secret against the attacker Eve.

Keys can be used in many ways to protect the communication.

In cryptography, it is typically used to map

the plaintext into different letters or characters,

so that they look like gibberish by the attacker.

Kerckhoff's principle states that the attacker knows the system except for the key

and this holds even when the rest of the system is

not necessarily advertised or explicitly stated.

In other words, the only disadvantage of the attacker is not

knowing the key which is intent to be secret.

For example, while the key itself,

what it is, is not known to the attacker,

the key length, or how it is used for information protection,

may be known to the attacker.

In Kerckhoff's principle, the values that are secret and require

protection against the attacker is

clearly defined when designing the cryptographic scheme.

And the rest of the system is believed to be compromisable by the attacker.

Therefore when designing a cryptographic scheme,

you have a clearly defined scope of trust or secrecy,

and you build on that for the security of the scheme.

History and real world incidents in cybersecurity support Kerckhoff's principle.

Attacker was persistent and intelligent enough.

For example, APT or advanced persistent threat,

do its homework to gather information about the system before attacking.

Reverse engineering techniques can be useful for the attacker

when there is not sufficient detail available on the Internet, for example.

Kerckhoff's Principle