Signify stem index delimiter

[kentbull]

When recovering from just the bran/passcode then there is no delimiter between the ridx and kidx which means that you have more than a single digit in ridx or kidx then indexes become ambiguous.

How should we solve this issue?

This started from Fergal's SignifyTS issue: WebOfTrust/signify-ts#300

A corresponding KERIpy issue is opened here: #928

[kentbull]

Question: Should we use a key derivation function (KDF)? Why did we not use a secure key derivation function?

Answer: What we used is a secure derivation function, it is just very simple. Typically KDFs are very complicated to implement.
Salty keys are a kind of key derivation function, a very simple default verification function. You could pass in your own stem.
An example KDF, BIP-39 is loose and supports insecure key derivation so typically BIP-39 is used with hardened keys.

In our case, we have something specific to KERIpy. Given a KEL you can recalculate the path.

The bug was the lack of a delimiter between the ridx and kidx because it makes the path ambiguous when you get high enough numbers.

There's nothing preventing support for other key derivation functions.