1 ==========================
2 Trusted and Encrypted Keys
3 ==========================
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.
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
26 * Root of trust for storage
28 (1) TPM (Trusted Platform Module: hardware device)
30 Rooted to Storage Root Key (SRK) which never leaves the TPM that
31 provides crypto operation to establish root of trust for storage.
33 (2) TEE (Trusted Execution Environment: OP-TEE based on Arm TrustZone)
35 Rooted to Hardware Unique Key (HUK) which is generally burnt in on-chip
36 fuses and is accessible to TEE only.
42 Fixed set of operations running in isolated execution environment.
46 Customizable set of operations running in isolated execution
47 environment verified via Secure/Trusted boot process.
49 * Optional binding to platform integrity state
53 Keys can be optionally sealed to specified PCR (integrity measurement)
54 values, and only unsealed by the TPM, if PCRs and blob integrity
55 verifications match. A loaded Trusted Key can be updated with new
56 (future) PCR values, so keys are easily migrated to new PCR values,
57 such as when the kernel and initramfs are updated. The same key can
58 have many saved blobs under different PCR values, so multiple boots are
63 Relies on Secure/Trusted boot process for platform integrity. It can
64 be extended with TEE based measured boot process.
70 TPMs have well-documented, standardized interfaces and APIs.
74 TEEs have well-documented, standardized client interface and APIs. For
75 more details refer to ``Documentation/staging/tee.rst``.
80 The strength and appropriateness of a particular TPM or TEE for a given
81 purpose must be assessed when using them to protect security-relevant data.
90 New keys are created from random numbers generated in the trust source. They
91 are encrypted/decrypted using a child key in the storage key hierarchy.
92 Encryption and decryption of the child key must be protected by a strong
93 access control policy within the trust source.
95 * TPM (hardware device) based RNG
97 Strength of random numbers may vary from one device manufacturer to
100 * TEE (OP-TEE based on Arm TrustZone) based RNG
102 RNG is customizable as per platform needs. It can either be direct output
103 from platform specific hardware RNG or a software based Fortuna CSPRNG
104 which can be seeded via multiple entropy sources.
109 Encrypted keys do not depend on a trust source, and are faster, as they use AES
110 for encryption/decryption. New keys are created from kernel-generated random
111 numbers, and are encrypted/decrypted using a specified ‘master’ key. The
112 ‘master’ key can either be a trusted-key or user-key type. The main disadvantage
113 of encrypted keys is that if they are not rooted in a trusted key, they are only
114 as secure as the user key encrypting them. The master user key should therefore
115 be loaded in as secure a way as possible, preferably early in boot.
121 Trusted Keys usage: TPM
122 -----------------------
124 TPM 1.2: By default, trusted keys are sealed under the SRK, which has the
125 default authorization value (20 bytes of 0s). This can be set at takeownership
126 time with the TrouSerS utility: "tpm_takeownership -u -z".
128 TPM 2.0: The user must first create a storage key and make it persistent, so the
129 key is available after reboot. This can be done using the following commands.
131 With the IBM TSS 2 stack::
133 #> tsscreateprimary -hi o -st
135 #> tssevictcontrol -hi o -ho 80000000 -hp 81000001
137 Or with the Intel TSS 2 stack::
139 #> tpm2_createprimary --hierarchy o -G rsa2048 -c key.ctxt
141 #> tpm2_evictcontrol -c key.ctxt 0x81000001
142 persistentHandle: 0x81000001
146 keyctl add trusted name "new keylen [options]" ring
147 keyctl add trusted name "load hex_blob [pcrlock=pcrnum]" ring
148 keyctl update key "update [options]"
152 keyhandle= ascii hex value of sealing key
153 TPM 1.2: default 0x40000000 (SRK)
154 TPM 2.0: no default; must be passed every time
155 keyauth= ascii hex auth for sealing key default 0x00...i
157 blobauth= ascii hex auth for sealed data default 0x00...
