This document discusses plans to create an Android app simulating an Enigma machine to raise funds for Bletchley Park Trust. It would educate users about the real WWII Enigma machines and cryptography. The app has been successful, with over 2,700 installs generating some donations and ad revenue. However, addressing Android fragmentation across many devices and versions took significant effort. Lessons on cryptography from Enigma include not underestimating attackers, using truly random numbers, and planning for key compromise. The document analyzes the app's install and revenue data in detail to evaluate the fundraising strategy.

1. Fund Raising with an Android Enigma Machine Simulator
02 June 2012 Franklin Heath Ltd

2. What Shall We Talk About?
 How the World War II Enigma machine works
 Potential for mobile apps to raise money for charities
 How bad is Android fragmentation and how best to
deal with it
 What mechanical cryptography can teach us about
today's security software
© Franklin Heath Ltd
02 June 2012 CC BY 3.0 2

3. Android Enigma Simulator: Why?
 2012 is Alan Turing’s centenary year
 One of Bletchley Park’s “Wicked Uncles” working on Enigma
 Real WWII Enigma machines can cost > $200,000
 Touch screen animation gives an idea of the real thing
 Education
 Strong cryptography existed before computers
 Fund raising
 Bletchley Park Trust needs matching funds to restore the site
 Experiment in Android development and monetisation
© Franklin Heath Ltd
02 June 2012 CC BY 3.0 3

4. Bletchley Park’s “Wicked Uncles”
 Senior codebreakers recruited in 1939
 Introduced mathematical and mechanised methods
 1941 memo delivered to P.M Winston Churchill
 Response: “Make sure they have all they want on extreme
priority and report to me that this had been done.”
Alan Turing 1912-1954
Stuart Milner-Barry 1906-1995 Hugh Alexander 1909-1974 Gordon Welchman 1906-1985
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02 June 2012 CC BY 3.0 4

5. Enigma Machine Components
Scrambler
Rotors and reflector
Output
Battery-powered lamps
Input
Keys (switch and lever)
Plug Board
Static, swaps letters
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02 June 2012 CC BY 3.0 5

6. Enigma Machine Components
Scrambler
Rotors and reflector
Output
Battery-powered lamps
Input
Keys (switch and lever)
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02 June 2012 CC BY 3.0 6

7. Fully Functional Paper Model
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8. Enigma Simulator App Demo
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02 June 2012 CC BY 3.0 8

9. Google Play International Reach
0% 5% 10% 15% 20% 25% 30%
USA 670
UK 450
Germany 264
Spain 151
Top 20 countries (of 99)
Italy 112
Poland 111
Netherlands 63
Australia 61
France 59
Canada 47
Czech Republic 46
Brazil 45
Croatia 44
Sweden 43
India 41
Turkey 39
Mexico 35
Hungary 33
Indonesia 30
0
Finland 27
100 200 300 400 500 600 700 800
© Franklin Heath Ltd
02 June 2012 CC BY 3.0 9

10. Revenue Stats (by 2012 Week Nos.)
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
300 £15.00
In-App. Payments
250 £12.50
Advert. Revenue
Nook Sales
200 £10.00
New Installs
Revenue
150 £7.50
100 £5.00
50 £2.50
0 £0.00
© Franklin Heath Ltd
02 June 2012 CC BY 3.0 10

11. How the Revenue Divides Up
 Google Play (user charged £1)
 17p to Her Majesty’s Revenue and Customs (V.A.T.)
 25p to Google (30% Android billing fee)
 29p to us (until development costs covered)
 29p to the Bletchley Park Trust
 AdMob (variable rates per click)
 Remitted in $US so no V.A.T. due
 50% to the Bletchley Park Trust
 Nook sales (user charged $2.49)
 $0.75 to Barnes & Noble (30% transaction fee)
 $0.87 to us
 $0.87 to the Bletchley Park Trust
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02 June 2012 CC BY 3.0 11

12. Fundraising:
What Have We Learned?
 People will donate using in-app billing
 ~ 2% of ~2,700 installs so far
 People do click on in-app advertisements
 ~ 0.8% of ~36,000 impressions so far
 Revenue so far is low
 Street corner tin rattling could have been more productive!
 BUT software revenue scales effortlessly, so good potential
 What we need to do better
 Marketing (increase awareness and donations)
 Continuing user engagement (increase ad revenue)
© Franklin Heath Ltd
02 June 2012 CC BY 3.0 12

13. Android Testing Strategy
 448 different device models in 2864 installs!
 Clearly impossible to test on every device
 What is the best mix of devices to test on?
 Use Android resource qualifiers to identify categories
 “small” → “xlarge”, “ldpi” → “xhdpi”
 Using Android’s layout engine to adapt to different screens
 Also need to cover all supported Android API levels
 We don’t support Android 1.5
 0.23% of category, mandates extra permissions for compatibility
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02 June 2012 CC BY 3.0 13

14. Android Device Variability:
Screen Size and Density
60% 1600
ldpi
Percentage of all EnigmaSim installs
1370
mdpi
1400
50%
hdpi 1200
40% xhdpi 1000
30% 800
600
20%
359 335 400
10%
142 129 120 119
200
67
1 16 0 0 4 26 0 0
0% 0
small normal large xlarge
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02 June 2012 CC BY 3.0 14

