0001 ==========================
0002 Trusted and Encrypted Keys
0003 ==========================
0004
0005 Trusted and Encrypted Keys are two new key types added to the existing kernel
0006 key ring service. Both of these new types are variable length symmetric keys,
0007 and in both cases all keys are created in the kernel, and user space sees,
0008 stores, and loads only encrypted blobs. Trusted Keys require the availability
0009 of a Trust Source for greater security, while Encrypted Keys can be used on any
0010 system. All user level blobs, are displayed and loaded in hex ASCII for
0011 convenience, and are integrity verified.
0012
0013
0014 Trust Source
0015 ============
0016
0017 A trust source provides the source of security for Trusted Keys. This
0018 section lists currently supported trust sources, along with their security
0019 considerations. Whether or not a trust source is sufficiently safe depends
0020 on the strength and correctness of its implementation, as well as the threat
0021 environment for a specific use case. Since the kernel doesn't know what the
0022 environment is, and there is no metric of trust, it is dependent on the
0023 consumer of the Trusted Keys to determine if the trust source is sufficiently
0024 safe.
0025
0026 * Root of trust for storage
0027
0028 (1) TPM (Trusted Platform Module: hardware device)
0029
0030 Rooted to Storage Root Key (SRK) which never leaves the TPM that
0031 provides crypto operation to establish root of trust for storage.
0032
0033 (2) TEE (Trusted Execution Environment: OP-TEE based on Arm TrustZone)
0034
0035 Rooted to Hardware Unique Key (HUK) which is generally burnt in on-chip
0036 fuses and is accessible to TEE only.
0037
0038 (3) CAAM (Cryptographic Acceleration and Assurance Module: IP on NXP SoCs)
0039
0040 When High Assurance Boot (HAB) is enabled and the CAAM is in secure
0041 mode, trust is rooted to the OTPMK, a never-disclosed 256-bit key
0042 randomly generated and fused into each SoC at manufacturing time.
0043 Otherwise, a common fixed test key is used instead.
0044
0045 * Execution isolation
0046
0047 (1) TPM
0048
0049 Fixed set of operations running in isolated execution environment.
0050
0051 (2) TEE
0052
0053 Customizable set of operations running in isolated execution
0054 environment verified via Secure/Trusted boot process.
0055
0056 (3) CAAM
0057
0058 Fixed set of operations running in isolated execution environment.
0059
0060 * Optional binding to platform integrity state
0061
0062 (1) TPM
0063
0064 Keys can be optionally sealed to specified PCR (integrity measurement)
0065 values, and only unsealed by the TPM, if PCRs and blob integrity
0066 verifications match. A loaded Trusted Key can be updated with new
0067 (future) PCR values, so keys are easily migrated to new PCR values,
0068 such as when the kernel and initramfs are updated. The same key can
0069 have many saved blobs under different PCR values, so multiple boots are
0070 easily supported.
0071
0072 (2) TEE
0073
0074 Relies on Secure/Trusted boot process for platform integrity. It can
0075 be extended with TEE based measured boot process.
0076
0077 (3) CAAM
0078
0079 Relies on the High Assurance Boot (HAB) mechanism of NXP SoCs
0080 for platform integrity.
0081
0082 * Interfaces and APIs
0083
0084 (1) TPM
0085
0086 TPMs have well-documented, standardized interfaces and APIs.
0087
0088 (2) TEE
0089
0090 TEEs have well-documented, standardized client interface and APIs. For
0091 more details refer to ``Documentation/staging/tee.rst``.
0092
0093 (3) CAAM
0094
0095 Interface is specific to silicon vendor.
0096
0097 * Threat model
0098
0099 The strength and appropriateness of a particular trust source for a given
0100 purpose must be assessed when using them to protect security-relevant data.
0101
0102
0103 Key Generation
0104 ==============
0105
0106 Trusted Keys
0107 ------------
0108
0109 New keys are created from random numbers. They are encrypted/decrypted using
0110 a child key in the storage key hierarchy. Encryption and decryption of the
0111 child key must be protected by a strong access control policy within the
0112 trust source. The random number generator in use differs according to the
0113 selected trust source:
0114
0115 * TPM: hardware device based RNG
0116
0117 Keys are generated within the TPM. Strength of random numbers may vary
0118 from one device manufacturer to another.
