who can get a replacement SIM card, and many don't. The problem in 2020 is rapid growth in attacks based on intercepting SMS authentication codes, which mostly seem to involve SIM swap, where the attacker pretends to be you to your mobile phone company and gets a replacement SIM card for your account. SIM-swap attacks started in South Africa in 2007, became the main form of bank fraud in Nigeria, then caught on in America – initially as a means of taking over valuable Instagram accounts, then to loot people's accounts at bitcoin exchanges, then for bank fraud more generally [1094]. I will discuss SIM-swap attacks in more detail in section 12.7.4.
Attackers have also exploited the SS7 signalling protocol to wiretap targets' mobile phones remotely and steal codes [485]. I'll discuss such attacks in more detail in the chapters on phones and on banking. The next step in the arms race will be moving customers from SMS messages for authentication and account recovery to an app; the same Google research shows that this improves these last two figures to 99% for bulk phishing and 90% for targeted attacks [574]. As for the targeted attacks, other research by Ariana Mirian along with colleagues from UCSD and Google approached gangs who advertised ‘hack-for-hire’ services online and asked them to phish Gmail passwords. Three of the gangs succeeded, defeating SMS-based 2fa with a middleperson attack; forensics then revealed 372 other attacks on Gmail users from the same IP addresses during March to October 2018 [1324]. This is still an immature criminal market, but to stop such attacks an app or authentication token is the way to go. It also raises further questions about account recovery. If I use a hardware security key on my Gmail, do I need a second one in a safe as a recovery mechanism? (Probably.) If I use one app on my phone to do banking and another as an authenticator, do I comply with rules on two-factor authentication? (See section 12.7.4 in the chapter on banking.)
Email notification is the default for telling people not just of suspicious login attempts, but of logins to new devices that succeeded with the help of a code. That way, if someone plants malware on your phone, you have some chance of detecting it. How a victim recovers then is the next question. If all else fails, a service provider may eventually let them speak to a real person. But when designing such a system, never forget that it's only as strong as the weakest fallback mechanism – be it a recovery email loop with an email provider you don't control, a phone code that's vulnerable to SIM swapping or mobile malware, or a human who's open to social engineering.
3.4.2 Password choice
Many accounts are compromised by guessing PINs or passwords. There are botnets constantly breaking into online accounts by guessing passwords and password-recovery questions, as I described in 2.3.1.4, in order to use email accounts to send spam and to recruit machines to botnets. And as people invent new services and put passwords on them, the password guessers find new targets. A recent example is cryptocurrency wallets: an anonymous ‘bitcoin bandit’ managed to steal $50m by trying lots of weak passwords for ethereum wallets [810]. Meanwhile, billions of dollars' worth of cryptocurrency has been lost because passwords were forgotten. So passwords matter, and there are basically three broad concerns, in ascending order of importance and difficulty:
1 Will the user enter the password correctly with a high enough probability?
2 Will the user remember the password, or will they have to either write it down or choose one that's easy for the attacker to guess?
3 Will the user break the system security by disclosing the password to a third party, whether accidentally, on purpose, or as a result of deception?
3.4.3 Difficulties with reliable password entry
The first human-factors issue is that if a password is too long or complex, users might have difficulty entering it correctly. If the operation they're trying to perform is urgent, this might have safety implications. If customers have difficulty entering software product activation codes, this can generate expensive calls to your support desk. And the move from laptops to smartphones during the 2010s has made password rules such as ‘at least one lower-case letter, upper-case letter, number and special character’ really fiddly and annoying. This is one of the factors pushing people toward longer but simpler secrets, such as passphrases of three or four words. But will people be able to enter them without making too many errors?
An interesting study was done for the STS prepayment meters used to sell electricity in many less-developed countries. The customer hands some money to a sales agent, and gets a 20-digit number printed out on a receipt. They take this receipt home, enter the numbers at a keypad in the meter, and the lights come on. The STS designers worried that since a lot of the population was illiterate, and since people might get lost halfway through entering the number, the system might be unusable. But illiteracy was not a problem: even people who could not read had no difficulty with numbers (‘everybody can use a phone’, as one of the engineers said). The biggest problem was entry errors, and these were dealt with by printing the twenty digits in two rows, with three groups of four digits in the first row followed by two in the second [94]. I'll describe this in detail in section 14.2.
A quite different application is the firing codes for US nuclear weapons. These consist of only 12 decimal digits. If they are ever used, the operators will be under extreme stress, and possibly using improvised or obsolete communications channels. Experiments suggested that 12 digits was the maximum that could be conveyed reliably in such circumstances. I'll discuss how this evolved in section 15.2.
3.4.4 Difficulties with remembering the password
Our second psychological issue is that people often find passwords hard to remember [2079]. Twelve to twenty digits may be easy to copy from a telegram or a meter ticket, but when customers are expected to memorize passwords, they either choose values that are easy for attackers to guess, or write them down, or both. In fact, standard password advice has been summed up as: “Choose a password you can't remember, and don't write it down”.
The problems are not limited to computer access. For example, one chain of cheap hotels in France introduced self service. You'd turn up at the hotel, swipe your credit card in the reception machine, and get a receipt with a numerical access code to unlock your room door. To keep costs down, the rooms did not have en-suite bathrooms. A common failure mode was that you'd get up in the middle of the night to go to the bathroom, forget your access code, and realise you hadn't taken the receipt with you. So you'd have to sleep on the bathroom floor until the staff arrived the following morning.
Password memorability can be discussed under five main headings: naïve choice, user abilities and training, design errors, operational failures and vulnerability to social-engineering attacks.
3.4.4.1 Naïve choice
Since the mid-1980s, people have studied what sort of passwords people choose, and found they use spouses' names, single letters, or even just hit carriage return giving an empty string as their password. Cryptanalysis of tapes from a 1980 Unix system showed that of the pioneers, Dennis Ritchie used ‘dmac’ (his middle name was MacAlistair); the later Google chairman Eric Schmidt used ‘wendy!!!’ (his wife's name) and Brian Kernighan used ‘/.,/.,’ [796]. Fred Grampp and Robert Morris's classic 1984 paper on Unix security [806] reports that after software became available which forced passwords to be at least six characters long and have at least one nonletter, they made a file of the 20 most common female names, each followed by a single digit.
