With BIP39, initial entropy becomes a mnemonic; the mnemonic and optional passphrase then produce a binary seed. BIP32 derives a master key and chain code from it. These values and steps are not interchangeable; other backup formats may use different processes.
BIP39 provides a readable transcription of computer-generated randomness, not sentences invented by people. Choosing a familiar quotation or a personal word sequence is therefore not equivalent to generating entropy through the specified process. [BIP39 — Mnemonic generation and seed conversion]
Linux getrandom() waits for source initialization with default settings. GRND_NONBLOCK may return EAGAIN instead of waiting. Implementations must respect status and returned length; a read failure must not be presented as successfully obtained input. [Linux getrandom — Initialization and return values]
Web Crypto getRandomValues() supplies cryptographically secure bytes to an integer array; above 65536 bytes it throws QuotaExceededError. Meeting this interface alone does not verify loaded code, subsequent processing or protection of the mnemonic against disclosure. [W3C Web Crypto — getRandomValues contract]
BIP39 appends 4 checksum bits to 128 input bits to make 12 words; for 256 bits it appends 8 bits to make 24 words. These bits derive from SHA256, rather than providing further independent entropy. [BIP39 — Mnemonic generation and seed conversion]
BIP39 derives a 512-bit seed using PBKDF2-HMAC-SHA512 with 2048 iterations. It uses the mnemonic and a salt consisting of the literal text mnemonic plus the passphrase, both in UTF-8 NFKD. An absent passphrase is empty; output length does not prove 512 bits of input entropy. [BIP39 — Mnemonic generation and seed conversion]
BIP32 processes the seed with HMAC-SHA512 keyed by Bitcoin seed, splitting the result into the master private key and chain code. A zero key value or one outside the allowed range is invalid; the whole seed is not used directly as one private key. [BIP32 — Master key generation]
BIP39 publishes vectors containing entropy, mnemonic and seed, including the test passphrase TREZOR. A match establishes conversion for that input, not unpredictability of the live generator. A public test input must not serve as a secret backup for real funds. [BIP39 — Mnemonic generation and seed conversion]
NIST SP 800-90C connects entropy sources and generation mechanisms. Wallet assessment must therefore also follow the actual version, error handling and transfer of values to subsequent steps. Evidence for one component alone establishes neither whole-process security nor later backup-storage security. [NIST SP 800-90C — Random bit generator constructions]
For the clearest picture, read this entry together with Random Number Generator, BIP 39, HD Wallet, Seed Phrase. The reverse links also lead from Random Number Generator, CSPRNG, Deterministic RNG, Entropy Mixing.