74 lines
3.1 KiB
ReStructuredText
74 lines
3.1 KiB
ReStructuredText
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==========================================
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Encrypted keys for the eCryptfs filesystem
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==========================================
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ECryptfs is a stacked filesystem which transparently encrypts and decrypts each
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file using a randomly generated File Encryption Key (FEK).
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Each FEK is in turn encrypted with a File Encryption Key Encryption Key (FEFEK)
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either in kernel space or in user space with a daemon called 'ecryptfsd'. In
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the former case the operation is performed directly by the kernel CryptoAPI
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using a key, the FEFEK, derived from a user prompted passphrase; in the latter
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the FEK is encrypted by 'ecryptfsd' with the help of external libraries in order
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to support other mechanisms like public key cryptography, PKCS#11 and TPM based
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operations.
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The data structure defined by eCryptfs to contain information required for the
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FEK decryption is called authentication token and, currently, can be stored in a
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kernel key of the 'user' type, inserted in the user's session specific keyring
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by the userspace utility 'mount.ecryptfs' shipped with the package
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'ecryptfs-utils'.
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The 'encrypted' key type has been extended with the introduction of the new
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format 'ecryptfs' in order to be used in conjunction with the eCryptfs
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filesystem. Encrypted keys of the newly introduced format store an
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authentication token in its payload with a FEFEK randomly generated by the
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kernel and protected by the parent master key.
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In order to avoid known-plaintext attacks, the datablob obtained through
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commands 'keyctl print' or 'keyctl pipe' does not contain the overall
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authentication token, which content is well known, but only the FEFEK in
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encrypted form.
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The eCryptfs filesystem may really benefit from using encrypted keys in that the
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required key can be securely generated by an Administrator and provided at boot
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time after the unsealing of a 'trusted' key in order to perform the mount in a
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controlled environment. Another advantage is that the key is not exposed to
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threats of malicious software, because it is available in clear form only at
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kernel level.
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Usage::
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keyctl add encrypted name "new ecryptfs key-type:master-key-name keylen" ring
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keyctl add encrypted name "load hex_blob" ring
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keyctl update keyid "update key-type:master-key-name"
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Where::
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name:= '<16 hexadecimal characters>'
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key-type:= 'trusted' | 'user'
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keylen:= 64
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Example of encrypted key usage with the eCryptfs filesystem:
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Create an encrypted key "1000100010001000" of length 64 bytes with format
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'ecryptfs' and save it using a previously loaded user key "test"::
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$ keyctl add encrypted 1000100010001000 "new ecryptfs user:test 64" @u
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19184530
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$ keyctl print 19184530
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ecryptfs user:test 64 490045d4bfe48c99f0d465fbbbb79e7500da954178e2de0697
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dd85091f5450a0511219e9f7cd70dcd498038181466f78ac8d4c19504fcc72402bfc41c2
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f253a41b7507ccaa4b2b03fff19a69d1cc0b16e71746473f023a95488b6edfd86f7fdd40
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9d292e4bacded1258880122dd553a661
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$ keyctl pipe 19184530 > ecryptfs.blob
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Mount an eCryptfs filesystem using the created encrypted key "1000100010001000"
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into the '/secret' directory::
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$ mount -i -t ecryptfs -oecryptfs_sig=1000100010001000,\
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ecryptfs_cipher=aes,ecryptfs_key_bytes=32 /secret /secret
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