Merkle Tree Certificates for TLS

🚨 Merkle Tree Certificates (MTC) is a moving target.

Implementation of Merkle Tree Certificates for TLS

At the moment only the Certification Authority (CA) side is implemented, and we differ from -01 and master branch, by including some unmerged PRs.

Demo

For a proper introduction and motivation, check out the draft specification and David's TLS working group presentation at IETF116.

Merkle Tree Certificates is an optimisation to the WebPKI (including Certificate Transparency) motivated by the large sizes of typical post-quantum signatures and public keys, to reduce the number of keys and signatures required for the common case where

1. Certificate issuance does not have to be immediate. For instance, because a certificate can be requested ahead of time for an existing domain by an ACME client like certbot.

2. The relying party (eg. browser) has a trusted update mechanism. There are also several ways to use MTC without trusted update mechanism, with various trade-offs: see the Transparency Services section of the spec.

MTC does not aim to replace the full WebPKI, but is a first meaningful step for improvement.

Intermezzo: mtc commandline tool

To play around with MTC, you can install the mtc commandline tool:

$ go install github.com/bwesterb/mtc/cmd/[email protected]

Assertions

In MTC CAs certify assertions, which bind a subject to a claim. An informal example of an assertion is:

For TLS, you can trust the P-256 public key a02342ff2…23ef when visiting example.com or 198.51.100.60.

The first part (TLS and the public key) is the subject, and the latter (domain and IP) are the claim. Roughly, an assertion is like a certificate without the signature.

To create an assertion, you can use the mtc new-assertion command. First, let's quickly create a P-256 public key to play with.

$ openssl ecparam -name prime256v1 -genkey -out p256.priv
$ openssl ec -in p256.priv -pubout -out p256.pub

Now we create an assertion that this P-256 public key should be valid for example.com and 198.51.100.60, and write it to the my-assertion.

$ mtc new-assertion --tls-pem p256.pub --dns example.com --ip4 198.51.100.60 -o my-assertion
checksum: 14bc907eafd02d5be8b8cc319d87ad5afe9266a6910a18cbdcbfcee1b7af696a

Let's check it using mtc inspect:

$ mtc inspect assertion my-assertion
subject_type     TLS
signature_scheme p256
public_key_hash  a02a1758e4c9d6511dc02f59301b9f29e41762d3d769c87a22333497984a41ef
dns              [example.com]
ip4              [198.51.100.60]

Batches, merkle trees and signed validity windows

An MTC CA doesn't give you a certificate for an assertion immediately. Instead, assertions are queued and issued in batches with a fixed rhythm, for instance a batch is issused once every hour. All assertions in a single batch are valid for the same period of time, the validity window, which is, for instance, two weeks. The CA publishes these batches publicly over HTTP.

For each batch, the CA computes a Merkle tree. This condenses all the assertions in that batch into a single root hash. For every batch, the CA signs that root together with all the roots of the currently valid batches. This signature, together with those signed roots is called the signed validity window for that batch, which is published alongside the assertions.

Creating a CA

Let's create an MTC CA.

$ mtc ca new --batch-duration 5m --lifetime 1h 62253.12.15 ca.example.com/path

This creates a new MTC CA called my-mtc-ca, and puts the data in the current working directory. A batch is issued every 5 minutes, and each batch is valid for one hour.

Let's have a look at the files created:

$ find .
.
./signing.key
./www
./www/mtc
./www/mtc/v1
./www/mtc/v1/ca-params
./www/mtc/v1/batches
./queue
./tmp

The signing.key file contains the private key of the keypair used by the CA.

The www folder contains the files that have to be served at https://ca.example.com/path. At the moment, the only file of interest is ca-params, which contains the information about the CA:

$ mtc inspect ca-params www/mtc/v1/ca-params
issuer                 62253.12.15
start_time             1705677477 2024-01-19 16:17:57 +0100 CET
batch_duration         300        5m0s
life_time              3600       1h0m0s
storage_window_size    24         2h0m0s
validity_window_size   12
http_server            ca.example.com/path
public_key fingerprint ml-dsa-87:85b5a617ef109e0a8d68a094c8b969f622ac4096c513fa0acd169c231ce2fad5

The batches folder is empty, because there are no batches issued yet.

