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/driver-api/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 $ evmkey=$(dd if=/dev/urandom bs=1 count=32 | xxd -c32 -p) 354 $ keyctl add encrypted evm "new default user:kmk 32 $evmkey" @u 355 794890253 356 357 $ keyctl print 794890253 358 default user:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b382d 359 bbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0247 360 17c64 5972dcb82ab2dde83376d82b2e3c09ffc 361 362Other uses for trusted and encrypted keys, such as for disk and file encryption 363are anticipated. In particular the new format 'ecryptfs' has been defined 364in order to use encrypted keys to mount an eCryptfs filesystem. More details 365about the usage can be found in the file 366``Documentation/security/keys/ecryptfs.rst``. 367 368Another new format 'enc32' has been defined in order to support encrypted keys 369with payload size of 32 bytes. This will initially be used for nvdimm security 370but may expand to other usages that require 32 bytes payload. 371 372 373TPM 2.0 ASN.1 Key Format 374------------------------ 375 376The TPM 2.0 ASN.1 key format is designed to be easily recognisable, 377even in binary form (fixing a problem we had with the TPM 1.2 ASN.1 378format) and to be extensible for additions like importable keys and 379policy:: 380 381 TPMKey ::= SEQUENCE { 382 type OBJECT IDENTIFIER 383 emptyAuth [0] EXPLICIT BOOLEAN OPTIONAL 384 parent INTEGER 385 pubkey OCTET STRING 386 privkey OCTET STRING 387 } 388 389type is what distinguishes the key even in binary form since the OID 390is provided by the TCG to be unique and thus forms a recognizable 391binary pattern at offset 3 in the key. The OIDs currently made 392available are:: 393 394 2.23.133.10.1.3 TPM Loadable key. This is an asymmetric key (Usually 395 RSA2048 or Elliptic Curve) which can be imported by a 396 TPM2_Load() operation. 397 398 2.23.133.10.1.4 TPM Importable Key. This is an asymmetric key (Usually 399 RSA2048 or Elliptic Curve) which can be imported by a 400 TPM2_Import() operation. 401 402 2.23.133.10.1.5 TPM Sealed Data. This is a set of data (up to 128 403 bytes) which is sealed by the TPM. It usually 404 represents a symmetric key and must be unsealed before 405 use. 406 407The trusted key code only uses the TPM Sealed Data OID. 408 409emptyAuth is true if the key has well known authorization "". If it 410is false or not present, the key requires an explicit authorization 411phrase. This is used by most user space consumers to decide whether 412to prompt for a password. 413 414parent represents the parent key handle, either in the 0x81 MSO space, 415like 0x81000001 for the RSA primary storage key. Userspace programmes 416also support specifying the primary handle in the 0x40 MSO space. If 417this happens the Elliptic Curve variant of the primary key using the 418TCG defined template will be generated on the fly into a volatile 419object and used as the parent. The current kernel code only supports 420the 0x81 MSO form. 421 422pubkey is the binary representation of TPM2B_PRIVATE excluding the 423initial TPM2B header, which can be reconstructed from the ASN.1 octet 424string length. 425 426privkey is the binary representation of TPM2B_PUBLIC excluding the 427initial TPM2B header which can be reconstructed from the ASN.1 octed 428string length. 429