Showcasing Google Cloud Workload Identity Federation from AWS using raw http requests

aws-raw.ts

import { Sha256 } from '@aws-crypto/sha256-universal'; // or @aws-crypto/sha256-js
import { AssumeRoleCommand, STSClient } from '@aws-sdk/client-sts';
import { SignatureV4 } from '@aws-sdk/signature-v4';
import axios from 'axios';

const AWS_REGION = 'eu-west-1';
const AWS_ROLE_ARN = 'arn:aws:iam::999999999999:role/my-federated-role';
const GCP_IDENTITY_PROVIDER = '//iam.googleapis.com/projects/PROJECTNR/locations/global/workloadIdentityPools/POOL-ID/providers/PROVIDER-ID';
const GCP_SERVICE_ACCOUNT = 'ID@PROJECT.iam.gserviceaccount.com';

(async () => {
    try {
        // use IAM user credentials to get temporary security credentials. Normally, this step is not necessary as using google workload
        // identity federation from an AWS identity is intended to be used from AWS infrastructure that already has an attached IAM Role
        const stsClient = new STSClient({ region: AWS_REGION });
        const sessionName = new Date().toISOString().replace(/[:.-]/g, '');
        const assumeRoleResponse = await stsClient.send(new AssumeRoleCommand({ RoleArn: AWS_ROLE_ARN, RoleSessionName: sessionName }));

        // create a signer utility from the AWS SDK to create a Signature Version 4 (SigV4)
        const signer = new SignatureV4({
            applyChecksum: false, // do not include a x-amz-content-sha256 header as google sts will throw a 400 when given unexpected headers
            service: 'sts',
            region: AWS_REGION,
            sha256: Sha256,
            credentials: {
                // use the temporary security credentials we got from 'sts assume-role'
                accessKeyId: assumeRoleResponse.Credentials.AccessKeyId,
                secretAccessKey: assumeRoleResponse.Credentials.SecretAccessKey,
                sessionToken: assumeRoleResponse.Credentials.SessionToken,
            },
        });

        // Build a signed request to AWS STS to get our own identity (which is the assumed IAM Role), but do not execute this request.
        // Instead, we are going to give this fully signed request to google so it can execute this request on our behalf as a means to
        // prove our identity.
        // Depending on your programming language and AWS SDK there might be better ways to build a SigV4 signed request.
        const signedRequest = await signer.sign({
            method: 'POST',
            protocol: 'https', // not used for signing
            hostname: `sts.${AWS_REGION}.amazonaws.com`, // not used for signing
            path: '/',
            query: {
                Action: 'GetCallerIdentity',
                Version: '2011-06-15',
            },
            headers: {
                'host': `sts.${AWS_REGION}.amazonaws.com`,
                'x-goog-cloud-target-resource': GCP_IDENTITY_PROVIDER,
            },
        });
        console.log('signed request:');
        console.log(signedRequest);
        /*
        {
            method: 'POST',
            protocol: 'https',
            hostname: 'sts.eu-west-1.amazonaws.com',
            path: '/',
            headers: {
                host: 'sts.eu-west-1.amazonaws.com',
                'x-goog-cloud-target-resource': '//iam.googleapis.com/....',
                'x-amz-date': '20210531T163536Z',
                'x-amz-security-token': 'IQoJ....1h/u',
                authorization: 'AWS4-HMAC-SHA256 Credential=ASIA....TYOH/20210531/eu-west-1/sts/aws4_request, SignedHeaders=host;x-amz-date;x-amz-security-token;x-goog-cloud-target-resource, Signature=187e....9d71'
            },
            query: { Action: 'GetCallerIdentity', Version: '2011-06-15' }
        }
        */

        // Convert the signed request to a format as google wants to receive it
        // see https://cloud.google.com/iam/docs/access-resources-aws#exchange-token
        const getCallerIdentityToken = {
            // https://sts.${AWS_REGION}.amazonaws.com?Action=GetCallerIdentity&Version=2011-06-15
            url: `${signedRequest.protocol}://${signedRequest.hostname}${signedRequest.path}?${new URLSearchParams(
                signedRequest.query as Record<string, string>,
            ).toString()}`,
            method: signedRequest.method,
            // google wants to receive the headers as an array of objects with a key and value property
            headers: Object.entries(signedRequest.headers).map(([key, value]) => ({ key, value })),
        };
        console.log('-'.repeat(80));
        console.log('getCallerIdentityToken:');
        console.log(getCallerIdentityToken);
        /*
        {
            url: 'https://sts.eu-west-1.amazonaws.com/?Action=GetCallerIdentity&Version=2011-06-15',
            method: 'POST',
            headers: [
                { key: 'host', value: 'sts.eu-west-1.amazonaws.com' },
                { key: 'x-goog-cloud-target-resource', value: '//iam.googleapis.com/....' },
                { key: 'x-amz-date', value: '20210531T163817Z' },
                { key: 'x-amz-security-token', value: 'IQoJ....1h/u' },
                { key: 'authorization', value: 'AWS4-HMAC-SHA256 Credential=ASIA....TYOH/20210531/eu-west-1/sts/aws4_request, SignedHeaders=host;x-amz-date;x-amz-security-token;x-goog-cloud-target-resource, Signature=187e....9d71' }
            ]
        }
        */

