Secure File Storage Nathanael Paul CRyptography Applications Bistro March 25, 2004

Choosing an Encrypted File System (EFS) Require kernel patch? –root needed How much control is root given? –Swap space Key management Backups and recovery options Very few files need encryption or entire file system? Sharing options?

Multitude of solutions Linux Crypto API Windows EFS CFS –Early UNIX implementation SiRiUS Steganographic file systems –not ready for use ppdd –Encrypts root partition, and swap space?

CFS Early implementation by Matt Blaze –First free UNIX EFS –Client NFS server listening on localhost interface Key for each directory –Uses passphrases Implemented in user-level

Accessing files main() { … read(); … } VFS NFS Local FileSys Network

CFS (a.k.a. /crypt) VFS CFS Local FileSys … Mount points

CFS NFS Encrypt/ Decrypt VFS call (e.g., read()) call VFS again, but go to file on storage media

Accessing files main() { … read(); … } VFS CFS Encrypted Local FileSys …

CFS Advantages/Drawbacks Key for each directory –Usability? Implemented in user-level (slow) –Makes it simpler –RPC calls –But most EFSs are slow Not possible to have different files under different groups in same directory –IV is stored in group id field in inode

Stackable v-nodes: CryptFS

Linux CryptoAPI File system mounted on loopback device which is mounted on directory mount point Loopback device intercepts kernel commands

So why SiRiUS? Assumes file server untrusted –No change to file server Distinguishes read/write access –Sharing Only a few keys needed Like CFS, users run user-level daemon Good for sharing among small groups Timely revocation –Rollback attacks

main() { … read(); … } VFS SiRiUS Local FileSys Network

SiRiUS Overview Intercepts NFS requests –Process requests and send to NFS server Could mimic CFS –Process requests and send to VFS of local file system SiRiUS faster with NFS (compared to CFS), since requests go straight to NFS server and not through VFS to regular NFS client on machine

Files in SiRiUS Files stored in 2 parts –md-file: file metadata Access control –d-file: file itself Encrypted with unique symmetric File Encryption Key (FEK) Signed with a unique File Signature Key (FSK) –To read, user needs FEK –To write, user needs FSK

md-files Encrypted Key Block (Owner) Encrypted Key Block (User 1) Encrypted Key Block (User n) … FSK Metdata last modified timestamp filename Owner’s hash of metadata MSK used MEK

Encrypted Key Blocks Username (or keyID) FEK FSK public key Plaintext Encrypted with MEK of user read/write read Username (or keyID) FEK Plaintext Encrypted with MEK of user

Freshness Guarantees Prevent rollback attacks –Alice replaces new md-file with an older saved md-file mdf-file: metadata freshness file –One in each directory of user’s file system –Stamped with unique Master Signing Key (MSK) of user –Contains root of hash tree of all md-files in current directory and mdf-files in immediate subdirectories

Creating mdf-files 1.Apply SHA-1 hash on each md-file in current directory (verifying md-file signatures as you go) 2.Concatenate resulting hashes together with mdf- files of immediate subdirectories and apply SHA-1 hash to concatenation 3.Place final hash and directory name in mdf-file Note: Timestamp used before final hash of concatenated hashes on root mdf-file

Verifying a file Files are guaranteed up to timestamp on root mdf- file Verifying a file in root directory –Compute mdf-file hash and check timestamps Verifying a file not in the root directory –Apply first 2 steps of creation of mdf-file recursively up to root directory comparing each mdf-file in its subdirectories Requires checking many hashes –What happens if a file in a non-related subtree’s hash doesn’t match up?

File swapping attack Bob wants access to Alice’s /home/alice/secret.txt, but Bob only has read access to /home/alice/readme.txt Bob switches filenames with secret.txt and readme.txt Would work if filename not included in md-file Directory included in mdf-file to prevent directory swapping

Creating a file Generate random DSA signature FSK Generate random AES FEK Generate encrypted key block Owner’s hash of metadata Create md-file Encrypt file data –Use FEK –Apply SHA-1 to encrypted data and sign with private key of FSK. Append hash to data. Update root mdf-file

File Sharing, Reading, Writing Use IBE (or other PKI) –Will need public key of those that will have shared access to create their encrypted key blocks –Will need public key of owner to verify signature and freshness of md-file

User keys MSK, MEK –Can be used without SiRiUS Revocation –Read: change FEK, remove user’s key block, update other key blocks with new FEK, reencrypt and sign d-file, update md-file signature, update root mdf-file –Write: same as read except create new FSK, and sign with new key (write implies read access)

Performance (ms) TestFile SizeKernel NFSDumbFSSiRiUS File Creation File Deletion Sequential Read 8 kb Sequential Write 8 kb Sequential Read 1 Mb Sequential Write 1 Mb

Performance Caching and optimizations pay off on larger files If working on smaller files, it’s much slower Read/Write –Encrypt data (decrypt for read), verify 3 signatures (2 for file integrity, one for freshness), generate a signature (not for a read)

Conclusions Encrypted file systems throw normal performance out the window Read/write capabilities of SiRiUS are nice Single user with just a few critical files –Program to manually perform encryption is probably sufficient

How to really protect your data… Burn it at 3,000 degrees...