hmac principle
Computing a hash function HMAC requires hash (or may be md5 sha-1) key and a key. L hash function output is represented by long strings (MD5 16), B is represented by the length of the data block (sha-1 and MD5 divided data block length is 64). The encryption key length may be less than equal to the data block length B, is greater than if the data block length, may use the hash function to the key conversion, the result is a long L key.
According to RFC 2316 (Report of the IAB, April 1998), HMAC (Hashed Message Authentication Code: Hashed Message Authentication Code) is considered, and IPSec security critical core protocol Interact. It is not a hash function, instead of using the MD5 message authentication mechanisms or SHA1 hash function with the shared secret key (public key / private key pair different) used together. Basically, in combination with the key message and runs a hash function. Then run the results combined with the key and run the hash function once again. The 128-bit result is truncated to 96 become MAC.
Then create two different long strings B:
- innerpad = length of B 0 × 36
- outterpad = length of B 0 × 5C
HMAC calculation input string str:
- hash(key ^ outterpad, hash(key ^ innerpad, str))
hmac applications
hmac mainly used in identity verification, its use is this:
- Client requests to sign (assume GET request is the browser)
- The server returns a random value and the random value recorded in the session
- The client random value as a key, the user password hmac operation, and submitted to the server
- User database server reads the user password and the random value transmitted in the step 2 and the client doing the same calculation hmac, and then compared with the results sent by the user, the user if the verification results are consistent legitimate
In this process, it is subject to security attacks are random values and hmac users to send the results sent by the server, and for the interception of the two values of hackers these two values makes no sense, no get user password introduced the possibility to make a random value hmac only valid in the current session, greatly enhancing the safety and practicality. Most languages have achieved hmac algorithm, such as php's mhash, python's hmac.py, java class of MessageDigest, hmac use in web verification is possible, md5 speed operation with js is also relatively fast.
Python
>>> import hmac
>>> message = b'Hello, world!'
>>> key = b'secret'
>>> h = hmac.new(key, message, digestmod='MD5')
>>> # 如果消息很长，可以多次调用h.update(msg)
>>> h.hexdigest()
'fa4ee7d173f2d97ee79022d1a7355bcf'
