Services/Sync/P2P Key Exchange And Rotation: Difference between revisions

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Revision as of 03:48, 17 January 2016

3,907 bytes removed ,  17 January 2016
Revised eXfio Peer v2 to reflect candidate B solution
(updated message structures)
(Revised eXfio Peer v2 to reflect candidate B solution)
 
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== eXfio Peer v2 ==
== 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>.
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 an adversary, Eve, having to select a public key without knowing the public key of either Alice or Bob, giving Eve a single opportunity to guess a 4 character base32 authcode with a probability of 1.05 x 10<sup>-6</sup>. Thus making it very difficult to launch a MITM attack without detection.


<span id="Messaging_Protocol_v2"></span>
<span id="Messaging_Protocol_v2"></span>
===Messaging Protocol===
===Messaging Protocol===
By requiring the responding party, Bob, to nominate a key in advance using a hash commitment, a MITM adversary, Eve, is required to select a public key without knowing the either Alice key. Thus making it very difficult to launch a MITM attack without detection. See [[#Registration_Protocol_2|Registration Protocol]]
Although version 2 of the eXfio Peer protocol requires two round trips it has been designed such that the user input for out-of-band verification takes place on the registered device, Client B, which can determine if the verification of Client A has been successful without exchanging additional messages between clients. However to detect a MITM adversary impersonating Client B an additional verification check is needed on the new device, Client A. Although this attack would not result in a compromise of the master key it could result in the sharing of confidential data. Thus if a user incorrectly enters the authcode Client A must assume the session is compromised or exchange additional messages between clients to give the user additional opportunities to enter the correct authcode.


'''Protocol Sequence'''
'''Protocol Sequence'''
<ol start="0">
<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>Each client publishes an identity key (AIp, BIp)</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 A: Generates a new ephemeral key (AEp) and to initiate a verified session sends a message to Client B including a digest of the key (AEd), thereby committing to a specific ephemeral key without revealing it</li>
<li>Client B: Using 3DHE and a KDF generates the shared secret (SK) then sends message including own ephemeral key (BEp) matching digest (BEd). Client A can now also generate the shared secret (SK) and hence the message body can be encrypted immediately.
<li>Client B: Generates a new ephemeral key (BEp) and sends a message to Client A including the key (not a digest). Both parties have now committed to a specific ephemeral key without first knowing the key of the other party</li>
<li>Client A: Sends a message to Client B including the ephemeral key (AEp)</li>
<li>Out-of-Band: Client B can now verify the session by confirming that Client A's ephemeral key (AEp) matches the digest (AEd), provided earlier, and using user input to avoid interception, i.e. out-of-band, that an authcode (VCS) derived from Client A's ephemeral key (AEp) and identity key (AIp) has the same value on Client A and Client B.</li>
<li>Client B: Derives a shared secret (SK) using 3DHE and sends an encrypted message to Client A including the status of the verification, i.e. okay|fail, and a random component of the authcode (VCRui) thus proving the response is from the registered device, i.e. Client B. If verification is successful the message can optionally include additional information such as account keys</li>
</ol>
</ol>


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


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


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


t1        AIs                                    AIp, BIp        BIs
t1        AIs                                    AIp, BIp        BIs
          AEs                                    AEp             BEs
Key        AEs                                    AEd              
                                                  BEd              BEp
Commit    AEp                                   


t2        AIs                                    AIp, BIp        BIs
t2        AIs                                    AIp, BIp        BIs
           AEs                                    AEp, BEp         BEs
           AEs                                    BEp             BEs
           SK = KDF(3DHE(AIs, AEs, BIp, BEp))    BEd              SK = KDF(3DHE(BIs, BEs, AIp, AEp)
           AEp                                    AEd             
          CT                                                      PT = <message>
          PT = DECRYPT(CT, SK)                                   CT = ENCRYPT(PT, SK)


t3        AIs                                    AIp, BIp        BIs
Key        AEs                                    AEp, BEp        BEs
Verify
t4        AIs                                    AIp, BIp        BIs
(OoB)      AEs                                    AEd              BEs
          AEp                                                    BEp
          VCS = DIGEST(AIp, AEp)                                  VCSui = <user input>
          VCR = RANDOM()                                          VCRui = <user input>
                                                                  Verified = (VCSui == DIGEST(AIp, AEp) + AEd == DIGEST(AEp))
t5        AIs                                    AIp, BIp        BIs
          SK = KDF(3DHE(AIs, AEs, BIp, BEp))                      SK = KDF(3DHE(BIs, BEs, AIp, AEp))
          VCR                                                    VCRui
          VCRui                                                 
          Verified = (VCRui == VCR)                             
</pre>
</pre>