159 pcrinfo= ascii hex of PCR_INFO or PCR_INFO_LONG (no default)
160 pcrlock= pcr number to be extended to "lock" blob
161 migratable= 0|1 indicating permission to reseal to new PCR values,
162 default 1 (resealing allowed)
163 hash= hash algorithm name as a string. For TPM 1.x the only
164 allowed value is sha1. For TPM 2.x the allowed values
165 are sha1, sha256, sha384, sha512 and sm3-256.
166 policydigest= digest for the authorization policy. must be calculated
167 with the same hash algorithm as specified by the 'hash='
169 policyhandle= handle to an authorization policy session that defines the
170 same policy and with the same hash algorithm as was used to
173 "keyctl print" returns an ascii hex copy of the sealed key, which is in standard
174 TPM_STORED_DATA format. The key length for new keys are always in bytes.
175 Trusted Keys can be 32 - 128 bytes (256 - 1024 bits), the upper limit is to fit
176 within the 2048 bit SRK (RSA) keylength, with all necessary structure/padding.
178 Trusted Keys usage: TEE
179 -----------------------
183 keyctl add trusted name "new keylen" ring
184 keyctl add trusted name "load hex_blob" ring
187 "keyctl print" returns an ASCII hex copy of the sealed key, which is in format
188 specific to TEE device implementation. The key length for new keys is always
189 in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
194 The decrypted portion of encrypted keys can contain either a simple symmetric
195 key or a more complex structure. The format of the more complex structure is
196 application specific, which is identified by 'format'.
200 keyctl add encrypted name "new [format] key-type:master-key-name keylen"
202 keyctl add encrypted name "load hex_blob" ring
203 keyctl update keyid "update key-type:master-key-name"
207 format:= 'default | ecryptfs | enc32'
208 key-type:= 'trusted' | 'user'
210 Examples of trusted and encrypted key usage
211 -------------------------------------------
213 Create and save a trusted key named "kmk" of length 32 bytes.
215 Note: When using a TPM 2.0 with a persistent key with handle 0x81000001,
216 append 'keyhandle=0x81000001' to statements between quotes, such as
217 "new 32 keyhandle=0x81000001".
221 $ keyctl add trusted kmk "new 32" @u
226 -3 --alswrv 500 500 keyring: _ses
227 97833714 --alswrv 500 -1 \_ keyring: _uid.500
228 440502848 --alswrv 500 500 \_ trusted: kmk
230 $ keyctl print 440502848
231 0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915
232 3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b
233 27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722
234 a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec
235 d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d
236 dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0
237 f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
238 e4a8aea2b607ec96931e6f4d4fe563ba
240 $ keyctl pipe 440502848 > kmk.blob
242 Load a trusted key from the saved blob::
244 $ keyctl add trusted kmk "load `cat kmk.blob`" @u
247 $ keyctl print 268728824
248 0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915
249 3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b
250 27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722
251 a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec
252 d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d
253 dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0
254 f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
255 e4a8aea2b607ec96931e6f4d4fe563ba
257 Reseal (TPM specific) a trusted key under new PCR values::
259 $ keyctl update 268728824 "update pcrinfo=`cat pcr.blob`"
260 $ keyctl print 268728824
261 010100000000002c0002800093c35a09b70fff26e7a98ae786c641e678ec6ffb6b46d805
262 77c8a6377aed9d3219c6dfec4b23ffe3000001005d37d472ac8a44023fbb3d18583a4f73
263 d3a076c0858f6f1dcaa39ea0f119911ff03f5406df4f7f27f41da8d7194f45c9f4e00f2e
264 df449f266253aa3f52e55c53de147773e00f0f9aca86c64d94c95382265968c354c5eab4
265 9638c5ae99c89de1e0997242edfb0b501744e11ff9762dfd951cffd93227cc513384e7e6
266 e782c29435c7ec2edafaa2f4c1fe6e7a781b59549ff5296371b42133777dcc5b8b971610
