[deliverable/linux.git] / Documentation / digsig.txt

Digital Signature Verification API

CONTENTS

1. Introduction
2. API
3. User-space utilities


1. Introduction

Digital signature verification API provides a method to verify digital signature.
Currently digital signatures are used by the IMA/EVM integrity protection subsystem.

Digital signature verification is implemented using cut-down kernel port of
GnuPG multi-precision integers (MPI) library. The kernel port provides
memory allocation errors handling, has been refactored according to kernel
coding style, and checkpatch.pl reported errors and warnings have been fixed.

Public key and signature consist of header and MPIs.

struct pubkey_hdr {
	uint8_t		version;	/* key format version */
	time_t		timestamp;	/* key made, always 0 for now */
	uint8_t		algo;
	uint8_t		nmpi;
	char		mpi[0];
} __packed;

struct signature_hdr {
	uint8_t		version;	/* signature format version */
	time_t		timestamp;	/* signature made */
	uint8_t		algo;
	uint8_t		hash;
	uint8_t		keyid[8];
	uint8_t		nmpi;
	char		mpi[0];
} __packed;

keyid equals to SHA1[12-19] over the total key content.
Signature header is used as an input to generate a signature.
Such approach insures that key or signature header could not be changed.
It protects timestamp from been changed and can be used for rollback
protection.

2. API

API currently includes only 1 function:

	digsig_verify() - digital signature verification with public key


/**
 * digsig_verify() - digital signature verification with public key
 * @keyring:	keyring to search key in
 * @sig:	digital signature
 * @sigen:	length of the signature
 * @data:	data
 * @datalen:	length of the data
 * @return:	0 on success, -EINVAL otherwise
 *
 * Verifies data integrity against digital signature.
 * Currently only RSA is supported.
 * Normally hash of the content is used as a data for this function.
 *
 */
int digsig_verify(struct key *keyring, const char *sig, int siglen,
						const char *data, int datalen);

3. User-space utilities

The signing and key management utilities evm-utils provide functionality
to generate signatures, to load keys into the kernel keyring.
Keys can be in PEM or converted to the kernel format.
When the key is added to the kernel keyring, the keyid defines the name
of the key: 5D2B05FC633EE3E8 in the example bellow.

Here is example output of the keyctl utility.

$ keyctl show
Session Keyring
       -3 --alswrv      0     0  keyring: _ses
603976250 --alswrv      0    -1   \_ keyring: _uid.0
817777377 --alswrv      0     0       \_ user: kmk
891974900 --alswrv      0     0       \_ encrypted: evm-key
170323636 --alswrv      0     0       \_ keyring: _module
548221616 --alswrv      0     0       \_ keyring: _ima
128198054 --alswrv      0     0       \_ keyring: _evm

$ keyctl list 128198054
1 key in keyring:
620789745: --alswrv     0     0 user: 5D2B05FC633EE3E8


Dmitry Kasatkin
06.10.2011
Commit	Line	Data
051dbb91 DK	1	Digital Signature Verification API
	2
	3	CONTENTS
	4
	5	1. Introduction
	6	2. API
	7	3. User-space utilities
	8
	9
	10	1. Introduction
	11
	12	Digital signature verification API provides a method to verify digital signature.
	13	Currently digital signatures are used by the IMA/EVM integrity protection subsystem.
	14
	15	Digital signature verification is implemented using cut-down kernel port of
	16	GnuPG multi-precision integers (MPI) library. The kernel port provides
	17	memory allocation errors handling, has been refactored according to kernel
	18	coding style, and checkpatch.pl reported errors and warnings have been fixed.
	19
	20	Public key and signature consist of header and MPIs.
	21
	22	struct pubkey_hdr {
	23	uint8_t version; /* key format version */
	24	time_t timestamp; /* key made, always 0 for now */
	25	uint8_t algo;
	26	uint8_t nmpi;
	27	char mpi[0];
	28	} __packed;
	29
	30	struct signature_hdr {
	31	uint8_t version; /* signature format version */
	32	time_t timestamp; /* signature made */
	33	uint8_t algo;
	34	uint8_t hash;
	35	uint8_t keyid[8];
	36	uint8_t nmpi;
	37	char mpi[0];
	38	} __packed;
	39
	40	keyid equals to SHA1[12-19] over the total key content.
	41	Signature header is used as an input to generate a signature.
	42	Such approach insures that key or signature header could not be changed.
	43	It protects timestamp from been changed and can be used for rollback
	44	protection.
	45
	46	2. API
	47
	48	API currently includes only 1 function:
	49
	50	digsig_verify() - digital signature verification with public key
	51
	52
	53	/**
	54	* digsig_verify() - digital signature verification with public key
	55	* @keyring: keyring to search key in
	56	* @sig: digital signature
	57	* @sigen: length of the signature
	58	* @data: data
	59	* @datalen: length of the data
	60	* @return: 0 on success, -EINVAL otherwise
	61	*
	62	* Verifies data integrity against digital signature.
	63	* Currently only RSA is supported.
	64	* Normally hash of the content is used as a data for this function.
65	*
66	*/
67	int digsig_verify(struct key keyring, const char sig, int siglen,
68	const char *data, int datalen);
69
70	3. User-space utilities
71
72	The signing and key management utilities evm-utils provide functionality
73	to generate signatures, to load keys into the kernel keyring.
74	Keys can be in PEM or converted to the kernel format.
75	When the key is added to the kernel keyring, the keyid defines the name
76	of the key: 5D2B05FC633EE3E8 in the example bellow.
77
78	Here is example output of the keyctl utility.
79
80	$ keyctl show
81	Session Keyring
82	-3 --alswrv 0 0 keyring: _ses
83	603976250 --alswrv 0 -1 \_ keyring: _uid.0
84	817777377 --alswrv 0 0 \_ user: kmk
85	891974900 --alswrv 0 0 \_ encrypted: evm-key
86	170323636 --alswrv 0 0 \_ keyring: _module
87	548221616 --alswrv 0 0 \_ keyring: _ima
88	128198054 --alswrv 0 0 \_ keyring: _evm
89
90	$ keyctl list 128198054
91	1 key in keyring:
92	620789745: --alswrv 0 0 user: 5D2B05FC633EE3E8
93
94
95	Dmitry Kasatkin
96	06.10.2011