Crypto spec: the Keybase filesystem (KBFS)

Version 2.0

ChangeLog

• Version 2.0
• Describe the new block encryption/hashing scheme, addressing the low-risk issue NCC-KB2018-005 in the NCC audit. We will eventually stop supporting new blocks being generated with the old scheme.
• Version 1.10
• Point to new teams documentation.
• Version 1.9
• Temporarily remove talk of orgs while we refactor the doc.
• Version 1.8
• More details and dates for the KBFS Merkle Trees.
• Version 1.7
• A threat model
• Version 1.6
• Say what's in progress on what's implemented.
• Last writer isn't encrypted.
• Version 1.5
• Remove pairwise HMACs, just used signatures.
• Rename file from crypto to kbfs-crypto.
• Remove browser/deterministic keys.
• Version 1.4
• Revert PRF-based per-block key generation
• Raise Jeremy's objection to MAC-ing TLFs with read-only members.
• Provide system overview
• Better key exchange specifics
• Version 1.3
• PRF-based per-block key generation system with per-epoch folder keys
• Try to use "TLF" consistently everywhere instead of "folder" or "directory"
• Version 1.2
• Describe directory IDs
• Detail $$(M_f, m_f)$$ generation
• Version 1.1
• Organizations
• Merkle Trees for Public and Private data
• Version 1.0: Initial Revision

Threat Model and Security Claims

End-to-end security: Keybase promises secrecy and integrity of file system data and metadata even in the case of total server compromise.

Our key cryptographic assumptions are: (1) weak collision-resistance of SHA256; (2) the security of the Go Implementation of the NaCl cryptographic library.

Best Effort via Access Control: If our server infrastructure isn't compromised, we provide only via access control:

• Forward-secrecy for lost or decommissioned devices.
• Confidentiality regarding which private folders have data, and which are empty.
• Confidentiality for quota data.

In other words, an adversary who has access to Keybase's server data could: learn about who is communicating with whom via KBFS; would have the same usage details we would eventually use for billing; and would be able to recover data encrypted for lost or decommissioned devices, if they additionally recover those devices.

Of course we are mere mortals and don't believe we can indefinitely stave off concerted attacks against our infrastructure, so we encourage our users to only depend on our end-to-end security claims.

Additionally, we don't guarantee availability of data or metadata in the case of server compromise.

System Overview

Sibling keys and subkeys. Each user in Keybase keeps two keypairs per device; one keypair for signing, and one pair for encryption. Secret keys never leave the device. The signing keys are known as "siblings" since they all are equally powerful. Any active sibling key can provision or revoke another device. Subkeys are signed by an active sibling key but can't delegate to other keys. The first of a series of sibling keys is called an "eldest" key and can be revoked without affecting its other live siblings.

Private Personal Directories Every keybase user gets a private personal directory, keyed so that all of her devices can read and write it.

Public Directories Groups of one-or-more users can create public directories. Items in public directories are signed by one of the devices of one of the directory's writers; they are not encrypted. Public directories are good places to release software or authenticated manifestos.

Private Group Directories The most interesting case, a group of people with read/write permission get together with some other people who have read-only permission to share a directory. This directory should use authenticated encryption to guarantee that only someone in the group could have written to it. It should further use some mechanism (currently vanilla signatures) to determine which writer is responsible for which write, so that users can't put words in each other's mouths.

Forest of Merkle Trees [Unimplemented] Keybase keeps public Merkle trees for managing: (1) each user's collection of keypairs and public identities; and (2) the progression of file system states, so that the server can't maliciously roll back to previous states.

Keys

Eldest Key

When a user Alice ($$A$$) joins, she starts off by generating or importing a keypair. For our current users, this is a PGP keypair, denoted ($$P_A,p_A$$). But there's nothing special about PGP keys, and we are free to change the format in the future. We call this keypair Alice's eldest keypair, since it's the first among potentially many sibling keys.

Per-Device Keys

For each of her devices, Alice generates two per-device keypairs. The signing key becomes a sibkey, and the encryption keypair is just a subkey.

