Menu

Expand
Rate this page:

Trusted Calling with SHAKEN/STIR

This page contains general information on SHAKEN/STIR, along with some implementation details.

If you're ready to enable SHAKEN/STIR on your account(s), go to the SHAKEN/STIR Onboarding page.

SHAKEN/STIR Overview

The Problem: Robocalls

Robocalls are calls that are created by an auto-dialer, and typically play pre-recorded messages. Fraudsters use robocalls in combination with spoofing (falsifying caller IDs) to acquire something of value from their victims. In 2020, there were 48.9 billion robocalls in the United States, leading to an erosion of trust in the telephone network and a decline in call answer rates from unidentified phone numbers.

The Solution: SHAKEN/STIR

SHAKEN stands for Signature-based Handling of Asserted Information using toKENs. It is a specification designed by the Alliance for Telecommunications Industry Solutions (ATIS) to fight caller ID spoofing. STIR (Secure Telephone Identity Revisited) is a protocol developed by the Internet Engineering Task Force (IETF) to enable end-to-end call authentication, but the protocol is very broad and doesn't ensure that different providers will be able to verify each others' calls. SHAKEN is a set of implementation details that follows the STIR protocol, but streamlines specifications to increase the likelihood of carrier interoperability. This is why you will often see the technology referred to as "SHAKEN/STIR" or "STIR/SHAKEN".

Essentially, SHAKEN/STIR uses encrypted digital signatures to share information about the caller to each provider along a call's path from caller to recipient, such as the caller's identity and whether the caller has the right to use the phone number they provided as the caller ID.

To learn more about the implementation of SHAKEN/STIR, read our blog post "Everything You Need to Know about SHAKEN/STIR Today".

Twilio, like all major carriers in the United States, has signing and verifying privileges. Keep reading to learn about the product changes you can expect when you to enable SHAKEN/STIR on your account(s).

Changes for Twilio Customers

As of 06/2021, support for the SHAKEN/STIR call authentication framework is being deployed in the United States only.

  • For Programmable Voice customers, a new parameter will be present in incoming webhooks and outgoing calls: StirVerstat.
  • For Elastic SIP Trunking customers, there is a new header called X-Twilio-VerStat, and a new Identity header with the SHAKEN PASSporT.

To understand the possible values for the StirVerstat parameter/X-Twilio-VerStat header, you will first need to understand the three different attestation levels:

  • A : the highest attestation given by the originating service provider to indicate that the caller is known and has the right to use the phone number as the caller ID
  • B : the customer is known, it is unknown if they have the right to use the caller ID being used
  • C : it doesn't meet the requirements of A or B including international calls.

The table below describes the possible values for the StirVerstat parameter/X-Twilio-VerStat header.

StirVerstat parameter / X-Twilio-VerStat header value Definition
TN-Validation-Passed-A

Twilio received the SIP INVITE, with a SHAKEN PASSporT, and was able to fetch the public certificate of the originating service provider from the Certificate Authority that signed the call to verify that no one tampered with the SIP INVITE during transit.

Attestation level A

TN-Validation-Passed-B

Twilio received the SIP INVITE, with a SHAKEN PASSporT, and was able to fetch the public certificate of the originating service provider from the Certificate Authority that signed the call to verify that no one tampered with the SIP INVITE during transit.

Attestation level B

TN-Validation-Passed-C

Twilio received the SIP INVITE, with a SHAKEN PASSporT, and was able to fetch the public certificate of the originating service provider from the Certificate Authority that signed the call to verify that no one tampered with the SIP INVITE during transit.

Attestation level C

TN-Validation-Failed-A

Twilio was unable to verify the contents of the SHAKEN PASSporT.

This could mean tampering, or simply that Twilio could not retrieve the public certificate of the originating service provider from the Certificate Authority.

Attestation level A

TN-Validation-Failed-B

Twilio was unable to verify the contents of the SHAKEN PASSporT.

This could mean tampering, or simply that Twilio could not retrieve the public certificate of the originating service provider from the Certificate Authority.

Attestation level B

TN-Validation-Failed-C

Twilio was unable to verify the contents of the SHAKEN PASSporT.

