commit d909cff98e419d0d168124a6d09e88ec7d8ec136 Author: Nick Mathewson <nickm@torproject.org> Date: Mon Aug 12 15:17:13 2013 -0400 Add an Ed25519 identities proposal and a kill-create_fast proposal --- proposals/000-index.txt | 6 + proposals/220-ecc-id-keys.txt | 518 ++++++++++++++++++++++++++++++ proposals/221-stop-using-create-fast.txt | 90 ++++++ 3 files changed, 614 insertions(+) diff --git a/proposals/000-index.txt b/proposals/000-index.txt index eaa103c..8c2d844 100644 --- a/proposals/000-index.txt +++ b/proposals/000-index.txt @@ -139,6 +139,9 @@ Proposals by number: 216 Improved circuit-creation key exchange [CLOSED] 217 Tor Extended ORPort Authentication [OPEN] 218 Controller events to better understand connection/circuit usage [OPEN] +219 Support for full DNS and DNSSEC resolution in Tor [DRAFT] +220 Migrate server identity keys to Ed25519 [DRAFT] +221 Stop using CREATE_FAST [OPEN] Proposals by status: @@ -153,6 +156,8 @@ Proposals by status: 182 Credit Bucket 195 TLS certificate normalization for Tor 0.2.4.x [for 0.2.4.x] 203 Avoiding censorship by impersonating an HTTPS server + 219 Support for full DNS and DNSSEC resolution in Tor [for 0.2.5.x] + 220 Migrate server identity keys to Ed25519 [for 0.2.5.x] NEEDS-REVISION: 131 Help users to verify they are using Tor 190 Bridge Client Authorization Based on a Shared Secret @@ -186,6 +191,7 @@ Proposals by status: 215 Let the minimum consensus method change with time 217 Tor Extended ORPort Authentication [for 0.2.5.x] 218 Controller events to better understand connection/circuit usage [for 0.2.5.x] + 221 Stop using CREATE_FAST [for 0.2.5.x] ACCEPTED: 140 Provide diffs between consensuses 147 Eliminate the need for v2 directories in generating v3 directories [for 0.2.4.x] diff --git a/proposals/220-ecc-id-keys.txt b/proposals/220-ecc-id-keys.txt new file mode 100644 index 0000000..1c112b9 --- /dev/null +++ b/proposals/220-ecc-id-keys.txt @@ -0,0 +1,518 @@ +Filename: 220-ecc-id-keys.txt +Title: Migrate server identity keys to Ed25519 +Authors: Nick Mathewson +Created: 12 August 2013 +Target: 0.2.5.x +Status: Draft + + [Note: This is a draft proposal; I've probably made some important + mistakes, and there are parts that need more thinking. I'm + publishing it now so that we can do the thinking together.] + +0. Introduction + + In current Tor designs, identity keys are limited to 1024-bit RSA + keys. + + Clearly, that should change, because RSA doesn't represent a good + performance-security tradeoff nowadays, and because 1024-bit RSA is + just plain too short. + + We've already got an improved circuit extension handshake protocol + that uses curve25519 in place of RSA1024, and we're using (where + supported) P256 ECDHE in our TLS handshakes, but there are more uses + of RSA1024 to replace, including: + + * Router identity keys + * TLS link keys + * Hidden service keys + + This proposal describes how we'll migrate away from using 1024-bit + RSA in the first two, since they're tightly coupled. Hidden service + crypto changes will be complex, and will merit their own proposal. + + In this proposal, we'll also (incidentally) be extirpating a number + of SHA1 usages. + +1. Overview + + When this proposal is implemented, every router will have an Ed25519 + identity key in addition to its current RSA1024 public key. + + Ed25519 (specifically, Ed25519-SHA-512 as described and specified at + http://ed25519.cr.yp.to/) is a desirable choice here: it's secure, + fast, has small keys and small signatures, is bulletproof in several + important ways, and supports fast batch verification. (It isn't quite + as fast as RSA1024 when it comes to public key operations, since RSA + gets to take advantage of small exponents when generating public + keys.) + + (For reference: In Ed25519 public keys are 32 bytes long, private keys + are 64 bytes long, and signatures are 64 bytes long.) + + To mirror the way that authority identity keys work, we'll fully + support keeping Ed25519 identity keys offline; they'll be used to + sign long-ish term signing keys, which in turn will do all of the + heavy lifting. A signing key will get used to sign the things that + RSA1024 identity keys currently sign. + +1.1. 