After designing some APIs, I found out that you must have at some point a *token* system to authenticate to your server. For the simplest design, the server attributes a unique token for each client, this token will then be sent over each request to authenticate.
An easy MITM attack could be to repeat a previously sent request while the token is still valid or even catch the client *token* and build a malicious request with its authentication.
> In this document we will assume that what travels through the network is public, so any MITM can store it. Obviously I highly recommend using TLS for communicating, but we look here for a consistent token system, it only can be better through TLS.
A good alternative could be to use a *one-time token* where each request features a unique token. This design implies that the token is sent from the server to the client before being used ; so any MITM can catch it. To overcome this flaw we could have a system where *one key* is used to generate all the tokens in a consistent manner. If so, the tokens do not have to be sent and can be guessed from the key. But this also implies that the key is shared at some point between the client and the server.
A better solution would be to generate a private key on each client and use it to generate a token for each request. Also, to avoid to share the key and allow the server to easily check tokens, we would like a system where each token can be checked from the previous token without having to know the private key (because it is private...). It would avoid attackers to repeat requests (because each token is unique) or guess the private key because it never travels over the network. In addition short-lived one-time passwords have a mechanism that we could use to build a time-dependent system.
1. Mixing 2 hashes in a way that without one of them, the other is *cryptographically impossible* to guess (*i.e. [one-time pad](https://en.wikipedia.org/wiki/One-time_pad)*).
2. Having a time-dependent unique feature, that could be found only a few seconds after sending it (as for *[TOTP](https://tools.ietf.org/html/rfc6238)*).
3. A cryptographic hash function that, from an input of any length, outputs a fixed-length digest in a way that is *impossible* to guess the input back from it.
1. a <u>Stateless Time Scrambling Protocol</u> to take care of the request's invalidation over time.
2. a <u>Stateless Cyclic Hash Algorithm</u> to use a private key as a one-time token generator in a way that no clue is given over published tokens (*i.e. one-way function*).
4. A <u>rescue protocol</u> to resynchronise the client with a new key in a way that no clue is given over the network and the client has to process a "proof of work".
| $h(m)$ | The digest of the message $m$ by a (consistent) secure cryptographic hash function $h()$ ; *e.g. sha512* |
| $h^n(m)$ | The digest of the $n$-recursive hash function $h()$ with the input data $m$ ; <br>$h^2(m) \equiv h(h(m))$, $h^1(m) \equiv h(m)$, and $h^0(m) \equiv m$. |
These variables are both on the server and clients. They are specific to the server so each client must match its values. These variables shape the system's **context** $(W, min, belt, max)$.
| $W$ | time window | A number of seconds that is typically the maximum transmission time from end to end. It will be used by the *time-scrambling protocol*. The lower the number, the less time an attacker has to try to brute-force the tokens. |
| $min$ | resynchonization range | A number that is used to resynchronize the client if there is a communication issue (*e.g. lost request, lost response, attack*). The higher the value, the higher the challenge for the client to recover the authentication, thus the harder for an attacker to guess it. |
| $belt$ | security range | A number that is used to resynchronize the client if there is a communication shift (*e.g. lost request, lost response, attack*). It corresponds of the number of desynchronizations the client can handle before never being able to gain the authentication again. |
| $max$ | maximum nonce | A number that is used to cap the value of client's nonces. A too high value will result on keys that will never be replaced, thus making them open to long-processing attacks (*e.g. brute-force*). |
Every client holds a **keyset** $(K, n, s)$. It represents its private key and is used to generate the tokens. The secure hash function is extended to a **one-way function** to generate all the tokens from the keyset. Note that the client may hold a secondary keyset between the generation of a new keyset and the server's validation of it.
| $s$ | key state | A number that reflects the state of the keyset. It is used to know what to do on the **next request** : <br>- $0$ : normal request<br>- $1$ : will switch to the new key<br>- $2$ : rescue proof of work sent, waiting for the server's acknowledgement |
- 1 : `SWITCH` - default protocol variation to switch to a new keyset when the current one is consumed (*i.e. when $n$ if less or equal to $min+belt$*).
When the client switches to a new key, it has to store the new keyset along the current one, in order not to lose its authentication if the network fails.
This protocol is processed when the server sends the 2 hashes $(y_1, y_2)$ to the client (instead of the standard response). It means that the server has received a wrong hash, so it sends the rescue challenge to the client.
When receiving an invalid token, the server may sent back to the client a challenge to get the authentication back. This challenge is formed by 2 hashes $(y_1, y_2)$.
