- =============================================
- ASYMMETRIC / PUBLIC-KEY CRYPTOGRAPHY KEY TYPE
- =============================================
+.. SPDX-License-Identifier: GPL-2.0
-Contents:
+=============================================
+Asymmetric / Public-key Cryptography Key Type
+=============================================
+
+.. Contents:
- Overview.
- Key identification.
- Keyring link restrictions.
-========
-OVERVIEW
+Overview
========
The "asymmetric" key type is designed to be a container for the keys used in
system (for example, a TPM).
-==================
-KEY IDENTIFICATION
+Key Identification
==================
If a key is added with an empty name, the instantiation data parsers are given
comparisons than just the straightforward comparison of the description with
the criterion string:
- (1) If the criterion string is of the form "id:<hexdigits>" then the match
+ 1) If the criterion string is of the form "id:<hexdigits>" then the match
function will examine a key's fingerprint to see if the hex digits given
- after the "id:" match the tail. For instance:
+ after the "id:" match the tail. For instance::
keyctl search @s asymmetric id:5acc2142
- will match a key with fingerprint:
+ will match a key with fingerprint::
1A00 2040 7601 7889 DE11 882C 3823 04AD 5ACC 2142
- (2) If the criterion string is of the form "<subtype>:<hexdigits>" then the
+ 2) If the criterion string is of the form "<subtype>:<hexdigits>" then the
match will match the ID as in (1), but with the added restriction that
only keys of the specified subtype (e.g. tpm) will be matched. For
- instance:
+ instance::
keyctl search @s asymmetric tpm:5acc2142
Looking in /proc/keys, the last 8 hex digits of the key fingerprint are
-displayed, along with the subtype:
+displayed, along with the subtype::
1a39e171 I----- 1 perm 3f010000 0 0 asymmetric modsign.0: DSA 5acc2142 []
-=========================
-ACCESSING ASYMMETRIC KEYS
+Accessing Asymmetric Keys
=========================
For general access to asymmetric keys from within the kernel, the following
-inclusion is required:
+inclusion is required::
#include <crypto/public_key.h>
This gives access to functions for dealing with asymmetric / public keys.
Three enums are defined there for representing public-key cryptography
-algorithms:
+algorithms::
enum pkey_algo
-digest algorithms used by those:
+digest algorithms used by those::
enum pkey_hash_algo
-and key identifier representations:
+and key identifier representations::
enum pkey_id_type
The operations defined upon a key are:
- (1) Signature verification.
+ 1) Signature verification.
Other operations are possible (such as encryption) with the same key data
required for verification, but not currently supported, and others
(eg. decryption and signature generation) require extra key data.
-SIGNATURE VERIFICATION
+Signature Verification
----------------------
An operation is provided to perform cryptographic signature verification, using
-an asymmetric key to provide or to provide access to the public key.
+an asymmetric key to provide or to provide access to the public key::
int verify_signature(const struct key *key,
const struct public_key_signature *sig);
The caller must have already obtained the key from some source and can then use
it to check the signature. The caller must have parsed the signature and
-transferred the relevant bits to the structure pointed to by sig.
+transferred the relevant bits to the structure pointed to by sig::
struct public_key_signature {
u8 *digest;
if the key argument is the wrong type or is incompletely set up.
-=======================
-ASYMMETRIC KEY SUBTYPES
+Asymmetric Key Subtypes
=======================
Asymmetric keys have a subtype that defines the set of operations that can be
the data required for it. The asymmetric key retains a reference on the
subtype module.
-The subtype definition structure can be found in:
+The subtype definition structure can be found in::
#include <keys/asymmetric-subtype.h>
-and looks like the following:
+and looks like the following::
struct asymmetric_key_subtype {
struct module *owner;
There are a number of operations defined by the subtype:
- (1) describe().
+ 1) describe().
Mandatory. This allows the subtype to display something in /proc/keys
against the key. For instance the name of the public key algorithm type
could be displayed. The key type will display the tail of the key
identity string after this.
- (2) destroy().
+ 2) destroy().
Mandatory. This should free the memory associated with the key. The
asymmetric key will look after freeing the fingerprint and releasing the
reference on the subtype module.
- (3) query().
+ 3) query().
Mandatory. This is a function for querying the capabilities of a key.
- (4) eds_op().
+ 4) eds_op().
Optional. This is the entry point for the encryption, decryption and
signature creation operations (which are distinguished by the operation ID
in the parameter struct). The subtype may do anything it likes to
implement an operation, including offloading to hardware.
- (5) verify_signature().
+ 5) verify_signature().
Optional. This is the entry point for signature verification. The
subtype may do anything it likes to implement an operation, including
offloading to hardware.
-
-==========================
-INSTANTIATION DATA PARSERS
+Instantiation Data Parsers
==========================
The asymmetric key type doesn't generally want to store or to deal with a raw
During key instantiation each parser in the list is tried until one doesn't
return -EBADMSG.
-The parser definition structure can be found in:
+The parser definition structure can be found in::
#include <keys/asymmetric-parser.h>
-and looks like the following:
+and looks like the following::
struct asymmetric_key_parser {
struct module *owner;
There is currently only a single operation defined by the parser, and it is
mandatory:
- (1) parse().
+ 1) parse().
This is called to preparse the key from the key creation and update paths.
In particular, it is called during the key creation _before_ a key is
The caller passes a pointer to the following struct with all of the fields
cleared, except for data, datalen and quotalen [see
- Documentation/security/keys/core.rst].
+ Documentation/security/keys/core.rst]::
struct key_preparsed_payload {
char *description;
public-key algorithm such as RSA and DSA this will likely be a printable
hex version of the key's fingerprint.
-Functions are provided to register and unregister parsers:
+Functions are provided to register and unregister parsers::
int register_asymmetric_key_parser(struct asymmetric_key_parser *parser);
void unregister_asymmetric_key_parser(struct asymmetric_key_parser *subtype);
displaying in debugging messages.
-=========================
-KEYRING LINK RESTRICTIONS
+Keyring Link Restrictions
=========================
Keyrings created from userspace using add_key can be configured to check the
Several restriction methods are available:
- (1) Restrict using the kernel builtin trusted keyring
+ 1) Restrict using the kernel builtin trusted keyring
- Option string used with KEYCTL_RESTRICT_KEYRING:
- "builtin_trusted"
rejected. The ca_keys kernel parameter also affects which keys are used
for signature verification.
- (2) Restrict using the kernel builtin and secondary trusted keyrings
+ 2) Restrict using the kernel builtin and secondary trusted keyrings
- Option string used with KEYCTL_RESTRICT_KEYRING:
- "builtin_and_secondary_trusted"
kernel parameter also affects which keys are used for signature
verification.
- (3) Restrict using a separate key or keyring
+ 3) Restrict using a separate key or keyring
- Option string used with KEYCTL_RESTRICT_KEYRING:
- "key_or_keyring:<key or keyring serial number>[:chain]"
certificate in order (starting closest to the root) to a keyring. For
instance, one keyring can be populated with links to a set of root
certificates, with a separate, restricted keyring set up for each
- certificate chain to be validated:
+ certificate chain to be validated::
# Create and populate a keyring for root certificates
root_id=`keyctl add keyring root-certs "" @s`
one of the root certificates.
A single keyring can be used to verify a chain of signatures by
- restricting the keyring after linking the root certificate:
+ restricting the keyring after linking the root certificate::
# Create a keyring for the certificate chain and add the root
chain2_id=`keyctl add keyring chain2 "" @s`
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