1 ==========================
2 Trusted and Encrypted Keys
3 ==========================
4
5 Trusted and Encrypted Keys are two new key types added to the existing kernel
6 key ring service. Both of these new types are variable length symmetric keys,
7 and in both cases all keys are created in the kernel, and user space sees,
8 stores, and loads only encrypted blobs. Trusted Keys require the availability
9 of a Trust Source for greater security, while Encrypted Keys can be used on any
10 system. All user level blobs, are displayed and loaded in hex ASCII for
11 convenience, and are integrity verified.
12
13
14 Trust Source
15 ============
16
17 A trust source provides the source of security for Trusted Keys. This
18 section lists currently supported trust sources, along with their security
19 considerations. Whether or not a trust source is sufficiently safe depends
20 on the strength and correctness of its implementation, as well as the threat
21 environment for a specific use case. Since the kernel doesn't know what the
22 environment is, and there is no metric of trust, it is dependent on the
23 consumer of the Trusted Keys to determine if the trust source is sufficiently
24 safe.
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
127 Key Generation
128 ==============
129
130 Trusted Keys
131 ------------
132
133 New keys are created from random numbers. They are encrypted/decrypted using
134 a child key in the storage key hierarchy. Encryption and decryption of the
135 child key must be protected by a strong access control policy within the
136 trust source. The random number generator in use differs according to the
137 selected 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
163 Users may override this by specifying ``trusted.rng=kernel`` on the kernel
164 command-line to override the used RNG with the kernel's random number pool.
165
166 Encrypted Keys
167 --------------
168
169 Encrypted keys do not depend on a trust source, and are faster, as they use AES
170 for encryption/decryption. New keys are created either from kernel-generated
171 random numbers or user-provided decrypted data, and are encrypted/decrypted
172 using a specified ‘master’ key. The ‘master’ key can either be a trusted-key or
173 user-key type. The main disadvantage of encrypted keys is that if they are not
174 rooted in a trusted key, they are only as secure as the user key encrypting
175 them. The master user key should therefore be loaded in as secure a way as
176 possible, preferably early in boot.
177
178
179 Usage
180 =====
181
182 Trusted Keys usage: TPM
183 -----------------------
184
185 TPM 1.2: By default, trusted keys are sealed under the SRK, which has the
186 default authorization value (20 bytes of 0s). This can be set at takeownership
187 time with the TrouSerS utility: "tpm_takeownership -u -z".
188
189 TPM 2.0: The user must first create a storage key and make it persistent, so the
190 key is available after reboot. This can be done using the following commands.
191
192 With the IBM TSS 2 stack::
193
194 #> tsscreateprimary -hi o -st
195 Handle 80000000
196 #> tssevictcontrol -hi o -ho 80000000 -hp 81000001
197
198 Or 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
205 Usage::
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
235 TPM_STORED_DATA format. The key length for new keys are always in bytes.
236 Trusted Keys can be 32 - 128 bytes (256 - 1024 bits), the upper limit is to fit
237 within the 2048 bit SRK (RSA) keylength, with all necessary structure/padding.
238
239 Trusted Keys usage: TEE
240 -----------------------
241
242 Usage::
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
249 specific to TEE device implementation. The key length for new keys is always
250 in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
251
252 Trusted Keys usage: CAAM
253 ------------------------
254
255 Usage::
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
262 CAAM-specific format. The key length for new keys is always in bytes.
263 Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
264
265 Trusted Keys usage: DCP
266 -----------------------
267
268 Usage::
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
275 specific to this DCP key-blob implementation. The key length for new keys is
276 always in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
277
278 Encrypted Keys usage
279 --------------------
280
281 The decrypted portion of encrypted keys can contain either a simple symmetric
282 key or a more complex structure. The format of the more complex structure is
283 application specific, which is identified by 'format'.
284
285 Usage::
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
294 Where::
295
296 format:= 'default | ecryptfs | enc32'
297 key-type:= 'trusted' | 'user'
298
299 Examples of trusted and encrypted key usage
300 -------------------------------------------
301
302 Create and save a trusted key named "kmk" of length 32 bytes.
303
304 Note: When using a TPM 2.0 with a persistent key with handle 0x81000001,
305 append '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
331 Load 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
346 Reseal (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
361 The initial consumer of trusted keys is EVM, which at boot time needs a high
362 quality symmetric key for HMAC protection of file metadata. The use of a
363 trusted key provides strong guarantees that the EVM key has not been
364 compromised by a user level problem, and when sealed to a platform integrity
365 state, protects against boot and offline attacks. Create and save an
366 encrypted key "evm" using the above trusted key "kmk":
367
368 option 1: omitting 'format'::
369
370 $ keyctl add encrypted evm "new trusted:kmk 32" @u
371 159771175
372
373 option 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
385 Load 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
395 Instantiate 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
406 Other uses for trusted and encrypted keys, such as for disk and file encryption
407 are anticipated. In particular the new format 'ecryptfs' has been defined
408 in order to use encrypted keys to mount an eCryptfs filesystem. More details
409 about the usage can be found in the file
410 ``Documentation/security/keys/ecryptfs.rst``.
411
412 Another new format 'enc32' has been defined in order to support encrypted keys
413 with payload size of 32 bytes. This will initially be used for nvdimm security
414 but may expand to other usages that require 32 bytes payload.
415
416
417 TPM 2.0 ASN.1 Key Format
418 ------------------------
419
420 The TPM 2.0 ASN.1 key format is designed to be easily recognisable,
421 even in binary form (fixing a problem we had with the TPM 1.2 ASN.1
422 format) and to be extensible for additions like importable keys and
423 policy::
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
433 type is what distinguishes the key even in binary form since the OID
434 is provided by the TCG to be unique and thus forms a recognizable
435 binary pattern at offset 3 in the key. The OIDs currently made
436 available 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
451 The trusted key code only uses the TPM Sealed Data OID.
452
453 emptyAuth is true if the key has well known authorization "". If it
454 is false or not present, the key requires an explicit authorization
455 phrase. This is used by most user space consumers to decide whether
456 to prompt for a password.
457
458 parent represents the parent key handle, either in the 0x81 MSO space,
459 like 0x81000001 for the RSA primary storage key. Userspace programmes
460 also support specifying the primary handle in the 0x40 MSO space. If
461 this happens the Elliptic Curve variant of the primary key using the
462 TCG defined template will be generated on the fly into a volatile
463 object and used as the parent. The current kernel code only supports
464 the 0x81 MSO form.
465
466 pubkey is the binary representation of TPM2B_PRIVATE excluding the
467 initial TPM2B header, which can be reconstructed from the ASN.1 octet
468 string length.
469
470 privkey is the binary representation of TPM2B_PUBLIC excluding the
471 initial TPM2B header which can be reconstructed from the ASN.1 octed
472 string length.
473
474 DCP 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
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