1. Intro
In this tutorial, we’ll discuss an essential aspect of secure data transfer and communication in the digital era. In addition, we’ll explore two commonly used techniques in cryptanalysis, namely Linear and Differential Cryptanalysis.
Finally, we’ll compare these techniques and analyse their strengths and weaknesses.
2. What Is Cryptography?
Cryptography is a technique that secures communication and data transmission in the digital world. It involves converting plain text into a secret code, which prevents unauthorised individuals from understanding the information. This ensures that the data stays confidential and protected, even if accessed by unauthorised parties.
Moreover, we’ve used cryptography for centuries to protect sensitive information, such as military secrets, financial transactions, and personal data. Indeed, as computers and digital communication became more common, cryptography evolved and became more complex.
3. What Is Cryptanalysis?
Cryptanalysis is the art of breaking or analysing codes to uncover hidden information. It finds vulnerabilities in encryption systems and decodes messages encoded using cryptographic techniques.
For centuries, cryptanalysis has played a crucial role in revealing hidden information in secret messages conveyed through basic codes. Cryptanalysis advanced with the emergence of computers & internet, evolving into a more sophisticated field with new techniques & tools to break complex encryption.
Furthermore, cryptanalysis is vital in information security, playing a critical role in attacking and defending against cyber threats.
4. Linear Cryptanalysis
Linear cryptanalysis finds linear approximations to predict input-output bit relationships in cryptographic functions and is used to analyse and break encryption.
Furthermore, linear cryptanalysis is a valuable tool in cryptology. It helps evaluate new encryption algorithms’ security and finding existing ones’ vulnerabilities.
4.1. How Does It Work?
Linear cryptanalysis examines the linear connection between the plaintext, ciphertext, and encryption key. Linear cryptanalysis is constructing linear equations using known plaintext and ciphertext combinations to obtain the encryption key.
In addition, to use linear cryptanalysis properly, we must build a strong correlation between the plaintext and the ciphertext. Moreover, this approach benefits block cyphers and can be applied to various cryptographic systems, such as DES and AES.
4.2. Advantages
Comparatively speaking to other cryptanalytic methods, linear cryptanalysis has the advantage of being reasonably simple to implement.
It requires minimal computational resources and can be performed with relatively small amounts of data. Likewise, we can also use it to target a wide variety of cryptographic systems and crack different block cyphers.
4.3. Limitations
Despite its many advantages, linear cryptanalysis has several limitations. For example, it’s less effective against cyphers that use a non-linear substitution box (S-box).
Nevertheless, it also requires a considerable amount of known plaintext-ciphertext pairs, which can be a limitation when trying to attack systems with limited data availability. Finally, it’s susceptible to noise and randomisation, significantly affecting the attack’s success rate.
4.4. Example
The attack on a small, two-round DES encryption is a simple example of linear cryptanalysis.
The plaintext and ciphertext pairs, as well as their linear approximations:
Plaintext
Ciphertext
Linear Approximation
0000 0000
1100 0011
1 
x0 x2
0000 1111
0011 0001
x0 x1
1111 0000
1010 1100
x3 x4
1111 1111
0111 1111
x1 x3 1
We can derive linear equations that approximate the key using known plaintext-ciphertext pairs. Moreover, we can solve these equations to obtain the key used for encryption.
5. Differential Cryptanalysis
Differential cryptanalysis is a technique used to break encryption systems by analysing the differences between pairs of plaintexts and ciphertext.
5.1. How Does It Work?
Differential cryptanalysis examines the differences between pairs of plaintext and ciphertext blocks to figure out how they differ. Finding pairs of plaintexts that only differ by one bit is the first step in the process. The second step involves examining the difference in the corresponding ciphertext blocks.
It must be remembered establishing a high correlation between the differences in plaintext and the differences in the ciphertext is essential for the technique to be successful. This method is particularly effective for attacking substitution-permutation networks-based cyphers like AES and DES.
