krb5: Add _krb5_evp_hmac_iov

Add a function which will perform an HMAC over a set of iovecs,
using the hcrypto provided HMAC functions.

Join contiguous iovecs together before passing them to the hash
function so we make as few calls into the hash as possible.

lib/krb5/crypto-evp.c

@@ -104,6 +104,49 @@ out:
     return ret;
 }
 
+krb5_error_code
+_krb5_evp_hmac_iov(krb5_context context,
+                   struct _krb5_key_data *key,
+                   const struct krb5_crypto_iov *iov,
+                   int niov,
+                   void *hmac,
+                   unsigned int *hmaclen,
+                   const EVP_MD *md,
+                   ENGINE *engine)
+{
+    HMAC_CTX *ctx;
+    krb5_data current = {0, 0};
+    int i;
+
+    ctx = HMAC_CTX_new();
+    if (ctx == NULL)
+        return krb5_enomem(context);
+
+    HMAC_Init_ex(ctx, key->key->keyvalue.data, key->key->keyvalue.length,
+                 md, engine);
+
+    for (i = 0; i < niov; i++) {
+        if (_krb5_crypto_iov_should_sign(&iov[i])) {
+	    if ((char *)current.data + current.length == iov[i].data.data) {
+		current.length += iov[i].data.length;
+	    } else {
+		if (current.data)
+		    HMAC_Update(ctx, current.data, current.length);
+		current = iov[i].data;
+	    }
+	}
+    }
+
+    if (current.data)
+	HMAC_Update(ctx, current.data, current.length);
+
+    HMAC_Final(ctx, hmac, hmaclen);
+
+    HMAC_CTX_free(ctx);
+
+    return 0;
+}
+
 krb5_error_code
 _krb5_evp_encrypt(krb5_context context,
 		struct _krb5_key_data *key,