The file default_wallet is currently stored in plain text. Do I have to secure it more? What's the use of the electrum's password?
Plaintext just means that your wallet file is not encrypted. However, an Electrum wallet can have THREE states:
1. Unencrypted file, unencrypted data - EVERYTHING is in plain text (including private keys/seed)
2. Unencrypted file, private keys/seed encrypted - File is in plain text, but important data like private keys/seed is encrypted using wallet password
3. Encrypted file, encrypted data - Entire file AND important data contained with in are both encrypted using wallet password.
Is it just encoded in base64 or both encoded and encrypted? (base64 is not an encryption protocol). And if encrypted, is the line "seed": "...", the result of the encryption of the set of the words with my electrum password?
If you have a wallet password set, then the private keys (and seed) WILL be encrypted... the file itself is only encrypted if you tick the "encrypt file" box when setting a password.
On the software, I did Wallet > Export my private keys.
Then, Electrum generate 32 lines like that :
"19RMXtXmXFuF2wgYigLRZswN9Jcw60qdt3" "5H45CGM8rMjxEXxiPuD5BDxh9ZxSdrM4q9qM6wiuGBdioGYMqGW" (those are not mine ^^)
(
https://en.bitcoin.it/wiki/Wallet_import_format)
Does it mean that the set of 12 words is able to generate multiple private keys?
Yes, if you have 12 words, you have an "HD" wallet... these are designed to generate an "infinite" number of private keys from the seed that is able to be restored from your 12 words... The same 12 words will always generate the same list of private keys.