15. Android Device Variability:
API Level / Android Version
1308
1400
Percentage of all EnigmaSim installs
50.0%
1200
40.0% 1000
30.0% 800
600
20.0% 424
383 400
10.0% 195
159 200
27 19 49 48
0 0 2
0.0% 0
API level: 4 5 6 7 8 9 10 11 12 13 14 15
Version: 1.6 2.0 2.1 2.2 2.3 3.0 3.1 3.2 4.0
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02 June 2012 CC BY 3.0 15

16. Device-Specific Android Issues
 Samsung Galaxy S app data directory bug
 Android 4 XT9 soft keyb. ignores “no suggestion” flag
 Android 1.6 ghost status bar on resume
 System font changed in Android 4 (size changes)
 Nook default dialog title colour different
 Android 1.6 in-app billing hangs
 Samsung Galaxy Note pixel density
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02 June 2012 CC BY 3.0 16

17. Android Testing Strategy:
Representative Devices
 Most popular devices in the commonly used categories
normal-hdpi 51.0% Samsung Galaxy S2
normal-mdpi 13.4% Samsung Galaxy Ace
xlarge-mdpi 12.5% Asus EeePad Transformer TF101
small-ldpi 5.3% Samsung Galaxy Mini
small-mdpi 4.8% Sony Ericsson Xperia X10 Mini Pro
normal-xhdpi 4.5% Samsung Galaxy Nexus
large-mdpi 4.4% Samsung Galaxy Tab
large-xhdpi 2.5% Samsung Galaxy Note
 How to avoid having to buy all these devices?
 Remote access testing services (DeviceAnywhere, TestDroid)
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02 June 2012 CC BY 3.0 17

18. Android Fragmentation:
What Have We Learned?
 Support for all versions/screens took 80% of our effort
 But 70% installed on Android 2.x, 51% on “normal-hdpi”
 Best strategy to release on a limited subset, at least initially?
 Automated tests are essential
 Overnight tests on 15 different emulated devices
 Android extensions to JUnit are extremely flaky
 Unpredictable data corruption caused by test framework,we gave up
 MonkeyRunner is usable but still unreliable
 We will be looking at Robotium in future
 Remote access could help with device-specific issues
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02 June 2012 CC BY 3.0 18

19. The Crypto Lessons that Enigma
and Bletchley Park Can Teach Us
 Don’t assume that key length is equivalent to security
 User-selected passwords may be the weakest link
 Plan for your keys to be compromised
 Use really random numbers, not random-looking ones
 Don't underestimate the attacker
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02 June 2012 CC BY 3.0 19

20. Enigma Machine Key Length
 4-rotor Enigma M4
 2 possible reflectors
 672 possible rotor choices
 676 possible notch positions
 532,985,208,200,576 possible combinations of plugs
 456,976 possible starting positions
 = 221,286,292,668,406,558,235,295,744 possible keys
 Log2 gives equivalent binary key length: ~88 bits
 This is still export-controlled!
 Yet it could be broken with 70-year old mechanical technology
 Key length isn’t the most important factor
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02 June 2012 CC BY 3.0 20

21. Users Pick Poor Passwords
 Many Enigma messages were read by guessing the
message key that the operator chose (“Cillies”)
 AAA BBB, QWE ASD, BER LIN, etc.
 This was addressed later in the war by operational
procedures
 Daily settings used as a pseudo-random generator
 Cryptographic keys need more entropy than users can
supply in the form of a password
 Salts, nonces, etc.
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02 June 2012 CC BY 3.0 21

22. Plan for Key Compromise
 “Pinches” were an important way into new Enigma networks
 1941 HMS Tartar: code books from weather ship Lauenberg
 1942 HMS Petard: machine and code books from U-559
 1940 HMS Gleaner: rotors VI and VII from U-33
 1941 HMS Somali: rotors and code books from armed trawler Krebs
 1940 HMS Griffin: settings and cribs from armed trawler Polares
 1941 HMS Somali: code books from weather ship München
 1941 HMS Bulldog: machine and code books from U-110
 They had emergency procedures to switch to other settings
 Modern security systems need to have “renewability” too
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02 June 2012 CC BY 3.0 22

23. Use Really Random Numbers
 Don’t be tempted to interfere to make it look random
 German cipher staff had rules for not repeating rotor
order and not plugging adjacent letters
 This significantly reduced the number of possible settings that
needed to be tried on the Bombe
 Many security vulnerabilities in modern systems are
due to poor randomness
 e.g. Debian OpenSSL vulnerability in 2008
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02 June 2012 CC BY 3.0 23

24. Don’t Underestimate the Enemy
 German high command told Enigma was “unbreakable”
 German cryptographers knew it was theoretically
breakable, but thought no one would put in that much
effort
 Bletchley Park’s mathematical approach and production
line methods led to industrial-scale cryptanalysis
 Today: it only needs a handful of bright and bored
attackers to find a “class break” and then millions of
“script kiddies” can use it.
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02 June 2012 CC BY 3.0 24

25. Questions?
 If we have time!
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02 June 2012 CC BY 3.0 25