linux-libre/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