1==========================
2Trusted and Encrypted Keys
3==========================
4
5Trusted and Encrypted Keys are two new key types added to the existing kernel
6key ring service.  Both of these new types are variable length symmetric keys,
7and in both cases all keys are created in the kernel, and user space sees,
8stores, and loads only encrypted blobs.  Trusted Keys require the availability
9of a Trust Source for greater security, while Encrypted Keys can be used on any
10system. All user level blobs, are displayed and loaded in hex ASCII for
11convenience, and are integrity verified.
12
13
14Trust Source
15============
16
17A trust source provides the source of security for Trusted Keys.  This
18section lists currently supported trust sources, along with their security
19considerations.  Whether or not a trust source is sufficiently safe depends
20on the strength and correctness of its implementation, as well as the threat
21environment for a specific use case.  Since the kernel doesn't know what the
22environment is, and there is no metric of trust, it is dependent on the
23consumer of the Trusted Keys to determine if the trust source is sufficiently
24safe.
25
26  *  Root of trust for storage
27
28     (1) TPM (Trusted Platform Module: hardware device)
29
30         Rooted to Storage Root Key (SRK) which never leaves the TPM that
31         provides crypto operation to establish root of trust for storage.
32
33     (2) TEE (Trusted Execution Environment: OP-TEE based on Arm TrustZone)
34
35         Rooted to Hardware Unique Key (HUK) which is generally burnt in on-chip
36         fuses and is accessible to TEE only.
37
38     (3) CAAM (Cryptographic Acceleration and Assurance Module: IP on NXP SoCs)
39
40         When High Assurance Boot (HAB) is enabled and the CAAM is in secure
41         mode, trust is rooted to the OTPMK, a never-disclosed 256-bit key
42         randomly generated and fused into each SoC at manufacturing time.
43         Otherwise, a common fixed test key is used instead.
44
45  *  Execution isolation
46
47     (1) TPM
48
49         Fixed set of operations running in isolated execution environment.
50
51     (2) TEE
52
53         Customizable set of operations running in isolated execution
54         environment verified via Secure/Trusted boot process.
55
56     (3) CAAM
57
58         Fixed set of operations running in isolated execution environment.
59
60  * Optional binding to platform integrity state
61
62     (1) TPM
63
64         Keys can be optionally sealed to specified PCR (integrity measurement)
65         values, and only unsealed by the TPM, if PCRs and blob integrity
66         verifications match. A loaded Trusted Key can be updated with new
67         (future) PCR values, so keys are easily migrated to new PCR values,
68         such as when the kernel and initramfs are updated. The same key can
69         have many saved blobs under different PCR values, so multiple boots are
70         easily supported.
71
72     (2) TEE
73
74         Relies on Secure/Trusted boot process for platform integrity. It can
75         be extended with TEE based measured boot process.
76
77     (3) CAAM
78
79         Relies on the High Assurance Boot (HAB) mechanism of NXP SoCs
80         for platform integrity.
81
82  *  Interfaces and APIs
83
84     (1) TPM
85
86         TPMs have well-documented, standardized interfaces and APIs.
87
88     (2) TEE
89
90         TEEs have well-documented, standardized client interface and APIs. For
91         more details refer to ``Documentation/staging/tee.rst``.
92
93     (3) CAAM
94
95         Interface is specific to silicon vendor.
96
97  *  Threat model
98
99     The strength and appropriateness of a particular trust source for a given
100     purpose must be assessed when using them to protect security-relevant data.
101
102
103Key Generation
104==============
105
106Trusted Keys
107------------
108
109New keys are created from random numbers. They are encrypted/decrypted using
110a child key in the storage key hierarchy. Encryption and decryption of the
111child key must be protected by a strong access control policy within the
112trust source. The random number generator in use differs according to the
113selected trust source:
114
115  *  TPM: hardware device based RNG
116
117     Keys are generated within the TPM. Strength of random numbers may vary
118     from one device manufacturer to another.
119
120  *  TEE: OP-TEE based on Arm TrustZone based RNG
121
122     RNG is customizable as per platform needs. It can either be direct output
123     from platform specific hardware RNG or a software based Fortuna CSPRNG
124     which can be seeded via multiple entropy sources.
