[tor-commits] [torspec/master] prop224: Revisit basic client authorization

commit 890779ffec60f067048c4e4a9895ccdd49d183a5 Author: George Kadianakis desnacked@riseup.net Date: Thu Dec 1 16:13:50 2016 -0500

prop224: Revisit basic client authorization

In the past prop224 used to embed the client authorization key in the subcredential. The problem here is that if we wanted to revoke a client, we would have to change the whole subcredential of the service, which means that we would have to announce it to all clients.

This patch makes it so that every client has an x25519 and an ed25519 which are used to perform client authorization.

To achieve this on the descriptor level, we change the descriptor format to a double-layer encryption where the first layer protects against entities who don't know the public key of the HS, and the second layer protects against unauthorized clients who don't know the x25519 key.

The intro level authorization remains as is and uses ed25519 for authentication.

Thanks to special for noticing this issue.

Thanks to Nick for sketching out the x25519 descriptor auth scheme. --- proposals/224-rend-spec-ng.txt | 446 +++++++++++++++++++++++++++++++++++------ 1 file changed, 384 insertions(+), 62 deletions(-)

diff --git a/proposals/224-rend-spec-ng.txt b/proposals/224-rend-spec-ng.txt index 680e5bc..4f05638 100644 --- a/proposals/224-rend-spec-ng.txt +++ b/proposals/224-rend-spec-ng.txt @@ -23,6 +23,7 @@ Table of contents: 1.7. In more detail: Keeping crypto keys offline 1.8. In more detail: Encryption Keys And Replay Resistance 1.9. In more detail: A menagerie of keys + 1.9.1. In even more detail: Client authorization [CLIENT-AUTH] 2. Generating and publishing hidden service descriptors [HSDIR] 2.1. Deriving blinded keys and subcredentials [SUBCRED] 2.2. Locating, uploading, and downloading hidden service descriptors @@ -36,9 +37,17 @@ Table of contents: 2.3.1. Client behavior in the absense of shared random values 2.3.2. Hidden services and changing shared random values 2.4. Hidden service descriptors: outer wrapper [DESC-OUTER] - 2.5. Hidden service descriptors: encryption format [ENCRYPTED-DATA] - 2.5.1. Number of introduction points [NUM_INTRO_POINT] - 3. The introduction protocol + 2.5. Hidden service descriptors: encryption format [HS-DESC-ENC] + 2.5.1. First layer of encryption [HS-DESC-FIRST-LAYER] + 2.5.1.1. First layer encryption logic + 2.5.1.2. First layer plaintext format + 2.5.1.3. Client behavior + 2.5.1.4. Obfuscating the number of authorized clients + 2.5.2. Second layer of encryption [HS-DESC-SECOND-LAYER] + 2.5.2.1. Second layer encryption keys + 2.5.2.2. Second layer plaintext format + 2.5.3. Deriving hidden service descriptor encryption keys [HS-DESC-ENCRYPTION-KEYS] + 3. The introduction protocol [INTRO-PROTOCOL] 3.1. Registering an introduction point [REG_INTRO_POINT] 3.1.1. Extensible ESTABLISH_INTRO protocol. [EST_INTRO] 3.1.2. Registering an introduction point on a legacy Tor node [LEGACY_EST_INTRO] @@ -424,34 +433,36 @@ Table of contents:

1.3. In more detail: Access control [IMD:AC]

- Access control for a hidden service is imposed at multiple points - through the process above. + Access control for a hidden service is imposed at multiple points through + the process above. Furthermore, there is also the option to impose + additional client authorization access control using pre-shared secrets + exchanged out-of-band between the hidden service and its clients.

- In order to download a descriptor, clients must know which blinded - signing key was used to sign it. (See the next section for more info - on key blinding.) This blinded signing key is derived from the - service's public key and, optionally, an additional secret that is - not part of the hidden service's onion address. The public key and - this secret together constitute the service's "credential". + The first stage of access control happens when downloading HS descriptors. + Specifically, in order to download a descriptor, clients must know which + blinded signing key was used to sign it. (See the next section for more info + on key blinding.)

