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I have used electrum for many years on my desktop pc, and have alway felt very secure in doing so because of my ability to use a very strong password to encrypt my seed phrase with. However I just started using the android version and have some concerns.
The android version requires me to select a PIN but it is only 6 digits long? How does the android version of electrum ensure that this tiny keyspace (1 million) stays secure?
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Due to the nature of typing on mobile devices you can't really expect the user to enter a long password. So you have to compromise on security somewhat.
Electrum relies on Android keeping apps from stepping on each other's toes. It expects the OS to isolate Electrum's files and prevent other apps from accessing them.
The wallet files can't be removed from the device either unless it has been rooted before the wallet was created. Otherwise if you could get the wallet file onto a PC bruteforcing the password would be trivial.
Some other wallet software let you unlock your wallet with your fingerprint. IDK whether that is more secure but it's definitely more convenient.
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The android version requires me to select a PIN but it is only 6 digits long? How does the android version of electrum ensure that this tiny keyspace (1 million) stays secure?
The PIN is not used as a master seed in this way. It may be used as an additional source of user-supplied entropy, but the wallet's seed will be generated in a cryptographically secure way, using other sources of randomness to provide sufficient security.
I am unfamiliar with the specifics of Electrum's implementation, but there are generally two possibilities here:
1) the password / PIN is simply used to encrypt the wallet, and is not used as input for the seed
2) the password/ PIN is used as a '13th/25th word' that acts as a password on the mnemonic. Find more info here. Also, from BIP 39:
A user may decide to protect their mnemonic with a passphrase. If a passphrase is not present, an empty string "" is used instead.
1Electrum uses option 1 - it has support for option 2, but you configure that during wallet set up, and don't need to reenter it unless you are restoring a wallet (I'm not sure if that applies to the mobile version, but since the desktop works that way, I imagine it does) – Raghav Sood – 2019-10-30T20:26:26.783
Option 1 is what I was afraid was happening, and what I meant when I asked if the private seed is secure. A 6 digit pin as an encryption key for the wallet, which contains the master private key, is not secure. I'm going to have to re evaluate using electrum on my android device because of this. If anyone ever gains access to the encrypted wallet file, it would be trivial to brute force the wallet file encryption. – user258667 – 2019-10-30T22:04:46.367
@RaghavSood ah, thats good info, thanks! – chytrik – 2019-10-30T22:27:51.450
@user258667 Oh! I understand the question better now. Your comment is correct, if anyone gains access to the encrypted wallet file, the 6-digit PIN encryption is not extremely strong. Nevertheless, the security model provided by such a mobile wallet may still be useful in some situations (ie a small amount of 'daily spend' BTC, not a large amount that you want to keep extremely secure). – chytrik – 2019-10-30T22:35:19.623