Cryptanalysis is a process of finding weaknesses in cryptographic
algorithms and using these weaknesses to decipher the ciphertext without
knowing the secret key (instance deduction). Sometimes the weakness is not
in the cryptographic algorithm itself, but rather in how it is applied that
makes cryptanalysis successful. An attacker may have other goals as well,
such as:
1. Total Break - Finding the secret key
2. Gobal Deduction - Finding a functionally equivalent algorithm for
encryption and decryption that does not require knowledge of the secret
key.
3. Information Deduction - Gaining some information about plaintexts
or ciphertexts that was not previously known
4. Distinguishing Algorithm - The attacker has the ability to
distinguish the output of the encryption (ciphertext) from a random
permutation of bits
The goal of the attacker performing cryptanalysis will depend on the
specific needs of the attacker in a given attack context. In most cases, if
cryptanalysis is successful at all, an attacker will not be able to go past
being able to deduce some information about the plaintext (goal 3). However,
that may be sufficient for an attacker, depending on the context.
Attack Execution Flow
An attacker discovers a weakness in the
cryptographic algorithm or a weakness in how it was
applied to a particular chunk of plaintext.
An attacker leverages the discovered weakness to
decrypt, partially decrypt or infer some information
about the contents of the encrypted message. All of
that is done without knowing the secret key.
Attack Prerequisites
The target software utilizes some sort fo cryptographic algorithm.
An underlying weaknesses exists either in the cryptographic algorithm used
or in the way that it was applied to a particular chunk of plaintext.
The encryption algorithm is known to the attacker.
An attacker has access to the ciphertext.
Typical Likelihood of Exploit
Likelihood: Very Low
Methods of Attack
Analysis
Brute Force
Examples-Instances
Description
A very easy to understand (but totally inapplicable to modern
cryptographic ciphers) example is a cryptanalysis technique called
frequency analysis that can be successfully applied to the very basic
classic encryption algorithms that performed monoalphabetic substitution
replacing each letter in the plaintext with its predetermined mapping
letter from the same alphabet. This was considered an improvement over a
more basic technique that would simply shift all of the letters of the
plaintext by some constant number of positions and replace the original
letters with the new letter with the resultant alphabet position. While
monoalphabetic substitution ciphers are resilient to blind brute force,
they can be broken easily with nothing more than a pen and paper.
Frequency analysis cryptanalysis uses the fact that natural language is
not random and monoalphabetic substitution does not hide the statistical
properties of the natural language. So if the letter "E" in an English
language occurs with a certain known frequency (about 12.7%), whatever
"E" was substituted with to get to the ciphertext, will occur with the
similar frequency. Having this frequency information allows the
cryptanalyst to quickly determine the substitutions and decipher the
ciphertext. Frequency analysis techniques are not applicable to modern
ciphers as they are all resilient to it (unless this is a very bad case
of a homegrown encryption algorithm). This example is just here to
illustrate a rudimentary example of cryptanalysis.
Attacker Skills or Knowledge Required
Skill or Knowledge Level: High
Cryptanalysis generally requires a very significant level of
understanding of mathematics and computation.
Resources Required
Computing resource requirements will vary based on the complexity of a given
cryptanalysis technique. Access to the encryption/decryption routines of the
algorithm is also required.
Solutions and Mitigations
Use proven cryptographic algorithms with recommended key sizes.
Ensure that the algorithms are used properly. That means:
1. Not rolling out your own crypto; Use proven algorithms and
implementations.
2. Choosing initialization vectors with sufficiently random
numbers
3. Generating key material using good sources of randomness and
avoiding known weak keys
4. Using proven protocols and their implementations.
5. Picking the most appropriate cryptographic algorithm for your usage
context and data
Description
