lockbitchat
7b703e4e65
The descriptor no longer carries key material. It carries what brings up DTLS
plus a 16-byte commitment; the ECDH and ECDSA public keys travel as the first
frame on the DataChannel and are checked against that commitment BEFORE they are
parsed or imported. Measured on the live site, the invitation went from 2274
characters across 4 animated QR frames to 151 characters in one frame.
What the split buys is that nothing is trusted on the strength of having
completed a handshake. The fingerprint arrives over the channel the user
authenticated by looking at it, so DTLS completes only with whoever showed the
code; the commitment makes substituted key material fail closed automatically
instead of relying on the human comparison; and the SAS is computed over a
transcript covering both descriptors verbatim and both key blobs, so anything an
attacker can change anywhere in the handshake changes the digits.
Consequences for the rest of the crypto:
- the HKDF salt is derived from the transcript rather than transmitted, binding
every session key to both DTLS fingerprints and every candidate;
- authProof is replaced by one ECDSA signature over the transcript, proving the
same possession without echoing a nonce back across seven fields;
- Double Ratchet support is implied by the format rather than advertised in it.
SBQ2 postdates the ratchet, so the "peer is old, fall back to static keys"
branch is unreachable here -- it used to fire silently, since SBQ2 has no `dr`
field for _initializeRatchet to find.
Fail closed throughout. _handshakeMode is latched per connection, so a session
cannot be pushed back onto SB1 halfway through; commitment mismatch, wrong role,
duplicate blob, bad proof and timeout all tear the connection down with a
specific message rather than degrade. SBQ2 handshake frames are refused outright
on a non-SBQ2 session, where there would be no commitment to check them against.
Reception of both formats is unconditional and split by first byte (0x02 vs
ASCII 'S') and text prefix. SBQ2_SEND_ENABLED is the single value that governs
what we emit; flipping it to false and redeploying reverts new invitations to
SB1, and the animated multi-frame QR path stays for them.
Verified end to end in real browsers -- 12/12 across {Chrome, Firefox} squared
and three network profiles -- through to a confirmed SAS, an active ratchet, a
64-byte transcript-derived salt and a decrypted message on the far side.
219 lines
9.7 KiB
Markdown
219 lines
9.7 KiB
Markdown
# Cryptography
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Everything here runs in the browser on the Web Crypto API. There are no
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hand-rolled primitives. What is written by hand is the composition: the key
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schedule, the ratchet, the verification flow and the framing, and that is what
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this document describes.
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| --- | --- |
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| Release | v5.9.0 |
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| Protocol version | 4.1 |
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| Ratchet wire version | 1 |
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## Primitives
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| Purpose | Algorithm |
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| --- | --- |
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| Key agreement | ECDH P-384, falling back to P-256 if the browser refuses P-384 |
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| Signatures | ECDSA P-384 with SHA-384, falling back to P-256 with SHA-256 |
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| Key derivation | HKDF-SHA256 |
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| Message encryption | AES-256-GCM |
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| Message authentication | HMAC-SHA256, and AES-GCM's own tag on the ratcheted path |
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| Password derivation | PBKDF2-SHA256, 310,000 iterations, 32-byte salt |
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Session keys are non-extractable `CryptoKey` handles. The exceptions are the
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values a ratchet has to chain itself, which Web Crypto cannot do behind an opaque
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handle; those are raw bytes and are overwritten when finished with.
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## Session establishment
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A session begins with one ECDH exchange. The public keys travel inside signed
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packages, and the receiving side validates the SPKI structure (algorithm OID,
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curve, point format and length) before importing anything.
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From the shared secret, HKDF-SHA256 derives five independent values, each under
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its own `info` label so that recovering one reveals nothing about the others:
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| Label | Use |
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| --- | --- |
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| `message-encryption-v4` | AES-256-GCM payload key on the static path |
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| `message-authentication-v4` | HMAC-SHA256 message authentication |
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| `metadata-protection-v4` | AES-256-GCM for message metadata |
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| `fingerprint-generation-v4` | Key fingerprint shown to the user and fed into the safety code |
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| `double-ratchet-root-v1` | Root key for the Double Ratchet |
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The raw ECDH output is produced with `deriveBits`, used as HKDF input material,
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and the buffer holding it is overwritten as soon as derivation completes. It is
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never exported through an extractable key.
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The 64-byte session salt is generated by the inviting peer and travels in the
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invitation, so both sides derive the same schedule.
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## Verification
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Both peers compute the same safety code with HKDF, from the ECDH-derived key
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fingerprint together with both DTLS fingerprints. The fingerprints are
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canonicalised and sorted so that each side reaches the same value regardless of
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role.
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Users compare the code through a channel an attacker cannot impersonate and enter
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it manually. Local success is not sufficient: the session becomes verified only
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after both peers confirm. Three incorrect entries end the session.
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This is the step that makes the rest meaningful. Completing the key exchange
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proves only that someone completed it; anyone able to rewrite the invitation can
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do that with both people at once. The safety code covers the keys actually in
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use, so a substitution changes the code the users read to each other.
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Verification is also a gate rather than a label. Before it completes, the session
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declines to act on control messages from the peer: reconnection signalling, call
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setup, message deletion and delivery receipts. The verification exchange itself is
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the deliberate exception, since it necessarily runs first.
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## Forward secrecy
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The session keys above would last the whole conversation on their own. The Double
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Ratchet, implemented in `src/crypto/DoubleRatchet.js`, replaces them for message
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traffic so that protection does not rest on a single set of keys.