For Alice's $$i$$th device, the procedure is:

1. Generate an Ed25519 Keypair at random, yielding the pair $$(E^i_A, e^i_A)$$. In Ed25519, the private key is a 64-byte string, and the public key is a 32-byte (derived) string.
2. Generate a Curve25519 Keypair suitable for use with NaCL's Box feature (or Go Crypto's box.Seal feature). Call it $$(M^i_A,m^i_A)$$. In Curve25519 (Montgomery form), the public and private key are both 32-byte strings.
3. Sign $$E^i_{A}$$ with a valid sibling key; and as in PGP, include a "reverse" signature of the delegating key, signed by the new key. Push this sibkey signature into Alice's signature chain. See our doc on key exchange for more specifics.
4. Sign $$M^i_{A}$$ with $$e^i_{A}$$. Push this subkey signature into Alice's signature chain.
5. Store $$e^i_{A}$$ and $$m^i_{A}$$ on device $$i$$

Key IDS (kids)

Keybase has an idea of a "Key ID". In the case of PGP keys, it's a hash of the public key materials. In the case of ECC keys, it's the public key itself. We describe here how to construct them in Version 1:

• For a PGP public key $$P$$, compute the serialization $$s(P)$$ as you would to compute a PGP fingerprint. Let $$t$$ be the one-byte type of the key in question. For example, or RSA, $$t = 1$$; for DSA $$t = 17$$. See RFC 4880 and RFC 6637 for a complete list. Then:
kid($$P$$) = 0x01 || $$t$$ || SHA-256($$s(P)$$) || 0x0a
• For a NaCl encryption key, $$E$$:
kid(E) = 0x01 || 0x21 || E || 0x0a
• For a NaCl EDDSA key, $$M$$:
kid(M) = 0x01 || 0x20 || M || 0x0a

Top-Level Folder (TLFs) in KBFS

A top-level folder (TLF) in KBFS has a fixed set of readers and writers, as specified by the name of the folder. Every file and folder recursively contained in a TLF has the same permissions as its parent. This organization will feel a little bit different from normal POSIX semantics, but greatly simplifies the system.

Private TLFs

We cover the case of private TLFs with multiple readers and writers first, since it's the most general. Home directories are a simple subcase of general private group TLFs.

Keying a Private Group TLF

Let's say Alice ($$A$$) is creating a new TLF for Bob ($$B$$) and Charlie ($$C$$). Bob has read/write access, and Charlie only gets read-only access. Alice keys the folder (call it $$f$$) for all other users when she creates it. The procedure is as follows:

1. Generate a random 15-byte directory ID, with the suffix 0x16
2. Generate a per-TLF Curve25519 DH key pair for inclusion of this folder's metadata in the site's private-data Merkle tree; call it $$(M_f, m_f)$$.
3. Generate a 32-byte random secret key (version=0 for the first one): $$s^{f,0}$$ = rand(32)
4. Generate an ephemeral Curve25519 Diffie-Helman keypair. Call it $$(M_e,m_e)$$.
5. For each user $$u$$ in $$\{A,B,C\}$$:
1. For each device $$i$$ in $$u$$'s set of devices:
1. generate $$s^{f,0,i}_{u} =$$ rand(32). This will be the server-side half of the key.
2. $$t^{f,0,i}_{u} = s^{f,0,i}_u \oplus s^{f,0}$$. That is, XOR the global per-TLF key together with the per-user-per-device key to get a masked key.
3. $$S^{f,0,i}_{u} = (M_e,$$Box$$(m_e, M^i_u, t^{f,0,i}_u))$$. That is, run the NaCl Box function with the ephemeral private key, with the user's public device key, and on the message $$t^{f,0,i}_u$$. This new key $$S^{f,0,i}_u$$ can now be published publicly. It can only be decrypted by $$u$$'s private device key, and with some server assist (via $$s^{f,0,i}_u$$). Note that we prepend the public key $$M_e$$ to the output of Box. Eventually, we might need to add new keys or replace existing keys, and we'll do so with a new ephemeral keypair. So $$M_e$$ might eventually be replaced.
6. Publish the following metadata ($$md_c$$) in the clear:
1. A block of reader keys. In this case it's just Charlie who is read-only:
readers = $$[ S^{f,0,1}_{C}, S^{f,0,2}_{C}, S^{f,0,3}_{C} ]$$
2. A block of writer keys. In this case it's Alice and Bob:
writers = $$[ S^{f,0,1}_{A}, S^{f,0,2}_{A}, S^{f,0,1}_{B},S^{f,0,2}_{B} ]$$
3. The public key $$M_f$$
7. Some of the metadata is encrypted. Construct a plaintext $$md_{e}$$ as:
1. The root TLF block (described more below).
2. The private key $$m_f$$.

Then, the encryption is computed as: SecretBox($$s^{f,0}, md_{e}$$)

Note that the server sees reader and writer blocks in the clear, and can therefore sanity check updates to them. For instance, the server can check that the readers and writers correspond to the TLF name, and that readers and writers aren't being dropped maliciously during updates.

Similarly, the last writer is known to the server, since the server should check the signatures on incoming updates.