0119
0120 * TEE: OP-TEE based on Arm TrustZone based RNG
0121
0122 RNG is customizable as per platform needs. It can either be direct output
0123 from platform specific hardware RNG or a software based Fortuna CSPRNG
0124 which can be seeded via multiple entropy sources.
0125
0126 * CAAM: Kernel RNG
0127
0128 The normal kernel random number generator is used. To seed it from the
0129 CAAM HWRNG, enable CRYPTO_DEV_FSL_CAAM_RNG_API and ensure the device
0130 is probed.
0131
0132 Users may override this by specifying ``trusted.rng=kernel`` on the kernel
0133 command-line to override the used RNG with the kernel's random number pool.
0134
0135 Encrypted Keys
0136 --------------
0137
0138 Encrypted keys do not depend on a trust source, and are faster, as they use AES
0139 for encryption/decryption. New keys are created either from kernel-generated
0140 random numbers or user-provided decrypted data, and are encrypted/decrypted
0141 using a specified ‘master’ key. The ‘master’ key can either be a trusted-key or
0142 user-key type. The main disadvantage of encrypted keys is that if they are not
0143 rooted in a trusted key, they are only as secure as the user key encrypting
0144 them. The master user key should therefore be loaded in as secure a way as
0145 possible, preferably early in boot.
0146
0147
0148 Usage
0149 =====
0150
0151 Trusted Keys usage: TPM
0152 -----------------------
0153
0154 TPM 1.2: By default, trusted keys are sealed under the SRK, which has the
0155 default authorization value (20 bytes of 0s). This can be set at takeownership
0156 time with the TrouSerS utility: "tpm_takeownership -u -z".
0157
0158 TPM 2.0: The user must first create a storage key and make it persistent, so the
0159 key is available after reboot. This can be done using the following commands.
0160
0161 With the IBM TSS 2 stack::
0162
0163 #> tsscreateprimary -hi o -st
0164 Handle 80000000
0165 #> tssevictcontrol -hi o -ho 80000000 -hp 81000001
0166
0167 Or with the Intel TSS 2 stack::
0168
0169 #> tpm2_createprimary --hierarchy o -G rsa2048 -c key.ctxt
0170 [...]
0171 #> tpm2_evictcontrol -c key.ctxt 0x81000001
0172 persistentHandle: 0x81000001
0173
0174 Usage::
0175
0176 keyctl add trusted name "new keylen [options]" ring
0177 keyctl add trusted name "load hex_blob [pcrlock=pcrnum]" ring
0178 keyctl update key "update [options]"
0179 keyctl print keyid
0180
0181 options:
0182 keyhandle= ascii hex value of sealing key
0183 TPM 1.2: default 0x40000000 (SRK)
0184 TPM 2.0: no default; must be passed every time
0185 keyauth= ascii hex auth for sealing key default 0x00...i
0186 (40 ascii zeros)
0187 blobauth= ascii hex auth for sealed data default 0x00...
0188 (40 ascii zeros)
0189 pcrinfo= ascii hex of PCR_INFO or PCR_INFO_LONG (no default)
0190 pcrlock= pcr number to be extended to "lock" blob
0191 migratable= 0|1 indicating permission to reseal to new PCR values,
0192 default 1 (resealing allowed)
0193 hash= hash algorithm name as a string. For TPM 1.x the only
0194 allowed value is sha1. For TPM 2.x the allowed values
0195 are sha1, sha256, sha384, sha512 and sm3-256.
0196 policydigest= digest for the authorization policy. must be calculated
0197 with the same hash algorithm as specified by the 'hash='
0198 option.
0199 policyhandle= handle to an authorization policy session that defines the
0200 same policy and with the same hash algorithm as was used to
0201 seal the key.
0202
0203 "keyctl print" returns an ascii hex copy of the sealed key, which is in standard
0204 TPM_STORED_DATA format. The key length for new keys are always in bytes.
0205 Trusted Keys can be 32 - 128 bytes (256 - 1024 bits), the upper limit is to fit
0206 within the 2048 bit SRK (RSA) keylength, with all necessary structure/padding.
