Java 加密过程中有一段代码看不明白希望高手指点

public static String hmacSign(String aValue, String aKey) {
byte k_ipad[] = new byte[64];
byte k_opad[] = new byte[64];
byte keyb[];
byte value[];
try {
keyb = aKey.getBytes(encodingCharset);
value = aValue.getBytes(encodingCharset);
} catch (UnsupportedEncodingException e) {
keyb = aKey.getBytes();
value = aValue.getBytes();
}

Arrays.fill(k_ipad, keyb.length, 64, (byte) 54);
Arrays.fill(k_opad, keyb.length, 64, (byte) 92);
for (int i = 0; i < keyb.length; i++) {
k_ipad[i] = (byte) (keyb[i] ^ 0x36);
k_opad[i] = (byte) (keyb[i] ^ 0x5c);
}

MessageDigest md = null;
try {
md = MessageDigest.getInstance("MD5");
} catch (NoSuchAlgorithmException e) {

return null;
}
md.update(k_ipad);
md.update(value);
byte dg[] = md.digest();
md.reset();
md.update(k_opad);