Based on kernel version 3.19. Page generated on 2015-02-13 21:22 EST.
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