View source for Services/Sync/P2P Key Exchange And Rotation

Support secure and intuitive key exchange and key rotation between Weave Sync clients.

Over a number of versions of Weave Sync different key exchange mechanisms have been implemented, however to date they have had significant weaknesses in either security or user experience (UX) or both. This wiki describes (yet another) proposal which attempts to find the balance between (good enough) security and an intuitive UX.

== Overview ==
The objective of the P2P Key Exchange protocol is to securely transfer a secret key to a new weave client during registration. Importantly, as such it is only relevant for Weave Sync versions which utilise a randomly generated secret key, namely [https://docs.services.mozilla.com/sync/storageformat5.html Weave Sync v1.1 API/v5 Data] and the [https://github.com/mozilla/fxa-auth-server/wiki/onepw-protocol#pairing-extension pairing extension] of Weave Sync v1.5 API/v5 Data.

Arguable the [http://en.wikipedia.org/wiki/Password_Authenticated_Key_Exchange_by_Juggling J-PAKE] based key exchange protocol used in Weave Sync v1.1/v5 already achieves this, however as discussed below a significant weakness is that it requires three round trips to complete and thus requires both the new and an already registered client to be online concurrently and with good connectivity. Also Weave Sync v1.1/v5, as currently implemented, does not support rotation of the secret key.

The P2P Key Exchange Protocol aims to address these issues by implementing a [http://lists.randombit.net/pipermail/cryptography/2013-October/005558.html 3DHE] based key exchange protocol with pre-generated ephemeral keys, allowing the key exchange to be completed asynchronously and in a single round trip. The same technique can also be leveraged for key rotation, although in this case only a single message needs to be sent by the initiating client, a response is not required but can be used to indicate success. The key exchange is also implemented within Weave Sync storage meaning no changes are required to the Weave Sync registration API to implement.

The P2P Key Exchange protocol is inspired by [https://github.com/WhisperSystems/TextSecure/ TextSecure].

'Pros'
* User only needs to know username and password to register device
* Secret key is randomly generated
* Secret key does not leave device
* Key exchange is implemented using Weave Sync storage and thus is compatible with both Weave Sync v1.1/v5 and the Weave Sync v1.5/v5 pairing extension

'Cons'
* Registering additional devices after the first requires access to an already registered device
* Performing key exchange within Weave Sync storage may have [[#Registration Protocol|security ramifications]].

== Existing Weave Sync Key Exchange Implementations ==

===Passphrase (v1.1 API/v3 Data)===
The [https://wiki.mozilla.org/index.php?title=Labs/Weave/Developer/Crypto&oldid=200527 Passphrase] implementation derives a secret key from a user selected passphrase independent of the account password.

'Pros'
* Registering additional devices does not require access to an already registered device
* Secret key does not leave device

'Cons'
* User needs to know passphrase in addition to username and password to register device
* Secret key is only as secure as passphrase

===Easy Setup (v1.1 API/v5 Data)===
The [https://wiki.mozilla.org/Services/KeyExchange Easy Setup] implementation generates a secret key on account registration and for additional clients performs key exchange using [http://en.wikipedia.org/wiki/Password_Authenticated_Key_Exchange_by_Juggling J-PAKE].

'Pros'
* User only needs to know username and password to register device
* Secret key is randomly generated
* Secret key does not leave device

'Cons'
* Registering additional devices after the first requires access to an already registered device
* J-PAKE key exchange requires three round trips and hence both devices must be connected concurrently and with good connectivity

===One Password (v1.5 API/v5 Data)===
Like the Passphrase implementation, the [https://github.com/mozilla/fxa-auth-server/wiki/onepw-protocol One Password (onepw)] implementation derives a secret key from a user selected passphrase, however a single password/passphrase is used for both authentication and key derivation to address useability issues with maintaining independent passwords/passphrases.

'Pros'
* User only needs to know username and password to register device
* Registering additional devices does not require access to an already registered device

'Cons'
* Secret key is derived from password which is partially known by authentication server
* Secret key is only as secure as password
* Registration and authentication protocol significantly more complex

== Design ==

=== User Stories ===

* Information is secure: As a user I want my data to be secure, hence the secret key must be secure
* Easy to register: As a user I want to be able to register a new device with only the username and password
* Easy to change key: As a user I want to be able to change the secret key and de-register a device in the case that it has become compromised, i.e. device lost or stolen

=== Desired Behaviour ===

1) Register a device using only username and password

2) If verification is required this must be no more than one additional step and involve no more than one additional device

3) A registered device can change the sync key and re-distribute it to other selected devices with minimal actions, i.e. see (2) above.

