# PKCE support for LINE Login

# What is PKCE?

PKCE (Proof Key for Code Exchange) is an OAuth2.0 extension specification defined in RFC7636 (opens new window), intended to combat authorization code interception attacks.

The OAuth2.0 authorization flow that doesn't use PKCE is vulnerable to user-specific access tokens being stolen if a malicious app somehow gets the custom URI containing the authorization code. By implementing the PKCE authorization flow into web apps that incorporate LINE Login, you can further improve the security of LINE Login v2.1 and prevent authorization code interception attacks.

# Benefits of implementing PKCE for LINE Login

The behavior against authorization code interception attacks differs depending on whether PKCE is implemented or not in the web app that uses LINE Login. We recommend implementing PKCE to make your web app more secure.

Without PKCE implemented With PKCE implemented
If a malicious app somehow gets a callback URL containing an authorization code, it can steal an access token.
Authorization code interception attack when PKCE isn't implemented
Even if a malicious application steals the information passed during the redirection, it can be checked against a unique code_challenge to prevent access tokens from being stolen.
Authorization code interception attack when PKCE is implemented
Another benefit of implementing PKCE

If you access a web app that incorporates PKCE-implemented LINE login from the Yahoo! JAPAN app (opens new window), the auto login function that lets you skip the login process using your email address and password will be enabled.

Auto login from the Yahoo! JAPAN app

# Implement PKCE for LINE Login

To implement PKCE for LINE Login, follow these four steps, as well as the Integrating LINE Login with your web app process.

How to implement PKCE

  1. Generate code_verifier.
  2. Generate code_challenge based on thecode_verifier generated in step 1.
  3. Redirect a user to the authorization URL given to the query parameter with the code_challenge and code_challenge_method generated in step 2.
  4. Add the code_verifier generated in step 1 to the request body of the "Issue access token" API endpoint and execute it.
New parameters for PKCE support

To support PKCE, these parameters have been added to the LINE Login "Authorization URL" and "Issue access token" API endpoints.

  • code_verifier
  • code_challenge
  • code_challenge_method

For more information about each parameter, see the detailed description of each step below.

# 1. Generate code_verifier

On the web app side, a unique code_verifier is generated when the user executes a LINE Login. The code_verifier spec is based on RFC7636 (opens new window).

Parameter

Parameter Specs Example
code_verifier Available character types: A random string consisting of half-width alphanumeric characters (a-z, A-Z, 0-9) and symbols (-._~)
Character count: 43-128 characters
wJKN8qz5t8SSI9lMFhBB6qwNkQBkuPZoCxzRhwLRUo1

Sample code

This is a sample code for generating code_verifier using Node.js:

// randomAlphaNumericString() is supported to be a function that generates and returns a random string consisting of
// available characters (half-width alphanumeric characters and symbols) for the integer specified in the argument (43 to 128).
const code_verifier = randomAlphaNumericString(43);

# 2. Generate code_challenge

You can generate a code_challenge by encrypting the generated code_verifier with SHA256 and then encoding it in Base64URL format.

Parameter

Parameter Specs Example
code_challenge The value of code_verifier encrypted with SHA256 and encoded in Base64URL format BSCQwo_m8Wf0fpjmwkIKmPAJ1A7tiuRSNDnXzODS7QI
Format for URL query parameters

The value of code_challenge needs to be deleted or replaced from the normal Base64 format string so that it can be used as a URL query parameter. For more information, see 5. Base 64 Encoding with URL and Filename Safe Alphabet (opens new window) in RFC 4648.

  • Remove padding (character stuffing =)
  • Replace + with -
  • Replace / with _
Base64 format example Deletion and replacement for code_challenge example
BSCQwo_m8Wf0fpjmwk+KmPAJ1A/tiuRSNDnXzODS7== BSCQwo_m8Wf0fpjmwk-KmPAJ1A_tiuRSNDnXzODS7

Sample code

This is a sample code for generating code_challenge using Node.js:

// This sample code uses the Node.js "crypto" module.
// See: https://nodejs.org/api/crypto.html#crypto_crypto
const crypto = require("crypto");

// Encode BASE64 format into BASE64URL format.
function base64UrlEncode(str) {
    return str
        .replace(/\+/g, '-')
        .replace(/\//g, '_')
        .replace(/=/g, '');
}

// Encrypt code_verifier with SHA256 and encode it into BASE64URL format to generate code_challenge.
const code_challenge = base64UrlEncode(crypto
    .createHash('sha256')
    .update(code_verifier)
    .digest('base64'));

# 3. Add code_challenge andcode_challenge_method in the query parameters of the authorization URL

Include code_challenge andcode_challenge_method in the query parameters of the normal LINE Login authorization URL.

Parameter

Parameter Type Required Description
code_challenge String Optional The code_challenge generated in step 2. The default value is null. If no value is specified, the request doesn't support PKCE.
code_challenge_method String Optional S256 (Represents the hash function SHA256.)

Note: The RFC7636 "Client Creates the Code Challenge" (opens new window) defines plain (unencrypted) as well as S256 as the method for generating code_challenge, but LINE Login only supports S256 for security reasons.

Authorization URL example

https://access.line.me/oauth2/v2.1/authorize?response_type=code&client_id=1234567890&redirect_uri=https%3A%2F%2Fexample.com%2Fauth&state=12345abcde&scope=profile%20openid&nonce=09876xyz
&code_challenge={The value of code_challenge calculated in step 2}&code_challenge_method=S256

For more information on other query parameters for authorization URLs, see Authenticating users and making authorization requests.

# 4. Issue an access token by specifying code_verifier in the request body

Run with code_verifier in the request body of the Issue access token API endpoint.

Request body

code_verifier

String

Optional

The code_verifier generated in step 1.
(Example: wJKN8qz5t8SSI9lMFhBB6qwNkQBkuPZoCxzRhwLRUo1)

Example request

curl -v -X POST https://api.line.me/oauth2/v2.1/token \
-H 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'grant_type=client_credentials' \
--data-urlencode 'client_assertion_type=urn:ietf:params:oauth:client-assertion-type:jwt-bearer' \
--data-urlencode 'client_assertion={JWT}'
--data-urlencode 'code_verifier={The code_verifier generated in step 1}'

For more information about the "Issue access token" API endpoint, see Issue access token in the LINE Login v2.1 API reference.