| ========================== |
| Trusted and Encrypted Keys |
| ========================== |
| |
| Trusted and Encrypted Keys are two new key types added to the existing kernel |
| key ring service. Both of these new types are variable length symmetric keys, |
| and in both cases all keys are created in the kernel, and user space sees, |
| stores, and loads only encrypted blobs. Trusted Keys require the availability |
| of a Trust Source for greater security, while Encrypted Keys can be used on any |
| system. All user level blobs, are displayed and loaded in hex ASCII for |
| convenience, and are integrity verified. |
| |
| |
| Trust Source |
| ============ |
| |
| A trust source provides the source of security for Trusted Keys. This |
| section lists currently supported trust sources, along with their security |
| considerations. Whether or not a trust source is sufficiently safe depends |
| on the strength and correctness of its implementation, as well as the threat |
| environment for a specific use case. Since the kernel doesn't know what the |
| environment is, and there is no metric of trust, it is dependent on the |
| consumer of the Trusted Keys to determine if the trust source is sufficiently |
| safe. |
| |
| * Root of trust for storage |
| |
| (1) TPM (Trusted Platform Module: hardware device) |
| |
| Rooted to Storage Root Key (SRK) which never leaves the TPM that |
| provides crypto operation to establish root of trust for storage. |
| |
| (2) TEE (Trusted Execution Environment: OP-TEE based on Arm TrustZone) |
| |
| Rooted to Hardware Unique Key (HUK) which is generally burnt in on-chip |
| fuses and is accessible to TEE only. |
| |
| (3) CAAM (Cryptographic Acceleration and Assurance Module: IP on NXP SoCs) |
| |
| When High Assurance Boot (HAB) is enabled and the CAAM is in secure |
| mode, trust is rooted to the OTPMK, a never-disclosed 256-bit key |
| randomly generated and fused into each SoC at manufacturing time. |
| Otherwise, a common fixed test key is used instead. |
| |
| * Execution isolation |
| |
| (1) TPM |
| |
| Fixed set of operations running in isolated execution environment. |
| |
| (2) TEE |
| |
| Customizable set of operations running in isolated execution |
| environment verified via Secure/Trusted boot process. |
| |
| (3) CAAM |
| |
| Fixed set of operations running in isolated execution environment. |
| |
| * Optional binding to platform integrity state |
| |
| (1) TPM |
| |
| Keys can be optionally sealed to specified PCR (integrity measurement) |
| values, and only unsealed by the TPM, if PCRs and blob integrity |
| verifications match. A loaded Trusted Key can be updated with new |
| (future) PCR values, so keys are easily migrated to new PCR values, |
| such as when the kernel and initramfs are updated. The same key can |
| have many saved blobs under different PCR values, so multiple boots are |
| easily supported. |
| |
| (2) TEE |
| |
| Relies on Secure/Trusted boot process for platform integrity. It can |
| be extended with TEE based measured boot process. |
| |
| (3) CAAM |
| |
| Relies on the High Assurance Boot (HAB) mechanism of NXP SoCs |
| for platform integrity. |
| |
| * Interfaces and APIs |
| |
| (1) TPM |
| |
| TPMs have well-documented, standardized interfaces and APIs. |
| |
| (2) TEE |
| |
| TEEs have well-documented, standardized client interface and APIs. For |
| more details refer to ``Documentation/staging/tee.rst``. |
| |
| (3) CAAM |
| |
| Interface is specific to silicon vendor. |
| |
| * Threat model |
| |
| The strength and appropriateness of a particular trust source for a given |
| purpose must be assessed when using them to protect security-relevant data. |
| |
| |
| Key Generation |
| ============== |
| |
| Trusted Keys |
| ------------ |
| |
| New keys are created from random numbers. They are encrypted/decrypted using |
| a child key in the storage key hierarchy. Encryption and decryption of the |
| child key must be protected by a strong access control policy within the |
| trust source. The random number generator in use differs according to the |
| selected trust source: |
| |
| * TPM: hardware device based RNG |
| |
