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 Trusted Platform Module (TPM) chip 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.
Trusted Keys use a TPM both to generate and to seal the keys. Keys are sealed under a 2048 bit RSA key in the TPM, and 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.
By default, trusted keys are sealed under the SRK, which has the default authorization value (20 zeros). This can be set at takeownership time with the trouser’s utility: “tpm_takeownership -u -z”.
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
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.
Encrypted keys do not depend on a TPM, and are faster, as they use AES for encryption/decryption. New keys are created from kernel generated random numbers, 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.
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’.
keyctl add encrypted name "new [format] key-type:master-key-name keylen" ring keyctl add encrypted name "load hex_blob" ring keyctl update keyid "update key-type:master-key-name"
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 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 specific boot PCR values, 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
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
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.