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


'''Client JSON'''
'''Notes:'''
* The authcode (VCS + VCR) is a digest of Client A's identity key and ephemeral key and a random value respectively. This code must be entered on a registered device to verify the user is in possession of the device requesting authorisation. If the authcode displayed by Client A matches the VCS value generated on Client B and if Client A's hash commit matches the digest of Client A's public key then a man-in-the-middle attack impersonating Client A is highly unlikely. Finally if the VCRui value returned to Client A matches the original VCR random value then a man-in-the-middle attack impersonating Client B is highly unlikely.
* To minimise errors during user input the authcode can be represented using a word list, i.e. [https://en.wikipedia.org/wiki/PGP_word_list PGP], [http://world.std.com/~reinhold/diceware.html Diceware], [https://tools.ietf.org/html/rfc1760 S/KEY] or optionally scanned using a QR code.
* By including Client A's identity key in the authcode provides the additional benefit of verifying both keys
 
 
'''Client JSON (compatible with v1 protocol)'''
<pre>
<pre>
{
{
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</pre>
</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>
<span id="Registration_Protocol_v2"></span>
===Registration Protocol===
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 a user 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.
====Candidate Solution A====
Candidate solution A has the benefit that all user input takes place on the new device, Client A, and thus the user experience is similar to second factor authentication (2FA) using software based one time password (OTP) application such as [https://en.wikipedia.org/wiki/Google_Authenticator Google Authenticator]. Out-of-band verification mitigates against the impersonation of Client A and Client B, however because only the registered device, Client B, can determine if verification of Client A has been successful if a user incorrectly enters the authcode additional messages need to be exchanged between clients potentially causing confusion as a result of the delay in notifying the user that verification has failed.
'''Note:''' When the new device is mobile a QR code could be used for the out-of-band verification, which would remove the need for user input.
<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 random value, 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 random value component of authcode (VCR), 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 the user is notified of the registration status and if successful client record is updated with status of 'approved'
<pre>
          Alice (Client A)                      Eve (MITM)      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
          SK = KDF(3DHE(AIs, AEs, BIp, BEp))    BEd              SK = KDF(3DHE(BIs, BEs, AIp, AEp)
          Verified = (BEd == DIGEST(BEp))                     
t3        AIs                                    AIp, BIp        BIs, MK
(OoB)      SK                                                      SK
          VCSui = <user input>                                    VCS = DIGEST(SK)
          VCRui = <user input>                                    VCR = RANDOM()
          Verified = (VCSui == DIGEST(SK))                                       
t4        AIs                                    AIp, BIp        BIs, MK
          SK                                                      SK
                                                                  VCR
                                                                  VCRui
                                                                  Verified = (VCRui == VCR)
t5        AIs, MK                                AIp, BIp        BIs, MK
</pre>
'''Notes:'''
* The authcode (VCS + VCR) is a digest of the session key and a random value 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 then a man-in-the-middle attack impersonating Client B is highly unlikely. Finally if the VCRui value returned to Client B matches the original VCR random value then a man-in-the-middle attack impersonating Client A is highly unlikely.
* To minimise errors during user input the authcode can be represented using a word list, i.e. [https://en.wikipedia.org/wiki/PGP_word_list PGP], [http://world.std.com/~reinhold/diceware.html Diceware], [https://tools.ietf.org/html/rfc1760 S/KEY] or optionally scanned using a QR code.
'''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>
====Candidate Solution B====