| Keys are generated within the TPM. Strength of random numbers may vary |
| from one device manufacturer to another. |
| |
| * TEE: OP-TEE based on Arm TrustZone based RNG |
| |
| RNG is customizable as per platform needs. It can either be direct output |
| from platform specific hardware RNG or a software based Fortuna CSPRNG |
| which can be seeded via multiple entropy sources. |
| |
| * CAAM: Kernel RNG |
| |
| The normal kernel random number generator is used. To seed it from the |
| CAAM HWRNG, enable CRYPTO_DEV_FSL_CAAM_RNG_API and ensure the device |
| is probed. |
| |
| Users may override this by specifying ``trusted.rng=kernel`` on the kernel |
| command-line to override the used RNG with the kernel's random number pool. |
| |
| Encrypted Keys |
| -------------- |
| |
| Encrypted keys do not depend on a trust source, and are faster, as they use AES |
| for encryption/decryption. New keys are created either from kernel-generated |
| random numbers or user-provided decrypted data, and are encrypted/decrypted |
| using a specified ‘master’ key. The ‘master’ key can either be a trusted-key or |
| user-key type. The main disadvantage of encrypted keys is that if they are not |
| rooted in a trusted key, they are only as secure as the user key encrypting |
| them. The master user key should therefore be loaded in as secure a way as |
| possible, preferably early in boot. |
| |
| |
| Usage |
| ===== |
| |
| Trusted Keys usage: TPM |
| ----------------------- |
| |
| TPM 1.2: By default, trusted keys are sealed under the SRK, which has the |
| default authorization value (20 bytes of 0s). This can be set at takeownership |
| time with the TrouSerS utility: "tpm_takeownership -u -z". |
| |
| TPM 2.0: The user must first create a storage key and make it persistent, so the |
| key is available after reboot. This can be done using the following commands. |
| |
| With the IBM TSS 2 stack:: |
| |
| #> tsscreateprimary -hi o -st |
| Handle 80000000 |
| #> tssevictcontrol -hi o -ho 80000000 -hp 81000001 |
| |
| Or with the Intel TSS 2 stack:: |
| |
| #> tpm2_createprimary --hierarchy o -G rsa2048 -c key.ctxt |
| [...] |
| #> tpm2_evictcontrol -c key.ctxt 0x81000001 |
| persistentHandle: 0x81000001 |
| |
| Usage:: |
| |
| keyctl add trusted name "new keylen [options]" ring |
| keyctl add trusted name "load hex_blob [pcrlock=pcrnum]" ring |
| keyctl update key "update [options]" |
| keyctl print keyid |
| |
| options: |
| keyhandle= ascii hex value of sealing key |
| TPM 1.2: default 0x40000000 (SRK) |
| TPM 2.0: no default; must be passed every time |
| keyauth= ascii hex auth for sealing key default 0x00...i |
| (40 ascii zeros) |
| blobauth= ascii hex auth for sealed data default 0x00... |
| (40 ascii zeros) |
| pcrinfo= ascii hex of PCR_INFO or PCR_INFO_LONG (no default) |
| pcrlock= pcr number to be extended to "lock" blob |
| migratable= 0|1 indicating permission to reseal to new PCR values, |
| default 1 (resealing allowed) |
| hash= hash algorithm name as a string. For TPM 1.x the only |
| allowed value is sha1. For TPM 2.x the allowed values |
| are sha1, sha256, sha384, sha512 and sm3-256. |
| policydigest= digest for the authorization policy. must be calculated |
| with the same hash algorithm as specified by the 'hash=' |
| option. |
| policyhandle= handle to an authorization policy session that defines the |
| same policy and with the same hash algorithm as was used to |
| seal the key. |
| |
| "keyctl print" returns an ascii hex copy of the sealed key, which is in standard |
| TPM_STORED_DATA format. The key length for new keys are always in bytes. |
| Trusted Keys can be 32 - 128 bytes (256 - 1024 bits), the upper limit is to fit |
| within the 2048 bit SRK (RSA) keylength, with all necessary structure/padding. |
| |
| Trusted Keys usage: TEE |
| ----------------------- |
| |
| Usage:: |
| |
| keyctl add trusted name "new keylen" ring |
| keyctl add trusted name "load hex_blob" ring |
| keyctl print keyid |
| |
| "keyctl print" returns an ASCII hex copy of the sealed key, which is in format |
| specific to TEE device implementation. The key length for new keys is always |