This could mean tampering, or simply that Twilio could not retrieve the public certificate of the originating service provider from the Certificate Authority.

Attestation level C

No-TN-Validation

Possible causes:

  • A malformed E.164 phone number
  • SHAKEN PASSporT format is invalid. It should consist of a header, payload, signature, and parameters.
  • SHAKEN PASSporT does not have required fields like ppt headers or info parameter
  • SHAKEN PASSporT orig field doesn't match with actual callerid in the SIP messages (P-Asserted-Identity, Remote-Party-Identity, or From header).
  • SHAKEN PASSporT dest field doesn't match with the actual destination of the call in the SIP Request-URI.
  • SHAKEN PASSporT iat field is too old – more than 1 minutes from current timestamp or the SIP Date header value.
TN-Validation-Failed

Twilio was unable to verify the contents of the SHAKEN PASSporT.

This could mean tampering, or simply that Twilio could not retrieve the public certificate of the originating service provider from the Certificate Authority.

No attestation level determined.

NULL

Incoming Calls

Twilio Programmable Voice and Elastic SIP Trunking now perform SHAKEN/STIR verification on incoming calls to your Twilio local phone numbers. It can be used to display a trust indicator or to make a routing decision, such as bypassing a voice captcha or IVR and directing the call to an end user.

A verified call that has been given the highest attestation under SHAKEN/STIR means that the carrier that originated the call both (1) knows the identity of the caller, and (2) knows the caller has the right to use the phone number as the caller ID.

Note: The new StirVerstatparameter/X-Twilio-VerStat header are only present for incoming calls with SHAKEN PASSporT identity headers. To take advantage of X-Twilio-VerStat header on inbound Elastic SIP Trunking calls you will need to reach out for it to be enabled.

Outgoing Calls

When your application receives a request webhook that has the new StirVerstat parameter, Twilio will implicitly pass the StirVerstat to the Client when you <Dial><Client>. The information in the StirVerstat parameter can be used to display a trust indicator to the recipient when an incoming call from the public telephone network has been verified under the SHAKEN/STIR framework.

The Javascript Client now has: Connection.CallerInfo.isVerified

The Android and iOS Mobile SDKs now have the CallerInfo object and TVOCallerInfo class to represent information about the caller.

Calls Status Callbacks

The Status Callback StirStatus optional parameter will inform you of the attestation Twilio gave your call to the public telephone network. If the call is forwarded (functionality coming soon), this will be attestation of the incoming call that was forwarded.

Call Forwarding - Coming Later

Anything marked as "Coming Later" is subject to change.

You may encounter situations in which an incoming call to your Twilio phone number needs to be forwarded to another number. If you want to maintain the original caller's CallerID for the forwarded leg of the call and facilitate SHAKEN/STIR verification, you will need to pass a CallToken from the parent leg of the call to the forwarded leg.

Use a CallToken when forwarding a call

When one of your Twilio phone numbers receives an incoming call, Twilio sends a webhook to your application. The request body of this webhook contains a CallToken property. The CallToken contains any SHAKEN/STIR and DIV (diversion) PASSporTs contained in the SIP headers of the incoming call.

In order to forward a call received by your Twilio number, you must:

  1. Use the value of the CallToken from the inbound call's incoming webhook as the CallToken parameter when creating a new Call Resource.
  2. Use the original caller's phone number as the From parameter when creating the new Call Resource.

Example call forwarding scenario

A caller (+12222222222) dials your Twilio phone number (+15555555555) and you need to forward the call to a new phone number (+18888888888).

