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ASYMMETRIC / PUBLIC-KEY CRYPTOGRAPHY KEY TYPE
=============================================
Contents:
- Overview.
- Key identification.
- Accessing asymmetric keys.
- Signature verification.
- Asymmetric key subtypes.
- Instantiation data parsers.
========
OVERVIEW
========
The "asymmetric" key type is designed to be a container for the keys used in
public-key cryptography, without imposing any particular restrictions on the
form or mechanism of the cryptography or form of the key.
The asymmetric key is given a subtype that defines what sort of data is
associated with the key and provides operations to describe and destroy it.
However, no requirement is made that the key data actually be stored in the
key.
A completely in-kernel key retention and operation subtype can be defined, but
it would also be possible to provide access to cryptographic hardware (such as
a TPM) that might be used to both retain the relevant key and perform
operations using that key. In such a case, the asymmetric key would then
merely be an interface to the TPM driver.
Also provided is the concept of a data parser. Data parsers are responsible
for extracting information from the blobs of data passed to the instantiation
function. The first data parser that recognises the blob gets to set the
subtype of the key and define the operations that can be done on that key.
A data parser may interpret the data blob as containing the bits representing a
key, or it may interpret it as a reference to a key held somewhere else in the
system (for example, a TPM).
==================
KEY IDENTIFICATION
==================
If a key is added with an empty name, the instantiation data parsers are given
the opportunity to pre-parse a key and to determine the description the key
should be given from the content of the key.
This can then be used to refer to the key, either by complete match or by
partial match. The key type may also use other criteria to refer to a key.
The asymmetric key type's match function can then perform a wider range of
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
function will examine a key's fingerprint to see if the hex digits given
after the "id:" match the tail. For instance:
keyctl search @s asymmetric id:5acc2142
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
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:
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:
1a39e171 I----- 1 perm 3f010000 0 0 asymmetri modsign.0: DSA 5acc2142 []
=========================
ACCESSING ASYMMETRIC KEYS
=========================
For general access to asymmetric keys from within the kernel, the following
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:
enum pkey_algo
digest algorithms used by those:
enum pkey_hash_algo
and key identifier representations:
enum pkey_id_type
Note that the key type representation types are required because key
identifiers from different standards aren't necessarily compatible. For
instance, PGP generates key identifiers by hashing the key data plus some
PGP-specific metadata, whereas X.509 has arbitrary certificate identifiers.
The operations defined upon a key are:
(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
----------------------
An operation is provided to perform cryptographic signature verification, using
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.
struct public_key_signature {
u8 *digest;
u8 digest_size;
enum pkey_hash_algo pkey_hash_algo : 8;
u8 nr_mpi;
union {
MPI mpi[2];
...
};
};
The algorithm used must be noted in sig->pkey_hash_algo, and all the MPIs that
make up the actual signature must be stored in sig->mpi[] and the count of MPIs
placed in sig->nr_mpi.
In addition, the data must have been digested by the caller and the resulting
hash must be pointed to by sig->digest and the size of the hash be placed in
sig->digest_size.
The function will return 0 upon success or -EKEYREJECTED if the signature
doesn't match.
The function may also return -ENOTSUPP if an unsupported public-key algorithm
or public-key/hash algorithm combination is specified or the key doesn't
support the operation; -EBADMSG or -ERANGE if some of the parameters have weird
data; or -ENOMEM if an allocation can't be performed. -EINVAL can be returned
if the key argument is the wrong type or is incompletely set up.
=======================
ASYMMETRIC KEY SUBTYPES
=======================
Asymmetric keys have a subtype that defines the set of operations that can be
performed on that key and that determines what data is attached as the key
payload. The payload format is entirely at the whim of the subtype.
The subtype is selected by the key data parser and the parser must initialise
the data required for it. The asymmetric key retains a reference on the
subtype module.
The subtype definition structure can be found in:
#include <keys/asymmetric-subtype.h>
and looks like the following:
struct asymmetric_key_subtype {
struct module *owner;
const char *name;
void (*describe)(const struct key *key, struct seq_file *m);
void (*destroy)(void *payload);
int (*verify_signature)(const struct key *key,
const struct public_key_signature *sig);
};
Asymmetric keys point to this with their type_data[0] member.
The owner and name fields should be set to the owning module and the name of
the subtype. Currently, the name is only used for print statements.
There are a number of operations defined by the subtype:
(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().
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) verify_signature().
Optional. These are the entry points for the key usage operations.
Currently there is only the one defined. If not set, the caller will be
given -ENOTSUPP. The subtype may do anything it likes to implement an
operation, including offloading to hardware.
==========================
INSTANTIATION DATA PARSERS
==========================
The asymmetric key type doesn't generally want to store or to deal with a raw
blob of data that holds the key data. It would have to parse it and error
check it each time it wanted to use it. Further, the contents of the blob may
have various checks that can be performed on it (eg. self-signatures, validity
dates) and may contain useful data about the key (identifiers, capabilities).
Also, the blob may represent a pointer to some hardware containing the key
rather than the key itself.
Examples of blob formats for which parsers could be implemented include:
- OpenPGP packet stream [RFC 4880].
- X.509 ASN.1 stream.
- Pointer to TPM key.
- Pointer to UEFI key.
During key instantiation each parser in the list is tried until one doesn't
return -EBADMSG.
The parser definition structure can be found in:
#include <keys/asymmetric-parser.h>
and looks like the following:
struct asymmetric_key_parser {
struct module *owner;
const char *name;
int (*parse)(struct key_preparsed_payload *prep);
};
The owner and name fields should be set to the owning module and the name of
the parser.
There is currently only a single operation defined by the parser, and it is
mandatory:
(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
allocated, and as such, is permitted to provide the key's description in
the case that the caller declines to do so.
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.txt].
struct key_preparsed_payload {
char *description;
void *type_data[2];
void *payload;
const void *data;
size_t datalen;
size_t quotalen;
};
The instantiation data is in a blob pointed to by data and is datalen in
size. The parse() function is not permitted to change these two values at
all, and shouldn't change any of the other values _unless_ they are
recognise the blob format and will not return -EBADMSG to indicate it is
not theirs.
If the parser is happy with the blob, it should propose a description for
the key and attach it to ->description, ->type_data[0] should be set to
point to the subtype to be used, ->payload should be set to point to the
initialised data for that subtype, ->type_data[1] should point to a hex
fingerprint and quotalen should be updated to indicate how much quota this
key should account for.
When clearing up, the data attached to ->type_data[1] and ->description
will be kfree()'d and the data attached to ->payload will be passed to the
subtype's ->destroy() method to be disposed of. A module reference for
the subtype pointed to by ->type_data[0] will be put.
If the data format is not recognised, -EBADMSG should be returned. If it
is recognised, but the key cannot for some reason be set up, some other
negative error code should be returned. On success, 0 should be returned.
The key's fingerprint string may be partially matched upon. For a
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:
int register_asymmetric_key_parser(struct asymmetric_key_parser *parser);
void unregister_asymmetric_key_parser(struct asymmetric_key_parser *subtype);
Parsers may not have the same name. The names are otherwise only used for
displaying in debugging messages.