125
126  *  CAAM: Kernel RNG
127
128     The normal kernel random number generator is used. To seed it from the
129     CAAM HWRNG, enable CRYPTO_DEV_FSL_CAAM_RNG_API and ensure the device
130     is probed.
131
132Users may override this by specifying ``trusted.rng=kernel`` on the kernel
133command-line to override the used RNG with the kernel's random number pool.
134
135Encrypted Keys
136--------------
137
138Encrypted keys do not depend on a trust source, and are faster, as they use AES
139for encryption/decryption. New keys are created either from kernel-generated
140random numbers or user-provided decrypted data, and are encrypted/decrypted
141using a specified ‘master’ key. The ‘master’ key can either be a trusted-key or
142user-key type. The main disadvantage of encrypted keys is that if they are not
143rooted in a trusted key, they are only as secure as the user key encrypting
144them. The master user key should therefore be loaded in as secure a way as
145possible, preferably early in boot.
146
147
148Usage
149=====
150
151Trusted Keys usage: TPM
152-----------------------
153
154TPM 1.2: By default, trusted keys are sealed under the SRK, which has the
155default authorization value (20 bytes of 0s).  This can be set at takeownership
156time with the TrouSerS utility: "tpm_takeownership -u -z".
157
158TPM 2.0: The user must first create a storage key and make it persistent, so the
159key is available after reboot. This can be done using the following commands.
160
161With the IBM TSS 2 stack::
162
163  #> tsscreateprimary -hi o -st
164  Handle 80000000
165  #> tssevictcontrol -hi o -ho 80000000 -hp 81000001
166
167Or with the Intel TSS 2 stack::
168
169  #> tpm2_createprimary --hierarchy o -G rsa2048 -c key.ctxt
170  [...]
171  #> tpm2_evictcontrol -c key.ctxt 0x81000001
172  persistentHandle: 0x81000001
173
174Usage::
175
176    keyctl add trusted name "new keylen [options]" ring
177    keyctl add trusted name "load hex_blob [pcrlock=pcrnum]" ring
178    keyctl update key "update [options]"
179    keyctl print keyid
180
181    options:
182       keyhandle=    ascii hex value of sealing key
183                       TPM 1.2: default 0x40000000 (SRK)
184                       TPM 2.0: no default; must be passed every time
185       keyauth=	     ascii hex auth for sealing key default 0x00...i
186                     (40 ascii zeros)
187       blobauth=     ascii hex auth for sealed data default 0x00...
188                     (40 ascii zeros)
189       pcrinfo=	     ascii hex of PCR_INFO or PCR_INFO_LONG (no default)
190       pcrlock=	     pcr number to be extended to "lock" blob
191       migratable=   0|1 indicating permission to reseal to new PCR values,
192                     default 1 (resealing allowed)
193       hash=         hash algorithm name as a string. For TPM 1.x the only
194                     allowed value is sha1. For TPM 2.x the allowed values
195                     are sha1, sha256, sha384, sha512 and sm3-256.
196       policydigest= digest for the authorization policy. must be calculated
197                     with the same hash algorithm as specified by the 'hash='
198                     option.
199       policyhandle= handle to an authorization policy session that defines the
200                     same policy and with the same hash algorithm as was used to
201                     seal the key.
202
203"keyctl print" returns an ascii hex copy of the sealed key, which is in standard
204TPM_STORED_DATA format.  The key length for new keys are always in bytes.
205Trusted Keys can be 32 - 128 bytes (256 - 1024 bits), the upper limit is to fit
206within the 2048 bit SRK (RSA) keylength, with all necessary structure/padding.
