## 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.
#include <rpc/util.h>
#include <wallet/rpc/util.h>
#include <wallet/wallet.h>
namespace wallet {
RPCHelpMan walletpassphrase()
{
    return RPCHelpMan{"walletpassphrase",
                "\nStores the wallet decryption key in memory for 'timeout' seconds.\n"
                "This is needed prior to performing transactions related to private keys such as sending bitcoins\n"
            "\nNote:\n"
            "Issuing the walletpassphrase command while the wallet is already unlocked will set a new unlock\n"
            "time that overrides the old one.\n",
                {
                    {"passphrase", RPCArg::Type::STR, RPCArg::Optional::NO, "The wallet passphrase"},
                    {"timeout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The time to keep the decryption key in seconds; capped at 100000000 (~3 years)."},
                },
                RPCResult{RPCResult::Type::NONE, "", ""},
                RPCExamples{
            "\nUnlock the wallet for 60 seconds\n"
            + HelpExampleCli("walletpassphrase", "\"my pass phrase\" 60") +
            "\nLock the wallet again (before 60 seconds)\n"
            + HelpExampleCli("walletlock", "") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("walletpassphrase", "\"my pass phrase\", 60")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    if (!wallet) return UniValue::VNULL;