'Personalized' signatures + + Each of the keys introduced here is used to sign more than one kind + of document. While these documents should be unambiguous, I'd going + to forward-proof the signatures by specifying each signature to be + generated, not on the document itself, but on the document prefixed + with some distinguishing string. + +2. Certificates and Router descriptors. + +2.1. Certificates + + When generating a signing key, we also generate a certificate for it. + Unlike the certificates for authorities' signing keys, these + certificates need to be sent around frequently, in significant + numbers. So we'll choose a compact representation. + + VERSION [1 Byte] + TYPE [1 Byte] + CERTIFIED_KEY [32 Bytes] + EXPIRATION_DATE [4 Bytes] + EXTRA_STUFF [variable length] + SIGNATURE [64 Bytes] + + The "VERSION" field holds the value [01]. The "TYPE" field holds the + value [01]. The CERTIFIED_KEY field is an Ed25519 public key. The + expiration date is a day, given in DAYS since the epoch, after which + this certificate isn't valid. The EXTRA_STUFF field is left for a + future version of this format. + + [XXXX Is "EXTRA_STUFF" a good idea? -NM] + + Before processing any certificate, parties MUST know which identity + key it is supposed to be signed by, and then check the signature. + The signature is formed by signing the first N-64 bytes of the + certificate prefixed with the string "Tor node signing key + certificate v1". + + We also specify a revocation document for revoking a signing key or an + identity key. Its format is: + FIXED_PREFIX [8 Bytes] + VERSION [1 Byte] + KEYTYPE [1 Byte] + IDENTITY_KEY [32 Bytes] + REVOKED_KEY [32 Bytes] + PUBLISHED [8 Bytes] + EXTRA_STUFF [variable length] + SIGNATURE [64 Bytes] + + FIXED_PREFIX is "REVOKEID" or "REVOKESK". VERSION is [01]. KEYTYPE is + [01] for revoking a signing key or [02] or revoking an identity key. + REVOKED_KEY is the key being revoked; IDENTITY_KEY is the node's + Ed25519 identity key. PUBLISHED is the time that the document was + generated, in seconds since the epoch. EXTRA_STUFF is left for a + future version of this document. The SIGNATURE is generated with + the same key as in IDENTITY_KEY, and covers the entire revocation, + prefixed with "Tor key revocation v1". + + Using these revocation documents is unspecified. + +2.2. Managing keys + + By default, we can keep the easy-to-setup key management properties + that Tor has now, so that node operators aren't required to have + offline public keys: + + * When a Tor node starts up with no Ed25519 identity keys, it + generates a new identity keypair. + * When a Tor node has an Ed25519 identity keypair, and it has + no signing key, or its signing key is going to expire within + the next 48 hours, it generates a new signing key to last + 30 days. + + But we also support offline identity keys: + + * When a Tor node starts with an Ed25519 public identity key + but no private identity key, it checks wither it has + a currently valid certified signing keypair. If it does, + it starts. Otherwise, it refuses to start. + * If a Tor node's signing key is going to expire soon, it starts + warning the user. If it is expired, then the node shuts down. + +2.3. Router descriptors + + We specify the following element that may appear at most once in + each router descriptor: + "identity-ed25519" SP identity-key SP certification NL + + The identity-key and certification are base64 encoded with + terminating =s removed. When this element is present, it MUST appear + as the first or second element in the router descriptor. + + [XXX The rationale here is to allow extracting the identity key and + signing key and checking the signature before fully parsing the rest + of the document. -NM] + + When an identity-ed25519 element is present, there must also be an + "router-signature-ed25519" element. It MUST be the next-to-last + element in the descriptor, appearing immediately before RSA + signature. It MUST contain an ed25519 signature of the entire + document, from the first character up to but not including the + "router-signature-ed25519" element, prefixed with the string "Tor + router descriptor signature v1". Its format is: + + "router-signature-ed25519" SP signature NL + + Were 'signature' is encoded in base64 with terminating =s removed. + + The identity key in the identity-ed25519 key MUST be the one used to + sign the certification, and the signing key in the certification MUST + be the one used to sign the document. + + + Note that these keys cross-certify as follows: the ed25519 identity + key signs the ed25519 signing key in the certificate. The ed25519 + signing key signs itself and the ed25519 