+/* Compute the HMAC keyed-hash function using the given hash algorithm, data,
+ * and key. */
+void compute_hmac(GChecksumType algo,
+ const guchar *data, gsize data_len,
+ const guchar *key, gsize key_len,
+ guint8 *buffer, gsize *digest_len)
+{
+ int block_size;
+
+ switch (algo) {
+ case G_CHECKSUM_MD5:
+ case G_CHECKSUM_SHA1:
+ case G_CHECKSUM_SHA256:
+ block_size = 64;
+ break;
+ default:
+ g_error("Unknown hash algorithm for HMAC: %d\n", algo);
+ }
+
+ gsize digest_size = g_checksum_type_get_length(algo);
+
+ guchar keybuf[block_size];
+ memset(keybuf, 0, block_size);
+ memcpy(keybuf, key, MIN(block_size, key_len));
+ for (int i = 0; i < block_size; i++)
+ keybuf[i] ^= 0x36;
+
+ GChecksum *csum1 = g_checksum_new(algo);
+ g_checksum_update(csum1, keybuf, block_size);
+ g_checksum_update(csum1, data, data_len);
+ guint8 digest[digest_size];
+ g_checksum_get_digest(csum1, digest, &digest_size);
+
+ memset(keybuf, 0, block_size);
+ memcpy(keybuf, key, MIN(block_size, key_len));
+ for (int i = 0; i < block_size; i++)
+ keybuf[i] ^= 0x5c;
+
+ GChecksum *csum2 = g_checksum_new(algo);
+ g_checksum_update(csum2, keybuf, block_size);
+ g_checksum_update(csum2, digest, digest_size);
+
+ g_checksum_get_digest(csum2, buffer, digest_len);
+
+ g_checksum_free(csum1);
+ g_checksum_free(csum2);
+}
+