| in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits). |
| |
| Trusted Keys usage: CAAM |
| ------------------------ |
| |
| Usage:: |
| |
| keyctl add trusted name "new keylen" ring |
| keyctl add trusted name "load hex_blob" ring |
| keyctl print keyid |
| |
| "keyctl print" returns an ASCII hex copy of the sealed key, which is in a |
| CAAM-specific format. The key length for new keys is always in bytes. |
| Trusted Keys can be 32 - 128 bytes (256 - 1024 bits). |
| |
| Encrypted Keys usage |
| -------------------- |
| |
| The decrypted portion of encrypted keys can contain either a simple symmetric |
| key or a more complex structure. The format of the more complex structure is |
| application specific, which is identified by 'format'. |
| |
| Usage:: |
| |
| keyctl add encrypted name "new [format] key-type:master-key-name keylen" |
| ring |
| keyctl add encrypted name "new [format] key-type:master-key-name keylen |
| decrypted-data" ring |
| keyctl add encrypted name "load hex_blob" ring |
| keyctl update keyid "update key-type:master-key-name" |
| |
| Where:: |
| |
| format:= 'default | ecryptfs | enc32' |
| key-type:= 'trusted' | 'user' |
| |
| Examples of trusted and encrypted key usage |
| ------------------------------------------- |
| |
| Create and save a trusted key named "kmk" of length 32 bytes. |
| |
| Note: When using a TPM 2.0 with a persistent key with handle 0x81000001, |
| append 'keyhandle=0x81000001' to statements between quotes, such as |
| "new 32 keyhandle=0x81000001". |
| |
| :: |
| |
| $ keyctl add trusted kmk "new 32" @u |
| 440502848 |
| |
| $ keyctl show |
| Session Keyring |
| -3 --alswrv 500 500 keyring: _ses |
| 97833714 --alswrv 500 -1 \_ keyring: _uid.500 |
| 440502848 --alswrv 500 500 \_ trusted: kmk |
| |
| $ keyctl print 440502848 |
| 0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915 |
| 3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b |
| 27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722 |
| a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec |
| d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d |
| dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0 |
| f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b |
| e4a8aea2b607ec96931e6f4d4fe563ba |
| |
| $ keyctl pipe 440502848 > kmk.blob |
| |
| Load a trusted key from the saved blob:: |
| |
| $ keyctl add trusted kmk "load `cat kmk.blob`" @u |
| 268728824 |
| |
| $ keyctl print 268728824 |
| 0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915 |
| 3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b |
| 27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722 |
| a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec |
| d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d |
| dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0 |
| f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b |
| e4a8aea2b607ec96931e6f4d4fe563ba |
| |
| Reseal (TPM specific) a trusted key under new PCR values:: |
| |
| $ keyctl update 268728824 "update pcrinfo=`cat pcr.blob`" |
| $ keyctl print 268728824 |
| 010100000000002c0002800093c35a09b70fff26e7a98ae786c641e678ec6ffb6b46d805 |
| 77c8a6377aed9d3219c6dfec4b23ffe3000001005d37d472ac8a44023fbb3d18583a4f73 |
| d3a076c0858f6f1dcaa39ea0f119911ff03f5406df4f7f27f41da8d7194f45c9f4e00f2e |
| df449f266253aa3f52e55c53de147773e00f0f9aca86c64d94c95382265968c354c5eab4 |
| 9638c5ae99c89de1e0997242edfb0b501744e11ff9762dfd951cffd93227cc513384e7e6 |
| e782c29435c7ec2edafaa2f4c1fe6e7a781b59549ff5296371b42133777dcc5b8b971610 |
| 94bc67ede19e43ddb9dc2baacad374a36feaf0314d700af0a65c164b7082401740e489c9 |
| 7ef6a24defe4846104209bf0c3eced7fa1a672ed5b125fc9d8cd88b476a658a4434644ef |
| df8ae9a178e9f83ba9f08d10fa47e4226b98b0702f06b3b8 |
| |
| |
| The initial consumer of trusted keys is EVM, which at boot time needs a high |
| quality symmetric key for HMAC protection of file metadata. The use of a |
| trusted key provides strong guarantees that the EVM key has not been |
| compromised by a user level problem, and when sealed to a platform integrity |
| state, protects against boot and offline attacks. Create and save an |
| encrypted key "evm" using the above trusted key "kmk": |
| |
| option 1: omitting 'format':: |
| |
| $ keyctl add encrypted evm "new trusted:kmk 32" @u |
| 159771175 |
| |
| option 2: explicitly defining 'format' as 'default':: |
| |
| $ keyctl add encrypted evm "new default trusted:kmk 32" @u |
| 159771175 |
| |
| $ keyctl print 159771175 |
| default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3 |
| 82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0 |
| 24717c64 5972dcb82ab2dde83376d82b2e3c09ffc |
| |
| $ keyctl pipe 159771175 > evm.blob |
| |
| Load an encrypted key "evm" from saved blob:: |
| |
| $ keyctl add encrypted evm "load `cat evm.blob`" @u |
| 831684262 |
| |
| $ keyctl print 831684262 |
| default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3 |
| 82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0 |
| 24717c64 5972dcb82ab2dde83376d82b2e3c09ffc |
| |
| Instantiate an encrypted key "evm" using user-provided decrypted data:: |
| |
| $ keyctl add encrypted evm "new default user:kmk 32 `cat evm_decrypted_data.blob`" @u |
| 794890253 |
| |
| $ keyctl print 794890253 |
| default user:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b382d |
| bbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0247 |
| 17c64 5972dcb82ab2dde83376d82b2e3c09ffc |
| |
| Other uses for trusted and encrypted keys, such as for disk and file encryption |
| are anticipated. In particular the new format 'ecryptfs' has been defined |
| in order to use encrypted keys to mount an eCryptfs filesystem. More details |
| about the usage can be found in the file |
| ``Documentation/security/keys/ecryptfs.rst``. |
| |
| Another new format 'enc32' has been defined in order to support encrypted keys |
| with payload size of 32 bytes. This will initially be used for nvdimm security |
| but may expand to other usages that require 32 bytes payload. |
| |
| |
| TPM 2.0 ASN.1 Key Format |
| ------------------------ |
| |
| The TPM 2.0 ASN.1 key format is designed to be easily recognisable, |
| even in binary form (fixing a problem we had with the TPM 1.2 ASN.1 |
| format) and to be extensible for additions like importable keys and |
| policy:: |
| |
| TPMKey ::= SEQUENCE { |
| type OBJECT IDENTIFIER |
| emptyAuth [0] EXPLICIT BOOLEAN OPTIONAL |
| parent INTEGER |
| pubkey OCTET STRING |
| privkey OCTET STRING |
| } |
| |
| type is what distinguishes the key even in binary form since the OID |
| is provided by the TCG to be unique and thus forms a recognizable |
| binary pattern at offset 3 in the key. The OIDs currently made |
| available are:: |
| |
| 2.23.133.10.1.3 TPM Loadable key. This is an asymmetric key (Usually |
| RSA2048 or Elliptic Curve) which can be imported by a |
| TPM2_Load() operation. |
| |
| 2.23.133.10.1.4 TPM Importable Key. This is an asymmetric key (Usually |
| RSA2048 or Elliptic Curve) which can be imported by a |
| TPM2_Import() operation. |
| |
| 2.23.133.10.1.5 TPM Sealed Data. This is a set of data (up to 128 |
| bytes) which is sealed by the TPM. It usually |
| represents a symmetric key and must be unsealed before |
| use. |
| |
| The trusted key code only uses the TPM Sealed Data OID. |
| |
| emptyAuth is true if the key has well known authorization "". If it |
| is false or not present, the key requires an explicit authorization |
| phrase. This is used by most user space consumers to decide whether |
| to prompt for a password. |
| |
| parent represents the parent key handle, either in the 0x81 MSO space, |
| like 0x81000001 for the RSA primary storage key. Userspace programmes |
| also support specifying the primary handle in the 0x40 MSO space. If |
| this happens the Elliptic Curve variant of the primary key using the |
| TCG defined template will be generated on the fly into a volatile |
| object and used as the parent. The current kernel code only supports |
| the 0x81 MSO form. |
| |
| pubkey is the binary representation of TPM2B_PRIVATE excluding the |
| initial TPM2B header, which can be reconstructed from the ASN.1 octet |
| string length. |
| |
| privkey is the binary representation of TPM2B_PUBLIC excluding the |
| initial TPM2B header which can be reconstructed from the ASN.1 octed |
| string length. |
