How to Compute a MAC of a File

A message authentication code, or MAC, computes a digest for the file and uses a secret key to further protect the digest. A MAC does not alter the original file.

1. List the available mechanisms.

   % mac -l
   Algorithm       Keysize:  Min   Max
   -----------------------------------
   des_mac                    64    64
   sha1_hmac                   8   512
   md5_hmac                    8   512
   sha224_hmac                 8   512
   sha256_hmac                 8   512
   sha384_hmac                 8  1024
   sha512_hmac                 8  1024

   ---

   Note -  Each supported algorithm is an alias to the most commonly used and least restricted version of a particular algorithm type. The output above shows available algorithm names and the keysize for each algorithm. Whenever possible, use a supported algorithm that matches a FIPS-approved algorithm with a FIPS-approved key length, listed at FIPS 140 Algorithms in the Cryptographic Framework in Using a FIPS 140 Enabled System in Oracle Solaris 11.2 .

   ---

2. Generate a symmetric key of the appropriate length.

   You can provide either a passphrase from which a key will be generated or you can provide a key.

   • If you provide a passphrase, you must store or remember the passphrase. If you store the passphrase online, the passphrase file should be readable only by you.

   • If you provide a key, it must be the correct size for the mechanism. You can use the pktool command. For the procedure and some examples, see How to Generate a Symmetric Key by Using the pktool Command.

3. Create a MAC for a file.

   Provide a key and use a symmetric key algorithm with the mac command.

   % mac [-v] -a algorithm [-k keyfile | -K key-label [-T token]] input-file

   –v

   Displays the output in the following format:

   algorithm (input-file) = mac

   –a algorithm

   The algorithm to use to compute the MAC. Type the algorithm as the algorithm appears in the output of the mac -l command.

   –k keyfile

   The file that contains a key of algorithm-specified length.

   –K key-label

   The label of a key in the PKCS #11 keystore.

   –T token

   The token name. By default, the token is Sun Software PKCS#11 softtoken. It is used only when the –K key-label option is used.

   input-file

   The input file for the MAC.

Example 3-5  Computing a MAC With SHA1_HMAC and a Passphrase

In the following example, the email attachment is authenticated with the SHA1_HMAC mechanism and a key that is derived from a passphrase. The MAC listing is saved to a file. If the passphrase is stored in a file, the file should not be readable by anyone but the user.

% mac -v -a sha1_hmac email.attach
Enter passphrase: Type passphrase
sha1_hmac (email.attach) = 2b31536d3b3c0c6b25d653418db8e765e17fe07b
% echo "sha1_hmac (email.attach) = 2b31536d3b3c0c6b25d653418db8e765e17fe07b" \
>> ~/sha1hmac.daily.05.12

Example 3-6  Computing a MAC With SHA1_HMAC and a Key File

In the following example, the directory manifest is authenticated with the SHA1_HMAC mechanism and a secret key. The results are placed in a file.

% mac -v -a sha1_hmac \
-k $HOME/keyf/05.07.mack64 docs/* > $HOME/mac.docs.legal.05.07
% more ~/mac.docs.legal.05.07
sha1_hmac (docs/legal1) = 9b31536d3b3c0c6b25d653418db8e765e17fe07a
sha1_hmac (docs/legal2) = 865af61a3002f8a457462a428cdb1a88c1b51ff5
sha1_hmac (docs/legal3) = 076c944cb2528536c9aebd3b9fbe367e07b61dc7
sha1_hmac (docs/legal4) = 7aede27602ef6e4454748cbd3821e0152e45beb4

Example 3-7  Computing a MAC With SHA1_HMAC and a Key Label

In the following example, the directory manifest is authenticated with the SHA1_HMAC mechanism and a secret key. The results are placed in the user's PKCS #11 keystore. The user initially created the keystore and the password to the keystore by using the pktool setpin command.

% mac -a sha1_hmac -K legaldocs0507 docs/*
Enter pin for Sun Software PKCS#11 softtoken:Type password

To retrieve the MAC from the keystore, the user uses the verbose option, and provides the key label and the name of the directory that was authenticated.

% mac -v -a sha1_hmac -K legaldocs0507  docs/*
Enter pin for Sun Software PKCS#11 softtoken:Type password
sha1_hmac (docs/legal1) = 9b31536d3b3c0c6b25d653418db8e765e17fe07a
sha1_hmac (docs/legal2) = 865af61a3002f8a457462a428cdb1a88c1b51ff5
sha1_hmac (docs/legal3) = 076c944cb2528536c9aebd3b9fbe367e07b61dc7
sha1_hmac (docs/legal4) = 7aede27602ef6e4454748cbd3821e0152e45beb4