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 102