Entropy calculations
Entropy is a measure of the randomness in a system. The more random the system, the less predictable it is and the higher its entropy.
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Entropy and cryptanalysis
Entropy is useful in a variety of different fields, including cryptography. A measure of the randomness in a system is a useful method of differentiating between strong encryption and weak or non-existent encryption.
One of the methods of determining if an encryption algorithm is effective is if the ciphertexts that it produces can be differentiated from a random binary string. A fully random binary string has maximal entropy, meaning that there is no information exposed.
This is desirable in an encryption algorithm because it means that the ciphertext leaks no information about the corresponding plaintext. Therefore, calculating the entropy of data can help to differentiate between the ciphertext created by a strong encryption algorithm or the use of potentially weak and broken encryption.
Calculating entropy
Entropy can be calculated in a number of different ways. In cryptography, the most commonly used type of entropy is Shannon entropy, which was created by Claude Shannon, the father of information theory.
Shannon entropy can be calculated based upon the observed probability that a particular event occurs. With cryptography, this is the number of occurrences of zeros and ones within the ciphertext. The more unusual the ciphertext, the lower the entropy and the more information that can be derived about the corresponding plaintext.
By looking for high-entropy data, it is possible to identify data encrypted by a strong encryption algorithm and if a particular ciphertext was created by a broken encryption algorithm. While it is possible to do this by hand, some tools, including radare2 and binwalk, offer built-in entropy calculators, which can help with identifying encrypted data within a particular file.
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Conclusion
Entropy calculations provide an easy shortcut for identifying encrypted data within a file. Encrypted data is high-entropy, making it easily identifiable from more ordered data, such as text or code.
On the other hand, poorly-encrypted data has lower entropy, providing a hint that a particular ciphertext may be breakable.
Sources
A Gentle Introduction to Information Entropy
In this Series
- Entropy calculations
- How does hashing work: Examples and video walkthrough
- How does encryption work? Examples and video walkthrough
- Planning for post-quantum cryptography: Impact, challenges and next steps
- Beginner’s guide to the basics of data encryption
- Structures of cryptography
- Role of digital signatures in asymmetric cryptography
- What is homomorphic encryption?
- Encryption and etcd: The key to securing Kubernetes
- Quantum cyberattacks: Preparing your organization for the unknown
- An Introduction to asymmetric vs symmetric cryptography
- Breaking misused stream ciphers
- Blockchain and asymmetric cryptography
- Security of the PKI ecosystem
- Elliptic curve cryptography
- Methods for attacking full disk encryption
- Introduction to Public Key Infrastructure (PKI)
- Introduction to the TLS/SSL cryptography protocol
- Introduction to Diffie-Hellman Key Exchange
- Introduction to the Rivest-Shamir-Adleman (RSA) encryption algorithm
- Introduction to full disk encryption
- 8 reasons you may not want to use VPNs
- Rivest Cipher 4 (RC4)
- Understanding stream ciphers in cryptography
- Cryptography Errors
- Understanding block ciphers in cryptography
- How to mitigate Credential Management Vulnerabilities
- How To Exploit Credential Management Vulnerabilities
- Poor Credential Management
- How Is Cryptography Used In Applications?
- Decrypting Downloaded Files
- Introduction to hash functions
- Introduction to Asymmetric Cryptography
- The Advanced Encryption Standard (AES)
- Fundamentals of symmetric and asymmetric cryptography
- Case Studies in Poor Password Management
- The ultimate guide to encryption key management
- Principles of cryptography
- Encryption vs Encoding
- Introduction to Cryptanalysis
- Secure Credential Management
- Introduction to Blockchain
- Blockchain Technology
- Virtual Private Networks (VPNs)
- Public Key Infrastructure: Architecture & Security
- Hash Functions
- Asymmetric Cryptography
- Fundamentals of Cryptography
- Symmetric Cryptography
- Introduction to cryptography
- Best tools to perform steganography [updated 2020]
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