Documentation / crypto / asymmetric-keys.txt

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Based on kernel version 3.9. Page generated on 2013-05-02 22:56 EST.

			=============================================
			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.

• [ crypto ]
• api-intro.txt
• asymmetric-keys.txt
• async-tx-api.txt
• descore-readme.txt
•