# Encryption of large files RSA

I know that asymmetric cipher shouldn't be used for encryption large files because it is slow. It is better to use symmetric cipher. But, what worse, I've read that it is unsecure to encrypt a large file with a public key. But, why?

• "I've read", where? – fkraiem Nov 22 '16 at 20:07
• Ok, maybe I missed some facts. So, is it unsecure? – user41405 Nov 22 '16 at 20:23
• @J.Doe, it depends on the specifics of how it is done. – mikeazo Nov 22 '16 at 20:27
• This seems very similar to an earlier question asked here. If the answers to that question (or to the ones linked from it) don't answer yours, you may want to edit your question to explain how it differs from the earlier one. – Ilmari Karonen Nov 22 '16 at 20:28

2. Safe RSA-based encryption schemes expand the size of the ciphertext. For example RSA-OAEP with SHA-256, when using 2048-bit RSA, uses 2048-bit ciphertext blocks (256 bytes) which convey at most $2048-2\cdot256-16=1520$ bits (190 bytes), loosing over 25% of the bandwidth. This can only be improved to some degree (for $n$-bit security, there must be at least $n$ bits lost per block).