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BOLT 8: make it clear that there are *two* chaining keys. #1079

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Jul 17, 2023
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BOLT 8: make it clear that there are *two* chaining keys. #1079

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13 changes: 9 additions & 4 deletions 08-transport.md
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Original file line number Diff line number Diff line change
Expand Up @@ -369,7 +369,9 @@ construction, and 16 bytes for a final authenticating tag.
receiving messages for the duration of the session, are generated.
7. `rn = 0, sn = 0`
* The sending and receiving nonces are initialized to 0.
8. Send `m = 0 || c || t` over the network buffer.
8. `rck = sck = ck`
* The sending and receiving chaining keys are initialized the same.
9. Send `m = 0 || c || t` over the network buffer.

**Receiver Actions:**

Expand Down Expand Up @@ -401,6 +403,8 @@ construction, and 16 bytes for a final authenticating tag.
receiving messages for the duration of the session, are generated.
10. `rn = 0, sn = 0`
* The sending and receiving nonces are initialized to 0.
11. `rck = sck = ck`
* The sending and receiving chaining keys are initialized the same.

## Lightning Message Specification

Expand Down Expand Up @@ -488,14 +492,15 @@ Changing keys regularly and forgetting previous keys is useful to
prevent the decryption of old messages, in the case of later key leakage (i.e.
backwards secrecy).

Key rotation is performed for _each_ key (`sk` and `rk`) _individually_. A key
Key rotation is performed for _each_ key (`sk` and `rk`) _individually_,
using `sck` and `rck` respectively. A key
is to be rotated after a party encrypts or decrypts 1000 times with it (i.e. every 500 messages).
This can be properly accounted for by rotating the key once the nonce dedicated
to it exceeds 1000.
to it reaches 1000.

Key rotation for a key `k` is performed according to the following steps:

1. Let `ck` be the chaining key obtained at the end of Act Three.
1. Let `ck` be the chaining key (i.e. `rck` for `rk` or `sck` for `sk`)
2. `ck', k' = HKDF(ck, k)`
3. Reset the nonce for the key to `n = 0`.
4. `k = k'`
Expand Down