== Implementation ==

Note for the purposes of describing the technical implementation the term ''client'' will be used in place of ''device'' and similarly ''authorised/unauthorised'' will be used in place of ''registered/unregistered''.

=== What Needs To Be Done ===

* Implement messaging protocol to support sending of messages between both authorised (registered) and unauthorised (unregistered) clients (devices)
* Implement registration protocol to authorise a new client (device) and exchange the master key for an existing Weave Sync account
* Implement key rotation protocol to change the master key and re-distribute to other authorised (registered) clients (devices)

== eXfio Peer v1 ==

===Messaging Protocol===
Version 1 of the eXfio Peer protocol leverages 3DHE key exchange. A bonus of using 3DHE is that it naturally abstracts in to a generalised messaging protocol that can be used to send an arbitrary messages encrypted with the session key derived during the 3DHE key exchange. In addition perfect forward secrecy can be achieved using [https://github.com/trevp/axolotl/wiki axolotl key ratcheting] as used in the Text Secure application. Although this is not implemented at this stage it presents an opportunity to extend functionality to include peer to peer key management that can be shared between multiple devices.

'''Protocol Sequence'''
<ol start="0">
<li>Each client publishes an identity key (AIp, BIp) and a set of ephemeral keys (AE{1..n}p, BE{1..n}p)</li>
<li>Client A generates a new ephemeral key (AEp) and randomly selects one of Client B's ephemeral keys (BEp). Using 3DHE and a KDF, client A generates the shared secret (SK) then sends message including own ephemeral key (AE) and an identifier for Client B's ephemeral key (BEp). Client B can now also generate the shared secret (SK) and hence the message body can be encrypted immediately.</li>
</ol>

Both Client A and Client B can now continue to communicate using the shared secret (SK) as a session key.

<pre>
           Alice (Client A)                       Eve (Public)     Bob (Client B)

t0         AIs                                    AIp, BIp         BIs
                                                  BE{1..n}p        BE{1..n}s

t1         AIs                                    AIp, BIp         BIs
           AEs                                    AEp, BEp         BEs
           SK = KDF(3DHE(AIs, AEs, BIp, BEp))                      SK = KDF(3DHE(BIs, BEs, AIp, AEp))
           PT = <message>                                          CT
           CT = ENCRYPT(PT, SK)                                    PT = DECRYPT(PT, SK)
                                                                   
</pre>

'''EphemeralKey JSON'''
<pre>
{
  keyid:     "id of ephemeral key",
  key:       "ephemeral public key"
}
</pre>

'''Client JSON'''
<pre>
{
  version:   "version of client record",
  clientid:  "id of client",
  name:      "name of client",
  key:       "identity public key",
  ekeys:     [EphemeralKey1, EphemeralKey2, ..., EphemeralKeyn],
  status:    "indicates if client is authorised, i.e. 'authorised' or 'pending'",
  authlevel: "will this client accept messages from unauthorised clients, 
             i.e. 'all' indicates all clients, 'authorised' indicates authorised clients only"
  hmac:      "HMAC of all other client fields"
}
</pre>

'''Message JSON'''
<pre>
{
  version:      "version of message record",
  srcclientid:  "id of sender",
  srckeyid:     "id of sender's ephemeral key",
  srckey:       "sender's ephemeral key (optional after first message, i.e. sequence > 1)",
  dstclientid:  "id of receiver",
  srckeyid:     "id of receiver's ephemeral key",
  sequence:     "sequence of this message in session",
  type:         "message type",
  content:      "message content"
  hmac:         "HMAC of all other message fields"
}
</pre>

=== Registration Protocol ===
'''IMPORTANT:''' If the attacker has full access to the storage, i.e. a hostile systems administrator, '''and''' knows the password a '''man-in-the-middle (MITM) attack is possible'''. At this time there is an implicit (and quite possibly incorrect) assumption that the systems administrator does not know the password. See [[#eXfio Peer v2|version 2]] of protocol for improved handling of this risk.

