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nut13 specify keyset ID integer size: 32 bits #189

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nut13 specify keyset ID integer size: 32 bits #189

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2 changes: 1 addition & 1 deletion 13.md
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Expand Up @@ -42,7 +42,7 @@ The wallet starts with `counter_k := 0` upon encountering a new keyset and incre

#### Keyset ID

The integer representation `keyset_id_int` of a keyset is calculated from its [hexadecimal ID][02] which has a length of 8 bytes or 16 hex characters. First, we convert the hex string to a big-endian sequence of bytes. This value is then modulo reduced by `2^31 - 1` to arrive at an integer that is a unique identifier `keyset_id_int`.
The 32 bit integer representation `keyset_id_int` of a keyset is calculated from its [hexadecimal ID][02] which has a length of 8 bytes or 16 hex characters. First, we convert the hex string to a big-endian sequence of bytes. This value is then modulo reduced by `2^31 - 1` to arrive at an integer that is a unique identifier `keyset_id_int`.
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I am noticing something else here that is not per-se an issue but it's awkward: why reduce by 2^31-1 instead of 2^31? But I think I know why:

q % 2^31 == q & (2^31-1). Normally when reducing modulo a power of 2 you can skip division and just use a mask to get the desired bits. Whoever wrote this first must have confused this or made a typo.
Now we can't change this back without breaking the protocol but I thought it was funny.


Example in Python:
```python
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