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     (4) DCP (Data Co-Processor: crypto accelerator of various i.MX SoCs)
46
47         Rooted to a one-time programmable key (OTP) that is generally burnt
48         in the on-chip fuses and is accessible to the DCP encryption engine only.
49         DCP provides two keys that can be used as root of trust: the OTP key
50         and the UNIQUE key. Default is to use the UNIQUE key, but selecting
51         the OTP key can be done via a module parameter (dcp_use_otp_key).
52
53  *  Execution isolation
54
55     (1) TPM
56
57         Fixed set of operations running in isolated execution environment.
58
59     (2) TEE
60
61         Customizable set of operations running in isolated execution
62         environment verified via Secure/Trusted boot process.
63
64     (3) CAAM
65
66         Fixed set of operations running in isolated execution environment.
67
68     (4) DCP
69
70         Fixed set of cryptographic operations running in isolated execution
71         environment. Only basic blob key encryption is executed there.
72         The actual key sealing/unsealing is done on main processor/kernel space.
73
74  * Optional binding to platform integrity state
75
76     (1) TPM
77
78         Keys can be optionally sealed to specified PCR (integrity measurement)
79         values, and only unsealed by the TPM, if PCRs and blob integrity
80         verifications match. A loaded Trusted Key can be updated with new
81         (future) PCR values, so keys are easily migrated to new PCR values,
82         such as when the kernel and initramfs are updated. The same key can
83         have many saved blobs under different PCR values, so multiple boots are
84         easily supported.
85
86     (2) TEE
87
88         Relies on Secure/Trusted boot process for platform integrity. It can
89         be extended with TEE based measured boot process.
90
91     (3) CAAM
92
93         Relies on the High Assurance Boot (HAB) mechanism of NXP SoCs
94         for platform integrity.
95
96     (4) DCP
97
98         Relies on Secure/Trusted boot process (called HAB by vendor) for
99         platform integrity.
100
101  *  Interfaces and APIs
102
103     (1) TPM
104
105         TPMs have well-documented, standardized interfaces and APIs.
106
107     (2) TEE
108
109         TEEs have well-documented, standardized client interface and APIs. For
110         more details refer to ``Documentation/driver-api/tee.rst``.
111
112     (3) CAAM
113
114         Interface is specific to silicon vendor.
115
116     (4) DCP
117
118         Vendor-specific API that is implemented as part of the DCP crypto driver in
119         ``drivers/crypto/mxs-dcp.c``.
120
121  *  Threat model
122
123     The strength and appropriateness of a particular trust source for a given
124     purpose must be assessed when using them to protect security-relevant data.
125
126
127Key Generation
128==============
129
130Trusted Keys
131------------
132
133New keys are created from random numbers. They are encrypted/decrypted using
134a child key in the storage key hierarchy. Encryption and decryption of the
135child key must be protected by a strong access control policy within the
136trust source. The random number generator in use differs according to the
137selected trust source:
138
139  *  TPM: hardware device based RNG
140
141     Keys are generated within the TPM. Strength of random numbers may vary
142     from one device manufacturer to another.
143
144  *  TEE: OP-TEE based on Arm TrustZone based RNG
145
146     RNG is customizable as per platform needs. It can either be direct output
147     from platform specific hardware RNG or a software based Fortuna CSPRNG
148     which can be seeded via multiple entropy sources.
149
150  *  CAAM: Kernel RNG
151
152     The normal kernel random number generator is used. To seed it from the
153     CAAM HWRNG, enable CRYPTO_DEV_FSL_CAAM_RNG_API and ensure the device
154     is probed.
155
156  *  DCP (Data Co-Processor: crypto accelerator of various i.MX SoCs)
157
158     The DCP hardware device itself does not provide a dedicated RNG interface,
159     so the kernel default RNG is used. SoCs with DCP like the i.MX6ULL do have
160     a dedicated hardware RNG that is independent from DCP which can be enabled
161     to back the kernel RNG.
162
163Users may override this by specifying ``trusted.rng=kernel`` on the kernel
164command-line to override the used RNG with the kernel's random number pool.
