State and nonce seem to be similar. But if you dig deep, you will find that they serve different purposes.
State is there to protect the end user from cross site request forgery(CSRF) attacks. It is introduced from OAuth 2.0 protocol RFC6749. Protocol states that,
Once authorization has been obtained from the end-user, the
authorization server redirects the end-user's user-agent back to the
client with the required binding value contained in the "state"
parameter. The binding value enables the client to verify the
validity of the request by matching the binding value to the
user-agent's authenticated state
And this is used in authorization request. It enables the client to validate that the authorization response is not altered and sent by the original server which auth. request was sent. In short, it allows the client to cross check the authorization request and response.
(More elaboration : To accept authorization code response, client need to accept a response from authorization server (ex:- In web app, this can be done by a redirect and a form post to back-end). This means, our client application have an endpoint which is open and accept requests. State parameter protect this endpoint by binding original authorization requests to responses. This is CSRF protection.)
Nonce serves a different purpose. It binds the tokens with the client. It serves as a token validation parameter and is introduced from OpenID Connect specification.
nonce - String value used to associate a Client session with an ID Token, and to mitigate replay attacks. The value is passed through unmodified from the Authentication Request to the ID Token. If present in the ID Token, Clients MUST verify that the nonce Claim Value is equal to the value of the nonce parameter sent in the Authentication Request. If present in the Authentication Request, Authorization Servers MUST include a nonce Claim in the ID Token with the Claim Value being the nonce value sent in the Authentication Request. Authorization Servers SHOULD perform no other processing on nonce values used. The nonce value is a case sensitive string
As you can see, nonce value originates from the authorization request and it is generated by the client. And if nonce is included, it will be present in the token. So the client can validate the token it received against the initial authorization request, thus ensuring the validity of the token.
Also, depending on the flow type, nonce can be a mandatory parameter. The implicit flow and hybrid flow mandate nonce value. Both values are generated and validated by client application.
Why state could not be reused?
If an authorization request is captured, then the malicious party can fake the authorization response. This can be avoided by altering state parameter.