- When the secret is in use, the hidden service gains protections - equivalent to the "stealth mode" in previous designs. + To learn the introduction points, clients must decrypt the body of the + hidden service descriptor. To do so, clients must know the _unblinded_ + public key of the service, which makes the descriptor unuseable by entities + without that knowledge (e.g. HSDirs that don't know the onion address).

- To learn the introduction points, the clients must decrypt the body - of the hidden service descriptor. The encryption key for these is - derived from the service's credential. + Also, if optional client authorization is enabled, hidden service + descriptors are superencrypted using each authorized user's identity x25519 + key, to further ensure that unauthorized entities cannot decrypt it.

In order to make the introduction point send a rendezvous request to the service, the client needs to use the per-introduction-point authentication key found in the hidden service descriptor.

- The final level of access control happens at the server itself, which - may decide to respond or not respond to the client's request - depending on the contents of the request. The protocol is extensible - at this point: at a minimum, the server requires that the client - demonstrate knowledge of the contents of the encrypted portion of the - hidden service descriptor. The service may additionally require a - user- or group-specific access token before it responds to requests. + The final level of access control happens at the server itself, which may + decide to respond or not respond to the client's request depending on the + contents of the request. The protocol is extensible at this point: at a + minimum, the server requires that the client demonstrate knowledge of the + contents of the encrypted portion of the hidden service descriptor. If + optional client authorization is enabled, the service may additionally + require the client to prove knowledge of a pre-shared private key.

1.4. In more detail: Distributing hidden service descriptors. [IMD:DIST]

@@ -511,7 +522,7 @@ Table of contents:

In order to operate a hidden service, the operator can generate in advance a number of blinded signing keys and descriptor signing - keys (and their credentials; see [DESC-OUTER] and [ENCRYPTED-DATA] + keys (and their credentials; see [DESC-OUTER] and [HS-DESC-ENC] below), and their corresponding descriptor encryption keys, and export those to the hidden service hosts.

@@ -597,6 +608,33 @@ Table of contents: part of the Tor circuit extension handshake, used to tie a request to a particular circuit.

+1.9.1. In even more detail: Client authorization keys [CLIENT-AUTH] + + When client authorization is enabled, each authorized client of a hidden + service has two more assymetric keypairs which are shared with the hidden + service. An entity without those keys is not able to use the hidden + service. Throughout this document, we assume that these pre-shared keys are + exchanged between the hidden service and its clients in a secure out-of-band + fashion. + + Specifically, each authorized client possesses: + + - An x25519 keypair used to compute decryption keys that allow the client to + decrypt the hidden service descriptor. See [HS-DESC-ENC]. + + - An ed25519 keypair which allows the client to compute signatures which + prove to the hidden service that the client is authorized. These + signatures are inserted into the INTRODUCE1 cell, and without them the + introduction to the hidden service cannot be completed. See [INTRO-AUTH]. + + The right way to exchange these keys is to have the client generate keys and + send the corresponding public keys to the hidden service out-of-band. An + easier but less secure way of doing this exchange would be to have the + hidden service generate the keypairs and pass the corresponding private keys + to its clients. See section [CLIENT-AUTH-MGMT] for more details on how these + keys should be managed. + + [TODO: Also specify stealth client authorization.]

2. Generating and publishing hidden service descriptors [HSDIR]

@@ -610,7 +648,7 @@ Table of contents:

2.1. Deriving blinded keys and subcredentials [SUBCRED]

- In each time period (see [TIME-PERIOD] for a definition of time + In each time period (see [TIME-PERIODS] for a definition of time periods), a hidden service host uses a different blinded private key to sign its directory information, and clients use a different blinded public key as the index for fetching that information. @@ -633,12 +671,10 @@ Table of contents:

In the above formula, credential corresponds to:

- credential = H(public-identity-key | authorization-key) + credential = H("credential" | public-identity-key)

where public-identity-key is the public identity master key of the hidden - service, and authorization-key is an optional secret used for client - authorization. If no client authorization is specified, authorization-key is - left blank. + service.

