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RFD 54 - Passwordless for macOS CLI

rfd/0054-passwordless-macos.md

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RFD 54 - Passwordless for macOS CLI

What

Passwordless features for native macOS CLIs, aka Touch ID support for CLI/tsh.

This is a part of the Passwordless RFD.

Passwordless is available as a preview in Teleport 10.

Why

Native, non-browser macOS clients lack support for Touch ID. This RFD explores how we can achieve that support for tsh in a secure way.

Details

Touch ID support is implemented via SecAccessControl-protected keys, which can be either a Keychain entry or a private key stored in the Secure Enclave. Both alternatives are Secure Enclave-protected, but in the latter the keys are generated in the Enclave and never leave it, making it our approach of choice. (See the alternatives considered section for other APIs evaluated for the design.)

In order to make use of the Keychain Sharing services, required for Secure Enclave protection, the tsh macOS binary needs to be:

  1. Code signed,
  2. Contain the necessary entitlements to use the Keychain,
  3. Embed a matching provisioning profile; and
  4. Notarized
<!-- TODO(codingllama): Add more details about notarization. -->

The requirements above mean that tsh needs to be packaged in a macOS .app for distribution. An account enrolled in the Apple Developer Program is also necessary.

See below for an example of the necessary entitlements:

xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>com.apple.developer.team-identifier</key>
  <string>TEAMID</string>
  <key>com.apple.application-identifier</key>
  <string>TEAMID.com.goteleport.tsh</string>

  <key>keychain-access-groups</key> <!-- aka Keychain Sharing -->
  <array>
    <string>TEAMID.com.goteleport.tsh</string>
  </array>
</dict>
</plist>

(CGO is used to bridge native ObjC code into the Go binaries.)

When running in a binary that isn't correctly signed or configured, tsh should disable Touch ID support.

Registration

Registration creates and saves a new key in the Secure Enclave, using a biometric-protected entry.

The proposed UX is similar to the current experience:

shell
$ tsh mfa add
> Choose device type [TOTP, WEBAUTHN, TOUCHID]: touchid
> Enter device name: touchid
> Tap any *registered* security key or enter a code from a *registered* OTP device: <taps>

> MFA device "touchid" added.

Under the hood, during the Touch ID prompt stage, the following happens:

  1. tsh creates a new Secure Enclave key, using the following parameters:

    objc
    // (Error handling and memory management omitted for simplicity.)

SecAccessControlRef access = SecAccessControlCreateWithFlags(
    kCFAllocatorDefault,
    kSecAttrAccessibleWhenUnlockedThisDeviceOnly,
    kSecAccessControlPrivateKeyUsage|kSecAccessControlBiometryAny,
    NULL /* error */);

// Use a context with a grace period so we don't ask for multiple touches
// in a single ceremony.
LAContext *context = [[LAContext alloc] init];
context.touchIDAuthenticationAllowableReuseDuration = 10; // seconds

NSDictionary attrs = @{
  // 256-bit elliptic curve keys are required by the Enclave.
  (id)kSecAttrKeyType:                (id)kSecAttrKeyTypeECSECPrimeRandom,
  (id)kSecAttrKeySizeInBits:          @256,
  (id)kSecAttrTokenID:                (id)kSecAttrTokenIDSecureEnclave,

  (id)kSecPrivateKeyAttrs: @{
    (id)kSecAttrIsPermanent:          @YES,
    (id)kSecAttrAccessControl:        (id)access,
    (id)kSecUseAuthenticationContext: (id)context,

    (id)kSecAttrApplicationLabel:     keyHandle,            // Generated UUID
    (id)kSecAttrApplicationTag:       @"[email protected]", // user@RPID, used to scope keys
  },
};
SecKeyRef key = SecKeyCreateRandomKey((CFDictionaryRef)attrs, NULL /* error */);

  2. tsh performs the key registration process, setting all parameters for a passwordless / resident key

    Note that Touch ID credentials are always considered (and function as) resident keys, even if the RPID/Teleport where to request ResidentKeyRequirement = "discouraged".

