Understanding the id_token in JWT

JWT (JSON Web Token) is a compact, URL-safe format for representing claims to be transferred between two parties. In this blog, we will break down the structure of a JWT id_token and explain its utility in the authentication process, particularly in enhancing security and streamlining user validation.

What is a JWT?

A JWT is a string made up of three parts: Header, Payload, and Signature. Each section is base64-url encoded and separated by dots (.). Here’s a breakdown of each part using a sample JWT:

Sample JWT

eyJraWQiOiJwazAxODMiLCJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJzdWIiOiI1MzEiLCJhd WQiOiIxMTQzMWNudiIsImF1dGhfdGltZSI6IjE2OTcyMDI4NzMiLCJpc3MiOiJodHRwczovL290c Gxlc3MuY29tIiwibmFtZSI6IkFzaHdpbiIsInBob25lX251bWJlcl92ZXJpZmllZCI6InRydWUiLCJhdX RoZW50aWNhdGlvbl9kZXRhaWxzIjoibnVsbCIsInBob25lX251bWJlciI6Iis5MTcwNDI1MDc2NDY iLCJleHAiOjE2OTcxODY4MjQsImlhdCI6MTY5NzE4MzIyNH0.m5BwCX59ZpMjhsv8WNxURMu dCjhrm0m7NqEncTqXBENEdMjxp4F7kWbgCfSFuVTzi0xvG3nj8d-LvX1tfXyHwn98Tj-JA4rQbV ORjmkM9gLpEag6c9B0VpHHewNmUBXDS0rOzBQhsui7-jTgB1V388EqSHe-c7dvXndkxETQy wwcPgnT9_AID5hRVlMChfF11mA2bhqwRyzHBAYTxbk2wzY7YbQIxk58NDRV7elSPSSO-uewe L_M4_opiEkjIX0xk4AvZbD_dLtzIknhQ26X7cmxaATeCHN9Jqzoy9eTCRO5l0aIIVYwOdl4Y-y9t-h 6TEVj80gTirN0CoB_NGmxUw

Decoded JWT

  • Header
      "kid": "pk0183",
      "typ": "JWT",
      "alg": "RS256"
  • Payload
      "sub": "531",
      "aud": "11431cnv",
      "auth_time": "1697202873",
      "iss": "https://otpless.com",
      "name": "Ashwin",
      "phone_number_verified": "true",
      "authentication_details": "null",
      "phone_number": "+917042507646",
      "exp": 1697186824,
      "iat": 1697183224
  • Signature Securely verifies the token using HMAC SHA256 algorithm.

Why Use JWT for Authentication?

JWTs are designed to carry a significant amount of information as claims. Claims are statements about an entity (typically, the user) and additional metadata. There are several benefits to using JWTs:

  • Compact: Can be sent through URL, POST parameter, or inside HTTP header.
  • Self-contained: The payload contains all the required information about the user, avoiding the need to query the database more than once.
  • Secure: Signature ensures that the token isn’t altered.

Verification of JWT

Verifying the JWT’s signature is crucial for ensuring that the token’s integrity and authenticity are maintained. Here’s what needs to be verified:

  • Signature: Validate it against the public key.
  • Issuer (iss): Confirm it matches the expected issuer.
  • Audience (aud): Ensure it matches the expected client ID.
  • Expiration (exp): The current date/time must be before the expiration date/time listed in the JWT.
  • Issued at (iat): Ensure that the issuance date is reasonable (e.g., the token was not issued in the future).


JWTs streamline the authentication process by minimizing the need for multiple database hits and providing a secure method to transfer user data. By understanding the structure and usage of id_token, developers can implement more efficient and secure web applications.