207
208Trusted Keys usage: TEE
209-----------------------
210
211Usage::
212
213    keyctl add trusted name "new keylen" ring
214    keyctl add trusted name "load hex_blob" ring
215    keyctl print keyid
216
217"keyctl print" returns an ASCII hex copy of the sealed key, which is in format
218specific to TEE device implementation.  The key length for new keys is always
219in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
220
221Trusted Keys usage: CAAM
222------------------------
223
224Usage::
225
226    keyctl add trusted name "new keylen" ring
227    keyctl add trusted name "load hex_blob" ring
228    keyctl print keyid
229
230"keyctl print" returns an ASCII hex copy of the sealed key, which is in a
231CAAM-specific format.  The key length for new keys is always in bytes.
232Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
233
234Encrypted Keys usage
235--------------------
236
237The decrypted portion of encrypted keys can contain either a simple symmetric
238key or a more complex structure. The format of the more complex structure is
239application specific, which is identified by 'format'.
240
241Usage::
242
243    keyctl add encrypted name "new [format] key-type:master-key-name keylen"
244        ring
245    keyctl add encrypted name "new [format] key-type:master-key-name keylen
246        decrypted-data" ring
247    keyctl add encrypted name "load hex_blob" ring
248    keyctl update keyid "update key-type:master-key-name"
249
250Where::
251
252	format:= 'default | ecryptfs | enc32'
253	key-type:= 'trusted' | 'user'
254
255Examples of trusted and encrypted key usage
256-------------------------------------------
257
258Create and save a trusted key named "kmk" of length 32 bytes.
259
260Note: When using a TPM 2.0 with a persistent key with handle 0x81000001,
261append 'keyhandle=0x81000001' to statements between quotes, such as
262"new 32 keyhandle=0x81000001".
263
264::
265
266    $ keyctl add trusted kmk "new 32" @u
267    440502848
268
269    $ keyctl show
270    Session Keyring
271           -3 --alswrv    500   500  keyring: _ses
272     97833714 --alswrv    500    -1   \_ keyring: _uid.500
273    440502848 --alswrv    500   500       \_ trusted: kmk
274
275    $ keyctl print 440502848
276    0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915
277    3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b
278    27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722
279    a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec
280    d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d
281    dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0
282    f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
283    e4a8aea2b607ec96931e6f4d4fe563ba
284
285    $ keyctl pipe 440502848 > kmk.blob
286
287Load a trusted key from the saved blob::
288
289    $ keyctl add trusted kmk "load `cat kmk.blob`" @u
290    268728824
291
292    $ keyctl print 268728824
293    0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915
294    3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b
295    27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722
296    a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec
297    d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d
298    dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0
299    f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
300    e4a8aea2b607ec96931e6f4d4fe563ba
301
302Reseal (TPM specific) a trusted key under new PCR values::
303
304    $ keyctl update 268728824 "update pcrinfo=`cat pcr.blob`"
305    $ keyctl print 268728824
306    010100000000002c0002800093c35a09b70fff26e7a98ae786c641e678ec6ffb6b46d805
307    77c8a6377aed9d3219c6dfec4b23ffe3000001005d37d472ac8a44023fbb3d18583a4f73
308    d3a076c0858f6f1dcaa39ea0f119911ff03f5406df4f7f27f41da8d7194f45c9f4e00f2e
309    df449f266253aa3f52e55c53de147773e00f0f9aca86c64d94c95382265968c354c5eab4
310    9638c5ae99c89de1e0997242edfb0b501744e11ff9762dfd951cffd93227cc513384e7e6
311    e782c29435c7ec2edafaa2f4c1fe6e7a781b59549ff5296371b42133777dcc5b8b971610
312    94bc67ede19e43ddb9dc2baacad374a36feaf0314d700af0a65c164b7082401740e489c9
313    7ef6a24defe4846104209bf0c3eced7fa1a672ed5b125fc9d8cd88b476a658a4434644ef
314    df8ae9a178e9f83ba9f08d10fa47e4226b98b0702f06b3b8
315
316
317The initial consumer of trusted keys is EVM, which at boot time needs a high
318quality symmetric key for HMAC protection of file metadata. The use of a
319trusted key provides strong guarantees that the EVM key has not been
