headerpayloadsignature

Headerpayloadsignature is commonly used to describe the three-part structure of a JSON Web Token (JWT): the header, the payload, and the signature, which are combined into a single string separated by dots. This three-part form is sometimes summarized as header.payload.signature.

In a typical JWT, the header contains metadata such as the token type (JWT) and the signing algorithm (for example, HS256 or RS256). The payload carries claims about the subject and other metadata, such as issuer, expiration time, and audience. The signature is produced by signing the base64url-encoded header and payload using a secret key (symmetric) or a private key (asymmetric) with the specified algorithm.

Encoding and construction involve converting the header and payload to JSON, base64url-encoding them without padding, and then creating the signature over the string formed by the encoded header, a dot, and the encoded payload. The final token takes the form: base64url(header).base64url(payload).base64url(signature).

Verification requires splitting the token into its three parts, recomputing the signature over the header and payload with the appropriate key and algorithm, and comparing it to the provided signature. Applications of headerpayloadsignature include authentication and authorization in OAuth 2.0 and OpenID Connect, where tokens carry claims and can be validated by the recipient.

Security considerations include protecting signing keys, enforcing proper algorithm usage, validating expiration and not-before times, and avoiding embedding sensitive data in the payload. Transport security (HTTPS) and careful token handling are essential.