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BIP 39

Week 39

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Being week 39, you decide you want to look once more at BIP-39 - the Bitcoin Implementation Protocol standard for defining how a series of words can be used to derive a user's private key representing their wallet - which acts as an easier alternative that a user can e.g. write down on a piece of paper, as opposed to copying and pasting a large hex/base 64 string and saving it on their device, which is arguably less secure

Here is a an array containing all 2048 (2^11) BIP-39 words:

Many crypto wallets will generate and allow you to restore your account (and hence funds) from a mnemonic - there are websites like this or this which can also generate mnemonics for you, if you want to familiarise yourself with the process

You decide you only want to go for maximum security, so will only generate 24-word phrases (rather than shorter e.g. 12 word phrases) yourself which will act as your wallets - as you can see in the "Generating the mnemonic" section from the official spec, it is a multistage process:

Generate 256 bits (32 bytes) of random values (0-255) - for this challenge, these random bytes will be given to you, so we can all get the same answer
Perform a SHA-256 hash of those 32 bytes - take the first byte in the hash result and append it to the end of your initial 32 random bytes (i.e. now 33 bytes total) - this byte will act as a checksum to ensure the sentence is valid
Split the entire 33 bytes into 11-bit sections (it might be easier to convert these bytes to a binary string like "010001..."), convert each 11 bits to an integer (will be 2048 (2^11) different values - i.e. range 0-2047) and use each integer as an index to get a word from the BIP 39 words listed above

Let's look at an example:

Here are 32 random bytes we generated:
[164, 215, 213, 131, 229, 33, 0, 124, 224, 79, 124, 203, 0, 22, 112, 152, 113, 93, 34, 121, 73, 117, 200, 163, 206, 127, 192, 184, 106, 6, 213, 237]

If we SHA256 those bytes, we get this byte array:
[148, 253, 163, 199, 5, 148, 187, 121, 135, 53, 246, 246, 17, 147, 87, 229, 113, 100, 158, 32, 239, 47, 252, 86, 88, 32, 1, 29, 125, 63, 141, 237]

If we take the first byte of our hash (148) and append it to our initial 32 bytes, we get this 33 byte array:
[164, 215, 213, 131, 229, 33, 0, 124, 224, 79, 124, 203, 0, 22, 112, 152, 113, 93, 34, 121, 73, 117, 200, 163, 206, 127, 192, 184, 106, 6, 213, 237, 148]

You will then want to figure out how to loop through 11 bits at a time, then use each 11 bit value as an index in our word array to get our 24-word mnemonic:

10100100110 = 1318 = pill (i.e. this binary number is the 8 bits of 164 (10100100) and the 3 most-significant bits (110) from 215)
10111110101 = 1525 = salt (this is the 5 LSB from 215 (10111) and the 6 MSB from 213 (110101))
01100000111 = 775 = genius
11001010010 = 1618 = ski
00010000000 = 128 = avoid
00111110011 = 499 = dinosaur
10000001001 = 1033 = license
11101111100 = 1916 = usage
11001011000 = 1624 = slam
00000000101 = 5 = absent
10011100001 = 1249 = order
00110000111 = 391 = couch
00010101110 = 174 = beyond
10010001001 = 1161 = muffin
11100101001 = 1833 = topple
00101110101 = 373 = concert
11001000101 = 1605 = silly
00011110011 = 243 = bunker
10011111111 = 1279 = paper
10000001011 = 1035 = lift
10000110101 = 1077 = mammal
00000011011 = 27 = address
01010111101 = 701 = fish
10110010100 = 1428 = raw

Our final mnemonic on a single line is:
pill salt genius ski avoid dinosaur license usage slam absent order couch beyond muffin topple concert silly bunker paper lift mammal address fish raw

To prove that your program is working, you should use the following 32 random bytes, then generate the 24 word phrase (same line, space-separated) from it

Hints

Hints will be released at the start of each of the following days - e.g. the start of day 3 is 48 hours after the challenge starts

Release Day	Hint
2