Why is a valid brain seed word count always divisible by 3?

[jcalfee]

I'm looking at bip-0039 and I'm not sure why you have this check to make sure the brain seed word count is divisible by 3:

https://github.com/bitcoinjs/bip39/blob/v2.3.0/index.js#L34

Maybe I missed something but I don't see this checking either:
https://github.com/bitpay/bitcore-mnemonic/blob/v1.2.5/lib/mnemonic.js#L110

[dcousens]

Its an intrinsic property of the encoding algorithm.

11 (1) acquire | ERR: Cannot read property 'map' of null
1111 (2) baby acquire | ERR: Cannot read property 'map' of null
111111 (3) baby mass ability | ERR: Invalid mnemonic checksum
11111111 (4) baby mass dutch
1111111111 (5) baby mass dust afford | ERR: Invalid mnemonic checksum
111111111111 (6) baby mass dust captain able | ERR: Invalid mnemonic checksum
11111111111111 (7) baby mass dust captain baby able | ERR: Invalid mnemonic checksum
1111111111111111 (8) baby mass dust captain baby material
111111111111111111 (9) baby mass dust captain baby mass ancient | ERR: Invalid mnemonic checksum
11111111111111111111 (10) baby mass dust captain baby mass dust absent | ERR: Invalid mnemonic checksum
1111111111111111111111 (11) baby mass dust captain baby mass dust captain ability | ERR: Invalid mnemonic checksum
111111111111111111111111 (12) baby mass dust captain baby mass dust captain badge
11111111111111111111111111 (13) baby mass dust captain baby mass dust captain baby ball | ERR: Invalid mnemonic checksum
1111111111111111111111111111 (14) baby mass dust captain baby mass dust captain baby mass acid | ERR: Invalid mnemonic checksum
111111111111111111111111111111 (15) baby mass dust captain baby mass dust captain baby mass dutch about | ERR: Invalid mnemonic checksum
11111111111111111111111111111111 (16) baby mass dust captain baby mass dust captain baby mass dust casino
1111111111111111111111111111111111 (17) baby mass dust captain baby mass dust captain baby mass dust captain acoustic | ERR: Invalid mnemonic checksum
111111111111111111111111111111111111 (18) baby mass dust captain baby mass dust captain baby mass dust captain baby add | ERR: Invalid mnemonic checksum
11111111111111111111111111111111111111 (19) baby mass dust captain baby mass dust captain baby mass dust captain baby mass about | ERR: Invalid mnemonic checksum
1111111111111111111111111111111111111111 (20) baby mass dust captain baby mass dust captain baby mass dust captain baby mass echo

I'm not sure how @weilu stumbled across it, but it is a good first pass approximation.
Another valid check would be to ensure that the entropy has a length multiple of 4.

[dcousens]

The mnemonic must encode entropy in a multiple of 32 bits.

Ref https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki#From_mnemonic_to_seed

[dcousens]

I'll add a comment and the multiple of 4 check too.

[dcousens]

entropyToMnemonic should probably error out rather than producing an invalid mnemonic though...

[dcousens]

See #47

[dcousens]

It'd be nice to derive an exact proof why this is the case ... otherwise I'd be tempted to remove it in favour of the actual constraint enforcement.

[dcousens]

#49