Writers can obviously update all of the metadata and data for a TLF. Readers can change two fields in the folder's metadata: they can push new keys onto the end of the reader key list; and they can flip the folder's "rekey" bit to "on." In either case, they sign these changes with any of their valid signing sibkeys. The server and clients can check that all updates to metadata are authorized. The server rejects unauthorized updates, and clients ignore them.

Encrypting a Block

Once she has established keys as above, Alice can start encrypting blocks. Take any arbitrary plaintext block like $$b_j$$ in TLF $$f$$. Assume version 0 of the key. To encrypt:

1. Generate a random 32-byte per-block key $$s_{j}$$.
2. Compute $$h_{j} = HMAC512(s^{f,0}, s_{j})$$.
3. Let $$h^{[0,31]}_{j}$$ be the first 32 bytes of $$h_{j}$$, to be used as the block encryption key.
4. Let $$n$$ be the next 24 bytes of $$h_{j}$$, to be used as a nonce.
5. Compute: $$B_j =$$ SecretBox$$(h^{[0,31]}_{j}, n, b_j)$$
6. For the initial rollout, store $$\{n,s_j\}$$ along with $$B_j$$ on the hosted storage provider. In the future, we hope to store $$s_{j}$$ on machines we host to get more secure block deletion.

The idea of some sort of per-block key is one we'll want to keep and expand upon in the future. It allows us to be quite liberal in distributing encrypted blocks to CDNs and local caches, without fearing that they might be never thrown away. If we apply more strict access controls on per-block keys $$s_j$$, we still retain some ability to throw away blocks, even after client key compromise.

We're not using convergent encryption or anything like it, so a block that's used twice will be encrypted differently both times. However, clients will keep per-folder caches that map hashes of block plaintexts to encrypted block IDs. So they shouldn't reencrypt/reupload blocks they know to be repeats. In practice, this local lookup might be good enough, since often data is read before it's copied, and many block copies will hit this cache. We don't want to allow block deduplication across top-level-directories for privacy reasons.

FS Structure

The files in a TLF form a Merkle Tree as in other secure distributed file systems (including SFSRO, SUNDR and Tahoe-LAFS). However, KBFS makes the same simplification as the ORI File system, and does away with iNode-style indirection. Thus, KBFS cannot implement hard-links without deeper architectural changes.

Block IDs are computed as the SHA-256 hashes of encrypted blocks and their nonces. Using the nonce $${n}$$ (which is derived from the block's secret key) prevents the server from colluding with another writer in the folder to compute another combination of secret key and plaintext data that will result in the same ciphertext for the block, thus allowing the server to serve alternate blocks (with identical block IDs) to some subset of readers. By deriving the nonce from the secret key, we guarantee that the block IDs would be different even if the attacker found a way to duplicate the ciphertext, and thus the attack would be detectable by the client. For more details, see NCC-KB2018-005 in the NCC audit

Directory nodes are, in turn, maps of file names to Block IDs. They are packed as blocks, and get the same encryption treatment as data blocks. This structure continues recursively up to the TLF root.

Signing The Root Block

Readers and writers in a private shared TLF know via authenticated encryption that someone who had access to $$s^{f,0}$$ wrote $$f$$, but they don't know who. Alice can try to steal credit for Bob's work, or Bob might frame Alice for saying something naughty.

A sledgehammer approach to this problem would be to have Alice sign the root block of $$f$$ on every update. But this is too strong of a statement. If Eve later compromises Charlie's key, she can blackmail Alice, since Alice proved to Bob, Charlie and everyone else that she wrote particular contents to $$f$$.

We experimented with various pairwise symmetric MAC schemes, to achieve repudiability as in OTR protocols. However, such schemes didn't work for folders with non-writing readers. So we do the simple thing which is just to sign the hash of the root block with the writer's per-device EdDSA signing key. The same technique is used in all three cases: public directories, private directories without non-writing readers, and private directories with non-writing readers.

Public Directories

If user $$u$$ updates a KBFS folder with his $$i$$th device, he must sign his changes to the whole folder state by signing its root block. This will in effect sign all data and metadata contained recursively in this folder.

To sign, $$u$$ computes the complete metadata $$md$$ as specified above, except without encrypting $$md_e$$. He then signs the block:

$$\sigma(md) =$$ Sign($$e^i_{u},md$$)

The Sign function outputs both the signature, and the KID of the key used to generate it. This way, verifiers know which key to use.

Home Directories

As described above, home directories are special cases of TLF group directories. When Alice sets up encryption keys for all of her devices, she uses the protocol described in 4.1.1, iterating all of her per-device keys. Alice can skip the signing step described in 4.1.3, because she is not worried about the "confused authorship" attack, but she still needs to sign $$md$$ as for public directories to prevent the server from spoofing rekeys.