The queue file contains the assertions that will be issued.

Issuing our first batch

Let's issue our first assertion. We can read the assertion from disk we've created earlier with mtc new-assertion:

$ mtc ca queue -i my-assertion 
$ mtc ca show-queue
checksum         14bc907eafd02d5be8b8cc319d87ad5afe9266a6910a18cbdcbfcee1b7af696a
subject_type     TLS
signature_scheme p256
public_key_hash  a02a1758e4c9d6511dc02f59301b9f29e41762d3d769c87a22333497984a41ef
dns              [example.com]
ip4              [198.51.100.60]

Total number of assertions in queue: 1

(We can pass the checksum from new-assertion with --checksum to make sure the assertion wasn't corrupted.)

We can also queue an assertion ad hoc:

$ mtc ca queue --tls-pem p256.pub -d other.example.com -d second.example.com
$ mtc ca show-queue | tail -n 8

checksum         fbdea936ae7795a3fa01d44230daba351d7480eceadd086ba32938ebc88a5458
subject_type     TLS
signature_scheme p256
public_key_hash  a02a1758e4c9d6511dc02f59301b9f29e41762d3d769c87a22333497984a41ef
dns              [other.example.com second.example.com]

Total number of assertions in queue: 2

Let's issue our first batch.

$ mtc ca issue   
2024/01/19 16:27:31 INFO Starting issuance time=2024-01-19T16:27:31.841+01:00
2024/01/19 16:27:31 INFO Current state expectedStored=0 expectedActive=0 existingBatches=⌀
2024/01/19 16:27:31 INFO To issue batches=0

And let's check:

$ find .
.
./signing.key
./www
./www/mtc
./www/mtc/v1
./www/mtc/v1/ca-params
./www/mtc/v1/batches
./www/mtc/v1/batches/0
./www/mtc/v1/batches/0/tree
./www/mtc/v1/batches/0/abridged-assertions
./www/mtc/v1/batches/0/signed-validity-window
./www/mtc/v1/batches/0/index
./www/mtc/v1/batches/latest
./queue
./tmp

We see a 0 batch has been created. latest is a symlink to 0.

The abridged-assertions is essentially the list of assertions: the difference between a regular and abridged assertion, is that with an abridged assertion, the public key has been replaced by the hash of the public key.

$ mtc inspect abridged-assertions www/mtc/v1/batches/0/abridged-assertions
key              28b2216e7905ab48d5444f5b7ebf3d2386bc0444c9721fff77b0b313e734dab4
subject_type     TLS
signature_scheme p256
public_key_hash  a02a1758e4c9d6511dc02f59301b9f29e41762d3d769c87a22333497984a41ef
dns              [example.com]
ip4              [198.51.100.60]

key              80944a1728bc7b4cd7e583c6b24a5f413ba50b7ef5ba9d214e26c1a1974f0a19
subject_type     TLS
signature_scheme p256
public_key_hash  a02a1758e4c9d6511dc02f59301b9f29e41762d3d769c87a22333497984a41ef
dns              [other.example.com second.example.com]

Total number of abridged assertions: 2

The signed-validity-window is the signed validity window: the roots of the currently valid batches:

$ mtc inspect -ca-params www/mtc/v1/ca-params signed-validity-window www/mtc/v1/batches/0/signed-validity-window 
signature       ✅
batch_number    0
tree_heads[-11] f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-10] f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-9]  f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-8]  f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-7]  f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-6]  f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-5]  f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-4]  f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-3]  f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-2]  f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-1]  f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[0]   c005dcdb53c4e41befcf3a294b815d8b8aa0a260e9f10bfd4e4cb52eb3724aa3

We need to pass the ca-params file to be able to parse the file, and check the signature therein. (As this is the first batch, the previous batches contain a placeholder value.)

The tree file contains the Merkle tree.

