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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	==================================
2	Digital Signature Verification API
3	==================================
4	
5	:Author: Dmitry Kasatkin
6	:Date: 06.10.2011
7	
8	
9	.. CONTENTS
10	
11	   1. Introduction
12	   2. API
13	   3. User-space utilities
14	
15	
16	Introduction
17	============
18	
19	Digital signature verification API provides a method to verify digital signature.
20	Currently digital signatures are used by the IMA/EVM integrity protection subsystem.
21	
22	Digital signature verification is implemented using cut-down kernel port of
23	GnuPG multi-precision integers (MPI) library. The kernel port provides
24	memory allocation errors handling, has been refactored according to kernel
25	coding style, and checkpatch.pl reported errors and warnings have been fixed.
26	
27	Public key and signature consist of header and MPIs::
28	
29		struct pubkey_hdr {
30			uint8_t		version;	/* key format version */
31			time_t		timestamp;	/* key made, always 0 for now */
32			uint8_t		algo;
33			uint8_t		nmpi;
34			char		mpi[0];
35		} __packed;
36	
37		struct signature_hdr {
38			uint8_t		version;	/* signature format version */
39			time_t		timestamp;	/* signature made */
40			uint8_t		algo;
41			uint8_t		hash;
42			uint8_t		keyid[8];
43			uint8_t		nmpi;
44			char		mpi[0];
45		} __packed;
46	
47	keyid equals to SHA1[12-19] over the total key content.
48	Signature header is used as an input to generate a signature.
49	Such approach insures that key or signature header could not be changed.
50	It protects timestamp from been changed and can be used for rollback
51	protection.
52	
53	API
54	===
55	
56	API currently includes only 1 function::
57	
58		digsig_verify() - digital signature verification with public key
59	
60	
61		/**
62		* digsig_verify() - digital signature verification with public key
63		* @keyring:	keyring to search key in
64		* @sig:	digital signature
65		* @sigen:	length of the signature
66		* @data:	data
67		* @datalen:	length of the data
68		* @return:	0 on success, -EINVAL otherwise
69		*
70		* Verifies data integrity against digital signature.
71		* Currently only RSA is supported.
72		* Normally hash of the content is used as a data for this function.
73		*
74		*/
75		int digsig_verify(struct key *keyring, const char *sig, int siglen,
76				  const char *data, int datalen);
77	
78	User-space utilities
79	====================
80	
81	The signing and key management utilities evm-utils provide functionality
82	to generate signatures, to load keys into the kernel keyring.
83	Keys can be in PEM or converted to the kernel format.
84	When the key is added to the kernel keyring, the keyid defines the name
85	of the key: 5D2B05FC633EE3E8 in the example bellow.
86	
87	Here is example output of the keyctl utility::
88	
89		$ keyctl show
90		Session Keyring
91		-3 --alswrv      0     0  keyring: _ses
92		603976250 --alswrv      0    -1   \_ keyring: _uid.0
93		817777377 --alswrv      0     0       \_ user: kmk
94		891974900 --alswrv      0     0       \_ encrypted: evm-key
95		170323636 --alswrv      0     0       \_ keyring: _module
96		548221616 --alswrv      0     0       \_ keyring: _ima
97		128198054 --alswrv      0     0       \_ keyring: _evm
98	
99		$ keyctl list 128198054
100		1 key in keyring:
101		620789745: --alswrv     0     0 user: 5D2B05FC633EE3E8
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