Thanks birr for explaining the private key conversion, I wasn't familiar with online tools that could do that (just knew they existed).
Apparently, passguardian shows a "reconstructed secret" with a couple of zeroes prepended. As I mentioned above, these extra characters aren't in the private key, properly defined. The private key consists of 64, count 'em, 64 hex characters for the statutory 256 bits.
Just want to clarify this bit.
In Mycelium Entropy, the 'secret' is the entire WIF-formatted sequence (without the checksum), not just the private key:
- for an uncompressed key, or
- '01' for a compressed key.
A "compressed" private key (actually, a private key which is to be used with a compressed public key) starts with K or L and is the most common type and the default. An "uncompressed" private key starts with a 5. Note the extra byte at the end when compressed keys are used.
The prefix is 80 for Bitcoin, EF for Bitcoin testnet, B0 for Litecoin etc.
The algorithm used by PassGuardian uses the first '1' bit for its own purposes and therefore removes it from the result, so the prefix becomes 00 for Bitcoin, 6F for testnet, 30 for Litecoin etc.
While the prefix can easily be discarded, it's important to watch out for the extra 01 byte at the end. If it is present, and the total length is 68 hex characters, then it's the compressed key marker and should also be discarded; in that case, the key should be converted to WIF with the 'compressed' option set.
Like you say, there are multiple online tools which can do that last step of converting the raw key to a base-58 encoded WIF. Some take the 64 hex digits of the key and ask you for additional information (coin type and compression). There might be others, which take the whole sequence including the prefix and optionally the suffix, and convert it to Base58Check; then the coin type and compression do not need to be decoded manually.
It would be nice if a tool existed to reassemble keys from Entropy shares. However, the algorithm is not something Mycelium-specific. If something happens with the wallet app, the keys can always be recovered using a combination of general purpose tools.