Security impact of signing small data using RSA I have an application which consists of a client and a server. Both have a pair of public and private key (RSA). For communication, both sign their messages and send them to the other part. So a message Looks like this:

_____________________
| HTTP Post request |
|___________________|
|     Post data     |
|___________________|
|     Signature     |
|___________________|


The post data is signed with the private key and the signature is added to the data. Sometimes the post data is very small (sometimes it just contains one number).

I know it can impact performance very much if I sign every little piece of data, but this time I'm not concerned too much about performance as the frequency of signing is very low.

Can it cause security problems if I sign very small amount of data such as just one number?
If you are using a secure signature algorithm, padding and all, then it must be secure for messages of any length. So in that sense you are good.

However, in many protocols your messages must include something to prevent replay attacks, like an incrementing counter, in which case you shouldn't be signing just a single number if the messages are meant to say anything at all.