File-copy from v4.4.100
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tag checked out (commit 26d6298789e695c9f627ce49a7bbd2286405798a) on
git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
Please refer to that tree for all history prior to this point.
Change-Id: I8a9ee2aea93cd29c52c847d0ce33091a73ae6afe
diff --git a/Documentation/security/keys-ecryptfs.txt b/Documentation/security/keys-ecryptfs.txt
new file mode 100644
index 0000000..c3bbeba
--- /dev/null
+++ b/Documentation/security/keys-ecryptfs.txt
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+ Encrypted keys for the eCryptfs filesystem
+
+ECryptfs is a stacked filesystem which transparently encrypts and decrypts each
+file using a randomly generated File Encryption Key (FEK).
+
+Each FEK is in turn encrypted with a File Encryption Key Encryption Key (FEFEK)
+either in kernel space or in user space with a daemon called 'ecryptfsd'. In
+the former case the operation is performed directly by the kernel CryptoAPI
+using a key, the FEFEK, derived from a user prompted passphrase; in the latter
+the FEK is encrypted by 'ecryptfsd' with the help of external libraries in order
+to support other mechanisms like public key cryptography, PKCS#11 and TPM based
+operations.
+
+The data structure defined by eCryptfs to contain information required for the
+FEK decryption is called authentication token and, currently, can be stored in a
+kernel key of the 'user' type, inserted in the user's session specific keyring
+by the userspace utility 'mount.ecryptfs' shipped with the package
+'ecryptfs-utils'.
+
+The 'encrypted' key type has been extended with the introduction of the new
+format 'ecryptfs' in order to be used in conjunction with the eCryptfs
+filesystem. Encrypted keys of the newly introduced format store an
+authentication token in its payload with a FEFEK randomly generated by the
+kernel and protected by the parent master key.
+
+In order to avoid known-plaintext attacks, the datablob obtained through
+commands 'keyctl print' or 'keyctl pipe' does not contain the overall
+authentication token, which content is well known, but only the FEFEK in
+encrypted form.
+
+The eCryptfs filesystem may really benefit from using encrypted keys in that the
+required key can be securely generated by an Administrator and provided at boot
+time after the unsealing of a 'trusted' key in order to perform the mount in a
+controlled environment. Another advantage is that the key is not exposed to
+threats of malicious software, because it is available in clear form only at
+kernel level.
+
+Usage:
+ keyctl add encrypted name "new ecryptfs key-type:master-key-name keylen" ring
+ keyctl add encrypted name "load hex_blob" ring
+ keyctl update keyid "update key-type:master-key-name"
+
+name:= '<16 hexadecimal characters>'
+key-type:= 'trusted' | 'user'
+keylen:= 64
+
+
+Example of encrypted key usage with the eCryptfs filesystem:
+
+Create an encrypted key "1000100010001000" of length 64 bytes with format
+'ecryptfs' and save it using a previously loaded user key "test":
+
+ $ keyctl add encrypted 1000100010001000 "new ecryptfs user:test 64" @u
+ 19184530
+
+ $ keyctl print 19184530
+ ecryptfs user:test 64 490045d4bfe48c99f0d465fbbbb79e7500da954178e2de0697
+ dd85091f5450a0511219e9f7cd70dcd498038181466f78ac8d4c19504fcc72402bfc41c2
+ f253a41b7507ccaa4b2b03fff19a69d1cc0b16e71746473f023a95488b6edfd86f7fdd40
+ 9d292e4bacded1258880122dd553a661
+
+ $ keyctl pipe 19184530 > ecryptfs.blob
+
+Mount an eCryptfs filesystem using the created encrypted key "1000100010001000"
+into the '/secret' directory:
+
+ $ mount -i -t ecryptfs -oecryptfs_sig=1000100010001000,\
+ ecryptfs_cipher=aes,ecryptfs_key_bytes=32 /secret /secret