  3. If registration is successful, tsh replaces any existing keys for the RPID+user pair with the newly-created key. This simplifies the authentication ceremony and allows re-registration as a fallback mechanism.

A few parameters specified in the code example deserve note:

  • kSecAttrAccessibleWhenUnlockedThisDeviceOnly requires the device to be unlocked and the user to have a password set. It is the more restrictive of the possible "Accessibility Values".

  • kSecAccessControlBiometryAny requires a biometric check (Touch ID on macOS). It is more restrictive than kSecAccessControlUserPresence (which allows passwords), but less restrictive than kSecAccessControlBiometryCurrentSet (doesn't work with newly enrolled fingerprints). kSecAccessControlBiometryAny seems to be the sweet spot of security and usability.

In case of a registration failure, tsh must do its best to delete the created-but-not-registered credential. If all fails, it is possible to use the hidden tsh support commands for a manual cleanup.

Authentication

Authentication offers a plethora of options, depending both on server settings (otp, webauthn, passwordless) and client state (FIDO2 keys present, Touch ID keys registered). In order to decide which flow to follow, tsh must first assess what is possible, preferably without asking for unnecessary user interaction.

Unlike FIDO2 keys, it is possible for tsh to discover if Touch ID keys are registered in the Enclave without user interaction. Because all Touch ID keys are functionally resident keys, as long as the server supports passwordless, then tsh is free to use it.

If Touch ID keys are present, then it's the preferred method of authentication, both for passwordless and MFA.

To allow users agency over the eager behaviors of Touch ID, tsh is augmented with the global --mfa-mode flag:

tsh --mfa-mode={auto,platform,cross-platform} - choose whether to use platform or cross-platform MFA

`auto` is the default behavior described above, which favors Touch ID

`platform` prefers platform authenticators, such as Touch ID, over OTP or
portable FIDO2 keys

`cross-platform` prefers FIDO2 or OTP (aka `tsh` behavior prior to this RFD)

Finally, if there are Touch ID credentials for multiple users and the login user is not known, tsh login may prompt the user to specify the --user flag.

Example of a passwordless Touch ID login:

shell
$ tsh login --proxy=example.com

> > Profile URL:        https://example.com
>   Logged in as:       codingllama
>   Cluster:            example.com
>   Roles:              access, editor
>   Logins:             codingllama
>   Kubernetes:         enabled
>   Valid until:        2021-10-04 23:32:29 -0700 PDT [valid for 12h0m0s]
>   Extensions:         permit-agent-forwarding, permit-port-forwarding, permit-pty

Detecting Touch ID support

Detecting Touch ID support is important so tsh may enable/disable related features as appropriate.

Apart from Go build tags, which are a rather coarse detection mechanism, we can take inspiration from Chromium's implementation and do the following checks:

<!-- TODO(codingllama): In theory it's all fine, but let's run some tests on older machines to be safe. -->

tsh support commands

The following support commands are added to tsh as hidden subcommands. They are useful to diagnose and manage certain aspects of Touch ID support.

The commands are only available on macOS builds.

tsh touchid diag - prints diagnostics about Touch ID support (for example, if the binary is signed, entitlements, macOS version and Touch ID availability)

tsh touchid ls - lists currently stored credentials

tsh touchid rm - deletes a stored credential

shell
$ tsh touchid diag  # diag output subject to change
> macOS version: 12.1
> Signed: yes
> Entitlements: {
>     "com.apple.application-identifier" = "K497G57PDJ.net.teleportdemo.codingllama-touchid";
>     "com.apple.developer.team-identifier" = K497G57PDJ;
>     "keychain-access-groups" =     (
>     );
> }
> LAContext check passed: yes
> Secure Enclave check passed: yes

$ tsh touchid ls

> RPID        User    Credential ID
> ----------- ------- ------------------------------------
> example.com llama   6ed2d2e4-7933-4988-9eeb-428e8531f122
> example.com alpaca  cbf251a3-0e44-4068-87cb-91a1eb241eaf

$ tsh touchid rm 6ed2d2e4-7933-4988-9eeb-428e8531f122

> Credential 6ed2d2e4-7933-4988-9eeb-428e8531f122 / [email protected] deleted.

