Can Knowing How an Encrypted File Changed Make it Vulnerable?
Introduction
In this article, we will explore the question of whether knowing how an encrypted file changed can make it vulnerable. We will discuss the fundamental properties of encryption schemes and examine the concept of semantic security. Additionally, we will address a specific scenario involving the use of PGP/GnuPG and provide insights on compression and disk encryption.
Understanding Encryption
The short answer is no, and this is a fundamental property of (non-broken) encryption schemes. Intuitively speaking, if you don’t know the decryption key, the only information you can get from a ciphertext is the length of the plaintext. The formal statement of this property is semantic security.
In your scenario, anyone can tell that test.txt is empty, since the ciphertext exposes the length of the plaintext. However, as soon as test.txt is non-empty, there is no way to tell its contents from the ciphertext, even if the attacker knows the ciphertext for related plaintexts. In fact, even if the attacker can choose ciphertexts and get any number of ciphertexts decrypted apart from test1.gpg itself (a chosen-ciphertext attack), they won’t be able to know anything about the plaintext of test1.gpg other than its length.
PGP/GnuPG Considerations
With PGP/GnuPG, you do need to be careful about compression. The encryption itself is secure, but the sequence compression+encryption is not, because the compressibility of the message, and therefore the length of what is encrypted, depends on the contents.
Disk Encryption
Most encryption schemes used in practice are semantically secure. However, there is one exception: disk encryption usually uses ciphers that are not semantically secure, for performance and storage space reasons. XTS is popular in disk encryption. This is generally okay because the typical threat model for disk encryption does not include chosen-ciphertext attacks, only chosen-plaintext attacks. Disk encryption is primarily intended to protect against theft of the storage media, and once the media is stolen, it doesn’t get used anymore. However, if an attacker manages to make copies of multiple versions of the ciphertext of a disk sector, they may be able to obtain partial information about its contents.
Conclusion
In conclusion, knowing how an encrypted file changed does not make it vulnerable. Encryption schemes, when properly implemented, provide semantic security, ensuring that the contents of a ciphertext remain confidential even if the attacker has knowledge of related plaintexts. However, it is important to be cautious when using compression in conjunction with encryption, as it can potentially leak information about the plaintext. Additionally, for disk encryption, the threat model and the chosen-ciphertext attack scenario may differ, which can impact the security of the encrypted data.
Follow-up Questions
Finally, the article poses two follow-up questions for further exploration: Does the vulnerability change depending on the type of edit made to the encrypted file, and what if many sample edits are available?