0207
0208 Trusted Keys usage: TEE
0209 -----------------------
0210
0211 Usage::
0212
0213 keyctl add trusted name "new keylen" ring
0214 keyctl add trusted name "load hex_blob" ring
0215 keyctl print keyid
0216
0217 "keyctl print" returns an ASCII hex copy of the sealed key, which is in format
0218 specific to TEE device implementation. The key length for new keys is always
0219 in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
0220
0221 Trusted Keys usage: CAAM
0222 ------------------------
0223
0224 Usage::
0225
0226 keyctl add trusted name "new keylen" ring
0227 keyctl add trusted name "load hex_blob" ring
0228 keyctl print keyid
0229
0230 "keyctl print" returns an ASCII hex copy of the sealed key, which is in a
0231 CAAM-specific format. The key length for new keys is always in bytes.
0232 Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
0233
0234 Encrypted Keys usage
0235 --------------------
0236
0237 The decrypted portion of encrypted keys can contain either a simple symmetric
0238 key or a more complex structure. The format of the more complex structure is
0239 application specific, which is identified by 'format'.
0240
0241 Usage::
0242
0243 keyctl add encrypted name "new [format] key-type:master-key-name keylen"
0244 ring
0245 keyctl add encrypted name "new [format] key-type:master-key-name keylen
0246 decrypted-data" ring
0247 keyctl add encrypted name "load hex_blob" ring
0248 keyctl update keyid "update key-type:master-key-name"
0249
0250 Where::
0251
0252 format:= 'default | ecryptfs | enc32'
0253 key-type:= 'trusted' | 'user'
0254
0255 Examples of trusted and encrypted key usage
0256 -------------------------------------------
0257
0258 Create and save a trusted key named "kmk" of length 32 bytes.
0259
0260 Note: When using a TPM 2.0 with a persistent key with handle 0x81000001,
0261 append 'keyhandle=0x81000001' to statements between quotes, such as
0262 "new 32 keyhandle=0x81000001".
0263
0264 ::
0265
0266 $ keyctl add trusted kmk "new 32" @u
0267 440502848
0268
0269 $ keyctl show
0270 Session Keyring
0271 -3 --alswrv 500 500 keyring: _ses
0272 97833714 --alswrv 500 -1 \_ keyring: _uid.500
0273 440502848 --alswrv 500 500 \_ trusted: kmk
0274
0275 $ keyctl print 440502848
0276 0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915
0277 3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b
0278 27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722
0279 a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec
0280 d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d
0281 dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0
0282 f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
0283 e4a8aea2b607ec96931e6f4d4fe563ba
0284
0285 $ keyctl pipe 440502848 > kmk.blob
0286
0287 Load a trusted key from the saved blob::
0288
0289 $ keyctl add trusted kmk "load `cat kmk.blob`" @u
0290 268728824
0291
0292 $ keyctl print 268728824
0293 0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915
0294 3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b
0295 27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722
0296 a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec
0297 d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d
0298 dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0
0299 f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
0300 e4a8aea2b607ec96931e6f4d4fe563ba
0301
0302 Reseal (TPM specific) a trusted key under new PCR values::
0303
0304 $ keyctl update 268728824 "update pcrinfo=`cat pcr.blob`"
0305 $ keyctl print 268728824
0306 010100000000002c0002800093c35a09b70fff26e7a98ae786c641e678ec6ffb6b46d805
0307 77c8a6377aed9d3219c6dfec4b23ffe3000001005d37d472ac8a44023fbb3d18583a4f73
0308 d3a076c0858f6f1dcaa39ea0f119911ff03f5406df4f7f27f41da8d7194f45c9f4e00f2e
0309 df449f266253aa3f52e55c53de147773e00f0f9aca86c64d94c95382265968c354c5eab4
0310 9638c5ae99c89de1e0997242edfb0b501744e11ff9762dfd951cffd93227cc513384e7e6
0311 e782c29435c7ec2edafaa2f4c1fe6e7a781b59549ff5296371b42133777dcc5b8b971610
0312 94bc67ede19e43ddb9dc2baacad374a36feaf0314d700af0a65c164b7082401740e489c9
0313 7ef6a24defe4846104209bf0c3eced7fa1a672ed5b125fc9d8cd88b476a658a4434644ef
0314 df8ae9a178e9f83ba9f08d10fa47e4226b98b0702f06b3b8
0315
0316
0317 The initial consumer of trusted keys is EVM, which at boot time needs a high
0318 quality symmetric key for HMAC protection of file metadata. The use of a
0319 trusted key provides strong guarantees that the EVM key has not been