165
166Encrypted Keys
167--------------
168
169Encrypted keys do not depend on a trust source, and are faster, as they use AES
170for encryption/decryption. New keys are created either from kernel-generated
171random numbers or user-provided decrypted data, and are encrypted/decrypted
172using a specified ‘master’ key. The ‘master’ key can either be a trusted-key or
173user-key type. The main disadvantage of encrypted keys is that if they are not
174rooted in a trusted key, they are only as secure as the user key encrypting
175them. The master user key should therefore be loaded in as secure a way as
176possible, preferably early in boot.
177
178
179Usage
180=====
181
182Trusted Keys usage: TPM
183-----------------------
184
185TPM 1.2: By default, trusted keys are sealed under the SRK, which has the
186default authorization value (20 bytes of 0s).  This can be set at takeownership
187time with the TrouSerS utility: "tpm_takeownership -u -z".
188
189TPM 2.0: The user must first create a storage key and make it persistent, so the
190key is available after reboot. This can be done using the following commands.
191
192With the IBM TSS 2 stack::
193
194  #> tsscreateprimary -hi o -st
195  Handle 80000000
196  #> tssevictcontrol -hi o -ho 80000000 -hp 81000001
197
198Or with the Intel TSS 2 stack::
199
200  #> tpm2_createprimary --hierarchy o -G rsa2048 -c key.ctxt
201  [...]
202  #> tpm2_evictcontrol -c key.ctxt 0x81000001
203  persistentHandle: 0x81000001
204
205Usage::
206
207    keyctl add trusted name "new keylen [options]" ring
208    keyctl add trusted name "load hex_blob [pcrlock=pcrnum]" ring
209    keyctl update key "update [options]"
210    keyctl print keyid
211
212    options:
213       keyhandle=    ascii hex value of sealing key
214                       TPM 1.2: default 0x40000000 (SRK)
215                       TPM 2.0: no default; must be passed every time
216       keyauth=	     ascii hex auth for sealing key default 0x00...i
217                     (40 ascii zeros)
218       blobauth=     ascii hex auth for sealed data default 0x00...
219                     (40 ascii zeros)
220       pcrinfo=	     ascii hex of PCR_INFO or PCR_INFO_LONG (no default)
221       pcrlock=	     pcr number to be extended to "lock" blob
222       migratable=   0|1 indicating permission to reseal to new PCR values,
223                     default 1 (resealing allowed)
224       hash=         hash algorithm name as a string. For TPM 1.x the only
225                     allowed value is sha1. For TPM 2.x the allowed values
226                     are sha1, sha256, sha384, sha512 and sm3-256.
227       policydigest= digest for the authorization policy. must be calculated
228                     with the same hash algorithm as specified by the 'hash='
229                     option.
230       policyhandle= handle to an authorization policy session that defines the
231                     same policy and with the same hash algorithm as was used to
232                     seal the key.
233
234"keyctl print" returns an ascii hex copy of the sealed key, which is in standard
235TPM_STORED_DATA format.  The key length for new keys are always in bytes.
236Trusted Keys can be 32 - 128 bytes (256 - 1024 bits), the upper limit is to fit
237within the 2048 bit SRK (RSA) keylength, with all necessary structure/padding.
238
239Trusted Keys usage: TEE
240-----------------------
241
242Usage::
243
244    keyctl add trusted name "new keylen" ring
245    keyctl add trusted name "load hex_blob" ring
246    keyctl print keyid
247
248"keyctl print" returns an ASCII hex copy of the sealed key, which is in format
249specific to TEE device implementation.  The key length for new keys is always
250in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
251
252Trusted Keys usage: CAAM
253------------------------
254
255Usage::
256
257    keyctl add trusted name "new keylen" ring
258    keyctl add trusted name "load hex_blob" ring
259    keyctl print keyid
260
261"keyctl print" returns an ASCII hex copy of the sealed key, which is in a
262CAAM-specific format.  The key length for new keys is always in bytes.
263Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
264
265Trusted Keys usage: DCP
266-----------------------
267
268Usage::
269
270    keyctl add trusted name "new keylen" ring
271    keyctl add trusted name "load hex_blob" ring
272    keyctl print keyid
273
274"keyctl print" returns an ASCII hex copy of the sealed key, which is in format
275specific to this DCP key-blob implementation.  The key length for new keys is
276always in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
277
278Encrypted Keys usage
279--------------------
280
281The decrypted portion of encrypted keys can contain either a simple symmetric
282key or a more complex structure. The format of the more complex structure is
283application specific, which is identified by 'format'.
284
285Usage::
286
287    keyctl add encrypted name "new [format] key-type:master-key-name keylen"
288        ring
289    keyctl add encrypted name "new [format] key-type:master-key-name keylen
290        decrypted-data" ring
291    keyctl add encrypted name "load hex_blob" ring
292    keyctl update keyid "update key-type:master-key-name"
293
294Where::
295
296	format:= 'default | ecryptfs | enc32'
297	key-type:= 'trusted' | 'user'
298
299Examples of trusted and encrypted key usage
300-------------------------------------------
301
302Create and save a trusted key named "kmk" of length 32 bytes.
303
304Note: When using a TPM 2.0 with a persistent key with handle 0x81000001,
305append 'keyhandle=0x81000001' to statements between quotes, such as
306"new 32 keyhandle=0x81000001".
307
308::
309
310    $ keyctl add trusted kmk "new 32" @u
311    440502848
312
313    $ keyctl show
314    Session Keyring
315           -3 --alswrv    500   500  keyring: _ses
316     97833714 --alswrv    500    -1   \_ keyring: _uid.500
317    440502848 --alswrv    500   500       \_ trusted: kmk
318
319    $ keyctl print 440502848
320    0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915
321    3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b
322    27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722
323    a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec
324    d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d
325    dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0
326    f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
327    e4a8aea2b607ec96931e6f4d4fe563ba
328
329    $ keyctl pipe 440502848 > kmk.blob
330
331Load a trusted key from the saved blob::
332
333    $ keyctl add trusted kmk "load `cat kmk.blob`" @u
334    268728824
335
336    $ keyctl print 268728824
337    0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915
338    3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b
339    27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722
340    a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec
341    d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d
342    dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0
343    f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
344    e4a8aea2b607ec96931e6f4d4fe563ba
345
346Reseal (TPM specific) a trusted key under new PCR values::
347
348    $ keyctl update 268728824 "update pcrinfo=`cat pcr.blob`"
349    $ keyctl print 268728824
350    010100000000002c0002800093c35a09b70fff26e7a98ae786c641e678ec6ffb6b46d805
351    77c8a6377aed9d3219c6dfec4b23ffe3000001005d37d472ac8a44023fbb3d18583a4f73
352    d3a076c0858f6f1dcaa39ea0f119911ff03f5406df4f7f27f41da8d7194f45c9f4e00f2e
353    df449f266253aa3f52e55c53de147773e00f0f9aca86c64d94c95382265968c354c5eab4
354    9638c5ae99c89de1e0997242edfb0b501744e11ff9762dfd951cffd93227cc513384e7e6
355    e782c29435c7ec2edafaa2f4c1fe6e7a781b59549ff5296371b42133777dcc5b8b971610
356    94bc67ede19e43ddb9dc2baacad374a36feaf0314d700af0a65c164b7082401740e489c9
357    7ef6a24defe4846104209bf0c3eced7fa1a672ed5b125fc9d8cd88b476a658a4434644ef
358    df8ae9a178e9f83ba9f08d10fa47e4226b98b0702f06b3b8
359
360
361The initial consumer of trusted keys is EVM, which at boot time needs a high
362quality symmetric key for HMAC protection of file metadata. The use of a
363trusted key provides strong guarantees that the EVM key has not been
364compromised by a user level problem, and when sealed to a platform integrity
365state, protects against boot and offline attacks. Create and save an
366encrypted key "evm" using the above trusted key "kmk":
367
368option 1: omitting 'format'::
369
370    $ keyctl add encrypted evm "new trusted:kmk 32" @u
371    159771175
372
373option 2: explicitly defining 'format' as 'default'::
374
375    $ keyctl add encrypted evm "new default trusted:kmk 32" @u
376    159771175
377
378    $ keyctl print 159771175
379    default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3
380    82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0
381    24717c64 5972dcb82ab2dde83376d82b2e3c09ffc
382
383    $ keyctl pipe 159771175 > evm.blob
384
385Load an encrypted key "evm" from saved blob::
386
387    $ keyctl add encrypted evm "load `cat evm.blob`" @u
388    831684262
389
390    $ keyctl print 831684262
391    default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3
392    82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0
393    24717c64 5972dcb82ab2dde83376d82b2e3c09ffc
394
395Instantiate an encrypted key "evm" using user-provided decrypted data::
396
397    $ evmkey=$(dd if=/dev/urandom bs=1 count=32 | xxd -c32 -p)
398    $ keyctl add encrypted evm "new default user:kmk 32 $evmkey" @u
399    794890253
400
401    $ keyctl print 794890253
402    default user:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b382d
403    bbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0247
404    17c64 5972dcb82ab2dde83376d82b2e3c09ffc
405
406Other uses for trusted and encrypted keys, such as for disk and file encryption
407are anticipated.  In particular the new format 'ecryptfs' has been defined
408in order to use encrypted keys to mount an eCryptfs filesystem.  More details
409about the usage can be found in the file
410``Documentation/security/keys/ecryptfs.rst``.
