How to Validate JWT Tokens from AWS Cognito in Go?

Validating JWT Tokens from AWS Cognito in Go

This article addresses the challenge of validating and extracting information from JWT tokens issued by Amazon Cognito. The integration process with Google authentication and Cognito's token endpoint is discussed, along with common pitfalls.

Obtaining the Public Key

To validate JWT tokens, a public key is required. Cognito provides a JSON Web Key (JWK) set containing public keys at:

https://cognito-idp.{region}.amazonaws.com/{userPoolId}/.well-known/jwks.json

This file structure can be manually parsed to generate public keys, but using a library like jwx (https://github.com/lestrrat-go/jwx) simplifies the process.

Token Verification Using JWT-Go

Once public keys are available, jwt-go (https://github.com/dgrijalva/jwt-go) can be leveraged to verify tokens. The following steps outline the process:

1. Parse the JWK set using jwx:

keySet, err := jwk.Fetch(THE_COGNITO_URL_DESCRIBED_ABOVE)

2. When parsing the token with jwt-go, utilize the "kid" field to locate the appropriate key for verification:

token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
    if _, ok := token.Method.(*jwt.SigningMethodRS256); !ok {
        return nil, fmt.Errorf("Unexpected signing method: %v", token.Header["alg"])
    }
    kid, ok := token.Header["kid"].(string)
    if !ok {
        return nil, errors.New("kid header not found")
    }
    keys := keySet.LookupKeyID(kid)
    if !ok {
        return nil, fmt.Errorf("key with specified kid is not present in jwks")
    }
    var publickey interface{}
    err = keys.Raw(&publickey)
    if err != nil {
        return nil, fmt.Errorf("could not parse pubkey")
    }
    return publickey, nil
})

By following these steps, developers can effectively validate and parse JWT tokens from AWS Cognito in Go, ensuring the authenticity and integrity of the tokens.

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