267 94bc67ede19e43ddb9dc2baacad374a36feaf0314d700af0a65c164b7082401740e489c9
268 7ef6a24defe4846104209bf0c3eced7fa1a672ed5b125fc9d8cd88b476a658a4434644ef
269 df8ae9a178e9f83ba9f08d10fa47e4226b98b0702f06b3b8
272 The initial consumer of trusted keys is EVM, which at boot time needs a high
273 quality symmetric key for HMAC protection of file metadata. The use of a
274 trusted key provides strong guarantees that the EVM key has not been
275 compromised by a user level problem, and when sealed to a platform integrity
276 state, protects against boot and offline attacks. Create and save an
277 encrypted key "evm" using the above trusted key "kmk":
279 option 1: omitting 'format'::
281 $ keyctl add encrypted evm "new trusted:kmk 32" @u
284 option 2: explicitly defining 'format' as 'default'::
286 $ keyctl add encrypted evm "new default trusted:kmk 32" @u
289 $ keyctl print 159771175
290 default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3
291 82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0
292 24717c64 5972dcb82ab2dde83376d82b2e3c09ffc
294 $ keyctl pipe 159771175 > evm.blob
296 Load an encrypted key "evm" from saved blob::
298 $ keyctl add encrypted evm "load `cat evm.blob`" @u
301 $ keyctl print 831684262
302 default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3
303 82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0
304 24717c64 5972dcb82ab2dde83376d82b2e3c09ffc
306 Other uses for trusted and encrypted keys, such as for disk and file encryption
307 are anticipated. In particular the new format 'ecryptfs' has been defined
308 in order to use encrypted keys to mount an eCryptfs filesystem. More details
309 about the usage can be found in the file
310 ``Documentation/security/keys/ecryptfs.rst``.
312 Another new format 'enc32' has been defined in order to support encrypted keys
313 with payload size of 32 bytes. This will initially be used for nvdimm security
314 but may expand to other usages that require 32 bytes payload.
317 TPM 2.0 ASN.1 Key Format
318 ------------------------
320 The TPM 2.0 ASN.1 key format is designed to be easily recognisable,
321 even in binary form (fixing a problem we had with the TPM 1.2 ASN.1
322 format) and to be extensible for additions like importable keys and
325 TPMKey ::= SEQUENCE {
326 type OBJECT IDENTIFIER
327 emptyAuth [0] EXPLICIT BOOLEAN OPTIONAL
333 type is what distinguishes the key even in binary form since the OID
334 is provided by the TCG to be unique and thus forms a recognizable
335 binary pattern at offset 3 in the key. The OIDs currently made
338 2.23.133.10.1.3 TPM Loadable key. This is an asymmetric key (Usually
339 RSA2048 or Elliptic Curve) which can be imported by a
340 TPM2_Load() operation.
342 2.23.133.10.1.4 TPM Importable Key. This is an asymmetric key (Usually
343 RSA2048 or Elliptic Curve) which can be imported by a
344 TPM2_Import() operation.
346 2.23.133.10.1.5 TPM Sealed Data. This is a set of data (up to 128
347 bytes) which is sealed by the TPM. It usually
348 represents a symmetric key and must be unsealed before
351 The trusted key code only uses the TPM Sealed Data OID.
353 emptyAuth is true if the key has well known authorization "". If it
354 is false or not present, the key requires an explicit authorization
355 phrase. This is used by most user space consumers to decide whether
356 to prompt for a password.
358 parent represents the parent key handle, either in the 0x81 MSO space,
359 like 0x81000001 for the RSA primary storage key. Userspace programmes
360 also support specifying the primary handle in the 0x40 MSO space. If
361 this happens the Elliptic Curve variant of the primary key using the
362 TCG defined template will be generated on the fly into a volatile
363 object and used as the parent. The current kernel code only supports
366 pubkey is the binary representation of TPM2B_PRIVATE excluding the
367 initial TPM2B header, which can be reconstructed from the ASN.1 octet
370 privkey is the binary representation of TPM2B_PUBLIC excluding the
371 initial TPM2B header which can be reconstructed from the ASN.1 octed