176 lines
8.0 KiB
ReStructuredText
176 lines
8.0 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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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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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 default 0x40000000 (SRK)
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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'
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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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$ 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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