2.2. Locating, uploading, and downloading hidden service descriptors [HASHRING] @@ -932,13 +968,13 @@ Table of contents: prevents an attacker from replacing a newer descriptor signed by a given key with a copy of an older version.)

- "encrypted" NL encrypted-string + "superencrypted" NL encrypted-string

[Exactly once.]

- An encrypted blob, whose format is discussed in [ENCRYPTED-DATA] - below. The blob is base-64 encoded and enclosed in -----BEGIN - MESSAGE---- and ----END MESSAGE---- wrappers. + An encrypted blob, whose format is discussed in [HS-DESC-ENC] below. The + blob is base64 encoded and enclosed in -----BEGIN MESSAGE---- and + ----END MESSAGE---- wrappers.

"signature" SP signature NL

@@ -950,37 +986,195 @@ Table of contents: the hidden service host does not need to have its private blinded key online.

-2.5. Hidden service descriptors: encryption format [ENCRYPTED-DATA] + HSDirs accept hidden service descriptors of up to 50k bytes (a consensus + parameter should also be introduced to control this value).

- The encrypted part of the hidden service descriptor is encrypted and - authenticated with symmetric keys generated as follows: +2.5. Hidden service descriptors: encryption format [HS-DESC-ENC]

- SALT = 16 bytes from H(random), changes each time we rebuld the - descriptor even if the content of the descriptor hasn't changed. - (So that we don't leak whether the intro point list etc. changed) + Hidden service descriptors are protected by two layers of encryption. + Clients need to decrypt both layers to connect to the hidden service.

- secret_input = blinded_public_key | subcredential | - INT_8(revision_counter) - keys = KDF(secret_input, salt, "hsdir-encrypted-data", - S_KEY_LEN + S_IV_LEN + MAC_KEY_LEN) + The first layer of encryption provides confidentiality against entities who + don't know the public key of the hidden service (e.g. HSDirs), while the + second layer of encryption is only useful when client authorization is enabled + and protects against entities that do not possess valid client credentials.

- SECRET_KEY = first S_KEY_LEN bytes of keys - SECRET_IV = next S_IV_LEN bytes of keys - MAC_KEY = last MAC_KEY_LEN bytes of keys +2.5.1. First layer of encryption [HS-DESC-FIRST-LAYER]

- The encrypted data has the format: + The first layer of HS descriptor encryption is designed to protect + descriptor confidentiality against entities who don't know the blinded + public key of the hidden service.

