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OAuth 2.0 This document defines the Nonce Endpoint for OAuth 2.0 implementations . It details how an Authorization Server generates and issues opaque Nonces and how a client can learn about this endpoint to obtain a Nonce generated by the Authorization Server. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Source for this draft and an issue tracker can be found at .

Introduction This specification presents a comprehensive guide to the Nonce endpoint in OAuth 2.0 implementations . It describes in detail how a client can request and receive a server-generated Nonce, which is a unique, one-time use, opaque string. This document provides in-depth insights into the cryptographic methods used in generating Nonces to protect the confidentiality of the information associated with them. In addition, it is a great resource for developers and system architects who desire to strengthen the scalability, security, and efficiency of their systems while using OAuth 2.0.

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Terminology

Nonce:

A random or pseudo-random number that is generated for a specific use, typically for cryptographic communication. The Nonce is used to protect against replay attacks by ensuring that a message or data cannot be reused or retransmitted. The term "Nonce" stands for "number used once" and it MUST be unique within some scope.

Nonce Issuer:

The entity that generates and provides the Nonce. In the context of OAuth 2.0, the Nonce Issuer would typically be the Authorization Server.

Nonce Endpoint:

The HTTP endpoint provided by the Nonce Issuer for the issuance of the Nonces.

Requirements The Nonce Endpoint MUST satisfy the following requirements:

• Employ TLS for securitung the Endpoint .

The Nonce MUST satisfy the following requirements:

• Pure opacity to recieving Clients.

The Nonce Issuer MUST satisfy the following requirements:

• Generate a unique Nonce for each request to ensure the Nonce Issuer never produces identical Nonces, regardless of whether they occur simultaneously or at different times;
• Encrypt the Nonce with an encryption key that:
  • MUST NOT supplied by the Nonce Issuer to the Client;
  • MUST NOT disclosed by the Nonce Issuer to any external entity beyond its domain.

The audiences of the Nonces satisfies the following requirements:

• The servers, within the Nonce Issuer's domain, SHOULD decrypt the Nonce and access its decrypted contents. No other entity might decrypt or know the decrypted contents of the Nonce.

Nonce Request When a Client needs a Nonce, it sends an HTTP GET request to the Nonce Endpoint. Below is a non normative example of the HTTP Request made by a Client to the Nonce Endpoint.

Nonce Response The Nonce Endpoint provides a Nonce to the Client, encapsulated within a JSON object . The response MUST use the HTTP Header Content-Type value set to application/json and MUST provide in the response message a JSON object with the member nonce. Below is a non-normative example of the response given by a Nonce Endpoint:

Nonce Endpoint Discovery When an Authorization Server requires the use of a Nonce in the request for a specific resource and the Client does not provide it in its request, the Authorization Server MUST return an HTTP response with the HTTP status code 400 and an error field with the value set to "nonce_required". This response MUST also contain the Nonce-Endpoint-URI HTTP header, with the value set to the URL corresponding to the Nonce Endpoint, where the Client SHOULD request and fetch a new Nonce. Once the Nonce is received, the Client MAY renew the request to the Authorization Server, including the obtained Nonce. Below is a non-normative example of an error response issued by an Authorization Server that requires the Nonce in the Client request, the response informs the Client about the Nonce Endpoint where the Nonce should be requested: In cases where, for some reasons, a correctly issued Nonce can no longer be considered valid by the Authorization Server that receives it, the Authorization Server MUST return the generic error "nonce_required" reporting the same description as "error_description", as if the Nonce had not been received. The cases when an issued Nonce is considered no longer valid MAY be caused by the rotation of the encryption keys, its expiration or other specific conditions internal to an implementation.

Non-normative Examples of a Nonce Payload The decrypted Nonce payload may use different formats and encodings, according to the different implemententative requirements, and contains any kind of implementation-specific claims, such as the issuance time, the time of expiration, the audiences and other where needed. Below are provided some non-normative examples, describing how a decrypted and JSON serialized Nonce payload may appear: Please note that the values represented in the previous examples may depend on domain specific requirements and MUST NOT be intended as normative.