0320 compromised by a user level problem, and when sealed to a platform integrity
0321 state, protects against boot and offline attacks. Create and save an
0322 encrypted key "evm" using the above trusted key "kmk":
0323
0324 option 1: omitting 'format'::
0325
0326 $ keyctl add encrypted evm "new trusted:kmk 32" @u
0327 159771175
0328
0329 option 2: explicitly defining 'format' as 'default'::
0330
0331 $ keyctl add encrypted evm "new default trusted:kmk 32" @u
0332 159771175
0333
0334 $ keyctl print 159771175
0335 default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3
0336 82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0
0337 24717c64 5972dcb82ab2dde83376d82b2e3c09ffc
0338
0339 $ keyctl pipe 159771175 > evm.blob
0340
0341 Load an encrypted key "evm" from saved blob::
0342
0343 $ keyctl add encrypted evm "load `cat evm.blob`" @u
0344 831684262
0345
0346 $ keyctl print 831684262
0347 default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3
0348 82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0
0349 24717c64 5972dcb82ab2dde83376d82b2e3c09ffc
0350
0351 Instantiate an encrypted key "evm" using user-provided decrypted data::
0352
0353 $ keyctl add encrypted evm "new default user:kmk 32 `cat evm_decrypted_data.blob`" @u
0354 794890253
0355
0356 $ keyctl print 794890253
0357 default user:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b382d
0358 bbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0247
0359 17c64 5972dcb82ab2dde83376d82b2e3c09ffc
0360
0361 Other uses for trusted and encrypted keys, such as for disk and file encryption
0362 are anticipated. In particular the new format 'ecryptfs' has been defined
0363 in order to use encrypted keys to mount an eCryptfs filesystem. More details
0364 about the usage can be found in the file
0365 ``Documentation/security/keys/ecryptfs.rst``.
0366
0367 Another new format 'enc32' has been defined in order to support encrypted keys
0368 with payload size of 32 bytes. This will initially be used for nvdimm security
0369 but may expand to other usages that require 32 bytes payload.
0370
0371
0372 TPM 2.0 ASN.1 Key Format
0373 ------------------------
0374
0375 The TPM 2.0 ASN.1 key format is designed to be easily recognisable,
0376 even in binary form (fixing a problem we had with the TPM 1.2 ASN.1
0377 format) and to be extensible for additions like importable keys and
0378 policy::
0379
0380 TPMKey ::= SEQUENCE {
0381 type OBJECT IDENTIFIER
0382 emptyAuth [0] EXPLICIT BOOLEAN OPTIONAL
0383 parent INTEGER
0384 pubkey OCTET STRING
0385 privkey OCTET STRING
0386 }
0387
0388 type is what distinguishes the key even in binary form since the OID
0389 is provided by the TCG to be unique and thus forms a recognizable
0390 binary pattern at offset 3 in the key. The OIDs currently made
0391 available are::
0392
0393 2.23.133.10.1.3 TPM Loadable key. This is an asymmetric key (Usually
0394 RSA2048 or Elliptic Curve) which can be imported by a
0395 TPM2_Load() operation.
0396
0397 2.23.133.10.1.4 TPM Importable Key. This is an asymmetric key (Usually
0398 RSA2048 or Elliptic Curve) which can be imported by a
0399 TPM2_Import() operation.
0400
0401 2.23.133.10.1.5 TPM Sealed Data. This is a set of data (up to 128
0402 bytes) which is sealed by the TPM. It usually
0403 represents a symmetric key and must be unsealed before
0404 use.
0405
0406 The trusted key code only uses the TPM Sealed Data OID.
0407
0408 emptyAuth is true if the key has well known authorization "". If it
0409 is false or not present, the key requires an explicit authorization
0410 phrase. This is used by most user space consumers to decide whether
0411 to prompt for a password.
0412
0413 parent represents the parent key handle, either in the 0x81 MSO space,
0414 like 0x81000001 for the RSA primary storage key. Userspace programmes
0415 also support specifying the primary handle in the 0x40 MSO space. If
0416 this happens the Elliptic Curve variant of the primary key using the
0417 TCG defined template will be generated on the fly into a volatile
0418 object and used as the parent. The current kernel code only supports
0419 the 0x81 MSO form.
0420
0421 pubkey is the binary representation of TPM2B_PRIVATE excluding the
0422 initial TPM2B header, which can be reconstructed from the ASN.1 octet
0423 string length.
0424
0425 privkey is the binary representation of TPM2B_PUBLIC excluding the
0426 initial TPM2B header which can be reconstructed from the ASN.1 octed
0427 string length.
