based on Lamport's scheme. Challenge–response The use of challenge–response one-time passwords requires a user to provide a response to a challenge. For example, this can be done by inputting the value that the token has generated into the token itself. To avoid duplicates, an additional counter is usually involved, so if one happens to get the same challenge twice, this still results in different one-time passwords. However, the computation does not usually involve the previous one-time password; that is, usually, this or another algorithm is used, rather than using both algorithms. Implementations SMS A common technology used for the delivery of OTPs is text messaging. Because text messaging is a ubiquitous communication channel, being directly available in nearly all mobile handsets and, through text-to-speech conversion, to any mobile or landline telephone, text messaging has a great potential to reach all consumers with a low total cost to implement. OTP over text messaging may be encrypted using an A5/x standard, which several hacking groups report can be successfully decrypted within minutes or seconds. Additionally, security flaws in the SS7 routing protocol can and have been used to redirect the associated text messages to attackers; in 2017, several O2 customers in Germany were breached in this manner in order to gain access to their mobile banking accounts. In July 2016, the U.S. NIST issued a draft of a special publication with guidance on authentication practices, which discourages the use of SMS as a method of implementing out-of-band two-factor authentication, due to the ability for SMS to be intercepted at scale. Text messages are also vulnerable to SIM swap scams—in which an attacker fraudulently transfers a victim's phone number to their own SIM card, which can then be used to gain access to messages being sent to it. Hardware tokens RSA Security's SecurID is one example of a time-synchronization type of token, along with HID Global's solutions. Like all tokens, these may be lost, damaged, or stolen; additionally, there is an inconvenience as batteries die, especially for tokens without a recharging facility or with a non-replaceable battery. A variant of the proprietary token was proposed by RSA in 2006 and was described as "ubiquitous authentication", in which RSA would partner with manufacturers to add physical SecurID chips to devices such as mobile phones. Recently, it has become possible to take the electronic components associated with regular keyfob OTP tokens and embed them in