Keeping the Server Honest

Unless we're careful, the server can selectively withhold file system updates or can serve two different versions of the FS to two different clients. In this section, we describe the precautions we take to prevent these sorts of attacks.

Our general approach is that the server creates a Merkle Tree out of all sensitive site data, then signs and publishes this tree for all to see. Third party monitors are invited to check on the progression of the site's Merkle trees and to check they obey the specified cryptographic consistency constraints. Clients check their views of the file system with those published in the Merkle trees to make sure the server isn't rolling back state or attempting a fork.

There are three separate Merkle trees to consider: (1) the Keybase Core Merkle tree that publicizes sibling keys, public identity proofs, and follower'' statements; (2) a public-data Merkle tree that captures all of the public data on KBFS; and (3) a private-data Merkle tree that encapsulates the state of all private data on KBFS.

The Keybase Core Merkle Tree

As described in our Server Security document, users sign statements about their keys, their identities and the identities of others. They commit these signatures to monotonically growing signature chains, which are collected into a sitewide Merkle Tree. As we describe later in this document, these signature chains will eventually cover organizations in addition to single users. These structure all exist outside of KBFS and should be accessible without KBFS software.

The Public-Data Merkle Tree

All public directories on KBFS are collected into a sitewide Merkle Tree. The leaves of this tree are dictionaries of key-value pairs. The key is the ID of the TLF, and the value is most recent signature of the TLF RootMetadata block. This signature contains, recursively: the writer keys for this directory, the root directory entry for this directory, and the hash of the previous RootMetadata block.

Interior nodes of the merkle tree are constructed upwards, covering shorter prefixes of the directory IDs listed in the leaves. The final root is signed, and published periodically, maybe every hour.

The Private-Data Merkle Tree

Private data deserves the same treatment as public data, but we have to be slightly more careful not to reveal: (1) how often private directories are modified; or (2) even worse, who is collaborating with whom. Naive solutions might allow an adversary to correlate device additions in public sigchains with changes in private TLFs to correlate share membership. We take a pretty blunt approach that should solve data leaks from the server to the public.

When a user Alice creates private TLF $$f$$, she generates a Curve25519 keypair $$(M_f, m_f)$$. She stores $$M_f$$ in the folder's public metadata, and $$m_f$$ in the folder's private metadata.

Every hour, the server makes a new Merkle Tree of the entire site's private data. The algorithm is:

1. Generate an ephemeral Curve25519 keypair $$(M_S, m_s)$$
2. For each private folder $$f$$:
1. Perform encryption $$C_f \leftarrow$$ Box($$m_s,M_f,\sigma_f$$), where $$\sigma_f$$ is the most-recent signature of $$f$$'s metadata.
2. Make a mapping of the ID of $$f$$ to $$C_f$$.
3. Construct a Merkle tree from these leaves as in the Public data case, and publish the tree along with $$M_s$$, the public ephemeral Curve25519 key.

When Alice's client goes to fetch TLF $$f$$, she gets the most recent root from the server, and also the most recently-published Merkle tree from the server. She traces a path down the from the root to the leaf for $$f$$, and then decrypts $$\sigma_f$$ using $$m_f$$. She finally checks that the current root links back to the root published in the merkle tree. She might of course need to traverse 100s or 1000s of links if there were many writes in the last hour, but the server should batch all of the data necessary to make this determination in a few RPC calls. Future implementations might also implement "skip list"-style fast-forward links to speed this process for periods of heavy writes.

Merkle Tree Integration

The roots of the two KBFS Merkle trees will be inserted into the root block of the main Keybase Merkle Tree. Doing so establishes ordering relationships between key manipulations and writes to KBFS. In turn, this knowledge allows KBFS clients to determine if writes to KBFS happened before or after a key is revoked, and therefore whether or not a corrupt server has exploited an exposed key.

Keying, Rekeying, and Revoking

Imagine Bob gets a new device. He now wants to provision it for access to all relevant KBFS folders. Here are the high-level steps he would take:

1. Key generation
2. Key certification: sign the new key with an existing sibling key
3. TLF keying: add the key to the relevant reader and writer blocks

The first step, key generation, was covered above, and is the same here. Key certification is covered in our key exchange document.

