Bip39 Wallet Android One illustration of manually building entropy might be flipping a coin. But when an attacker gives you a coin that may be weighted slightly in favor of heads, your First source of entropy should have been corrupted. You'll be able to see the entire source code in https://thebookmarkid.com/story18611300/word-bib-the-smart-trick-of-bip39-phrase-that-no-one-is-discussing