identity key and the RSA + identity key as part of signing the descriptor. And the RSA identity + key also signs all three keys as part of signing the descriptor. + + +2.3.1. Checking descriptor signatures. + + Current versions of Tor will handle these new formats by ignoring the + new fields, and not checking any ed25519 information. + + New version of Tor will have a flag that tells them whether to check + ed25519 information. When it is set, they must check: + + * All RSA information and signatures that Tor implementations + currently check. + * If the identity-ed25519 line is present, it must be well-formed, + and the certificate must be well-formed and correctly signed, + and there must be a valid. + * If we require an ed25519 key for this node (see 3.1 below), the + ed25519 key must be present. + + Authorities and directory caches will have this flag always-on. For + clients, it will be controlled by a torrc option and consensus + option, to be set to "always-on" in the future once enough clients + support it. + +2.3.2. Extra-info documents + + Extrainfo documents now include "identity-ed25519" and + "router-signature-ed25519" fields in the same positions in which they + appear in router descriptors. + + Additionally, we add the base64-encoded, =-stripped SHA256 digest of + a node's extra-info document field to the extra-info-digest line. + (All versions of Tor that recognize this line allow an extra field + there.) + +2.3.3. A note on signature verification + + Here and elsewhere, we're receiving a certificate and a document + signed with the key certified by that certificate in the same step. + This is a fine time to use the batch signature checking capability of + Ed25519, so that we can check both signatures at once without (much) + additional overhead over checking a single signature. + +3. Consensus documents and authority operation + +3.1. Handling router identity at the authority + + When receiving router descriptors, authorities must track mappings + between RSA and Ed25519 keys. + + Rule 1: Once an authority has seen an Ed25519 identity key and an RSA + identity key together on the same (valid) descriptor, it should no + longer accept any descriptor signed by that RSA key with a different + Ed25519 key, or that Ed25519 key with a different RSA key. + + Rule 2: Once an authority has seen an Ed25519 identity key and an RSA + identity key on the same descriptor, it should no longer accept any + descriptor signed by that RSA key unless it also has that Ed25519 + key. + + + These rules together should enforce the property that, even if an + attacker manages to steal or factor a node's RSA identity key, the + attacker can't impersonate that node to the authorities, even when + that node is identified by its RSA key. + + + Enforcement of Rule 1 should be advisory-only for a little while (a + release or two) while node operators get experience having Ed25519 + keys, in case there are any bugs that cause or force identity key + replacement. Enforcement of Rule 2 should be advisory-only for + little while, so that node operators can try 0.2.5 but downgrade to + 0.2.4 without being de-listed from the consensus. + + + [XXX I could specify a way to do a signed "I'm downgrading for a + while!" statement, and kludge some code back into 0.2.4.x to better + support that?] + +3.2. Formats + + Vote and consensus documents now contain an optional "id" field for each + routerstatus section. Its format is: + + "id" SP ed25519-identity NL + + where ed25519-identity is base-64 encoded, with trailing = characters + omitted. In vote documents, it may be replaced by the format: + + "id" SP "none" NL + + which indicates that the node does not have an ed25519 identity. (In + the consensus, a lack of "id" line means that the node has no ed25519 + identity.) + + [XXXX Should the id entries in consensuses go into microdescriptors + instead? I think perhaps so. -NM] + + A vote or consensus document is ill-formed if it includes the same + ed25519 identity key twice. + +3.3. Generating votes + + An authority should pick which descriptor to choose for a node as + before, and include the ed25519 identity key for the descriptor if + it's present. + + As a transition, before Rule 1 and Rule 2 in 3.1 are fully enforced, + authorities need a way to deal with the possibility that there might + be two nodes with the same ed25519 key but different RSA keys. In + that case, it votes for