Security Considerations The Nonce Endpoint MUST be protected by TLS to prevent eavesdropping and man-in-the-middle attacks, therefore the practices defined in should be followed. The Nonce Issuer MUST securely generate and store the encryption key used to encrypt the Nonce. The robustness of the encryption key plays a crucial role in the security of the Nonce Endpoint. The following considerations MUST be taken into account:

1. Key Strength: The cryptographic key used to encrypt the Nonce requires sufficient length to withstand brute-force attacks. A key length of 256 bits has been proposed as a common practice to ensure a minimum level of security.
2. Key Management: The cryptographic key requires secure management, which includes secure generation, storage, and revocation. Access to the key necessitates strict control, with access granted only to authorized entities.
3. Key Rotation: Regular key rotation is a good practice to mitigate the risk of key compromise. The frequency of key rotation depends on the specific requirements and threat model, but a common practice is to rotate keys frequently.
4. Randomness: To assure the randomness of the cryptographic key, it requires the usage of a safe random number generator. Attackers can simply guess predictable keys.
5. Secure Transmission: If the cryptographic key needs to be transmitted over a network and within the Nonce Issuer domain, it requires the usage of secure protocols such as TLS.
6. Backup and Recovery: Cryptographic keys require secure backup and recovery mechanisms. This ensures that the key can be retrieved in the event of its loss while also prohibiting unauthorised access to the backup.

The security of the Nonce Endpoint is only as strong as the security of the encryption key. Therefore, proper key management practices are essential.

Considerations about Nonce vs. jti This section provides some thought about the main differences and scopes of the Nonce in compared to the jti claim defined in . Both jti and Nonces are used to prevent replay attacks, however Nonces offer more implementation flexibility and are considered best practice. They can be created and managed stateless (e.g., by issuing the hmac over the current time as the Nonce), as this document outlines. The main differences between the use of the jti and the Nonces can be summarized as follows:

1. Generation: Nonces are generated by the server, while jti is generated by the Client.
2. Storage: Nonces can be self-authenticating and self-contained and therefore need not be stored. A common way to achieve this is for the Nonce to contain content encrypted to the Authorization Server that creates it. On the other hand, checking jti properly definitely requires a store that is shared across all domains that the associated JWT can be presented in.
3. Lifetime: The life span difference between a Nonce and a jti is significant. Nonces are kept just until the Client responds, which happens practically immediately after they are obtained, resulting in a very short lifespan. A jti, on the other hand, must be stored until the expiration window of its associated JWT expires, which is a substantially longer length than that of a Nonce.
4. Security: Nonces prevent replay attacks by ensuring that the proof of possession is fresh. On the other hand, jti does not guarantee freshness and using client-generated timestamps has problems, even for non-attacking Clients (e.g. devices with incorrect time-zones or daylight saving settings).

IANA Considerations This document has no IANA actions.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] The OAuth 2.0 authorization framework enables a third-party application to obtain limited access to an HTTP service, either on behalf of a resource owner by orchestrating an approval interaction between the resource owner and the HTTP service, or by allowing the third-party application to obtain access on its own behalf. This specification replaces and obsoletes the OAuth 1.0 protocol described in RFC 5849. [STANDARDS-TRACK] JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document formally deprecates Transport Layer Security (TLS) versions 1.0 (RFC 2246) and 1.1 (RFC 4346). Accordingly, those documents have been moved to Historic status. These versions lack support for current and recommended cryptographic algorithms and mechanisms, and various government and industry profiles of applications using TLS now mandate avoiding these old TLS versions. TLS version 1.2 became the recommended version for IETF protocols in 2008 (subsequently being obsoleted by TLS version 1.3 in 2018), providing sufficient time to transition away from older versions. Removing support for older versions from implementations reduces the attack surface, reduces opportunity for misconfiguration, and streamlines library and product maintenance. This document also deprecates Datagram TLS (DTLS) version 1.0 (RFC 4347) but not DTLS version 1.2, and there is no DTLS version 1.1. This document updates many RFCs that normatively refer to TLS version 1.0 or TLS version 1.1, as described herein. This document also updates the best practices for TLS usage in RFC 7525; hence, it is part of BCP 195. Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are used to protect data exchanged over a wide range of application protocols and can also form the basis for secure transport protocols. Over the years, the industry has witnessed several serious attacks on TLS and DTLS, including attacks on the most commonly used cipher suites and their modes of operation. This document provides the latest recommendations for ensuring the security of deployed services that use TLS and DTLS. These recommendations are applicable to the majority of use cases. RFC 7525, an earlier version of the TLS recommendations, was published when the industry was transitioning to TLS 1.2. Years later, this transition is largely complete, and TLS 1.3 is widely available. This document updates the guidance given the new environment and obsoletes RFC 7525. In addition, this document updates RFCs 5288 and 6066 in view of recent attacks.

Acknowledgments TODO acknowledge.