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### Symmetric ratchet
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Each message key comes from the current chain key through `KDF_CK`, which is
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HMAC-SHA256 over the chain key with one constant for the message key and another
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for the next chain key. The message key is used once and destroyed. Because the
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construction is one-way, holding the current chain key yields no earlier message
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key.
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### DH ratchet
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Each time the conversation changes direction, the replying peer generates a fresh
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ECDH key pair, and both sides mix the new shared secret into the root key with
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`KDF_RK` (HKDF-SHA256, root key as salt, producing the next root and a new chain
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key). A session therefore re-keys continuously as messages go back and forth, and
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an attacker who captured the full state is excluded again after one message in
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each direction.
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### Initialisation
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No extra handshake data is exchanged. Both peers already hold each other's
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authenticated ECDH public key, which is exactly what the safety code covers.
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The inviting peer starts with a fresh ratchet key pair against the peer's
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handshake key and steps the root once, so even its first message has left the
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handshake key behind. The joining peer keeps its handshake key pair as its
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current ratchet pair, which is what the inviting peer derived against, and takes
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no chain until the first message arrives.
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That asymmetry is inherent to the ratchet, not an implementation shortcut: both
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sides must derive the first chain from the same exchange. The consequence is that
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the joining peer has no sending chain until it receives something. The
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application sends a presence update from both sides as soon as verification
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completes, so those first frames use the session keys, and everything after them
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is ratcheted.
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### Frame format
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A ratcheted message is `{ type: "ratchet_message", h, c }`, where `h` is a header
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string and `c` is the base64 body.
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The header carries the sender's current ratchet public key, the length of the
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previous sending chain, and the message number in the current one. It travels in
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the clear because the receiver needs it before it can derive a key, and it is
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passed to AES-GCM as additional authenticated data. Modifying any field causes
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decryption to fail rather than redirecting the ratchet.
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The header must be handed back to the decrypt call exactly as received. It is the
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authenticated data itself, so re-serialising it can change a byte and fail
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authentication for no reason.
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### Out-of-order messages
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Keys for messages that have not yet arrived are retained so they can still be
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read, within fixed bounds:
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| Bound | Value |
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| --- | --- |
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| Maximum skip within one chain | 512 |
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| Total retained keys | 1024, oldest evicted first |
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| Retention period | 5 minutes |
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The message number comes off the wire, so the distance a single frame may claim
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has to be limited. Without a cap, one frame claiming a number in the millions
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would force the receiver to derive and hold that many keys.
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Replay protection is intrinsic here. A message key is destroyed on use, so a
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number behind the current chain has no key left to open it.
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### State is committed only after authentication
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Receiving stages the chain advance and any DH step, attempts decryption, and
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commits only on success. A frame that fails authentication leaves the ratchet
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exactly as it was.
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This matters because the header is reachable by anyone on the channel. Advancing
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the chains before verifying would let a single bad frame push the receiver past
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the sender and break the session permanently, which would be a remote denial of
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service against an established conversation.
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### Negotiation
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Support is advertised in the invitation and in the response, and the ratchet runs
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only when both sides advertise it. A peer on an earlier release negotiates it
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away and the session uses the per-session keys described above.
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The fallback is deliberate. With no server there is no way to update both ends at
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once, and a one-sided ratchet decrypts nothing. The security panel reports which
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of the two is in force for the current connection rather than what the client is
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capable of.
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## Message protection on the static path
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Messages encrypted with the session keys carry their metadata (identifier,
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timestamp, sequence number, original length) encrypted separately under the
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metadata key, and the whole payload is covered by an HMAC. Sequence numbers are
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checked against a sliding window: a number behind the expected one is rejected as
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a replay, and a gap beyond the window is rejected as well.
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Payloads are padded to a 16-byte boundary with random bytes, with the true length
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carried in the encrypted metadata.
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## Rendering
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Decrypted text is sanitized with DOMPurify configured to allow no tags and no
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attributes at all, then rendered through React text nodes. Fenced code blocks are
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tokenised by Prism, which escapes its input before highlighting and never
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evaluates it. The content security policy permits no inline or remote scripts.
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Chat content reaches the interface through one authenticated path. Frames that
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are not authenticated are rejected rather than displayed, so nothing can appear
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in a conversation that did not come from the peer holding the session keys.
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## Local storage
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Sensitive IndexedDB metadata is stored in encrypted envelopes. Legacy plaintext
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records remain readable through a migration path and are rewritten encrypted when
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next accessed. Corrupted encrypted metadata fails closed.
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The master key for persistent storage is derived from a password with PBKDF2 and
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is non-extractable. The application supplies the password interface; there is no
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browser dialog fallback.
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## Memory handling
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Values that can be overwritten are overwritten: the ECDH output, HKDF
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intermediates, the ratchet root and chain keys, and retained message keys.
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Values that cannot be overwritten are documented rather than reported as cleared.
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JavaScript strings are immutable, so a secret held as a string can only be
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dereferenced. A non-extractable `CryptoKey` has no bytes visible to JavaScript at
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all, so dropping the handle is the only available action and non-extractability is
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what protects it. Functions that cannot wipe say so in their logs instead of
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reporting success, because a cleanup path that reports work it did not do is
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worse than one that reports nothing.
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## Scope
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This describes the browser implementation as it stands in v5.9.0. It is not a
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substitute for independent cryptographic review.
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