- SALT hashed random bytes from above [16 bytes] - ENCRYPTED The plaintext encrypted with S [variable] - MAC MAC of both above fields [32 bytes] +2.5.1.1. First layer encryption logic + + The encryption keys and format for the first layer of encryption are + generated as specified in [HS-DESC-ENCRYPTION-KEYS] with customization + parameters: + + SECRET_DATA = blinded-public-key + STRING_CONSTANT = "hsdir-superencrypted-data" + + The ciphertext is placed on the "superencrypted" field of the descriptor. + + Before encryption the plaintext is padded with NUL bytes to the nearest + multiple of 10k bytes. + +2.5.1.2. First layer plaintext format + + After clients decrypt the first layer of encryption, they need to parse the + plaintext to get to the second layer ciphertext which is contained in the + "encrypted" field. + + If client auth is enabled, the hidden service generates a fresh + descriptor_cookie key (32 random bytes) and encrypts it using each + authorized client's identity x25519 key. Authorized clients can use the + descriptor cookie to decrypt the second layer of encryption. Our encryption + scheme requires the hidden service to also generate an ephemeral x25519 + keypair for each new descriptor. + + If client auth is disabled, fake data is placed in each of the fields below + to obfuscate whether client authorization is enabled. + + Here are all the supported fields: + + "desc-auth-type" SP type NL + + [Exactly once] + + This field contains the type of authorization used to protect the + descriptor. The only recognized type is "x25519" and specifies the + encryption scheme described in this section. + + If client authorization is disabled, the value here should be "x25519". + + "desc-auth-ephemeral-key" SP key NL + + [Exactly once] + + This field contains an ephemeral x25519 public key generated by the + hidden service and encoded in base64. The key is used by the encryption + scheme below. + + If client authorization is disabled, the value here should be a fresh + x25519 pubkey that will remain unused. + + "auth-client" SP client-id SP iv SP encrypted-cookie + + [Any number] + + When client authorization is enabled, the hidden service inserts an + "auth-client" line for each of its authorized clients. If client + authorization is disabled, the fields here can be populated with random + data of the right size (that's 8 bytes for 'client-id', 16 bytes for 'iv' + and 16 bytes for 'encrypted-cookie' all encoded with base64). + + When client authorization is enabled, each "auth-client" line contains + the descriptor cookie encrypted to each individual client. We assume that + each authorized client possesses a pre-shared x25519 keypair which is + used to decrypt the descriptor cookie. + + We now describe the descriptor cookie encryption scheme. Here are the + relevant keys: + + client_x = private x25519 key of authorized client + client_X = public x25519 key of authorized client + hs_y = private key of ephemeral x25519 keypair of hidden service + hs_Y = public key of ephemeral x25519 keypair of hidden service + descriptor_cookie = descriptor cookie used to encrypt the descriptor + + And here is what the hidden service computes: + + SECRET_SEED = x25519(hs_y, client_X) + KEYS = KDF(SECRET_SEED, 40) + CLIENT-ID = fist 8 bytes of KEYS + COOKIE-KEY = last 32 bytes of KEYS + + Here is a description of the fields in the "auth-client" line: + + - The "client-id" field is CLIENT-ID from above encoded in base64. + + - The "iv" field is 16 random bytes encoded in base64. + + - The "encrypted-cookie" field contains the descriptor cookie ciphertext + as follows and is encoded in base64: + encrypted-cookie = STREAM(iv, COOKIE-KEY) XOR descriptor_cookie + + See section [FIRST-LAYER-CLIENT-BEHAVIOR] for the client-side logic of + how to decrypt the descriptor cookie. + + "encrypted" NL encrypted-string + + [Exactly once] + + An encrypted blob containing the second layer ciphertext, whose format is + discussed in [HS-DESC-SECOND-LAYER] below. The blob is base64 encoded + and enclosed in -----BEGIN MESSAGE---- and ----END MESSAGE---- wrappers. + +2.5.1.3. Client behavior [FIRST-LAYER-CLIENT-BEHAVIOR] + + The goal of clients at this stage is to decrypt the "encrypted" field as + described in [HS-DESC-SECOND-LAYER]. + + If client authorization is enabled, authorized clients need to extract the + descriptor cookie to proceed with decryption of the second layer as + follows: + + An authorized client parsing the first layer of an encrypted descriptor, + extracts the ephemeral key from "desc-auth-ephemeral-key" and calculates + CLIENT-ID and COOKIE-KEY as described in the section above using their + x25519 private key. The client then uses CLIENT-ID to find the right + "auth-client" field which contains the ciphertext of the descriptor + cookie. The client then uses COOKIE-KEY and the iv to decrypt the + descriptor_cookie, which is used to decrypt the second layer of descriptor + encryption as described in [HS-DESC-SECOND-LAYER]. + +2.5.1.4. Hiding client authorization data + + Hidden services should avoid leaking whether client authorization is + enabled or how many authorized clients there are. + + Hence even when client authorization is disabled, the hidden service adds + fake "desc-auth-type", "desc-auth-ephemeral-key" and "auth-client" lines to + the descriptor, as described in [HS-DESC-FIRST-LAYER].

