Safe 5 Question

matejcik · July 18, 2024, 8:35am

this is how a mnemonic is generated from the entropy:

trezor/trezor-firmware/blob/main/crypto/bip39.c#L66-L97


      
          const char *mnemonic_from_data(const uint8_t *data, int len) {
            if (len % 4 || len < 16 || len > 32) {
              return 0;
            }
          
            uint8_t bits[32 + 1] = {0};
          
            sha256_Raw(data, len, bits);
            // checksum
            bits[len] = bits[0];
            // data
            memcpy(bits, data, len);
          
            int mlen = len * 3 / 4;
          
            int i = 0, j = 0, idx = 0;
            char *p = mnemo;
            for (i = 0; i < mlen; i++) {
              idx = 0;
              for (j = 0; j < 11; j++) {

This file has been truncated. show original

it’s a pretty straightforward implementation of the BIP-39 spec

as for how the entropy is gathered, here’s some code:

github.com

trezor/trezor-firmware/blob/main/core/src/apps/management/reset_device/init.py#L72-L83


      
          # generate and display internal entropy
          int_entropy = random.bytes(32, True)
          if __debug__:
              storage.debug.reset_internal_entropy = int_entropy
          if msg.display_random:
              await layout.show_internal_entropy(int_entropy)
          
          # request external entropy and compute the master secret
          entropy_ack = await call(EntropyRequest(), EntropyAck)
          ext_entropy = entropy_ack.entropy
          # For SLIP-39 this is the Encrypted Master Secret
          secret = _compute_secret_from_entropy(int_entropy, ext_entropy, msg.strength)

and here’s some accompanying explanation I typed up earlier: What library does Trezor use?

You’ll see the “external” entropy in the verbose output (as a content of EntropyAck message), and the “internal” entropy shown on the Trezor screen.
Then you can use the following script to check that the seed comes out right: trezor-firmware/python/tools/mnemonic_check.py at main · trezor/trezor-firmware · GitHub