Upon receiving the incoming call, your application will receive a webhook with the following request body:

{
  Called: '+15555555555',
  ToState: 'AL',
  CallerCountry: 'US',
  Direction: 'inbound',
  CallerState: 'PA',
  ToZip: '33333',
  CallSid: 'CAXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX',
  To: '+15555555555',
  CallerZip: '88888',
  ToCountry: 'US',
  StirVerstat: 'TN-Validation-Passed-A',
  CalledZip: '33333',
  ApiVersion: '2010-04-01',
  CalledCity: 'DOTHAN',
  CallStatus: 'ringing',
  From: '+12222222222',
  AccountSid: 'ACXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX',
  CalledCountry: 'US',
  CallerCity: 'PHILADELPHIA',
  StirPassportToken: 'STIR_TOKEN_IN_JWT_FORM',
  ToCity: 'DOTHAN',
  FromCountry: 'US',
  Caller: '+12222222222',
  FromCity: 'PHILADELPHIA',
  CalledState: 'AL',
  FromZip: '88888',
  FromState: 'PA'
  CallToken: '%7B%20%22parentCallInfoToken%22%3A%22eyJhbGciOiJFUzI1NiJ9.eyJjYWxsU2lkIjoiQ0FYWFhYLi4uLiIsImZyb20iOiIrMTIyMjIyMjIyMjIiLCJ0byI6IisxNTU1NTU1NTU1NSIsImlhdCI6IjE2MzU5NTc0MjMifQ.jVOxmCbSxxKg2fuzvDT_-PTStRRw_TrWgdh2QaZNzHQwvgwO6Qk_FRPFPYguJQn19x0yZqltPQfHwql4FJt_7g%22%2C%0A%20%20%22identityHeaderTokens%22%3A%5B%0A%20%20%20%20%20%20%22eyJhbGciOiJFUzI1NiIsInBwdCI6InNoYWtlbiIsInR5cCI6InBhc3Nwb3J0IiwieDV1IjoiaHR0cHM6Ly9leGFtcGxlLmNvbS9zb21lQ2VydGlmaWNhdGUucGVtIn0.eyJhdHRlc3QiOiJBIiwiZGVzdCI6eyJ0biI6WyIxNTU1NTU1NTU1NSJdfSwiaWF0IjoxNjM1OTU3NDIzLCJvcmlnIjp7InRuIjoiMTIyMjIyMjIyMjIifSwib3JpZ2lkIjoic29tZS1HVUlELWhlcmUifQ.ygPO2sImJR9MSqoRVD0CvnB2euv3RUYdupNEFS3wpgecs-yi8SU8FtYkkDypn7DC-siwdPY6vvVIx39y5Nb1sg%3Binfo%3D%3Chttps%3A%2F%2Fexample.com%2FsomeCertificate.pem%3E%3Balg%3DES256%3Bppt%3Dshaken%22%0A%20%20%20%20%20%20%5D%0A%7D'
}

To forward the call, you will retrieve the CallToken value from the incoming webhook (above) and use this value as the CallToken parameter when you create a new Call Resource.

The code sample below shows how to do that with your chosen Helper Library, cURL, and the Twilio CLI.

Note: You use the original caller's number (+12222222222) as the From parameter.

        
        
        

        Attestation with call forwarding

        If an incoming call does not have a SHAKEN PASSporT, Twilio will sign the call with its own SHAKEN PASSporT with Attestation C with DIV signing.

        Twilio will use the CallToken in the outgoing leg to verify the CallerID. If the CallerID doesn't match the CallToken, Twilio will reject the call with an error.

        The following conditions must be met in order for Twilio to forward the inbound SHAKEN PASSporT along with a DIV PASSporT for the outgoing (forwarded) call:

        • The CallerID of the outgoing call matches the CallerID in the CallToken of the outgoing call
        • The incoming call contains a SHAKEN PASSporT
        • The outbound carrier endpoint supports SHAKEN and DIV PASSporTs

        Carrier support for DIV PASSPorTs is limited at this time.

        Anatomy of a CallToken

        The CallToken property in an incoming call webhook is a string containing a URL-encoded JSON object.

