Suitability of encryption algorithms for different content types

Question

Are there encryption algorithms that are "more suited for" (i.e. provide greater security when) encrypting certain types of content?

For example, if I have a PDF file that I need to encrypt, is the fact that the content to be encrypted is in PDF format (as opposed to plain ASCII text or MSWord or .xlsx) a factor in deciding which encryption algorithm to use?

Answer 1

The specific file format is irrelevant in encryption. What matters are factors like the medium on which the file is located, the mechanism of access, and to some extent, the size of the file (which applies more to the difference between a hard drive image and a small document file, not so much to the difference between a PDF and a DOCX). If the file is extremely large, ciphers with a small block size (such as Blowfish, IDEA, and CAST5) begin to become less secure. Encrypting any more than 4 GiB worth of data with these ciphers can be dangerous.

While the cipher itself may be quite strong, the mode of operation can make or break a cryptosystem, and these modes are very context-dependent:

• ECB - Essentially no mode. Each block is encrypted with the same key. Unless you are encrypting a single block, this mode is dangerous and can give rise to the ECB penguin.
• CBC - Provides little malleability protection, but is usually good for simple confidentiality. There are many attacks against CBC, but they only apply in specific contexts.
• CTR - Is highly sensitive to nonce reuse and malleability attacks, but is fast and is good for parallelization. This mode turns a block cipher into a stream cipher. CTR actually uses encryption for both encryption and decryption, so two CTR encryptions with the same key cancel out.
• GCM - Similar to CTR, but authenticated. This provides integrity on top of confidentiality at the expense of some extra space for each message. This is used in networks.
• XTS - A "tweakable narrow block mode" which is commonly used for block device encryption. It requires double the key size to function, but provides some minimal malleability resistance.

These are the common ones. Some less common ones are XEX, LRW, PCBC, CFB, OCB, OFB, CTS, EME, EME2, and plenty more. They are either much more limited in purpose (PCBC for example is like CBC, but designed such that a single error in encryption or decryption propagates and mangles all future output), or are patented or otherwise hard to use in a broad context legally (such as EME2 and OCB).

To answer your actual question, I would use CBC for that. If your threat model does not involve someone intentionally messing with the encrypted files, and only with preventing someone from viewing the contents, CBC is fine. It is the most common mode for file encryption.

Do not implement crypto on your own

The specific nuances are very hard to get right, and you can easily end up with a very broken scheme despite using well-established primitives. You should use an existing library or program to encrypt your files, like GnuPG. Even using 7zip's encryption feature would suffice.

Answer 2

No. All modern algorithms (eg. AES) will provide ciphertext indistinguishability, so -if everything is done properly- it doesn't matter the kind of content that you are storing.

Not specific to any encryption algorithm, as it would happen with all of them, but kinda related to the question:

• Knowing that it is a PDF (or MSWord, or Excel) would allow some known plaintext for someone attacking the encryption (eg. if I was to bruteforce the encrypted file, I could check if the decrypted content with the candidate key starts like PDFs do).

• Simply applying an "encryption algorithm" may not hide the length of your data. You may want to add some padding to the file (or at least use a block cipher mode).

• You must use an iv on the files.

• Remember that you must verify the contents, not only check if it decrypts to "something". On certain filetypes, a blind attack would be possible where the encrypted file could be modified in order to obtain a different output.