Candidate solution B has the benefit that user input for out-of-band verification takes place on the registered device, Client B, which can determine if the verification of Client A has been successful without exchanging additional messages between clients. However to detect a MITM adversary impersonating Client B an additional verification check is needed on the new device, Client A. Although this attack would not result in a compromise of the master key it could result in the sharing of confidential data. Thus if a user incorrectly enters the authcode additional messages need to be exchanged between clients, however the out-of-band verification flow and user input flow would be in alignment, thus resulting in improved user experience.
'''Note:''' Similarly to Candidate solution A, when the new device is mobile a QR code could be used for the out-of-band verification, which would remove the need for user input and mitigate against the impersonation of Client A and Client B.
<ol start="0">
<li>Client A: Authenticate to sync server and create client record with status of 'pending' and including digest of ephemeral key (AEd)</li>
<li>Out-of-Band: User sights authcode displayed on Client A, generated from identity and key ephemeral key and random value, and enters it on Client B</li>
<li>Client B: Send SessionRequestMessage to new device, providing an ephemeral key (BEp) and nominating an ephemeral key (AEd) of other client, Client A (only one option available)</li>
<li>Client A: Send SessionResponseMessage including ephemeral key (AEp) nominated by Client B</li>
<li>Client B: If the identity and ephemeral key component (VCS) of authcode is verified then send ClientAuthV2ResponseMessage including the master key and the random value component of authcode (VCR), thus proving the response is from the registered device, i.e. Client B</li>
</ol>
Finally the user is notified of the registration status and if successful client record is updated with status of 'approved'
<pre>
          Alice (Client A)                      Eve (MITM)      Bob (Client B)
t0        AIs                                    AIp, BIp        BIs, MK                                                   
t1        AIs                                    AIp, BIp        BIs, MK
Key        AEs                                    AEd             
Commit    AEp                                   
t2        AIs                                    AIp, BIp        BIs, MK
(OoB)      AEs                                    AEd              BEs
          AEp                                                    BEp
          VCS = DIGEST(AIp, AEp)                                  VCSui = <user input>
          VCR = RANDOM()                                          VCRui = <user input>
t3        AIs                                    AIp, BIp        BIs, MK
          AEs                                    BEp              BEs
          AEp                                    AEd              VCSui   
          VCS                                                    VCRui                   
t4        AIs                                    AIp, BIp        BIs, MK
Key        AEs                                    AEp, BEp        BEs
Verify    SK = KDF(3DHE(AIs, AEs, BIp, BEp))    AEd              SK = KDF(3DHE(BIs, BEs, AIp, AEp))
          VCS                                                    VCSui
          VCR                                                    VCRui
                                                                  Verified = (VCSui == DIGEST(AIp, AEp) + AEd == DIGEST(AEp))
t5        AIs, MK                                AIp, BIp        BIs, MK
          SK                                                      SK
          VCR                                                    VCRui
          VCRui                                                 
          Verified = (VCRui == VCR)                             
</pre>
'''Notes:'''
* The authcode (VCS + VCR) is a digest of Client A's identity key and ephemeral key and a random value respectively. This code must be entered on a registered device to verify the user is in possession of the device requesting authorisation. If the authcode displayed by Client A matches the VCS value generated on Client B and if Client A's hash commit matches the digest of Client A's public key then a man-in-the-middle attack impersonating Client A is highly unlikely. Finally if the VCRui value returned to Client A matches the original VCR random value then a man-in-the-middle attack impersonating Client B is highly unlikely.
* To minimise errors during user input the authcode can be represented using a word list, i.e. [https://en.wikipedia.org/wiki/PGP_word_list PGP], [http://world.std.com/~reinhold/diceware.html Diceware], [https://tools.ietf.org/html/rfc1760 S/KEY] or optionally scanned using a QR code.
* A QR code including Client A's entire key (AEp) would allow step 2, 3 and 4 to be collapsed into a single step. See [[#Signal (TextSecure) Registration Protocol|Signal (TextSecure) Registration Protocol]] below.