411
412Another new format 'enc32' has been defined in order to support encrypted keys
413with payload size of 32 bytes. This will initially be used for nvdimm security
414but may expand to other usages that require 32 bytes payload.
415
416
417TPM 2.0 ASN.1 Key Format
418------------------------
419
420The TPM 2.0 ASN.1 key format is designed to be easily recognisable,
421even in binary form (fixing a problem we had with the TPM 1.2 ASN.1
422format) and to be extensible for additions like importable keys and
423policy::
424
425    TPMKey ::= SEQUENCE {
426        type		OBJECT IDENTIFIER
427        emptyAuth	[0] EXPLICIT BOOLEAN OPTIONAL
428        parent		INTEGER
429        pubkey		OCTET STRING
430        privkey		OCTET STRING
431    }
432
433type is what distinguishes the key even in binary form since the OID
434is provided by the TCG to be unique and thus forms a recognizable
435binary pattern at offset 3 in the key.  The OIDs currently made
436available are::
437
438    2.23.133.10.1.3 TPM Loadable key.  This is an asymmetric key (Usually
439                    RSA2048 or Elliptic Curve) which can be imported by a
440                    TPM2_Load() operation.
441
442    2.23.133.10.1.4 TPM Importable Key.  This is an asymmetric key (Usually
443                    RSA2048 or Elliptic Curve) which can be imported by a
444                    TPM2_Import() operation.
445
446    2.23.133.10.1.5 TPM Sealed Data.  This is a set of data (up to 128
447                    bytes) which is sealed by the TPM.  It usually
448                    represents a symmetric key and must be unsealed before
449                    use.
450
451The trusted key code only uses the TPM Sealed Data OID.
452
453emptyAuth is true if the key has well known authorization "".  If it
454is false or not present, the key requires an explicit authorization
455phrase.  This is used by most user space consumers to decide whether
456to prompt for a password.
457
458parent represents the parent key handle, either in the 0x81 MSO space,
459like 0x81000001 for the RSA primary storage key.  Userspace programmes
460also support specifying the primary handle in the 0x40 MSO space.  If
461this happens the Elliptic Curve variant of the primary key using the
462TCG defined template will be generated on the fly into a volatile
463object and used as the parent.  The current kernel code only supports
464the 0x81 MSO form.
465
466pubkey is the binary representation of TPM2B_PRIVATE excluding the
467initial TPM2B header, which can be reconstructed from the ASN.1 octet
468string length.
469
470privkey is the binary representation of TPM2B_PUBLIC excluding the
471initial TPM2B header which can be reconstructed from the ASN.1 octed
472string length.
473
474DCP Blob Format
475---------------
476
477.. kernel-doc:: security/keys/trusted-keys/trusted_dcp.c
478   :doc: dcp blob format
479
480.. kernel-doc:: security/keys/trusted-keys/trusted_dcp.c
481   :identifiers: struct dcp_blob_fmt
482