When Alice registers a new device with the Weave Sync server the client first checks if there are other authorised clients, if not it initialises the storage, if so it requests authorisation by following the procedure below.

<ol start="0">
<li>Client A: Authenticate to sync server and create client record with status of 'pending'</li>
<li>Client A: Send ClientAuthRequestMessage to existing authorised clients, i.e. Client B, including authcode digest (AD)</li>
<li>Out-of-Band: Client A displays authcode (AC) and user enters it on Client B
<li>Client B: If authcode (AC) is verified send ClientAuthResponseMessage with sync key</li>
</ol>

Finally Alice is notified of registration status and if successful client record is updated with status of 'approved'.


<pre>
           Alice (Client A)                       Eve (Public)     Bob (Client B)

t0         AIs                                    AIp, BIp         BIs, MK
                                                  BEp              BEs

t1         AIs                                    AIp, BIp         BIs, MK
           AEs                                    AEp, BEp         BEs
           SK = KDF(3DHE(AIs, AEs, BIp, BEp))                      SK = KDF(3DHE(BIs, BEs, AIp, AEp))
           VC = RANDOM()                                           VD
           VD = DIGEST(VC + DIGEST(password))

t2         AIs, MK                                AIp, BIp         BIs, MK
(OoB)      VC                                                      VD
                                                                   VCui = <user input>
                                                                   Verified = (VD == DIGEST(VCui + DIGEST(password)))

t3         AIs, MK                                AIp, BIp         BIs, MK

</pre>

'''Notes:'''
* The authcode is a random alphanumeric string which needs to be entered on the authorising device to verify the authenticity of the request. If the authcode is entered correctly then a man-in-the-middle attack is highly unlikely.

'''ClientAuthVerifier JSON'''
<pre>
{
  innersalt:  "Salt value for inner hash, i.e. hash(password)
  salt:       "Salt value for outer hash
  digest:     "hash(authcode + hash(password))
}
</pre>

'''ClientAuthRequestMessage JSON'''
<pre>
{
  clientid:  "id of client",
  name:      "name of client",
  auth:      ClientAuthVerifier
}
</pre>

'''ClientAuthResponseMessage JSON'''
<pre>
{
  clientid:  "id of client",
  name:      "name of client",
  status:    "okay|fail",
  message:   "authorisation approved|authorisation rejected",
  synckey:   "Master key for Weave Sync account (required if status 'okay')"
}
</pre>

===Key Rotation Protocol===
TODO

== eXfio Peer v2 ==
Version 2 of the eXfio Peer protocol improves the protection against a man in the middle (MITM) attack, at the expense of a more complex protocol, requiring two round-trips. Importantly the starting assumption is that an adversary has full access to the storage '''and''' knows the password, i.e. a hostile systems administrator. Fortunately an elegant means to mitigate against this threat is to use a [https://www.silentcircle.com/products-and-solutions/technology/scimp/ SCIMP] style hash commitment, which results in the adversary having a single opportunity to guess the authcode with a probability of 1.05 x 10<sup>-6</sup>.

===Messaging Protocol v2===
By first requiring each party to make a hash commitment an adversary is required to select a public key without knowing the other party’s key. Thus making it very difficult to launch a MITM attack without detection. See [[#Registration Protocol v2]]

'''Protocol Sequence'''
<ol start="0">
<li>Each client publishes an identity key (AI, BI) and a set of ephemeral key digests (AE{1..n}, BE{1..n}d)</li>
<li>Client A generates a new ephemeral key (AEp) and sends a message to Client B nominating one of client B's ephemeral key digests (BEd).</li>
<li>Client B receives client A's ephemeral key (AEp) and digest for own ephemeral key (BEd). Using 3DHE and a KDF Client B generates the session key (SK) then sends message including own ephemeral key (BEp) matching digest (BEd). Client B can now encrypt/decrypt the message body with session key.</li>
<li>Client A receives client B's ephemeral key (BEp). Using 3DHE and a KDF, client A generates the session key (SK). Client A can now encrypt/decrypt the message body with the session key.</li>
</ol>