        Once decoded, the CallToken will look like this:

        {
        
           "parentCallInfoToken": "eyJhbGciOiJFUzI1NiJ9.eyJjYWxsU2lkIjoiQ0FYWFhYLi4uLiIsImZyb20iOiIrMTIyMjIyMjIyMjIiLCJ0byI6IisxNTU1NTU1NTU1NSIsImlhdCI6IjE2MzU5NTc0MjMifQ.jVOxmCbSxxKg2fuzvDT_-PTStRRw_TrWgdh2QaZNzHQwvgwO6Qk_FRPFPYguJQn19x0yZqltPQfHwql4FJt_7g",
           "identityHeaderTokens": [
               "eyJhbGciOiJFUzI1NiIsInBwdCI6InNoYWtlbiIsInR5cCI6InBhc3Nwb3J0IiwieDV1IjoiaHR0cHM6Ly9leGFtcGxlLmNvbS9zb21lQ2VydGlmaWNhdGUucGVtIn0.eyJhdHRlc3QiOiJBIiwiZGVzdCI6eyJ0biI6WyIxNTU1NTU1NTU1NSJdfSwiaWF0IjoxNjM1OTU3NDIzLCJvcmlnIjp7InRuIjoiMTIyMjIyMjIyMjIifSwib3JpZ2lkIjoic29tZS1HVUlELWhlcmUifQ.ygPO2sImJR9MSqoRVD0CvnB2euv3RUYdupNEFS3wpgecs-yi8SU8FtYkkDypn7DC-siwdPY6vvVIx39y5Nb1sg;info=<https://example.com/someCertificate.pem>;alg=ES256;ppt=\"shaken\""
           ]
        
        }

        parentCallInfoToken property of a CallToken

        The parentCallInfoToken is a JSON Web Token (JWT). When decoded, the header and payload of the JWT will have the following shape:

        // Header
         
           {
             "alg": "ES256"
           }
        
        // Payload
         
           {
             "callSid": "CAXXXX....",
             "from": "+12222222222",
             "to": "+15555555555",
             "iat": "1635957423"
           }
        
        • The callSid value is the Call SID for the parent incoming Call Resource.
        • The from value is the caller's phone number
        • The to value is your Twilio number that received the call.
        • The iat is a claim that specifies the timestamp for when the parent call's PASSporT was created, which is necessary for further SHAKEN/STIR validation.

        identityHeaderTokens property of a CallToken

        In order to verify forwarded calls, terminating service providers need the original SHAKEN PASSporT and the DIV PASSporTs for each diversion. The identityHeaderTokens property contains the information needed for Twilio to add a DIV PASSporT to the outgoing call if necessary (See Attestation section above for more information).

        The identityHeaderTokens value is an array of all of the SHAKEN and DIV PASSporTs that were included as identity headers in the inbound leg of the call. If the call is signed, the array will contain one SHAKEN PASSporT, along with any DIV PASSporTs (if the call was diverted/forwarded) present on the inbound call.

        Below is an example of the decoded JWT from the identityHeaderTokens array. This represents the original SHAKEN PASSporT from the incoming call.

        // Headers
        
        {
           "alg": "ES256",
           "ppt": "shaken",
           "typ": "passport",
           "x5u": "https://example.com/someCertificate.pem"
         }
        
        // Payload
        
        {
           "attest": "A",
           "dest": {
             "tn": [
               "15555555555"
             ]
           },
           "iat": 1635957423,
           "orig": {
             "tn": "12222222222"
           },
           "origid": "some-GUID-here"
         }

        Since there is only one string in the CallToken's identityHeaderTokens array, this means the call was not diverted before it reached your Twilio number. If the call was diverted before it reached your Twilio number, you would see a DIV PASSporT for each diversion in the identityHeaderTokens array, as well.

        Rate this page:

        Need some help?

        We all do sometimes; code is hard. Get help now from our support team, or lean on the wisdom of the crowd by visiting Twilio's Community Forums or browsing the Twilio tag on Stack Overflow.

        Thank you for your feedback!

        We are always striving to improve our documentation quality, and your feedback is valuable to us. Please select the reason(s) for your feedback or provide additional information about how we can improve:

        Sending your feedback...
        🎉 Thank you for your feedback!
        Something went wrong. Please try again.

        Thanks for your feedback!

        Refer us and get $10 in 3 simple steps!

        Step 1

        Get link

        Get a free personal referral link here

        Step 2

        Give $10

        Your user signs up and upgrade using link

        Step 3

        Get $10

        1,250 free SMSes
        OR 1,000 free voice mins
        OR 12,000 chats
        OR more