+++ /dev/null
- =============================================
- ASYMMETRIC / PUBLIC-KEY CRYPTOGRAPHY KEY TYPE
- =============================================
-
-Contents:
-
- - Overview.
- - Key identification.
- - Accessing asymmetric keys.
- - Signature verification.
- - Asymmetric key subtypes.
- - Instantiation data parsers.
- - Keyring link restrictions.
-
-
-========
-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 asymmetric 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 (*query)(const struct kernel_pkey_params *params,
- struct kernel_pkey_query *info);
- int (*eds_op)(struct kernel_pkey_params *params,
- const void *in, void *out);
- int (*verify_signature)(const struct key *key,
- const struct public_key_signature *sig);
- };
-
-Asymmetric keys point to this with their payload[asym_subtype] 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) query().
-
- Mandatory. This is a function for querying the capabilities of a key.
-
- (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().
-
- 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
-==========================
-
-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.
- - PKCS#8 private key [RFC 5208].
- - PKCS#5 encrypted private key [RFC 2898].
-
-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/core.rst].
-
- struct key_preparsed_payload {
- char *description;
- void *payload[4];
- 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, ->payload[asym_subtype] should be
- set to point to the subtype to be used, ->payload[asym_crypto] should be
- set to point to the initialised data for that subtype,
- ->payload[asym_key_ids] should point to one or more hex fingerprints and
- quotalen should be updated to indicate how much quota this key should
- account for.
-
- When clearing up, the data attached to ->payload[asym_key_ids] and
- ->description will be kfree()'d and the data attached to
- ->payload[asm_crypto] will be passed to the subtype's ->destroy() method
- to be disposed of. A module reference for the subtype pointed to by
- ->payload[asym_subtype] 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.
-
-
-=========================
-KEYRING LINK RESTRICTIONS
-=========================
-
-Keyrings created from userspace using add_key can be configured to check the
-signature of the key being linked. Keys without a valid signature are not
-allowed to link.
-
-Several restriction methods are available:
-
- (1) Restrict using the kernel builtin trusted keyring
-
- - Option string used with KEYCTL_RESTRICT_KEYRING:
- - "builtin_trusted"
-
- The kernel builtin trusted keyring will be searched for the signing key.
- If the builtin trusted keyring is not configured, all links will be
- 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
-
- - Option string used with KEYCTL_RESTRICT_KEYRING:
- - "builtin_and_secondary_trusted"
-
- The kernel builtin and secondary trusted keyrings will be searched for the
- signing key. If the secondary trusted keyring is not configured, this
- restriction will behave like the "builtin_trusted" option. The ca_keys
- kernel parameter also affects which keys are used for signature
- verification.
-
- (3) Restrict using a separate key or keyring
-
- - Option string used with KEYCTL_RESTRICT_KEYRING:
- - "key_or_keyring:<key or keyring serial number>[:chain]"
-
- Whenever a key link is requested, the link will only succeed if the key
- being linked is signed by one of the designated keys. This key may be
- specified directly by providing a serial number for one asymmetric key, or
- a group of keys may be searched for the signing key by providing the
- serial number for a keyring.
-
- When the "chain" option is provided at the end of the string, the keys
- within the destination keyring will also be searched for signing keys.
- This allows for verification of certificate chains by adding each
- 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:
-
- # Create and populate a keyring for root certificates
- root_id=`keyctl add keyring root-certs "" @s`
- keyctl padd asymmetric "" $root_id < root1.cert
- keyctl padd asymmetric "" $root_id < root2.cert
-
- # Create and restrict a keyring for the certificate chain
- chain_id=`keyctl add keyring chain "" @s`
- keyctl restrict_keyring $chain_id asymmetric key_or_keyring:$root_id:chain
-
- # Attempt to add each certificate in the chain, starting with the
- # certificate closest to the root.
- keyctl padd asymmetric "" $chain_id < intermediateA.cert
- keyctl padd asymmetric "" $chain_id < intermediateB.cert
- keyctl padd asymmetric "" $chain_id < end-entity.cert
-
- If the final end-entity certificate is successfully added to the "chain"
- keyring, we can be certain that it has a valid signing chain going back to
- 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:
-
- # Create a keyring for the certificate chain and add the root
- chain2_id=`keyctl add keyring chain2 "" @s`
- keyctl padd asymmetric "" $chain2_id < root1.cert
-
- # Restrict the keyring that already has root1.cert linked. The cert
- # will remain linked by the keyring.
- keyctl restrict_keyring $chain2_id asymmetric key_or_keyring:0:chain
-
- # Attempt to add each certificate in the chain, starting with the
- # certificate closest to the root.
- keyctl padd asymmetric "" $chain2_id < intermediateA.cert
- keyctl padd asymmetric "" $chain2_id < intermediateB.cert
- keyctl padd asymmetric "" $chain2_id < end-entity.cert
-
- If the final end-entity certificate is successfully added to the "chain2"
- keyring, we can be certain that there is a valid signing chain going back
- to the root certificate that was added before the keyring was restricted.
-
-
-In all of these cases, if the signing key is found the signature of the key to
-be linked will be verified using the signing key. The requested key is added
-to the keyring only if the signature is successfully verified. -ENOKEY is
-returned if the parent certificate could not be found, or -EKEYREJECTED is
-returned if the signature check fails or the key is blacklisted. Other errors
-may be returned if the signature check could not be performed.
projects / linux-2.6-block.git / blobdiff