Security

A few security tradeoffs, in particular in relation to the chosen flags, are discussed in the Registration section.

The security of the system is predicated in two main components: the Secure Enclave and WebAuthn. As long as keys are created with the correct settings, it is not possible to employ them via tsh unless the user passes the biometric check. tsh can't exfiltrate or access key material by itself.

The server communication protocol is based on WebAuthn, as described by the WebAuthn and Passwordless RFDs.

UX

UX is discussed throughout the design, but here is a summary of changes:

tsh login --proxy=example.com will automatically do passwordless Touch ID login, if appropriate (server allows passwordless, hardware present, credential registered for "example.com")

tsh login --proxy=example.com --user=llama behaves as above, but using a specific user

tsh login --auth=passwordless --mfa-mode=platform --proxy=example.com --user=llama is the zero ambiguity, (needlessly) long form of the above.

tsh mfa add adds support for Touch ID, both for authentication and registering new credentials.

The following hidden maintenance commands are added:

  • tsh touchid diag
  • tsh touchid ls
  • tsh touchid rm

Regular users shouldn't need to touch those commands, but they are available for troubleshooting and credential management.

Alternatives considered

LAContext's evaluatePolicy

The LAContext's evaluatePolicy method may be used to trigger a Touch ID prompt. It takes a policy to evaluate (for example, LAPolicyDeviceOwnerAuthenticationWithBiometrics), plus a reason string, and replies with a boolean (success/failure) and an error.

There are a few issues that make it unsafe: evaluatePolicy returns only a boolean, offering no features to gate access to a resource. We must tackle key storage and management ourselves. A boolean check in a user-controlled binary is easy to bypass, and in the case of a bypass there is no actual security provided by the biometric check. In general, solutions based on LAContext evaluatePolicy are security theater.

The shortcomings of evaluatePolicy highlight a few desirable properties of an actual secure solution:

  • The biometric check must offer more than a boolean result: it must gate access to resources and/or supply information that can't be acquired otherwise (eg, perform a digital signature)
  • Ideally, the biometric solution stores secret information itself and never lets those secrets be exfiltrated (eg, Secure Enclave keys)

ASAuthorizationPlatformPublicKeyCredentialProvider / Authentication Services

The public-private key authentication APIs, released in Monterey, add native WebAuthn capabilities to macOS. They are, at first glance, an ideal fit for our needs, except for a single requirement: the binaries using them must have a matching associated domain entitlement.

Simplifying Apple's documentation, declaring an associated domain such as example.com has two components:

  1. A server-side XML declaring the apps with access to the webcredentials service:

    https://example.com/apple-app-site-association

    xml
    <!-- See https://developer.apple.com/documentation/xcode/supporting-associated-domains. -->
{
  "applinks": {
      "details": [{...}]
  },
  "webcredentials": {
      "apps": [ "TEAMID.com.example.app" ] <-- this is what we care about
  },
  "appclips": {...}
}

  2. A client-side entitlement for webcredentials, signed into the binary

    Example:

    xml
    <?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <!-- See https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_developer_associated-domains. -->
  <key>com.apple.developer.associated-domains</key>
  <array>
    <string>webcredentials:example.com</string>
  </array>
</dict>
</plist>

Client apps query the server-side entitlements directly from Apple servers, the server themselves hit the corresponding domains periodically (or on first load) and cache the entitlements.

The issue with entitlements is simple: we can't know beforehand the domains for all tsh installations. Usage of the API could be possible, but would likely require different entitlements per customer (an arrangement that might not be allowed by Apple). It is likely possible to make use of those APIs for Teleport Cloud, but we would need a solution for other installations regardless.

A final consequence of the above is that Passkey support (aka iCloud-stored credentials) for CLIs is out of the roadmap for the foreseeable future (but Passkeys can be used for Safari-based access).

References:

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