320compromised by a user level problem, and when sealed to a platform integrity
321state, protects against boot and offline attacks. Create and save an
322encrypted key "evm" using the above trusted key "kmk":
323
324option 1: omitting 'format'::
325
326    $ keyctl add encrypted evm "new trusted:kmk 32" @u
327    159771175
328
329option 2: explicitly defining 'format' as 'default'::
330
331    $ keyctl add encrypted evm "new default trusted:kmk 32" @u
332    159771175
333
334    $ keyctl print 159771175
335    default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3
336    82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0
337    24717c64 5972dcb82ab2dde83376d82b2e3c09ffc
338
339    $ keyctl pipe 159771175 > evm.blob
340
341Load an encrypted key "evm" from saved blob::
342
343    $ keyctl add encrypted evm "load `cat evm.blob`" @u
344    831684262
345
346    $ keyctl print 831684262
347    default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3
348    82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0
349    24717c64 5972dcb82ab2dde83376d82b2e3c09ffc
350
351Instantiate an encrypted key "evm" using user-provided decrypted data::
352
353    $ keyctl add encrypted evm "new default user:kmk 32 `cat evm_decrypted_data.blob`" @u
354    794890253
355
356    $ keyctl print 794890253
357    default user:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b382d
358    bbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0247
359    17c64 5972dcb82ab2dde83376d82b2e3c09ffc
360
361Other uses for trusted and encrypted keys, such as for disk and file encryption
362are anticipated.  In particular the new format 'ecryptfs' has been defined
363in order to use encrypted keys to mount an eCryptfs filesystem.  More details
364about the usage can be found in the file
365``Documentation/security/keys/ecryptfs.rst``.
366
367Another new format 'enc32' has been defined in order to support encrypted keys
368with payload size of 32 bytes. This will initially be used for nvdimm security
369but may expand to other usages that require 32 bytes payload.
370
371
372TPM 2.0 ASN.1 Key Format
373------------------------
374
375The TPM 2.0 ASN.1 key format is designed to be easily recognisable,
376even in binary form (fixing a problem we had with the TPM 1.2 ASN.1
377format) and to be extensible for additions like importable keys and
378policy::
379
380    TPMKey ::= SEQUENCE {
381        type		OBJECT IDENTIFIER
382        emptyAuth	[0] EXPLICIT BOOLEAN OPTIONAL
383        parent		INTEGER
384        pubkey		OCTET STRING
385        privkey		OCTET STRING
386    }
387
388type is what distinguishes the key even in binary form since the OID
389is provided by the TCG to be unique and thus forms a recognizable
390binary pattern at offset 3 in the key.  The OIDs currently made
391available are::
392
393    2.23.133.10.1.3 TPM Loadable key.  This is an asymmetric key (Usually
394                    RSA2048 or Elliptic Curve) which can be imported by a
395                    TPM2_Load() operation.
396
397    2.23.133.10.1.4 TPM Importable Key.  This is an asymmetric key (Usually
398                    RSA2048 or Elliptic Curve) which can be imported by a
399                    TPM2_Import() operation.
400
401    2.23.133.10.1.5 TPM Sealed Data.  This is a set of data (up to 128
402                    bytes) which is sealed by the TPM.  It usually
403                    represents a symmetric key and must be unsealed before
404                    use.
405
406The trusted key code only uses the TPM Sealed Data OID.
407
408emptyAuth is true if the key has well known authorization "".  If it
409is false or not present, the key requires an explicit authorization
410phrase.  This is used by most user space consumers to decide whether
411to prompt for a password.
412
413parent represents the parent key handle, either in the 0x81 MSO space,
414like 0x81000001 for the RSA primary storage key.  Userspace programmes
415also support specifying the primary handle in the 0x40 MSO space.  If
416this happens the Elliptic Curve variant of the primary key using the
417TCG defined template will be generated on the fly into a volatile
418object and used as the parent.  The current kernel code only supports
419the 0x81 MSO form.
420
421pubkey is the binary representation of TPM2B_PRIVATE excluding the
422initial TPM2B header, which can be reconstructed from the ASN.1 octet
423string length.
424
425privkey is the binary representation of TPM2B_PUBLIC excluding the
426initial TPM2B header which can be reconstructed from the ASN.1 octed
427string length.
428