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