- [Instructor] Knowledge-based attacks go
beyond the simplicity of brute-force attacks
and combine other information available to the attacker
with cryptanalytic techniques
to break the security of encrypted data.
The first knowledge-based attack
is the frequency analysis attack.
In this attack, the person trying to break the cipher
does statistical analysis of the cipher text
to try to detect patterns.
The analyst might make use of many
of the common characteristics of the English language
to help with this analysis.
You may know that the most common letters
in the English language are E, T, O, A, I, and N.
If you suspect that a simple substitution cipher was used
and see the letter X coming up repeatedly
in the cipher text,
there's a good chance that X was substituted for E.
There are also other lesser known rules
that can assist with frequency analysis.
For example, cryptanalysts can also use pairs of letters
that often appear together known as digraphs.
If they see the same two letters popping up repeatedly
in cipher text, they may then guess
that those two letters correspond
to a common English digraph,
such as T-H, H-E, I-N, or E-R.
There are many other rules like this
that can assist with frequency analysis.
Fortunately, you won't need to know how
to use these techniques on the CISSP exam.
You just need to know that frequency analysis studies
the patterns of letters in cipher text.
In some cases, the analyst may have access
to both the encrypted
and unencrypted versions of a message.
In these cases, this additional information
allows something called a known-plaintext attack
where the attacker uses this knowledge
to try to crack the decryption key for other messages.
Cryptanalysts can gain a further advantage
when they have the ability to encrypt a message
that they select using the algorithm and key.
In this type of attack, called a chosen-plaintext attack,
the attacker can study how the algorithm works
in greater detail and attempt to learn the key being used.