1 .. SPDX-License-Identifier: GPL-2.0
2
3 =============================================
4 Asymmetric / Public-key Cryptography Key Type
5 =============================================
6
7 .. Contents:
8
9 - Overview.
10 - Key identification.
11 - Accessing asymmetric keys.
12 - Signature verification.
13 - Asymmetric key subtypes.
14 - Instantiation data parsers.
15 - Keyring link restrictions.
16
17
18 Overview
19 ========
20
21 The "asymmetric" key type is designed to be a container for the keys used in
22 public-key cryptography, without imposing any particular restrictions on the
23 form or mechanism of the cryptography or form of the key.
24
25 The asymmetric key is given a subtype that defines what sort of data is
26 associated with the key and provides operations to describe and destroy it.
27 However, no requirement is made that the key data actually be stored in the
28 key.
29
30 A completely in-kernel key retention and operation subtype can be defined, but
31 it would also be possible to provide access to cryptographic hardware (such as
32 a TPM) that might be used to both retain the relevant key and perform
33 operations using that key. In such a case, the asymmetric key would then
34 merely be an interface to the TPM driver.
35
36 Also provided is the concept of a data parser. Data parsers are responsible
37 for extracting information from the blobs of data passed to the instantiation
38 function. The first data parser that recognises the blob gets to set the
39 subtype of the key and define the operations that can be done on that key.
40
41 A data parser may interpret the data blob as containing the bits representing a
42 key, or it may interpret it as a reference to a key held somewhere else in the
43 system (for example, a TPM).
44
45
46 Key Identification
47 ==================
48
49 If a key is added with an empty name, the instantiation data parsers are given
50 the opportunity to pre-parse a key and to determine the description the key
51 should be given from the content of the key.
52
53 This can then be used to refer to the key, either by complete match or by
54 partial match. The key type may also use other criteria to refer to a key.
55
56 The asymmetric key type's match function can then perform a wider range of
57 comparisons than just the straightforward comparison of the description with
58 the criterion string:
59
60 1) If the criterion string is of the form "id:<hexdigits>" then the match
61 function will examine a key's fingerprint to see if the hex digits given
62 after the "id:" match the tail. For instance::
63
64 keyctl search @s asymmetric id:5acc2142
65
66 will match a key with fingerprint::
67
68 1A00 2040 7601 7889 DE11 882C 3823 04AD 5ACC 2142
69
70 2) If the criterion string is of the form "<subtype>:<hexdigits>" then the
71 match will match the ID as in (1), but with the added restriction that
72 only keys of the specified subtype (e.g. tpm) will be matched. For
73 instance::
74
75 keyctl search @s asymmetric tpm:5acc2142
76
77 Looking in /proc/keys, the last 8 hex digits of the key fingerprint are
78 displayed, along with the subtype::
79
80 1a39e171 I----- 1 perm 3f010000 0 0 asymmetric modsign.0: DSA 5acc2142 []
81
82
83 Accessing Asymmetric Keys
84 =========================
85
86 For general access to asymmetric keys from within the kernel, the following
87 inclusion is required::
88
89 #include <crypto/public_key.h>
90
91 This gives access to functions for dealing with asymmetric / public keys.
92 Three enums are defined there for representing public-key cryptography
93 algorithms::
94
95 enum pkey_algo
96
97 digest algorithms used by those::
98
99 enum pkey_hash_algo
100
101 and key identifier representations::
102
103 enum pkey_id_type
104
105 Note that the key type representation types are required because key
106 identifiers from different standards aren't necessarily compatible. For
107 instance, PGP generates key identifiers by hashing the key data plus some
108 PGP-specific metadata, whereas X.509 has arbitrary certificate identifiers.
109
110 The operations defined upon a key are:
111
112 1) Signature verification.
113
114 Other operations are possible (such as encryption) with the same key data
115 required for verification, but not currently supported, and others
116 (eg. decryption and signature generation) require extra key data.
