projects / deliverable / linux.git / blame
| Commit | Line | Data |
|---|---|---|
| 79a73d18 RS |
1 | Encrypted keys for the eCryptfs filesystem |
| 2 | ||
| 3 | ECryptfs is a stacked filesystem which transparently encrypts and decrypts each | |
| 4 | file using a randomly generated File Encryption Key (FEK). | |
| 5 | ||
| 6 | Each FEK is in turn encrypted with a File Encryption Key Encryption Key (FEFEK) | |
| 7 | either in kernel space or in user space with a daemon called 'ecryptfsd'. In | |
| 8 | the former case the operation is performed directly by the kernel CryptoAPI | |
| 9 | using a key, the FEFEK, derived from a user prompted passphrase; in the latter | |
| 10 | the FEK is encrypted by 'ecryptfsd' with the help of external libraries in order | |
| 11 | to support other mechanisms like public key cryptography, PKCS#11 and TPM based | |
| 12 | operations. | |
| 13 | ||
| 14 | The data structure defined by eCryptfs to contain information required for the | |
| 15 | FEK decryption is called authentication token and, currently, can be stored in a | |
| 16 | kernel key of the 'user' type, inserted in the user's session specific keyring | |
| 17 | by the userspace utility 'mount.ecryptfs' shipped with the package | |
| 18 | 'ecryptfs-utils'. | |
| 19 | ||
| 20 | The 'encrypted' key type has been extended with the introduction of the new | |
| 21 | format 'ecryptfs' in order to be used in conjunction with the eCryptfs | |
| 22 | filesystem. Encrypted keys of the newly introduced format store an | |
| 23 | authentication token in its payload with a FEFEK randomly generated by the | |
| 24 | kernel and protected by the parent master key. | |
| 25 | ||
| 26 | In order to avoid known-plaintext attacks, the datablob obtained through | |
| 27 | commands 'keyctl print' or 'keyctl pipe' does not contain the overall | |
| 28 | authentication token, which content is well known, but only the FEFEK in | |
| 29 | encrypted form. | |
| 30 | ||
| 31 | The eCryptfs filesystem may really benefit from using encrypted keys in that the | |
| 32 | required key can be securely generated by an Administrator and provided at boot | |
| 33 | time after the unsealing of a 'trusted' key in order to perform the mount in a | |
| 34 | controlled environment. Another advantage is that the key is not exposed to | |
| 35 | threats of malicious software, because it is available in clear form only at | |
| 36 | kernel level. | |
| 37 | ||
| 38 | Usage: | |
| 39 | keyctl add encrypted name "new ecryptfs key-type:master-key-name keylen" ring | |
| 40 | keyctl add encrypted name "load hex_blob" ring | |
| 41 | keyctl update keyid "update key-type:master-key-name" | |
| 42 | ||
| 43 | name:= '<16 hexadecimal characters>' | |
| 44 | key-type:= 'trusted' | 'user' | |
| 45 | keylen:= 64 | |
| 46 | ||
| 47 | ||
| 48 | Example of encrypted key usage with the eCryptfs filesystem: | |
| 49 | ||
| 50 | Create an encrypted key "1000100010001000" of length 64 bytes with format | |
| 51 | 'ecryptfs' and save it using a previously loaded user key "test": | |
| 52 | ||
| 53 | $ keyctl add encrypted 1000100010001000 "new ecryptfs user:test 64" @u | |
| 54 | 19184530 | |
| 55 | ||
| 56 | $ keyctl print 19184530 | |
| 57 | ecryptfs user:test 64 490045d4bfe48c99f0d465fbbbb79e7500da954178e2de0697 | |
| 58 | dd85091f5450a0511219e9f7cd70dcd498038181466f78ac8d4c19504fcc72402bfc41c2 | |
| 59 | f253a41b7507ccaa4b2b03fff19a69d1cc0b16e71746473f023a95488b6edfd86f7fdd40 | |
| 60 | 9d292e4bacded1258880122dd553a661 | |
| 61 | ||
| 62 | $ keyctl pipe 19184530 > ecryptfs.blob | |
| 63 | ||
| 64 | Mount an eCryptfs filesystem using the created encrypted key "1000100010001000" | |
| 65 | into the '/secret' directory: | |
| 66 | ||
| 67 | $ mount -i -t ecryptfs -oecryptfs_sig=1000100010001000,\ | |
| 68 | ecryptfs_cipher=aes,ecryptfs_key_bytes=32 /secret /secret |