| Kyle Swenson | 8d8f654 | 2021-03-15 11:02:55 -0600 | [diff] [blame] | 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 |