117
118
119 Signature Verification
120 ----------------------
121
122 An operation is provided to perform cryptographic signature verification, using
123 an asymmetric key to provide or to provide access to the public key::
124
125 int verify_signature(const struct key *key,
126 const struct public_key_signature *sig);
127
128 The caller must have already obtained the key from some source and can then use
129 it to check the signature. The caller must have parsed the signature and
130 transferred the relevant bits to the structure pointed to by sig::
131
132 struct public_key_signature {
133 u8 *digest;
134 u8 digest_size;
135 enum pkey_hash_algo pkey_hash_algo : 8;
136 u8 nr_mpi;
137 union {
138 MPI mpi[2];
139 ...
140 };
141 };
142
143 The algorithm used must be noted in sig->pkey_hash_algo, and all the MPIs that
144 make up the actual signature must be stored in sig->mpi[] and the count of MPIs
145 placed in sig->nr_mpi.
146
147 In addition, the data must have been digested by the caller and the resulting
148 hash must be pointed to by sig->digest and the size of the hash be placed in
149 sig->digest_size.
150
151 The function will return 0 upon success or -EKEYREJECTED if the signature
152 doesn't match.
153
154 The function may also return -ENOTSUPP if an unsupported public-key algorithm
155 or public-key/hash algorithm combination is specified or the key doesn't
156 support the operation; -EBADMSG or -ERANGE if some of the parameters have weird
157 data; or -ENOMEM if an allocation can't be performed. -EINVAL can be returned
158 if the key argument is the wrong type or is incompletely set up.
159
160
161 Asymmetric Key Subtypes
162 =======================
163
164 Asymmetric keys have a subtype that defines the set of operations that can be
165 performed on that key and that determines what data is attached as the key
166 payload. The payload format is entirely at the whim of the subtype.
167
168 The subtype is selected by the key data parser and the parser must initialise
169 the data required for it. The asymmetric key retains a reference on the
170 subtype module.
171
172 The subtype definition structure can be found in::
173
174 #include <keys/asymmetric-subtype.h>
175
176 and looks like the following::
177
178 struct asymmetric_key_subtype {
179 struct module *owner;
180 const char *name;
181
182 void (*describe)(const struct key *key, struct seq_file *m);
183 void (*destroy)(void *payload);
184 int (*query)(const struct kernel_pkey_params *params,
185 struct kernel_pkey_query *info);
186 int (*eds_op)(struct kernel_pkey_params *params,
187 const void *in, void *out);
188 int (*verify_signature)(const struct key *key,
189 const struct public_key_signature *sig);
190 };
191
192 Asymmetric keys point to this with their payload[asym_subtype] member.
193
194 The owner and name fields should be set to the owning module and the name of
195 the subtype. Currently, the name is only used for print statements.
196
197 There are a number of operations defined by the subtype:
198
199 1) describe().
200
201 Mandatory. This allows the subtype to display something in /proc/keys
202 against the key. For instance the name of the public key algorithm type
203 could be displayed. The key type will display the tail of the key
204 identity string after this.
205
206 2) destroy().
207
208 Mandatory. This should free the memory associated with the key. The
209 asymmetric key will look after freeing the fingerprint and releasing the
210 reference on the subtype module.
211
212 3) query().
213
214 Mandatory. This is a function for querying the capabilities of a key.
215
216 4) eds_op().
217
218 Optional. This is the entry point for the encryption, decryption and
219 signature creation operations (which are distinguished by the operation ID
220 in the parameter struct). The subtype may do anything it likes to
221 implement an operation, including offloading to hardware.
222
223 5) verify_signature().
224
225 Optional. This is the entry point for signature verification. The
226 subtype may do anything it likes to implement an operation, including
227 offloading to hardware.
