I'm trying to encrypt files like PDFs or executables, but I can't pad it.
I try to read the file in this method:
with('file', 'rb') as file_read:
line = file_read.read(n) --> n multple of 16
encrypt(line, key)
Then I pass the line to my encrypt function, but when I reach a line that isn't a multiple of 16, my program crashes.
How can I correctly pad a sequence of bytes?
This is a simplified (edited) version of a previous answer.
Assuming a file is read in a loop using line = f.read(N) (where N is the block size) until EOF.
1) Trivial zero padding; just add this after the read:
elen = len(line) % N
if elen:
line += bytes(N - elen)
Zero padding is the simplest, but has drawbacks.
2) PKCS#7 padding, N < 256; add this after the read and make sure the loop will be exited afterward:
if len(line) < N:
elen = 1 + (len(line) - 1) % N
line += bytes(elen for _ in range(elen))
Please note that reading from files differs from reading from network sockets. A buffer is needed to read blocks of fixed length from network.
read()s. Let's concentrate on the main question. Please be more specific regarding the padding - could you answer zaph's question what encrypt function are you using?Most encryption implementations support a padding option, usually PKCS#7 (née PKCS#5) that adds the padding on encryption and removed it on decryption.
Note: mcrypt does not support PKCS#7 padding, stay away from it.
encryptfunction?