[tor-dev] Proposal 220 (revised): Migrate server identity keys to Ed25519
Nick Mathewson
nickm at torproject.org
Tue Feb 25 16:18:57 UTC 2014
I've revised proposal 220 based on commentary from Roger. The biggest
changes is tweaking all of the things called "certificates" to make
them actually follow the same format to greatest the extent possible.
To see diffs, you can use git, or browse the gitweb site at
https://gitweb.torproject.org/torspec.git/history/HEAD:/proposals/220-ecc-id-keys.txt
Filename: 220-ecc-id-keys.txt
Title: Migrate server identity keys to Ed25519
Authors: Nick Mathewson
Created: 12 August 2013
Target: 0.2.x.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, router 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'm 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]
CERT_TYPE [1 Byte]
EXPIRATION_DATE [3 Bytes]
CERT_KEY_TYPE [1 byte]
CERTIFIED_KEY [32 Bytes]
EXTENSIONS [variable length, up to length of certificate
minus 64 bytes.]
SIGNATURE [64 Bytes]
The "VERSION" field holds the value [01]. The "CERT_TYPE" field
holds a value depending on the type of certificate. (See appendix
A.1.) The CERTIFIED_KEY field is an Ed25519 public key if
CERT_KEY_TYPE is [01], or a SHA256 hash of some other key type
depending on the value of CERT_KEY_TYPE. The EXPIRATION_DATE is a
date, given in HOURS since the epoch, after which this
certificate isn't valid. (A three-byte field here will work fine
until 5797 A.D.)
The EXTENSIONS field contains zero or more extensions, each of
the format:
ExtLength [1 or 2 bytes]
ExtType [1 or 2 bytes]
ExtData [Length bytes]
The ExtLength and ExtType fields can represent values between 0
and 2^15-1, representing values under 128 as "0xxxxxxx" and
values over 128 as "1xxxxxxx yyyyyyyy". The meaning of the
ExtData field in an extension is type-dependent.
It is an error for an extension to be truncated; such a
certificate is invalid.
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".
2.2. Basic extensions
2.2.1. Signed-with-ed25519-key extension [type 04]
In several places, it's desirable to bundle the key signing a
certificate along with the certificate. We do so with this
extension.
ExtLength = 32
ExtData =
An ed25519 key [32 bytes]
When this extension is present, it MUST match the key used to
sign the certificate.
This
2.3. Revoking keys.
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]
REV_EXTENSIONS [variable length, up to length of revocation
document minus 64 bytes]
SIGNATURE [64 Bytes]
FIXED_PREFIX is "REVOKEID" or "REVOKESK". VERSION is [01]. KEYTYPE is
[01] for revoking a signing key or [02] for 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. REV_EXTENSIONS 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 left for a later specification.
2.4. 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 whether 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.
2.5. Router descriptors
We specify the following element that may appear at most once in
each router descriptor:
"identity-ed25519" SP certificate NL
The identity-key and certificate 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]
The certificate has CERT_TYPE of [04]. It must include a
signed-with-ed25519-key extension (see section 2.2.1), so that we
can extract the identity key.
When an identity-ed25519 element is present, there must also be a
"router-signature-ed25519" element. It MUST be the next-to-last
element in the descriptor, appearing immediately before the 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
Where 'signature' is encoded in base64 with terminating =s removed.
The signing key in the certificate 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.
2.5.1. Checking descriptor signatures.
Current versions of Tor will handle these new formats by ignoring the
new fields, and not checking any ed25519 information.
New versions 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 router-signature-ed25519 signature.
* 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.
2.5.2. Extra-info documents
Extra-info 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 in
the router descriptor. (All versions of Tor that recognize this line
allow an extra field there.)
2.5.3. 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 microdescriptor documents now contain an optional "id"
field for each routerstatus section. Its format is:
"id" SP "ed25519" SP ed25519-identity NL
where ed25519-identity is base64-encoded, with trailing = characters
omitted. In vote documents, it may be replaced by the format:
"id" SP "ed25519" SP "none" NL
which indicates that the node does not have an ed25519 identity. (In
a microdescriptor, 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
4.1. Overview of the status quo
This section won't make much sense unless you grok the v3
link protocol as described in tor-spec.txt, first proposed in
proposal 195. So let's review.
In the v3 link protocol, the client completes a TLS handshake
with the server, in which the server uses an arbitrary
certificate signed with an RSA key. The client then sends a
VERSIONS cell. The server replies with a VERSIONS cell to
negotiate version 3 or higher. The server also sends a CERTS
cell and an AUTH_CHALLENGE cell and a NETINFO cell.