the one with the most recent publication + date. + + (The existing rules already prevent an authority from voting for two + servers with the same RSA identity key.) + +3.4. Generating a consensus from votes + + This proposal requires a new consensus vote method. When we deploy + it, we'll pick the next available vote method in sequence to use for + this. + + When the new consensus method is in use, we must choose nodes first + by ECC key, then by RSA key. + + First, for every {ECC identity key, RSA identity key} pair listed by + over half of the voting authorities, list it, unless some other RSA + identity key digest is listed more popularly for the ECC key. Break + ties in favor of low RSA digests. Treat all routerstatus entries that + mention this <ECC,RSA> pair as being for the same router, and all + routerstatus entries that mention the same RSA key with an + unspecified ECC key as being for the same router. + + Then, for every node that has previously not been listed, perform the + current routerstatus algorithm: listing a node if it has been listed + by at least N/2 voting authorities, and treating all routerstatuses + containing the same identity as the same router. + + In other words: + + Let Entries = [] + + for each ECC ID listed by any voter: + Find the RSA key associated with that ECC ID by the most voters, + breaking ties in favor of low RSA keys. + + If that ECC ID and RSA key ID are listed by > N/2 voting authorities: + Add the consensus of the routerstatus entries for those + voters, along with the routerstatus entry for every voter + that included that RSA key with no ECC key, to Entries. + Include the ECC ID in the consensus. + + For each RSA key listed by any voter: + If that RSA key is already in Entries, skip it. + + If the RSA key is listed by > N/2 voting authorities: + Add the consensus of the routerstatus entries for those + voters to Entries. Do not include an ECC key in the + consensus. + + [XXX Think about this even more.] + +4. The link protocol + + [XXX This section won't make much sense unless you grok the v3 + connection protocol as described in tor-spec.txt, first proposed in + proposal 195.] + + We add four new CertType values for use in CERTS cells: + 4: Ed25519 signing key + 5: Link key certificate certified by Ed25519 signing key + 6: TLS authentication key certified by Ed25519 signing key + 7: RSA cross-certification for Ed25519 identity key + + The content of certificate type 4 is: + Ed25519 identity key [32 byets] + Signing key certificate as in 2.1 above [variable length] + + The content of certificate type 5 is: + CERT_DIGEST [32 bytes] + EXPIRATION_DATE [4 bytes] + EXTRA_STUFF [variable] + SIGNATURE [64 bytes] + where CERT_DIGEST is a SHA256 digest of the X.509 certificate used + for the TLS link, EXPIRATION_DATE is a date in *days* since the epoch + starting on which the certificate is invalid, and SIGNATURE is + a signature using the signing key of the above two fields, prefixed + with "Tor TLS link certificate check v1". + + The content of certificate type 6 is the same as the signing key in + 2.1 above, except that the TYPE field is 02 and the fixed string used + in signing is "". The Ed25519 key + certified by this key can be used in AUTHENTICATE cells. + + The content of certificate type 7 is: + ED25519_KEY [32 bytes] + EXPIRATION_DATE [4 bytes] + SIGNATURE [128 bytes] + Here, the Ed25519 identity key is signed with router's identity key, + to indicate that authenticating with a key certified by the Ed25519 + key counts as certifying with RSA identity key. (The signature is + computed on the SHA256 hash of the non-signature parts of the + certificate, prefixed with the string "Tor TLS RSA/Ed25519 + cross-certification".) + + (There's no reason to have a corresponding Ed25519-signed-RSA-key + certificate here, since we do not treat authenticating with an RSA + key as proving ownership of the Ed25519 identity.) + + Relays with Ed25519 keys should always send these certificate types + in addition to their other certificate types. + + Non-bridge relays with Ed25519 keys should generate TLS link keys of + appropriate strength, so that the certificate chain from the Ed25519 + key to the link key is strong enough. + + + We add a new authentication type for AUTHENTICATE cells: + "Ed25519-TLSSecret", with AuthType value 2. Its format is the same as + "RSA-SHA256-TLSSecret", except that the CID and SID fields support + more key