- The encryption format is ENCRYPTED = - STREAM(SECRET_IV,SECRET_KEY) XOR Plaintext + The hidden service also avoids leaking the number of authorized clients by + adding fake "auth-client" entries to its descriptor. Specifically, + descriptors always contain a number of authorized clients that is a + multiple of 16 by adding fake "auth-client" entries if needed. + [XXX consider randomization of the value 16]

- Before encryption, the plaintext must be padded to a multiple of 4096 bytes - with NUL bytes. + Clients MUST accept descriptors with any number of "auth-client" lines as + long as the total descriptor size is within the max limit of 50k (also + controlled with a consensus parameter).

- The plaintext format is: +2.5.2. Second layer of encryption [HS-DESC-SECOND-LAYER] + + The second layer of descriptor encryption is designed to protect descriptor + confidentiality against unauthorized clients. If client authorization is + enabled, it's encrypted using the descriptor_cookie, and contains needed + information for connecting to the hidden service, like the list of its + introduction points. + + If client authorization is disabled, then the second layer of HS encryption + does not offer any additional security, but is still used. + +2.5.2.1. Second layer encryption keys + + The encryption keys and format for the second layer of encryption are + generated as specified in [HS-DESC-ENCRYPTION-KEYS] with customization + parameters as follows: + + SECRET_DATA = blinded-public-key | descriptor_cookie + STRING_CONSTANT = "hsdir-encrypted-data" + + If client authorization is disabled the 'descriptor_cookie' field is left blank. + + The ciphertext is placed on the "encrypted" field of the descriptor. + +2.5.2.2. Second layer plaintext format + + After decrypting the second layer ciphertext, clients can finally learn the + list of intro points etc. The plaintext has the following format:

"create2-formats" SP formats NL

@@ -991,14 +1185,14 @@ Table of contents: tor-spec.txt. See tor-spec section 5.1 for a list of recognized handshake types.

- "authentication-required" SP types NL + "intro-auth-required" SP types NL

[At most once]

- A space-separated list of authentication types. A client that does - not support at least one of these authentication types will not be - able to contact the host. Recognized types are: 'password' and - 'ed25519'. See [INTRO-AUTH] below. + A space-separated list of introduction-layer authentication types; see + section [INTRO-AUTH] for more info. A client that does not support at + least one of these authentication types will not be able to contact the + host. Recognized types are: 'password' and 'ed25519'.

"single-onion-service"

@@ -1069,7 +1263,43 @@ Table of contents: Other encryption and authentication key formats are allowed; clients should ignore ones they do not recognize.

-2.5.1. Number of introduction points [NUM_INTRO_POINT] + Clients who manage to extract the introduction points of the hidden service + can prroceed with the introduction protocol as specified in [INTRO-PROTOCOL]. + +2.5.3. Deriving hidden service descriptor encryption keys [HS-DESC-ENCRYPTION-KEYS] + + In this section we present the generic encryption format for hidden service + descriptors. We use the same encryption format in both encryption layers, + hence we introduce two customization parameters SECRET_DATA and + STRING_CONSTANT which vary between the layers. + + The SECRET_DATA parameter specifies the secret data that are used during + encryption key generation, while STRING_CONSTANT is merely a string constant + that is used as part of the KDF. + + Here is the key generation logic: + + SALT = 16 bytes from H(random), changes each time we rebuld the + descriptor even if the content of the descriptor hasn't changed. + (So that we don't leak whether the intro point list etc. changed) + + secret_input = SECRET_DATA | subcredential | INT_8(revision_counter) + + keys = KDF(secret_input | salt | STRING_CONSTANT, S_KEY_LEN + S_IV_LEN + MAC_KEY_LEN) + + SECRET_KEY = first S_KEY_LEN bytes of keys + SECRET_IV = next S_IV_LEN bytes of keys + MAC_KEY = last MAC_KEY_LEN bytes of keys + + The encrypted data has the format: + + SALT hashed random bytes from above [16 bytes] + ENCRYPTED The ciphertext [variable] + MAC MAC of both above fields [32 bytes] + + The final encryption format is ENCRYPTED = STREAM(SECRET_IV,SECRET_KEY) XOR Plaintext + +2.5.4. Number of introduction points [NUM_INTRO_POINT]

This section defines how many introduction points an hidden service descriptor can have at minimum, by default and the maximum: @@ -1085,7 +1315,7 @@ Table of contents: HSDirs) but also in order for the descriptor size to not overwhelmed hidden service directories with user defined values that could be gigantic.