The CERTS cell from the server contains a set of one or more
certificates that authenticate the RSA key used in the TLS
handshake. (Right now there's one self-signed RSA identity key
certificate, and one certificate signing the RSA link key with
the identity key. These certificates are X509.)
Having received a CERTS cell, the client has enough information
to authenticate the server. At this point, the client may send a
NETINFO cell to finish the handshake. But if the client wants to
authenticate as well, it can send a CERTS cell and an AUTENTICATE
cell.
The client's CERTS cell also contains certs of the same general
kinds as the server's key file: a self-signed identity
certificate, and an authentication certificate signed with the
identity key. The AUTHENTICATE cell contains a signature of
various fields, including the contents of the AUTH_CHALLENGE
which the server sent cell, using the client's authentication
key. These cells allow the client to authenticate to the server.
4.2. Link protocol changes for ECC ID keys
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: Ed25519 TLS authentication key certified by Ed25519 signing key
7: RSA cross-certificate for Ed25519 identity key
These correspond to types used in the CERT_TYPE field of
the certificates.
The content of certificate type [04] (Ed25519 signing key)
is as in section 2.5 above, containing an identity key and the
signing key, both signed by the identity key.
Certificate type [05] (Link certificate signed with Ed25519
signing key) contains a SHA256 digest of the X.509 link
certificate used on the TLS connection in its key field; it is
signed with the signing key.
Certificate type [06] (Ed25519 TLS authentication signed with
Ed25519 signing key) has the signing key used to sign the
AUTHENTICATE cell described later in this section.
Certificate type [07] (Cross-certification of Ed25519 identity
with RSA key) contains the following data:
ED25519_KEY [32 bytes]
EXPIRATION_DATE [4 bytes]
SIGNATURE [128 bytes]
Here, the Ed25519 identity key is signed with router's RSA
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-certificate".)
(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. Clients can send this AUTHENTICATE type if the server
lists it in its AUTH_CHALLENGE cell.
Modified values and new fields below are marked with asterisks.
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]
RAND: A 24 byte value, randomly chosen by the initiator. [24 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 by length.
When we need to indicate an Ed25519 identity key in a hostname
format (as in a .exit address), we use the lowercased version of the
name, and perform a case-insensitive match. (This loses us a little
less than one bit per byte of name, leaving plenty of bits to make
sure we choose the right node.)
Nodes must not list Ed25519 identities in their family lines; clients and
authorities must not honor them there. (Doing so would make different
clients change paths differently in a possibly manipulatable way.)
Clients shouldn't accept .exit addresses with Ed25519 names on SOCKS
or DNS ports by default, even when AllowDotExit is set. We can add
another option for them later if there's a good reason to have this.
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 to identify 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 the release with these changes is in beta or rc, turn on the
consensus option for everyone who receives descriptors with
Ed25519 identity keys to check them.
Once the release with these changes is in beta or rc, turn on the
consensus option for clients to generate EXTEND2 requests with
Ed25519 identity keys.
Once the release with these changes 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
A.1. List of certificate types
The values marked with asterisks are not types corresponding to
the certificate format of section 2.1. Instead, they are
reserved for RSA-signed certificates to avoid conflicts between
the certificate type enumeration of the CERTS cell and the
certificate type enumeration of in our Ed25519 certificates.
**[00],[01],[02],[03] - Reserved to avoid conflict with types used
in CERTS cells.
[04] - signing a signing key with an identity key (Section 2.5)
[05] - TLS link certificate signed with ed25519 signing key
(Section 4.2)
[06] - Ed25519 authentication key signed with ed25519 signing key
(Section 4.2)
**[07] - reserved for RSA identity cross-certification (Section 4.2)
A.2. List of extension types
[01] - signed-with-ed25519-key (section 2.2.1)
A.3. List of signature prefixes
We describe various documents as being signed with a prefix. Here
are those prefixes:
"Tor router descriptor signature v1" (section 2.5)
"Tor node signing key certificate v1" (section 2.1)
A.4. List of certified key types
[01] ed25519 key
[02] SHA256 hash of an RSA key
[03] SHA256 hash of an X.509 certificate
A.5. Reserved numbers
We need a new consensus algorithm number to encompass checking
ed25519 keys and putting them in microdescriptors.
We need new CertType values for use in CERTS cells. We reserved
in section 4.2.
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-certificate for Ed25519 identity key
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