Do many small files encrypted with the same cipher and key pose a security vulnerability?

Question

I am curious, if I have many files, some as big as a few k, some as little as 2, 4, 8 bytes. Will there be a security hole? What if I have 10,000 files on a single disc all encrypted with the same algorithm (lets say SHA512 since I know SHA512CryptoServiceProvider exist)

Would that be a security vulnerability? Since so many files use the same cipher and key? Is is it bad that they are small also? The files may be in known formats like pdf, png, doc, jpg, etc. Does that affect anything since the first few bytes of those files are known?

Answer 1

I recall some danger involving files of less than roughly 30 or 40 bytes or so. This was also in the early days where DES was still considered unbreakable (it was until CPU power became sufficient). The danger had something to do with guessing a couple of messages completely from their length alone and thereby extracting the key.

Answer 2

My question was about files not how to encrypt. I dont understand. I thought everything that had CryptoServiceProvider at the end was a cipher

A hash algorithm like SHA is not a symmetric cipher. The main worry about cryptographic hash functions is someone purposefully generating collisions. Having said that, I do not believe you will need to worry about people generating collisions for your images.

Answer 3

If properly encrypted with a good cipher mode (not ECB) - with a random initialization vector (IV) - even with many small files using the same key, the encrypted files will be sufficiently different that no cryptanalysis is possible. Indeed, that is the definition of a sound cryptographic algorithm.

Answer 4

Like everyone said, SHA is not a cipher, its a HASH. This alone should be a good indication that you should NOT implement your own cipher.

Crypto is hard, even if you were an expert, I'd say still go with something public, all crypto implementations have had flaws, we consider them secure after many many years, because crypto is really hard, even expert make mistakes.

Also you said you want to encrypt different files with the same key ?, most cyphers encrypt blocks of equal sizes, some of this files, like pdf, the attacker might know say the first 64 bits. If he knows the plain text(the first 64 bits) if you cypher is deterministic, recovering the key is trivial.

My suggestion, stick with a well known cypher, with a public implementation, like AES, and with a good cypher mode, since your first blocks are likely to be equal, ECB suggested above will yield something probably not secure. CBC or CFB are likely your best bet.