diff --git a/backend/boltathon/util/blockstack.py b/backend/boltathon/util/blockstack.py
new file mode 100644
index 0000000..f056583
--- /dev/null
+++ b/backend/boltathon/util/blockstack.py
@@ -0,0 +1,27 @@
+import requests
+from jose import jwt
+from flask import current_app
+from boltathon.util.errors import RequestError
+
+# Given a Blockstack private key, validate that it was signed by the same
+# key as owns a username
+def validate_blockstack_auth(id, username, token):
+    try:
+        # Match up data with arguments
+        token = jwt.decode(token, [], options={ 'verify_signature': False })
+        if username != token.get('username'):
+            return False
+        if id not in token.get('profile_url'):
+            return False
+
+        # Match up data with Blockstack's atlas node
+        res = requests.get(f'https://core.blockstack.org/v1/names/{username}')
+        res = res.json()
+        if res.get('address') != id:
+            return False
+
+        return True
+    except Exception as e:
+        print(e)
+        current_app.logger.error(f'Could not lookup username {username} from blockstack.org: {e}')
+        raise RequestError(code=500, message='Failed to lookup Blockstack identity')
diff --git a/backend/boltathon/views/api.py b/backend/boltathon/views/api.py
index 83789f1..7d7d445 100644
--- a/backend/boltathon/views/api.py
+++ b/backend/boltathon/views/api.py
@@ -8,6 +8,7 @@
 from boltathon.util.node import get_pubkey_from_credentials, make_invoice, lookup_invoice
 from boltathon.util.errors import RequestError
 from boltathon.util.mail import send_email_once
+from boltathon.util.blockstack import validate_blockstack_auth
 from boltathon.models.user import User, self_user_schema, public_user_schema, public_users_schema
 from boltathon.models.connection import Connection, public_connections_schema
 from boltathon.models.tip import Tip, tip_schema, tips_schema
@@ -164,10 +165,14 @@ def get_tip(tip_id):
 @use_args({
   'id': fields.Str(required=True),
   'username': fields.Str(required=True),
+  'token': fields.Str(required=True),
 })
 def blockstack_auth(args):
-  # TODO: Server-side verification
-   # Find or create a new user and add the connection
+  # Assert that they generated a valid token
+  if not validate_blockstack_auth(args.get('id'), args.get('username'), args.get('token')):
+    raise RequestError(code=400, message='Invalid Blockstack token provided')
+
+  # Find or create a new user and add the connection
   user = Connection.get_user_by_connection(
       site='blockstack',
       site_id=args.get('id'),
diff --git a/backend/requirements.txt b/backend/requirements.txt
index ed7fd39..3013539 100644
--- a/backend/requirements.txt
+++ b/backend/requirements.txt
@@ -19,3 +19,6 @@ bitstring==3.1.5
 webargs==5.2.0
 flask-cors==3.0.7
 sendgrid==6.0.4
+
+# JWT package that supports ES256K for blockstack
+git+git://github.com/pohutukawa/python-jose@28cd302d#egg=python-jose
diff --git a/frontend/src/api.ts b/frontend/src/api.ts
index 88cc876..7c17d06 100644
--- a/frontend/src/api.ts
+++ b/frontend/src/api.ts
@@ -93,6 +93,7 @@ class API {
     return this.request<SelfUser>('POST', '/auth/blockstack', {
       id: data.identityAddress,
       username: data.username,
+      token: data.authResponseToken,
     });
   }
 
diff --git a/frontend/src/pages/BlockstackAuth.tsx b/frontend/src/pages/BlockstackAuth.tsx
index 2d7d9ff..104014a 100644
--- a/frontend/src/pages/BlockstackAuth.tsx
+++ b/frontend/src/pages/BlockstackAuth.tsx
@@ -1,20 +1,39 @@
 import React from 'react';
-import { Loader } from 'semantic-ui-react';
+import { Loader, Message } from 'semantic-ui-react';
 import { withRouter, RouteComponentProps } from 'react-router';
 import * as blockstack from 'blockstack.js';
 import { UserData } from 'blockstack.js/lib/auth/authApp';
-import api, { User } from '../api';
+import api from '../api';
+
+interface State {
+  error: string | null;
+}
+
+class BlockstackAuth extends React.Component<RouteComponentProps, State> {
+  state: State = {
+    error: null,
+  };
 
-class BlockstackAuth extends React.Component<RouteComponentProps> {
   componentDidMount() {
     if (blockstack.isUserSignedIn()) {
       this.auth(blockstack.loadUserData());
     } else if (blockstack.isSignInPending()) {
-      blockstack.handlePendingSignIn().then(this.auth)
+      blockstack.handlePendingSignIn().then(this.auth);
     }
   }
 
   render() {
+    const { error } = this.state;
+
+    if (error) {
+      return (
+        <Message negative>
+          <Message.Header>Failed to authenticate with Blockstack</Message.Header>
+          <Message.Content>{error}</Message.Content>
+        </Message>
+      );
+    }
+
     return <Loader size="huge" active>Connecting to Blockstack...</Loader>;
   }
 
@@ -23,9 +42,9 @@ class BlockstackAuth extends React.Component<RouteComponentProps> {
     api.blockstackAuth(data).then(res => {
       history.replace('/user/me');
     }).catch(err => {
-      alert(err.message);
+      this.setState({ error: err.message });
     });
   };
 };
 
-export default withRouter(BlockstackAuth);
\ No newline at end of file
+export default withRouter(BlockstackAuth);