Root/Documentation/digsig.txt

1Digital Signature Verification API
2
3CONTENTS
4
51. Introduction
62. API
73. User-space utilities
8
9
101. Introduction
11
12Digital signature verification API provides a method to verify digital signature.
13Currently digital signatures are used by the IMA/EVM integrity protection subsystem.
14
15Digital signature verification is implemented using cut-down kernel port of
16GnuPG multi-precision integers (MPI) library. The kernel port provides
17memory allocation errors handling, has been refactored according to kernel
18coding style, and checkpatch.pl reported errors and warnings have been fixed.
19
20Public key and signature consist of header and MPIs.
21
22struct 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
30struct 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
40keyid equals to SHA1[12-19] over the total key content.
41Signature header is used as an input to generate a signature.
42Such approach insures that key or signature header could not be changed.
43It protects timestamp from been changed and can be used for rollback
44protection.
45
462. API
47
48API 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 */
67int digsig_verify(struct key *keyring, const char *sig, int siglen,
68                        const char *data, int datalen);
69
703. User-space utilities
71
72The signing and key management utilities evm-utils provide functionality
73to generate signatures, to load keys into the kernel keyring.
74Keys can be in PEM or converted to the kernel format.
75When the key is added to the kernel keyring, the keyid defines the name
76of the key: 5D2B05FC633EE3E8 in the example bellow.
77
78Here is example output of the keyctl utility.
79
80$ keyctl show
81Session Keyring
82       -3 --alswrv 0 0 keyring: _ses
83603976250 --alswrv 0 -1 \_ keyring: _uid.0
84817777377 --alswrv 0 0 \_ user: kmk
85891974900 --alswrv 0 0 \_ encrypted: evm-key
86170323636 --alswrv 0 0 \_ keyring: _module
87548221616 --alswrv 0 0 \_ keyring: _ima
88128198054 --alswrv 0 0 \_ keyring: _evm
89
90$ keyctl list 128198054
911 key in keyring:
92620789745: --alswrv 0 0 user: 5D2B05FC633EE3E8
93
94
95Dmitry Kasatkin
9606.10.2011
97

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