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readme: Update description of key generation 

In a recent commit, I updated the key generation to Lucus' solution, in which
we don't have to do any format-preserving encryption, but where the
probability of collisions is amortized away (basically by doing exponential
backoff) by adding a character every time we get a collision.
This commit is contained in:
Daan Sprenkels 2019-08-25 21:30:10 +02:00
parent 061bb23625
commit 7926df7410
1 changed files with 9 additions and 8 deletions
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README.md
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@ -36,15 +36,16 @@ namespace. Ideas:
## Shorten keys and collisions
For the first version of the keys, we will use 3 bytes encoded as base64url,
as described in [RFC4648, par. 5]. To allow for truncate-resistant upgrading
of the URL key format, the first bit is set to `0`. That means the first base64
character will always be in `/[A-Za-f]/`.
First of all: A sexted is a value of 6 bits.
This domain contains `8388608` values. If the service is somewhat used, we will
get collisions early when we generate those keys at random. Instead, we will
use format-preserving-encryption to construct a counter that visits every value
in an unpredictable order. Daan will fix this.
For generating keys, we will initially generate a random value of 4 sextets,
where the first bit is set to `0`. If this collides with an existing key, we
will generate a new one made out of 5 sextexts, and set the prefix bits to
`0b10`. We will keep doing this until we don't have any collisions anymore.
To get proper-looking keys, we format the key to characters using the
base64url alphabet described in ([RFC4648, par. 5]). The encoded value will
be saved in the database.
[RFC4648, par. 5]: https://tools.ietf.org/html/rfc4648#section-5