'''Message JSON (compatible with v1 protocol)'''
'''Message JSON (compatible with v1 protocol)'''
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   name:        "name of client",
   name:        "name of client",
   keydigest:    "digest of sender's ephemeral key",
   keydigest:    "digest of sender's ephemeral key",
   getkeys:      "when true and session verified receiver will return account keys, optional"
   getkeys:      "when true and session verified account keys will be included in SessionKeyVerifyResponseMessage, optional. See Registration protocol"
}
}
</pre>
</pre>
Line 514: Line 350:
{
{
   authcode:  "Random component of authcode entered by user, i.e. VCRui"
   authcode:  "Random component of authcode entered by user, i.e. VCRui"
}
</pre>
'''AccountKeys JSON'''
<pre>
{
  masterkey:  "time used as input in to verification"
}
}
</pre>
</pre>
Line 541: Line 370:
   message:    "ephemeral key okay|ephemeral key mismatch",
   message:    "ephemeral key okay|ephemeral key mismatch",
   auth:        SessionVerifier (if status okay),
   auth:        SessionVerifier (if status okay),
   payload:    AccountKeys (if status okay),
   payload:    Optional. See Registration protocol,
}
}
</pre>
</pre>
Line 562: Line 391:
   status:    "verified|failed",
   status:    "verified|failed",
   message:  "session verification okay|session verification failed",
   message:  "session verification okay|session verification failed",
}
</pre>
<span id="Registration_Protocol_v2"></span>
===Registration Protocol===
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 a user 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. A key difference from the messaging protocol is that to improve user experience we want to avoid the user having to wait for messages to be exchanged, thus the out-of-band step is brought forward as early as possible and is able to take place immediately after Client A has sent the SesssionKeyCommitRequestMessage although it cannot be completed until Client B has received the message, hence good connectivity is required on both Client A and Client B, but not necessarily simultaneously.
'''Note:''' When the new device is mobile a QR code could be used for the out-of-band verification, which would remove the need for user input and mitigate against the impersonation of Client A and Client B.
<ol start="0">
<li>Client A: Authenticate to sync server and create client record with status of 'pending'</li>
<li>Client A: Send SessionKeyCommitRequestMessage to existing device, Client B, including digest of ephemeral key (AEd) and with a getkeys value of 'true'</li>
<li>Out-of-Band: User sights authcode displayed on Client A, generated from identity and key ephemeral key and random value, and enters it on Client B</li>
<li>Client B: Send SessionKeyCommitResponseMessage to new device, Client A, providing an ephemeral key (BEp)</li>
<li>Client A: Send SessionKeyVerifyRequestMessage to Client B including ephemeral key (AEp)</li>
<li>Client B: Send SessionKeyVerifyResponseMessage to Client A encrypted with a session key derived from the shared secret (SK) and including a status indicating if the identity and ephemeral key component (VCS) of authcode have been verified. If verification is successful include the AccountKeys payload, i.e. master key (MK)</li>
</ol>
Finally the user is notified of the registration status and if successful client record is updated with status of 'approved'
<pre>
          Alice (Client A)                      Eve (MITM)      Bob (Client B)
t0        AIs                                    AIp, BIp        BIs, MK                                                   
t1        AIs                                    AIp, BIp        BIs, MK
Key        AEs                                    AEd             
Commit    AEp                                   
t2        AIs                                    AIp, BIp        BIs, MK
(OoB)      AEs                                    AEd              BEs
          AEp                                                    BEp
          VCS = DIGEST(AIp, AEp)                                  VCSui = <user input>
          VCR = RANDOM()                                          VCRui = <user input>
t3        AIs                                    AIp, BIp        BIs, MK
          AEs                                    BEp              BEs
          AEp                                    AEd              VCSui   
          VCS                                                    VCRui                   
t4        AIs                                    AIp, BIp        BIs, MK
Key        AEs                                    AEp, BEp        BEs
Verify    VCS                                                    VCSui
          VCR                                                    VCRui
                                                                  Verified = (VCSui == DIGEST(AIp, AEp) + AEd == DIGEST(AEp))
t5        AIs, MK                                AIp, BIp        BIs, MK
          SK = KDF(3DHE(AIs, AEs, BIp, BEp))                      SK = KDF(3DHE(BIs, BEs, AIp, AEp))
          VCR                                                    VCRui
          VCRui                                                 
          Verified = (VCRui == VCR)                             
</pre>
'''Notes:'''
* A QR code including Client A's entire key (AEp) would allow step 2, 3 and 4 to be collapsed into a single step. See [[#Signal (TextSecure) Registration Protocol|Signal (TextSecure) Registration Protocol]] below.
'''AccountKeys JSON'''
<pre>
{
  masterkey:  "time used as input in to verification"
}
}
</pre>
</pre>
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