<pre>
           Alice (Client A)                       Eve (Public)     Bob (Client B)

t0         AIs                                    AIp, BIp         BIs
                                                  BE{1..n}d        BE{1..n}s
                                                                   BE{1..n}p

t1         AIs                                    AIp, BIp         BIs
           AEs                                    AEp              BEs
                                                  BEd              BEp

t2         AIs                                    AIp, BIp         BIs
           AEs                                    AEp, BEp         BEs
           SK = KDF(3DHE(AIs, AEs, BIp, BEp))     BEd              SK = KDF(3DHE(BIs, BEs, AIp, AEp)

t3         AIs                                    AIp, BIp         BIs
           SK                                                      SK
           PT = <message>                                          CT
           CT = ENCRYPT(PT, SK)                                    PT = DECRYPT(PT, SK)
</pre>

'''EphemeralKey JSON'''
<pre>
{
  keyid:     "id of ephemeral key",
  keydigest: "digest of ephemeral public key"
}
</pre>

'''Client JSON'''
<pre>
{
  version:   "version of client record",
  clientid:  "id of client",
  name:      "name of client",
  key:       "identity public key",
  ekeys:     [EphemeralKey1, EphemeralKey2, ..., EphemeralKeyn],
  status:    "indicates if client is authorised, i.e. 'authorised' or 'pending'",
  authlevel: "will this client accept messages from unauthorised clients, 
             i.e. 'all' indicates all clients, 'authorised' indicates authorised clients only"
  hmac:      "HMAC of all other client fields"
}
</pre>

'''Message JSON'''
<pre>
{
  version:      "version of message record",
  srcclientid:  "id of sender",
  srckeyid:     "id of sender's ephemeral key",
  dstclientid:  "id of receiver",
  dstkeyid:     "id of receiver's ephemeral key",
  sequence:     "sequence of this message in session",
  type:         "message type",
  content:      "message content"
  hmac:         "HMAC of all other message fields"
}
</pre>

'''SessionRequestMessage JSON'''
<pre>
{
  clientid:     "id of client",
  name:         "name of client"
  srckey:       "sender's ephemeral key",
  dstkeydigest: "digest of receiever's ephemeral key"
}
</pre>

'''SessionVerifier JSON'''
<pre>
{
  timestamp:  "time used as input in to verification"
  salt:       "salt used as input in to verification"
}
</pre>

'''SessionResponseMessage JSON'''
<pre>
{
  clientid:  "id of client",
  name:      "name of client",
  status:    "okay|fail",
  message:   "ephemeral key okay|ephemeral key mismatch",
  dstkey:    "ephemeral key matching dstkeydigest (if status okay)",
  auth:      SessionVerifier (if status okay)
}
</pre>

=== Registration Protocol v2 ===
The objective of the registration protocol is for a user, i.e. Alice, to authorise a new device and transfer to it the master key (sync key) thus allowing it to read and write encrypted data to and from the storage. To maintain the security of the master key the protocol must defend against a man-in-the-middle (MITM) attack from an adversary. Importantly for version 2 of the eXfio Peer protocol the starting assumption is that an adversary has full access to the storage '''and''' knows the password, i.e. a hostile systems administrator.

When Alice registers a new device with the Weave Sync server the client first checks if any devices have already been registered. If there are no registered devices it generates a master key (MK) and initialises the storage. If there are existing registered devices it requests authorisation by following the procedure below.

<ol start="0">
<li>Client A: Authenticate to sync server and create client record with status of 'pending'</li>
<li>Client A: Send SessionRequestMessage to registered devices, providing an ephemeral key (AEp) and nominating an ephemeral key digest (BEd) of other device, i.e. Client B</li>
<li>Client B: Send SessionResponseMessage including ephemeral key (BEp) nominated by Client A</li>
<li>Out-of-Band: User sights authcode displayed on Client B, generated from session key and master key, and enters it on Client A</li>
<li>Client A: If the session key component (VCS) of authcode is verified then send ClientAuthV2RequestMessage including the master key component of authcode (VCM), thus proving the user is in possession of a registered device, i.e. Client B</li>
<li>Client B: If Client A responds with proof of possessing registered device then send ClientAuthV2ResponseMessage including the master key</li>
</ol>
Finally Alice is notified of registration status and if successful client record is updated with status of 'approved'

