373 строки
14 KiB
ReStructuredText
373 строки
14 KiB
ReStructuredText
==========================
|
||
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.
|
||
|
||
* 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.
|
||
|
||
* 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.
|
||
|
||
* 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``.
|
||
|
||
|
||
* Threat model
|
||
|
||
The strength and appropriateness of a particular TPM or TEE 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 generated in the trust source. 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.
|
||
|
||
* TPM (hardware device) based RNG
|
||
|
||
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.
|
||
|
||
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 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.
|
||
|
||
|
||
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).
|
||
|
||
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 "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
|
||
|
||
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.
|