5.2. Advantages
The ability to attack cyphers that use non-linear S-boxes is one of the key advantages of the differential method. It’s also relatively efficient and requires fewer plaintext-ciphertext pairs than other cryptanalytic techniques.
Additionally, differential cryptanalysis has shown to be successful in breaching a variety of block cyphers and successfully attacking a wide range of cryptographic systems.
5.3. Limitations
Despite its many advantages, the differential technique has several limitations. For instance, this method is less effective against cyphers with a small block size or a limited number of rounds.
Furthermore, this method requires many plaintext-ciphertext pairs with specific characteristics. This can be a limitation when trying to attack systems with limited data. Finally, it’s susceptible to noise and randomisation, significantly affecting the attack’s success rate.
5.4. Example
The attack on a tiny two-round AES cypher is a simple example of differential cryptanalysis.
The differences in plaintext, ciphertext blocks, and their corresponding probabilities:
Plaintext Difference
Ciphertext Difference
Probability
0000 0001
1111 0011
0.25
0000 0010
0011 1101
0.125
0000 0100
0101 1110
0.125
0000 1000
1001 1110
0.125
0001 0000
0010 1011
0.0625
0010 0000
0000 1111
0.0625
0100 0000
0110 1110
0.0625
1000 0000
1101 1000
0.0625
We may develop differential characteristics that can be used to find the encryption key by using the differences between plaintext and ciphertext blocks. We can then use this information to attack the cypher and obtain plaintext messages.
6. Comparison Between Linear and Differential Cryptanalysis
Let’s compare linear and differential techniques in terms of their strengths and weaknesses, applicability and suitability, and effectiveness.
6.1. Strengths and Weaknesses
Both linear and differential cryptanalysis have their strengths and weaknesses. Linear cryptanalysis offers several strengths that make it a valuable tool in cryptanalysis. This technique can be easily implemented and applied to various cryptographic algorithms.
Attackers can efficiently perform the technique on modern computers, making it a practical choice for breaking encryption systems. However, one of its main weaknesses is that it requires a large amount of data to be effective.
Limited data availability or noisy data can be a problem for linear cryptanalysis. Unlike linear, differential cryptanalysis is highly practical at breaking block cyphers, commonly used in modern encryption systems. It’s also very efficient and requires less data than linear cryptanalysis.
However, one of its main weaknesses is that it’s more complex to implement and requires more knowledge of the inner workings of the cypher.
6.2. Application and Suitability
The particular encryption system being used decides the suitability of each cryptanalysis technique. The linear method is more effective on symmetric cyphers that have many rounds. In contrast, the differential process is better suited for block cyphers with fewer rounds.
In addition, linear cryptanalysis is particularly useful for analysing encryption systems that have diffusion. This means that changes in the plaintext only affect a few ciphertext bits.
On the other hand, differential cryptanalysis is more effective for analysing encryption systems with diffusion. This means that changes in the plaintext affect many bits of the ciphertext.
6.3. Effectiveness
Linear and differential approaches are both effective in breaking encryption systems. However, the effectiveness of each approach depends on the specific system being analysed.
While the differential approach is generally considered more powerful than linear cryptanalysis, there may be situations where this isn’t the case.
The effectiveness of each technique also depends on the quality and amount of data available for analysis:
Plaintext Difference
Ciphertext Difference
Probability
0000 0001
1111 0011
0.25
0000 0010
0011 1101
0.125
0000 0100
0101 1110
0.125
0000 1000
1001 1110
0.125
0001 0000
0010 1011
0.0625
0010 0000
0000 1111
0.0625
0100 0000
0110 1110
0.0625
1000 0000
1101 1000
0.0625
7. Conclusion
In this article, we discussed the concepts of linear and differential cryptanalysis. These techniques are commonly used to break encrypted messages and reveal underlying information.
While both methods have their strengths and weaknesses, combining them can lead to a more comprehensive assessment of the strength of a cryptographic system. By carefully considering the pros and cons of each approach, readers can make informed decisions on improving the security and confidentiality of their sensitive information.