$ mtc inspect tree www/mtc/v1/batches/0/tree       
number of leaves 2
number of nodes  3
root             c005dcdb53c4e41befcf3a294b815d8b8aa0a260e9f10bfd4e4cb52eb3724aa3

Finally, the index file allows a quick lookup in abridged-assertions by key (hash of the assertion):

$ mtc inspect index www/mtc/v1/batches/0/index
                                                             key   seqno  offset
28b2216e7905ab48d5444f5b7ebf3d2386bc0444c9721fff77b0b313e734dab4       0       0
80944a1728bc7b4cd7e583c6b24a5f413ba50b7ef5ba9d214e26c1a1974f0a19       1      69

total number of entries: 2

Issuing more batches

As we just issued a new batch, we need to wait a while before the next batch is ready to issue.

Let's queue some more assertions, wait a bit, and issue a new batch.

$ mtc ca queue --tls-pem p256.pub -d 1.example.com
$ mtc ca queue --tls-pem p256.pub -d 2.example.com
$ mtc ca queue --tls-pem p256.pub -d 3.example.com
$ mtc ca issue
2024/01/19 16:33:57 INFO Starting issuance time=2024-01-19T16:33:57.860+01:00
2024/01/19 16:33:57 INFO Current state expectedStored=0,…,2 expectedActive=0,…,2 existingBatches=0
2024/01/19 16:33:57 INFO To issue batches=1,2
$ find .
.
./signing.key
./www
./www/mtc
./www/mtc/v1
./www/mtc/v1/ca-params
./www/mtc/v1/batches
./www/mtc/v1/batches/0
./www/mtc/v1/batches/0/tree
./www/mtc/v1/batches/0/abridged-assertions
./www/mtc/v1/batches/0/signed-validity-window
./www/mtc/v1/batches/0/index
./www/mtc/v1/batches/latest
./www/mtc/v1/batches/1
./www/mtc/v1/batches/1/tree
./www/mtc/v1/batches/1/abridged-assertions
./www/mtc/v1/batches/1/signed-validity-window
./www/mtc/v1/batches/1/index
./www/mtc/v1/batches/2
./www/mtc/v1/batches/2/tree
./www/mtc/v1/batches/2/abridged-assertions
./www/mtc/v1/batches/2/signed-validity-window
./www/mtc/v1/batches/2/index
./queue
./tmp

As we waited a bit longer, the current batch is 2, which will contain the queued assertions. The batch 1 in between will be empty.

Now latest points to 2, and its signed validity window is more interesting.

$ mtc inspect -ca-params www/mtc/v1/ca-params signed-validity-window www/mtc/v1/batches/2/signed-validity-window
signature      ✅
batch_number   2
tree_heads[-9] f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-8] f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-7] f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-6] f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-5] f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-4] f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-3] f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-2] f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[-1] f2f65b0486c8cad3876475c9c509afdf3f51dc073b1d2d2d261ff9883d63f98e
tree_heads[0]  c005dcdb53c4e41befcf3a294b815d8b8aa0a260e9f10bfd4e4cb52eb3724aa3
tree_heads[1]  98a421741cf06a19b56d7b52436f686885bd798611426f638ffcdb6b5a65c42c
tree_heads[2]  ab3cb1262fc084be0447c2b3d175d63f6ec2782dcc1443888b12f685976093d5

Creating a certificate

In MTC, a certificate is an assertion, together with the TrustAnchorIdentifier (consisting of an OID for the CA and the batch number), and an authentication path in the Merkle tree. Let's create one for our initial assertion.

$ mtc ca cert -i my-assertion -o my-cert

If we inspect the certificate, it can recompute the root from the authentication path and CA parameters:

$ mtc inspect -ca-params www/mtc/v1/ca-params cert my-cert
subject_type     TLS
signature_scheme p256
public_key_hash  a02a1758e4c9d6511dc02f59301b9f29e41762d3d769c87a22333497984a41ef
dns              [example.com]
ip4              [198.51.100.60]

proof_type      merkle_tree_sha256
CA OID          62253.12.15
batch           0
index           0
recomputed root c005dcdb53c4e41befcf3a294b815d8b8aa0a260e9f10bfd4e4cb52eb3724aa3
authentication path
 00b17df8d909fd3e77005486a16ca00fdc9af38f92a23351359fd420d9f2ef78

This is indeed the root of the 0th batch, and so this certificate is valid.