## Problem

Encrypting a wallet through JSON RPC or Qt *appears to* allow a user to include null characters in the passphrase, but silently ignores everything including and after the first null character.

For instance (on regtest), trying to set a passphrase of "`a{null character}b`":

`curl --user __cookie__ --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "encryptwallet", "params": ["a\u0000b"]}' -H 'content-type: text/plain;' http://127.0.0.1:18443/`

This will succeed, but allow the user to unlock with the passphrase "`a`", instead of the expected full passphrase (which also works):

`curl --user __cookie__ --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "walletpassphrase", "params": ["a",10]}' -H 'content-type: text/plain;' http://127.0.0.1:18443/`

I am also able to replicate it in Qt on my macOS machine by running `printf 'a\0b' | pbcopy` and pasting the result into the passphrase dialog. 

My main concern is a user thinking that they're generating, say, 32 random bytes as a passphrase, and if they're unlucky and get a zero in the first few bytes, it unexpectedly cuts their security down to almost nothing. 

## Root Cause

The reason is due to our `SecureString` type. `SecureString` is a `std::string` specialization with a secure allocator. However, when assigned, it's treated like a C- string (no explicit length and null-terminated). See the [original PR](https://github.com/bitcoin/bitcoin/pull/666) for more details.

## Potential Solutions

I think there are two plausible approaches to take. The first (and my preference) is to allow and support null characters, and I will submit a PR that enables that (by making `SecureString` use the entire string). The second is to explicitly reject any passphrases that contain null characters.

One significant complication may be that, if anyone is *already* using a passphrase with a null, then my first solution would stop their wallet from unlocking. However, it would still be unlockable just by trimming the null and any subsequent characters.
pwallet->nRelockTime = 0;
    return UniValue::VNULL;
},
    };
}
RPCHelpMan encryptwallet()
{
    return RPCHelpMan{"encryptwallet",
                "\nEncrypts the wallet with 'passphrase'. This is for first time encryption.\n"
                "After this, any calls that interact with private keys such as sending or signing \n"
                "will require the passphrase to be set prior the making these calls.\n"
                "Use the walletpassphrase call for this, and then walletlock call.\n"
                "If the wallet is already encrypted, use the walletpassphrasechange call.\n",
                {
                    {"passphrase", RPCArg::Type::STR, RPCArg::Optional::NO, "The pass phrase to encrypt the wallet with. It must be at least 1 character, but should be long."},
                },
                RPCResult{RPCResult::Type::STR, "", "A string with further instructions"},
                RPCExamples{
            "\nEncrypt your wallet\n"
            + HelpExampleCli("encryptwallet", "\"my pass phrase\"") +
            "\nNow set the passphrase to use the wallet, such as for signing or sending bitcoin\n"
            + HelpExampleCli("walletpassphrase", "\"my pass phrase\"") +
            "\nNow we can do something like sign\n"
            + HelpExampleCli("signmessage", "\"address\" \"test message\"") +
            "\nNow lock the wallet again by removing the passphrase\n"
            + HelpExampleCli("walletlock", "") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("encryptwallet", "\"my pass phrase\"")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue