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Pull integrity updates from James Morris: "In Linux 4.19, a new LSM hook named security_kernel_load_data was upstreamed, allowing LSMs and IMA to prevent the kexec_load syscall. Different signature verification methods exist for verifying the kexec'ed kernel image. This adds additional support in IMA to prevent loading unsigned kernel images via the kexec_load syscall, independently of the IMA policy rules, based on the runtime "secure boot" flag. An initial IMA kselftest is included. In addition, this pull request defines a new, separate keyring named ".platform" for storing the preboot/firmware keys needed for verifying the kexec'ed kernel image's signature and includes the associated IMA kexec usage of the ".platform" keyring. (David Howell's and Josh Boyer's patches for reading the preboot/firmware keys, which were previously posted for a different use case scenario, are included here)" * 'next-integrity' of git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security: integrity: Remove references to module keyring ima: Use inode_is_open_for_write ima: Support platform keyring for kernel appraisal efi: Allow the "db" UEFI variable to be suppressed efi: Import certificates from UEFI Secure Boot efi: Add an EFI signature blob parser efi: Add EFI signature data types integrity: Load certs to the platform keyring integrity: Define a trusted platform keyring selftests/ima: kexec_load syscall test ima: don't measure/appraise files on efivarfs x86/ima: retry detecting secure boot mode docs: Extend trusted keys documentation for TPM 2.0 x86/ima: define arch_get_ima_policy() for x86 ima: add support for arch specific policies ima: refactor ima_init_policy() ima: prevent kexec_load syscall based on runtime secureboot flag x86/ima: define arch_ima_get_secureboot integrity: support new struct public_key_signature encoding field
209 lines
9.0 KiB
ReStructuredText
209 lines
9.0 KiB
ReStructuredText
==========================
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Trusted and Encrypted Keys
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==========================
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Trusted and Encrypted Keys are two new key types added to the existing kernel
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key ring service. Both of these new types are variable length symmetric keys,
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and in both cases all keys are created in the kernel, and user space sees,
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stores, and loads only encrypted blobs. Trusted Keys require the availability
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of a Trusted Platform Module (TPM) chip for greater security, while Encrypted
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Keys can be used on any system. All user level blobs, are displayed and loaded
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in hex ascii for convenience, and are integrity verified.
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Trusted Keys use a TPM both to generate and to seal the keys. Keys are sealed
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under a 2048 bit RSA key in the TPM, and optionally sealed to specified PCR
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(integrity measurement) values, and only unsealed by the TPM, if PCRs and blob
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integrity verifications match. A loaded Trusted Key can be updated with new
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(future) PCR values, so keys are easily migrated to new pcr values, such as
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when the kernel and initramfs are updated. The same key can have many saved
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blobs under different PCR values, so multiple boots are easily supported.
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TPM 1.2
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-------
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By default, trusted keys are sealed under the SRK, which has the default
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authorization value (20 zeros). This can be set at takeownership time with the
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trouser's utility: "tpm_takeownership -u -z".
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TPM 2.0
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-------
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The user must first create a storage key and make it persistent, so the key is
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available after reboot. This can be done using the following commands.
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With the IBM TSS 2 stack::
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#> tsscreateprimary -hi o -st
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Handle 80000000
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#> tssevictcontrol -hi o -ho 80000000 -hp 81000001
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Or with the Intel TSS 2 stack::
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#> tpm2_createprimary --hierarchy o -G rsa2048 -o key.ctxt
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[...]
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handle: 0x800000FF
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#> tpm2_evictcontrol -c key.ctxt -p 0x81000001
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persistentHandle: 0x81000001
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Usage::
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keyctl add trusted name "new keylen [options]" ring
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keyctl add trusted name "load hex_blob [pcrlock=pcrnum]" ring
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keyctl update key "update [options]"
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keyctl print keyid
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options:
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keyhandle= ascii hex value of sealing key
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TPM 1.2: default 0x40000000 (SRK)
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TPM 2.0: no default; must be passed every time
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keyauth= ascii hex auth for sealing key default 0x00...i
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(40 ascii zeros)
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blobauth= ascii hex auth for sealed data default 0x00...
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(40 ascii zeros)
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pcrinfo= ascii hex of PCR_INFO or PCR_INFO_LONG (no default)
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pcrlock= pcr number to be extended to "lock" blob
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migratable= 0|1 indicating permission to reseal to new PCR values,
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default 1 (resealing allowed)
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hash= hash algorithm name as a string. For TPM 1.x the only
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allowed value is sha1. For TPM 2.x the allowed values
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are sha1, sha256, sha384, sha512 and sm3-256.
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policydigest= digest for the authorization policy. must be calculated
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with the same hash algorithm as specified by the 'hash='
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option.
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policyhandle= handle to an authorization policy session that defines the
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same policy and with the same hash algorithm as was used to
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seal the key.
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"keyctl print" returns an ascii hex copy of the sealed key, which is in standard
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TPM_STORED_DATA format. The key length for new keys are always in bytes.
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Trusted Keys can be 32 - 128 bytes (256 - 1024 bits), the upper limit is to fit
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within the 2048 bit SRK (RSA) keylength, with all necessary structure/padding.
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Encrypted keys do not depend on a TPM, and are faster, as they use AES for
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encryption/decryption. New keys are created from kernel generated random
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numbers, and are encrypted/decrypted using a specified 'master' key. The
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'master' key can either be a trusted-key or user-key type. The main
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disadvantage of encrypted keys is that if they are not rooted in a trusted key,
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they are only as secure as the user key encrypting them. The master user key
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should therefore be loaded in as secure a way as possible, preferably early in
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boot.
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The decrypted portion of encrypted keys can contain either a simple symmetric
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key or a more complex structure. The format of the more complex structure is
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application specific, which is identified by 'format'.
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Usage::
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keyctl add encrypted name "new [format] key-type:master-key-name keylen"
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ring
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keyctl add encrypted name "load hex_blob" ring
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keyctl update keyid "update key-type:master-key-name"
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Where::
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format:= 'default | ecryptfs | enc32'
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key-type:= 'trusted' | 'user'
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Examples of trusted and encrypted key usage:
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Create and save a trusted key named "kmk" of length 32 bytes::
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Note: When using a TPM 2.0 with a persistent key with handle 0x81000001,
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append 'keyhandle=0x81000001' to statements between quotes, such as
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"new 32 keyhandle=0x81000001".
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$ keyctl add trusted kmk "new 32" @u
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440502848
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$ keyctl show
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Session Keyring
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-3 --alswrv 500 500 keyring: _ses
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97833714 --alswrv 500 -1 \_ keyring: _uid.500
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440502848 --alswrv 500 500 \_ trusted: kmk
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$ keyctl print 440502848
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0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915
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3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b
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27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722
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a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec
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d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d
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dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0
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f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
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e4a8aea2b607ec96931e6f4d4fe563ba
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$ keyctl pipe 440502848 > kmk.blob
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Load a trusted key from the saved blob::
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$ keyctl add trusted kmk "load `cat kmk.blob`" @u
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268728824
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$ keyctl print 268728824
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0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915
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3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b
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27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722
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a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec
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d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d
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dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0
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f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
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e4a8aea2b607ec96931e6f4d4fe563ba
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Reseal a trusted key under new pcr values::
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$ keyctl update 268728824 "update pcrinfo=`cat pcr.blob`"
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$ keyctl print 268728824
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010100000000002c0002800093c35a09b70fff26e7a98ae786c641e678ec6ffb6b46d805
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77c8a6377aed9d3219c6dfec4b23ffe3000001005d37d472ac8a44023fbb3d18583a4f73
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d3a076c0858f6f1dcaa39ea0f119911ff03f5406df4f7f27f41da8d7194f45c9f4e00f2e
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df449f266253aa3f52e55c53de147773e00f0f9aca86c64d94c95382265968c354c5eab4
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9638c5ae99c89de1e0997242edfb0b501744e11ff9762dfd951cffd93227cc513384e7e6
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e782c29435c7ec2edafaa2f4c1fe6e7a781b59549ff5296371b42133777dcc5b8b971610
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94bc67ede19e43ddb9dc2baacad374a36feaf0314d700af0a65c164b7082401740e489c9
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7ef6a24defe4846104209bf0c3eced7fa1a672ed5b125fc9d8cd88b476a658a4434644ef
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df8ae9a178e9f83ba9f08d10fa47e4226b98b0702f06b3b8
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The initial consumer of trusted keys is EVM, which at boot time needs a high
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quality symmetric key for HMAC protection of file metadata. The use of a
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trusted key provides strong guarantees that the EVM key has not been
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compromised by a user level problem, and when sealed to specific boot PCR
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values, protects against boot and offline attacks. Create and save an
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encrypted key "evm" using the above trusted key "kmk":
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option 1: omitting 'format'::
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$ keyctl add encrypted evm "new trusted:kmk 32" @u
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159771175
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option 2: explicitly defining 'format' as 'default'::
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$ keyctl add encrypted evm "new default trusted:kmk 32" @u
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159771175
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$ keyctl print 159771175
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default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3
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82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0
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24717c64 5972dcb82ab2dde83376d82b2e3c09ffc
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$ keyctl pipe 159771175 > evm.blob
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Load an encrypted key "evm" from saved blob::
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$ keyctl add encrypted evm "load `cat evm.blob`" @u
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831684262
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$ keyctl print 831684262
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default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3
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82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0
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24717c64 5972dcb82ab2dde83376d82b2e3c09ffc
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Other uses for trusted and encrypted keys, such as for disk and file encryption
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are anticipated. In particular the new format 'ecryptfs' has been defined in
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in order to use encrypted keys to mount an eCryptfs filesystem. More details
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about the usage can be found in the file
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``Documentation/security/keys/ecryptfs.rst``.
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Another new format 'enc32' has been defined in order to support encrypted keys
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with payload size of 32 bytes. This will initially be used for nvdimm security
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but may expand to other usages that require 32 bytes payload.
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