        // Invoke google sts service to get a federated access token.
        const federated = await axios.post<FederatedToken>('https://sts.googleapis.com/v1/token', {
            audience: GCP_IDENTITY_PROVIDER,
            grantType: 'urn:ietf:params:oauth:grant-type:token-exchange',
            requestedTokenType: 'urn:ietf:params:oauth:token-type:access_token',
            scope: 'https://www.googleapis.com/auth/cloud-platform',
            subjectTokenType: 'urn:ietf:params:aws:token-type:aws4_request',
            // send the aws sts request as an uri-encoded JSON object
            subjectToken: encodeURIComponent(JSON.stringify(getCallerIdentityToken)),
        });
        console.log('-'.repeat(80));
        console.log('federated access token:');
        console.log(federated.data);
        /*
        {
            access_token: 'ya29.d.Kt0E....iHPg',
            issued_token_type: 'urn:ietf:params:oauth:token-type:access_token',
            token_type: 'Bearer',
            expires_in: 900
        }
        */

        // Sign a custom token with the federated service account
        // see https://cloud.google.com/iam/docs/reference/credentials/rest/v1/projects.serviceAccounts/signJwt
        // The token will be signed by the service account and the associated public key that is needed to validate the token can be
        // retrieved in several formats from the following endpoints:
        // - RSA public key wrapped in an X.509 v3 certificate: https://www.googleapis.com/service_accounts/v1/metadata/x509/{ACCOUNT_EMAIL}
        // - Raw key in JSON format: https://www.googleapis.com/service_accounts/v1/metadata/raw/{ACCOUNT_EMAIL}
        // - JSON Web Key (JWK): https://www.googleapis.com/service_accounts/v1/metadata/jwk/{ACCOUNT_EMAIL}
        const customTokenPayload = {
            sub: 'user@example.com',
            aud: 'https://example.com',
            iat: Math.floor(Date.now() / 1000),
            exp: Math.floor(Date.now() / 1000) + 300,
        };
        const signedCustomToken = await axios.post<{ keyId: string; signedJwt: string }>(
            `https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/${GCP_SERVICE_ACCOUNT}:signJwt`,
            {
                payload: JSON.stringify(customTokenPayload),
            },
            { headers: { authorization: `Bearer ${federated.data.access_token}` } },
        );
        console.log('-'.repeat(80));
        console.log('signJwt result:');
        console.log(signedCustomToken.data);
        /*
            decoded token header:
            {
                "alg": "RS256",
                "kid": "e8c4....e35a",
                "typ": "JWT"
            }
            decoded token payload:
            {
                "sub": "user@example.com",
                "aud": "https://example.com",
                "iat": 1622480215,
                "exp": 1622480515
            }
        */

        // Get an OIDC id-token for the federated service account
        // see https://cloud.google.com/iam/docs/reference/credentials/rest/v1/projects.serviceAccounts/generateIdToken
        const idtoken = await axios.post<{ token: string }>(
            `https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/${GCP_SERVICE_ACCOUNT}:generateIdToken`,
            {
                audience: 'https://example.com',
                includeEmail: false,
            },
            { headers: { authorization: `Bearer ${federated.data.access_token}` } },
        );
        console.log('-'.repeat(80));
        console.log('generateIdToken result:');
        console.log(idtoken.data);
        /*
            decoded token header:
            {
                "alg": "RS256",
                "kid": "1719....08de",
                "typ": "JWT"
            }
            decoded token payload:
            {
                "aud": "https://example.com",
                "azp": "1000....6293", // numerical unique id of the service account
                "exp": 1622483939,
                "iat": 1622480339,
                "iss": "https://accounts.google.com",
                "sub": "1000....6293" // numerical unique id of the service account
            }
        */

        // Only a limited number of Google Cloud APIs support federated tokens. Call `generateAccessToken` to get a "service account
        // access token" which can be used to invoke any Google Cloud API this service account has access to.
        // see https://cloud.google.com/iam/docs/reference/credentials/rest/v1/projects.serviceAccounts/generateAccessToken
        // But if you want to invoke Google API's, it is much more convenient to use the `google-auth-library` for your programming
        // language as that will handle all the fetching (and refreshing) of tokens.
        const serviceAccountToken = await axios.post<{ accessToken: string; expireTime: string }>(
            `https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/${GCP_SERVICE_ACCOUNT}:generateAccessToken`,
            {
                // see https://developers.google.com/identity/protocols/oauth2/scopes
                scope: ['https://www.googleapis.com/auth/cloud-platform'],
            },
            { headers: { authorization: `Bearer ${federated.data.access_token}` } },
        );
        // Example of invoking a google cloud api with the service account access token
        const projects = await axios.get('https://cloudresourcemanager.googleapis.com/v1/projects', {
            headers: { authorization: `Bearer ${serviceAccountToken.data.accessToken}` },
        });
        console.log('-'.repeat(80));
        console.log('GET projects result (with service account access token):');
        console.log(projects.data);
    } catch (e) {
        if (axios.isAxiosError(e)) {
            console.log('ERROR RESPONSE', e.response?.data);
        }
        throw e;
    }
})().catch(e => {
    console.log(e);
    process.exit(1);
});

// see https://cloud.google.com/iam/docs/reference/sts/rest/v1/TopLevel/token#response-body
interface FederatedToken {
    access_token: string;
    issued_token_type: 'urn:ietf:params:oauth:token-type:access_token'; // same as requestedTokenType from token request
    token_type: 'Bearer';
    expires_in: number;
}