crypto: digital signature verification support

This patch implements RSA digital signature verification using GnuPG library.

The format of the signature and the public key is defined by their respective
headers. The signature header contains version information, algorithm,
and keyid, which was used to generate the signature.
The key header contains version and algorythim type.
The payload of the signature and the key are multi-precision integers.

The signing and key management utilities evm-utils provide functionality
to generate signatures and load keys into the kernel keyring.
When the key is added to the kernel keyring, the keyid defines the name
of the key.

Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Acked-by: Mimi Zohar <zohar@us.ibm.com>

1 files changed, 96 insertions, 0 deletions

diff --git a/Documentation/digsig.txt b/Documentation/digsig.txt
new file mode 100644
index 000000000000..3f682889068b
--- /dev/null
+++ b/Documentation/digsig.txt
@@ -0,0 +1,96 @@
+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