types; some strings are different, and the signature is + performed with Ed25519 using the authentication key from a type-6 + cert. + + TYPE: The characters "AUTH0002" [8 octets] + CID: A SHA256 hash of the initiator's RSA1024 identity key [32 octets] + SID: A SHA256 hash of the responder's RSA1024 identity key [32 octets] + CID_ED: The initiator's Ed25519 identity key [32 octets] + SID_ED: The responder's Ed25519 identity key, or all-zero. [32 octets] + SLOG: A SHA256 hash of all bytes sent from the responder to the + initiator as part of the negotiation up to and including the + AUTH_CHALLENGE cell; that is, the VERSIONS cell, the CERTS cell, + the AUTH_CHALLENGE cell, and any padding cells. [32 octets] + CLOG: A SHA256 hash of all bytes sent from the initiator to the + responder as part of the negotiation so far; that is, the + VERSIONS cell and the CERTS cell and any padding cells. [32 + octets] + SCERT: A SHA256 hash of the responder's TLS link certificate. [32 + octets] + TLSSECRETS: A SHA256 HMAC, using the TLS master secret as the + secret key, of the following: + - client_random, as sent in the TLS Client Hello + - server_random, as sent in the TLS Server Hello + - the NUL terminated ASCII string: + "Tor V3 handshake TLS cross-certification with Ed25519" + [32 octets] + TIME: The time of day in seconds since the POSIX epoch. [8 octets] + RAND: A 16 byte value, randomly chosen by the initiator. [16 octets] + SIG: A signature of all previous fields using the initiator's + Ed25519 authentication flags. + [variable length] + + + If you've got a consensus that lists an ECC key for a node, but the + node doesn't give you an ECC key, then refuse this connection. + +5. The extend protocol + + We add a new NSPEC node specifier for use in EXTEND2 cells, with + LSTYPE value [03]. Its length must be 32 bytes; its content is the + Ed25519 identity key of the target node. + + Clients should use this type only when: + * They know an Ed25519 identity key for the destination node. + * The source node supports EXTEND2 cells + * A torrc option is set, _or_ a consensus value is set. + + We'll leave the consensus value off for a while until more clients + support this, and then turn it on. + + When picking a channel for a circuit, if this NSPEC value is + provided, then the RSA identity *and* the Ed25519 identity must + match. + + If we have a channel with a given Ed25519 ID and RSA identity, and we + have a request for that Ed25519 ID and a different RSA identity, we + do not attempt to make another connection: we just fail and DESTROY + the circuit. + + If we receive an EXTEND or EXTEND2 request for a node listed in the + consensus, but that EXTEND/EXTEND2 request does not include an + Ed25519 identity key, the node SHOULD treat the connection as failed + if the Ed25519 identity key it receives does not match the one in the + consensus. + +6. Naming nodes in the interface + + Anywhere in the interface that takes an $identity should be able to + take an ECC identity too. ECC identities are case-sensitive base64 + encodings of Ed25519 identity keys. You can use $ to indicate them as + well; we distinguish RSA identity digests length. + + When we need to indicate an Ed25519 identity key in an hostname + format (as in a .exit address), we use the lowercased version of the + name, and perform a case-insensitive match. (This loses us one bit + per byte of name, + + Nodes must not list Ed25519 identities in their family lines; clients + and authorities must not honor them there. + + Clients shouldn't accept .exit addresses with Ed25519 names on SOCKS + or DNS ports by default, even when AllowDotExit is set. + + We need an identity-to-node map for ECC identity and for RSA + identity. + + The controller interface will need to accept and report Ed25519 + identity keys as well as (or instead of) RSA identity keys. That's a + separate proposal, though. + +7. Hidden service changes out of scope + + Hidden services need to be able identity nodes by ECC keys, just as + they will need to include ntor keys as well as TAP keys. Not just + yet though. This needs to be part of a bigger hidden service + revamping strategy. + +8. Proposed migration steps + + Once a few versions have shipped with Ed25519 key support, turn on + "Rule 1" on the authorities. (Don't allow an Ed25519<->RSA pairing + to change.) + + Once 0.2.5.x is in beta or rc, turn on the consensus option for + everyone who receives