1==================================
2Digital 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
16Introduction
17============
18
19Digital signature verification API provides a method to verify digital signature.
20Currently digital signatures are used by the IMA/EVM integrity protection subsystem.
21
22Digital signature verification is implemented using cut-down kernel port of
23GnuPG multi-precision integers (MPI) library. The kernel port provides
24memory allocation errors handling, has been refactored according to kernel
25coding style, and checkpatch.pl reported errors and warnings have been fixed.
26
27Public 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
47keyid equals to SHA1[12-19] over the total key content.
48Signature header is used as an input to generate a signature.
49Such approach insures that key or signature header could not be changed.
50It protects timestamp from been changed and can be used for rollback
51protection.
52
53API
54===
55
56API 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
78User-space utilities
79====================
80
81The signing and key management utilities evm-utils provide functionality
82to generate signatures, to load keys into the kernel keyring.
83Keys can be in PEM or converted to the kernel format.
84When the key is added to the kernel keyring, the keyid defines the name
85of the key: 5D2B05FC633EE3E8 in the example bellow.
86
87Here 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
102