228
229 Instantiation Data Parsers
230 ==========================
231
232 The asymmetric key type doesn't generally want to store or to deal with a raw
233 blob of data that holds the key data. It would have to parse it and error
234 check it each time it wanted to use it. Further, the contents of the blob may
235 have various checks that can be performed on it (eg. self-signatures, validity
236 dates) and may contain useful data about the key (identifiers, capabilities).
237
238 Also, the blob may represent a pointer to some hardware containing the key
239 rather than the key itself.
240
241 Examples of blob formats for which parsers could be implemented include:
242
243 - OpenPGP packet stream [RFC 4880].
244 - X.509 ASN.1 stream.
245 - Pointer to TPM key.
246 - Pointer to UEFI key.
247 - PKCS#8 private key [RFC 5208].
248 - PKCS#5 encrypted private key [RFC 2898].
249
250 During key instantiation each parser in the list is tried until one doesn't
251 return -EBADMSG.
252
253 The parser definition structure can be found in::
254
255 #include <keys/asymmetric-parser.h>
256
257 and looks like the following::
258
259 struct asymmetric_key_parser {
260 struct module *owner;
261 const char *name;
262
263 int (*parse)(struct key_preparsed_payload *prep);
264 };
265
266 The owner and name fields should be set to the owning module and the name of
267 the parser.
268
269 There is currently only a single operation defined by the parser, and it is
270 mandatory:
271
272 1) parse().
273
274 This is called to preparse the key from the key creation and update paths.
275 In particular, it is called during the key creation _before_ a key is
276 allocated, and as such, is permitted to provide the key's description in
277 the case that the caller declines to do so.
278
279 The caller passes a pointer to the following struct with all of the fields
280 cleared, except for data, datalen and quotalen [see
281 Documentation/security/keys/core.rst]::
282
283 struct key_preparsed_payload {
284 char *description;
285 void *payload[4];
286 const void *data;
287 size_t datalen;
288 size_t quotalen;
289 };
290
291 The instantiation data is in a blob pointed to by data and is datalen in
292 size. The parse() function is not permitted to change these two values at
293 all, and shouldn't change any of the other values _unless_ they are
294 recognise the blob format and will not return -EBADMSG to indicate it is
295 not theirs.
296
297 If the parser is happy with the blob, it should propose a description for
298 the key and attach it to ->description, ->payload[asym_subtype] should be
299 set to point to the subtype to be used, ->payload[asym_crypto] should be
300 set to point to the initialised data for that subtype,
301 ->payload[asym_key_ids] should point to one or more hex fingerprints and
302 quotalen should be updated to indicate how much quota this key should
303 account for.
304
305 When clearing up, the data attached to ->payload[asym_key_ids] and
306 ->description will be kfree()'d and the data attached to
307 ->payload[asm_crypto] will be passed to the subtype's ->destroy() method
308 to be disposed of. A module reference for the subtype pointed to by
309 ->payload[asym_subtype] will be put.
310
311
312 If the data format is not recognised, -EBADMSG should be returned. If it
313 is recognised, but the key cannot for some reason be set up, some other
314 negative error code should be returned. On success, 0 should be returned.
315
316 The key's fingerprint string may be partially matched upon. For a
317 public-key algorithm such as RSA and DSA this will likely be a printable
318 hex version of the key's fingerprint.
319
320 Functions are provided to register and unregister parsers::
321
322 int register_asymmetric_key_parser(struct asymmetric_key_parser *parser);
323 void unregister_asymmetric_key_parser(struct asymmetric_key_parser *subtype);
324
325 Parsers may not have the same name. The names are otherwise only used for
326 displaying in debugging messages.
327
328
329 Keyring Link Restrictions
330 =========================
331
332 Keyrings created from userspace using add_key can be configured to check the
333 signature of the key being linked. Keys without a valid signature are not
334 allowed to link.