descriptors with Ed25519 identity keys to check + them. + + Once 0.2.5.x is in beta or rc, turn on the consensus option for + clients to generate EXTEND2 requests with Ed25519 identity keys. + + Once 0.2.5.x has been stable for a month or two, turn on "Rule 2" on + authorities. (Don't allow nodes that have advertised an Ed25519 key + to stop.) + +9. Future proposals + + * Ed25519 identity support on the controller interface + * Supporting nodes without RSA keys + * Remove support for nodes without Ed25519 keys + * Ed25519 support for hidden services + * Bridge identity support. + * Ed25519-aware family support + * diff --git a/proposals/221-stop-using-create-fast.txt b/proposals/221-stop-using-create-fast.txt new file mode 100644 index 0000000..26ba5b7 --- /dev/null +++ b/proposals/221-stop-using-create-fast.txt @@ -0,0 +1,90 @@ +Filename: 221-stop-using-create-fast.txt +Title: Stop using CREATE_FAST +Authors: Nick Mathewson +Created: 12 August 2013 +Target: 0.2.5.x +Status: Open + +0. Summary + + I propose that in 0.2.5.x, Tor clients stop sending CREATE_FAST + cells, and use CREATE or CREATE2 cells instead as appropriate. + +1. Introduction + + The CREATE_FAST cell was created to avoid the performance hit of + using the TAP handshake on a TLS session that already provided what + TAP provided: authentication with RSA1024 and forward secrecy with + DH1024. But thanks to the introduction of the ntor onionskin + handshake in Tor 0.2.4.x, for nodes with older versions of OpenSSL, + the TLS handshake strength lags behind with the strength of the onion + handshake, and the arguments against CREATE no longer apply. + + Similarly, it's good to have an argument for circuit security that + survives possible breakdowns in TLS. But when CREATE_FAST is in use, + this is impossible: we can only argue forward-secrecy at the first + hop of each circuit by assuming that TLS has succeeded. + + So let's simply stop sending CREATE_FAST cells. + +2. Proposed design + + Currently, only clients will send CREATE_FAST, and only when they + have FastFirstHopPK set to its default value, 1. + + I propose that we change "FastFirstHopPK" from a boolean to also + allow a new default "auto" value that tells Tor to take a value from + the consensus. I propose a new consensus parameter, "usecreatefast", + default value taken to be 1. + + Once enough versions of Tor support this proposal, the authorities + should set the value for "usecreatefast" to be 0. + + In the series after that (0.2.6.x?), the default value for + "FastFirstHopPK" should be 0. + + (Note that CREATE_FAST must still be used in the case where a client + has connected to a guard node or bridge without knowing any onion + keys for it, and wants to fetch directory information from it.) + +3. Alternative designs + + We might make some choices to preserve CREATE_FAST under some + circumstances. For example, we could say that CREATE_FAST is okay if + we have a TLS connection with a cipher, public key, and ephemeral key + algorithm of a given strength. + + We might try to trust the TLS handshake for authentication but not + forward secrecy, and come up with a first-hop handshake that did a + simple curve25519 diffie-hellman. + + We might use CREATE_FAST only whenever ntor is not available. + + I'm rejecting all of the above for complexity reasons. + + We might just change the default for FastFirstHopPK to 1 in + 0.2.5.x-alpha. It would make early users of that alpha easy for + their guards to distinguish. + +4. Performance considerations + + This will increase the CPU requirements on guard nodes; their + cpuworkers would be more heavily loaded as 0.2.5.x is more + adopted. + + I believe that, if guards upgrade to 0.2.4.x as 0.2.5.x is under + development, the commensurate benefits of ntor will outweigh the + problems here. This holds even more if we wind up with a better ntor + implementation or replacement. + +5. Considerations on client detection + + Right now, in a few places, Tor nodes assume that any connection on + which they have received a CREATE_FAST cell is probably from a + non-relay node, since relays never do that. Implementing this + proposal would make that signal unreliable. + + We should do this proposal anyway. CREATE_FAST has never been a + reliable signal, since "FastFirstHopPK 0" is easy enough to type, and + the source code is easy enough to edit. Proposal 163 and its + successors have better ideas here anyway.