Elementary S02E06 - S02E12

Elementary S02E06 - An Unnatural Arrangement

In this episode the villain goes to the wrong house because of an error in "Maps Earth" (a fictive map page). The victim's address isn't correctly mapped and shows shows the neighboring house.
Getting addresses right is a big undertaking, and these kind of mistakes aren't uncommon for all big map services. Lots of countries don't provide this information, and mapping services, like Google Maps, therefore take photos (street-view pictures) to extract the house numbers from them.
For a long time, there weren't algorithms that could do this automatically and one had to use humans to analyze the pictures and update the databases.
Since computers were unable to analyze the difficult pictures, Google started to use it as a test to detect humans: show a picture of a house-number, and see if the other end can read the number. If it can, then a human is in front of the screen. Otherwise it's probably a computer. This test, called CAPTCHA, is crucial to stop automated bots from creating fake accounts which are then used for spam and other nefarious activities.

CAPTCHA with a house number on the right.

Not only was Google able to weed out bots from their services, they also used the responses to update their own databases. From time to time Google would show an non-analyzed picture and use the response to improve their database. (Obviously there were safeguards to avoid polluting the database with bad responses). Here is an article that was written when Google started to use this technique in 2012: http://techcrunch.com/2012/03/29/google-now-using-recaptcha-to-decode-street-view-addresses/
Since then, artificial intelligence has improved, and computers are now able to read house numbers accurately: http://www.technologyreview.com/view/523326/how-google-cracked-house-number-identification-in-street-view/. As a result, being sent to the wrong house should become less common.

Elementary S02E07 - The Marchioness

In the middle of this episode (at around 31:00) Holmes states that one of the simplest ways to monitor someone's movements is to listen in on someones phone conversations. "Anyone with 50 pounds to spend can buy the software which lets them listen in on virtually any wireless call."
There are two kinds of attacks to listen in on calls:
- one that modifies the victim's phone, and
- one that intercepts all wireless calls.
We already discussed a modified phone in the review of episode S01E22. It's easily imaginable that a generic spyware costs only 50 pounds. It's not clear how the attacker could then install it on the victim's phone, though. In any case, it's not as easy as Holmes makes it out to be.
The far superior attack consists of intercepting all phone calls with so called IMSI-catchers. This is sometimes done by law enforcement agencies. The idea consists of setting up a fake phone tower, and let the victim connect to it. There are two disadvantages with IMSI-catchers: since phones need to connect to it, it has to advertise itself and is therefore active. That means that one can detect and find these devices. Furthermore, IMSI-catchers aren't cheap. Stingrays (some of the best known devices) cost around $100,000: http://arstechnica.com/tech-policy/2013/09/meet-the-machines-that-steal-your-phones-data/

Elementary S02E08 - S02E11

No computers.

Elementary S02E12 - The Diabolical Kind

Again, Holmes has to find and decrypt hidden messages. As discussed in my review for S01E11, hiding secret messages in plain sight is called steganography.
An interesting article on this subject can be found here: http://www.strangehorizons.com/2001/20011008/steganography.shtml. It partially covers the techniques that are used by the protagonists in this episode. Moriarty uses different shadings and drawing techniques to hide numbers in her sketches. According to Holmes, the kidnappers, use the Ave-Maria cipher to hide their messages in innocent messages in the classified ads.

 A secret message hidden in an innocent-looking classified.

It is not clear which one contains the secret message, but the first reads:
...lent Charity Auction
Top-line items available for bid benefits cancer awareness programs. This weekend in the park ... contact Kathy at 21 ... information or ...portunities

The Ave-Maria cipher hides messages in prayers. As such, it's hard to see how any secret could be hidden in these classified ads. Even with other techniques it would be hard to hide significant messages in them. Stenography is a very low-bandwidth way of sending messages where a single character can sometimes be hidden inside multiple words. It is very surprising to hear that these classified ads apparently contain the following secret messages:
"You know what I want.
Tell me where it is."
"Be prompt. Your daughter's life
hangs in the balance."