335
336 Several restriction methods are available:
337
338 1) Restrict using the kernel builtin trusted keyring
339
340 - Option string used with KEYCTL_RESTRICT_KEYRING:
341 - "builtin_trusted"
342
343 The kernel builtin trusted keyring will be searched for the signing key.
344 If the builtin trusted keyring is not configured, all links will be
345 rejected. The ca_keys kernel parameter also affects which keys are used
346 for signature verification.
347
348 2) Restrict using the kernel builtin and secondary trusted keyrings
349
350 - Option string used with KEYCTL_RESTRICT_KEYRING:
351 - "builtin_and_secondary_trusted"
352
353 The kernel builtin and secondary trusted keyrings will be searched for the
354 signing key. If the secondary trusted keyring is not configured, this
355 restriction will behave like the "builtin_trusted" option. The ca_keys
356 kernel parameter also affects which keys are used for signature
357 verification.
358
359 3) Restrict using a separate key or keyring
360
361 - Option string used with KEYCTL_RESTRICT_KEYRING:
362 - "key_or_keyring:<key or keyring serial number>[:chain]"
363
364 Whenever a key link is requested, the link will only succeed if the key
365 being linked is signed by one of the designated keys. This key may be
366 specified directly by providing a serial number for one asymmetric key, or
367 a group of keys may be searched for the signing key by providing the
368 serial number for a keyring.
369
370 When the "chain" option is provided at the end of the string, the keys
371 within the destination keyring will also be searched for signing keys.
372 This allows for verification of certificate chains by adding each
373 certificate in order (starting closest to the root) to a keyring. For
374 instance, one keyring can be populated with links to a set of root
375 certificates, with a separate, restricted keyring set up for each
376 certificate chain to be validated::
377
378 # Create and populate a keyring for root certificates
379 root_id=`keyctl add keyring root-certs "" @s`
380 keyctl padd asymmetric "" $root_id < root1.cert
381 keyctl padd asymmetric "" $root_id < root2.cert
382
383 # Create and restrict a keyring for the certificate chain
384 chain_id=`keyctl add keyring chain "" @s`
385 keyctl restrict_keyring $chain_id asymmetric key_or_keyring:$root_id:chain
386
387 # Attempt to add each certificate in the chain, starting with the
388 # certificate closest to the root.
389 keyctl padd asymmetric "" $chain_id < intermediateA.cert
390 keyctl padd asymmetric "" $chain_id < intermediateB.cert
391 keyctl padd asymmetric "" $chain_id < end-entity.cert
392
393 If the final end-entity certificate is successfully added to the "chain"
394 keyring, we can be certain that it has a valid signing chain going back to
395 one of the root certificates.
396
397 A single keyring can be used to verify a chain of signatures by
398 restricting the keyring after linking the root certificate::
399
400 # Create a keyring for the certificate chain and add the root
401 chain2_id=`keyctl add keyring chain2 "" @s`
402 keyctl padd asymmetric "" $chain2_id < root1.cert
403
404 # Restrict the keyring that already has root1.cert linked. The cert
405 # will remain linked by the keyring.
406 keyctl restrict_keyring $chain2_id asymmetric key_or_keyring:0:chain
407
408 # Attempt to add each certificate in the chain, starting with the
409 # certificate closest to the root.
410 keyctl padd asymmetric "" $chain2_id < intermediateA.cert
411 keyctl padd asymmetric "" $chain2_id < intermediateB.cert
412 keyctl padd asymmetric "" $chain2_id < end-entity.cert
413
414 If the final end-entity certificate is successfully added to the "chain2"
415 keyring, we can be certain that there is a valid signing chain going back
416 to the root certificate that was added before the keyring was restricted.
417
418
419 In all of these cases, if the signing key is found the signature of the key to
420 be linked will be verified using the signing key. The requested key is added
421 to the keyring only if the signature is successfully verified. -ENOKEY is
422 returned if the parent certificate could not be found, or -EKEYREJECTED is
423 returned if the signature check fails or the key is blacklisted. Other errors
424 may be returned if the signature check could not be performed.
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