<pre>
           Alice (Client A)                       Eve (Public)     Bob (Client B)

t0         AIs                                    AIp, BIp         BIs, MK
                                                  BEd              BEs
                                                                   BEp

t1         AIs                                    AIp, BIp         BIs, MK
           AEs                                    AEp              BEs
                                                  BEd              BEp

t2         AIs                                    AIp, BIp         BIs, MK
           AEs                                    AEp, BEp         BEs
           BT                                     BEd              BT
           SK = KDF(3DHE(AIs, AEs, BIp, BEp))                      SK = KDF(3DHE(BIs, BEs, AIp, AEp)
           Verified = (BEd == DIGEST(BEp))                       

t3         AIs                                    AIp, BIp         BIs, MK
(OoB)      BT                                                      BT
           SK                                                      SK
           VCSui = <user input>                                    VCS = DIGEST(SK, BT)
           VCMui = <user input>                                    VCM = DIGEST(MK, BT)
           Verified = (VCSui == DIGEST(SK, BT))                                        

t4         AIs, MK                                AIp, BIp         BIs, MK
           SK                                                      SK
                                                                   VCM
                                                                   VCMui
                                                                   Verified = (VCMui == VCM)

t5         AIs, MK                                AIp, BIp         BIs, MK

</pre>

'''Notes:'''
* The authcode (VCS + VCM) is a digest of the session key and the master key respectively. This code needs to be entered on the device requesting authorisation to verify the user is in possession of registered device. If the authcode displayed by Client B matches the VCS value generated on Client A and the VCM value returned to Client B then a man-in-the-middle attack is highly unlikely.

'''ClientAuthV2Verifier JSON'''
<pre>
{
  proof:  "Proof of receiving authcode through out of band channel"
}
</pre>

'''ClientAuthV2RequestMessage JSON'''
<pre>
{
  clientid:  "id of client",
  name:      "name of client",
  auth:      ClientAuthV2Verifier
}
</pre>

'''ClientAuthV2ResponseMessage JSON'''
<pre>
No change from V1
</pre>

===Key Rotation Protocol v2===
TODO

===Vulnerabilities===
On deeper inspection of eXfio Peer v2 there are two vectors of attack that present themselves, a honeypot and a targeted phishing attack.

====Honeypot====
An adversary could set up a sync server and invite users to join, with say an offer of unlimited stage. When the user creates an account the hostile sync server initialises the storage with a fictitious authorised device, thus giving the impression that the user had previously registered with the service. The user could be presented with an authcode to enter on their device this transferring a pre-generated master key, known by the server, to the newly registered device.

'''Countermeasures'''

TODO

====Targeted Phishing====
Similar to a honeypot, but targeted at a user that has already registered one or more devices, an adversary with control of the sync server could request the user to authorise the registration of a fictitious device, i.e. with a similar name to a device familiar to the user. If the user then enters the authcode as requested, i.e. into a web page, the master key will be transferred to the fictitious device.

'''Countermeasures'''

TODO

=Other Key Exchange Protocols=
==SCIMP Messaging Protocol==

<pre>
           Alice (Client A)                       Eve (Public)     Bob (Client B)

t1         AEs                                    AEd              
           AEp

t2         AEs                                    AEd              BEs
           AEp                                    BEp              

t3         AEs                                    AEd              BEs
                                                  AEp, BEp         Verified = (DIGEST(AEp) == AEd)

t4         AEs                                    AEd              BEs
           SK = ECDH(BEp, AEp, AEs)               AEp, BEp         SK = ECDH(AEp, BEp, BEs)

t5         SK                                                      SK
(OoB)      AVC = DIGEST(SK)                                        BVC = DIGEST(SK)
           Verified = (AVC == BVC)                                 Verified = (AVC == BVC) 

</pre>

==Signal (TextSecure) Registration Protocol==

<pre>
           Alice (Client A)                       Eve (Public)     Bob (Client B)
t0                                                AUID, BUID       BIs, MK
                                                  BIp

t1         AIs                                    AUID, BUID       BIs, MK
(OoB)      AIp                                    BIp              AIp

t2         AIs, MK                                AUID, BUID       AIp, MK
                                                  AIp, BIp          

</pre>