Crypto Engineering

Challenges

• What data to encrypt?
• Where to store keys?
• Trade-off between confidentiality and availability

Why encrypt files:

AGAINST THE THEVIES!

• File system or disk encryption is useful if the threat is compromise from outside the boundaries of the machine: physical theft, remote file system, backup media, etc.
• It is not useful for intra-machine threats - an enemy who can bypass access controls can steal the key or the plaintext.

What can encryption not do?

File type is not well-protected

• Size distribution of different file types are predictable.

Files of 5 MB or 6 MB are — on my disk — very likely to be NEFs; files under 5 MB or over 7 MB are never NEFs. Files of 1–3 MB are probably JPG; files 7 MB and larger are almost never JPG.