RSA PKCS#1 compliant signature

Question

I am using PKCS#1 2.0 (OAEP) standard (signature with appendix), but there are some issues not clear to me.

1. What is the physical object that is beeing signed? I know it's hash function value and so on (I do know the algorithm), but is it calculated from the binary fform of the file, no matter what is the content?

2. What is the physical result of signing? A file containing the signed hash? Should this file be placed in a specified location? What is the format or extension of such thing?

3. If I have several files that I want to sign, should this operation be performed separately for each of them? Or should they be concatenated? Once again - what is the result of such operation (file?) ?

Answer 1

PKCS#1 is sometimes called 'raw RSA' and is a low-level cryptographic primitive: it doesn't work on files and doesn't produce files, it works on raw data: input is a number smaller than the public key and output is a number of the size of the public key (e.g. 1024 bit for RSA-1024).

If you want a signature file, you probably want to use PKCS#7/CMS format, as that's the most used signature format both for attached and detached signatures (even signatures in PDF files are usually PKCS#7 envelopes actually).

PS: I don't know much about OAEP, but from what I read it seems to be a padding scheme (something you do to data before the raw signature) so my argument should be still valid.