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Day 14: One-Time Pad
In order to communicate securely with Santa while you’re on this mission, you’ve been using a one-time pad that you generate using a pre-agreed algorithm. Unfortunately, you’ve run out of keys in your one-time pad, and so you need to generate some more.
To generate keys, you first get a stream of random data by taking the MD5 of a pre-arranged
salt (your puzzle input) and an increasing integer index (starting with 0, and represented
in decimal); the resulting MD5 hash should be represented as a string of lowercase
hexadecimal digits.
However, not all of these MD5 hashes are keys, and you need 64 new keys for your one-time
pad. A hash is a key only if:
- It contains three of the same character in a row, like
777. Only consider the first such triplet in a hash. - One of the next
1000hashes in the stream contains that same character five times in a row, like77777.
Considering future hashes for five-of-a-kind sequences does not cause those hashes to be skipped; instead, regardless of whether the current hash is a key, always resume testing for keys starting with the very next hash.
For example, if the pre-arranged salt is abc:
- The first index which produces a triple is
18, because the MD5 hash ofabc18contains...cc38887a5....However, index18does not count as a key for your one-time pad, because none of the next thousand hashes (index19through index1018) contain88888. - The next index which produces a triple is
39; the hash ofabc39containseee. It is also the first key: one of the next thousand hashes (the one at index 816) containseeeee. - None of the next six triples are keys, but the one after that, at index
92, is: it contains999and index200contains99999. - Eventually, index
22728meets all of the criteria to generate the64th key.
So, using our example salt of abc, index 22728 produces the 64th key.
Given the actual salt in your puzzle input, what index produces your 64th one-time pad
key?