TLF Keying

Now Bob must go through and add write access for his device on all encrypted Keybase directories that he can write. Call his new device $$n$$ and his old device $$d$$. Device $$d$$ does the following:

1. Generate an ephemeral Curve25519 Diffie-Helman keypair. Call it $$(M_e,m_e)$$.
2. For every TLF $$f$$ that Bob can read:
1. Use $$m^{d}_{B}$$ to decrypt $$t^{f,0,d}_B$$
2. Query the server for $$s^{f,0,d}_{B}$$ and XOR to recover $$s^{f,0}$$
3. Encrypt $$s^{f,0}$$ using sender key $$m_e$$ and receiver key $$M^{n}_{B}$$, which was generated on device $$n$$ and securely transferred over to $$d$$ in previous steps.
4. Add $$S^{f,0,n}_B$$ to the writers (or readers) list for $$f$$ and sign with $$e^{d}_B$$

Bob pushes $$(M_e, S^{f,0,n}_B)$$ to TLF $$f$$'s reader or writer list. If he is a reader, he is allowed to push onto the reader list in the metadata. If he is a writer, he can edit $$f$$'s metadata at will.

Freezing a Device [Unimplemented]

If Alice loses a device $$d$$, the first thing to do is to "freeze" access to the device via the server. Any device that can establish a session as the user can do so. Once a device is frozen, the server will refuse to honor requests for key-halves of the form $$s^{f,0,d}_{A}$$. Any device but $$d$$ can ask for an unfreeze.

Alice can take an additional (optional) step. She can sign a key freeze into her signature chain for each signing key on the lost device. Other clients should refuse to honor signatures from frozen keys during the freeze window. Alice can later cancel the freeze using the same key that ordered the freeze. The server can't assist here, so Alice can only freeze a specific key if she has another provisioned device.

Decommissioning a Device (And Optionally Adding a New One)

To decommision a device, Alice should first freeze it as above, issuing a final revocation statement for the key into her sigchain. Her client should then iterate over all TLFs that she has read or write access to, and delete the lost keys from the keyblocks, and then rekey. She of course needs an active keypair in order to do this. Let's say $$r$$ is Alice's lost device, and $$d$$ is her currently active device. Also, Alice can optionally provision a new device $$n$$ as a result of this process:

1. For each TLF $$f$$ Alice can write to:
1. Generate an ephemeral Curve25519 Diffie-Helman keypair. Call it $$(M_e,m_e)$$.
2. Delete $$S^{f,0,r}_{A}$$
3. Ask the server to delete $$s^{f,0,r}_A$$.
4. Generate a new 32-byte random secret key (version=1 for the next one): $$s^{f,1}$$ = rand(32)
5. For each user $$u$$ who can access $$f$$
1. For each device $$i$$ that $$u$$ has:
1. As before, generate $$s^{f,1,i}_{u} =$$ rand(32), for the server-side of the key
2. As before, encrypt: $$S^{f,1,i}_{u} =$$Box$$(m_e, M^i_u, s^{f,1,i}_u \oplus s^{f,1})$$
6. If provisioning $$n$$, generate and encrypt $$S^{f,1,n}_{A}$$.
7. Store the rekeyed key blocks to the server, signed with $$e^d_A$$
8. Store $$M_e$$ to the server
9. Don't bother to reencrypt old blocks, so leave the old decryption materials around (except for the compromised key)
2. For each TLF $$f$$ Alice can read:
1. Ask the server to delete $$s^{f,0,r}_A$$.
2. Flip the rekey flag on TLF $$f$$ to on''. The next writer who updates the directory will perform the above protocol that Alice used for her writable directories.
3. Rekey any organizations that Alice is a member of (see below).

Eldest Key Update (or Deletion) [Unimplemented]

If Alice fears all of her sibling keys have been compromised, she should start from scratch. The protocol is:

1. Generate a new eldest key pair.
2. Generate a new per-device keys for the current device.
3. Decommission all old keys, while provisioning the device keys.

If simply deleting her account, Alice will do as above but will not generate new keys or provision new devices.

Organizations

See our discussion of Teams.

Other Topics

File Wipe [Unimplemented]

The server maintains a list of which block-specific keys are in use for each folder. It doesn't know which key corresponds to which files, since the TLF structure cannot be recreated without decryption keys.

With A Provisioned Device

Say the user wants to delete a file $$f$$. A provisioned device can compute a set $$I$$ such that for any $$i \in I$$, $$b_i$$ is or was a part of $$f$$. The client then asks the server to delete $$s_i$$ for $$i \in I$$. The server honors this request as long as the user can write to the parent TLF.

Without a Provisioned Device

If a user loses all of his provisioned devices, he can ask the server to wipe a TLF by deleting all keys for blocks in that TLF. The server keeps a list, so can perform the task. Of course, the server will only honor such a request if the user can write to this TLF.