-3. The introduction protocol +3. The introduction protocol [INTRO-PROTOCOL]

The introduction protocol proceeds in three steps.

@@ -1858,3 +2088,95 @@ Appendix E. Reserved numbers

Note: The value "0A" is skipped because it's reserved for the onion key cross-certifying ntor identity key from proposal 228. + +Appendix F. Managing authorized client data [CLIENT-AUTH-MGMT] + + Hidden services and clients can configure their authorized client data either + using the torrc, or using the control port. This section presents a suggested + scheme for configuring client authorization. + + F.1. Configuring client authorization using torrc + + F.1.1. Hidden Service side + + A hidden service that wants to perform client authorization, adds a new + option HiddenServiceAuthorizeClient to its torrc file: + + HiddenServiceAuthorizeClient auth-type client-name,client-name,... + + The only recognized auth-type value is "basic" which describes the scheme in + section [CLIENT-AUTH]. The rest of the line is a comma-separated list of + human-readable authorized client names. + + Let's consider that one of the listed client names is "alice". In this case, + Tor checks the directory at "DataDirectory/hidden_service/client_auth/" for + any files with filename alice.key or alice.pub . + + Files ending in .key contain private keys for authorized clients, whereas + .pub files contain public keys for authorized clients. In general, authorized + clients should send their public keys to the hidden service operator, and the + operator should place them in the filesystem as .pub files. Only .pub files + are useful to the hidden service, whereas .key files are there only if the + hidden service had to generate its client's keypairs as described below. + + If no alice.key or alice.pub files exist, Tor is tasked with generating + client keys for Alice. To do so, Tor generates x25519 and ed25519 keypairs + for Alice, then makes an alice.key file and writes the private keys inside; + it also makes an alice.pub file and writes the public keys inside. + [XXX what format? it should be convenient so that client can just copy-paste] + + In this case, the hidden service operator has the responsibility to pass the + .key file to Alice in a secure out-of-band way. After the file is passed to + Alice, it can be shredded from the filesystem, as only the public keys are + required for the hidden service to function. + + F.1.2. Client side + + A client who wants to register client authorization data for a hidden service + needs to add the following line to their torrc: + + HidServAuth onion-address x25519-private-key ed25519-private-key + + The keys above are either generated by Alice using a key generation utility, + or they are extracted from a .key file provided by the hidden service. + + In the former case, the client is also tasked with transfering the public + keys to the hidden service in a secure out-of-band way. + + F.2. Configuring client authorization using the control port + + F.2.1. Service side + + A hidden service also has the option to configure authorized clients + using the control port. The idea is that hidden service operators can use + controller utilities that manage their access control instead of using + the filesystem to register client keys. + + Specifically, we require a new control port command ADD_ONION_CLIENT_AUTH + which is able to register x25519/ed25519 public keys tied to a specific + authorized client. + [XXX figure out control port command format] + + Hidden services who use the control port interface for client auth need + to perform their own key management. + + F.2.2. Client side + + There should also be a control port interface for clients to register + authorization data for hidden services without having to use the + torrc. It should allow both generation of client authorization private + keys, and also to import client authorization data provided by a hidden + service + + This way, Tor Browser can present "Generate client auth keys" and "Import + client auth keys" dialogs to users when they try to visit a hidden service + that is protected by client authorization. + + Specifically, we require two new control port commands: + IMPORT_ONION_CLIENT_AUTH_DATA + GENERATE_ONION_CLIENT_AUTH_DATA + which import and generate client authorization data respectively. + + [XXX how does key management work here?] + [XXX what happens when people use both the control port interface and the + filesystem interface?]