Cryptanalysis Example

1
Cryptanalysis Example
2
Cipher text

• cop raqv obl eqmp qipa cop kppr pmk
• hxlc obip b hxcymw kxayme gzbll
• ptpgxcp bc obzv rblc vqxa lbw
• bahbkyzzq cq gbzz yc qvv

3
Suppose e is the most common

• cop raqv obl eqmp qipa cop kppr pmk
• e e e e ee e
• fp hxlc obip b hxcymw kxayme gzbll
• e e
• ptpgxcp bc obzv rblc vqxa lbw
• e e e
• bahbkyzzq cq gbzz yc qvv

4
the may appear twice

• cop raqv obl eqmp qipa cop kppr pmk
• the h e e the ee e
• fp hxlc obip b hxcymw kxayme gzbll
• e t h e t
• ptpgxcp bc obzv rblc vqxa lbw
• e e te t h t
• bahbkyzzq cq gbzz yc qvv
• t t

5
a is also common

• cop raqv obl eqmp qipa cop kppr pmk
• the ha e e the ee e
• fp hxlc obip b hxcymw kxayme gzbll
• e t ha e a t a
• ptpgxcp bc obzv rblc vqxa lbw
• e e te at ha a t a
• bahbkyzzq cq gbzz yc qvv
• a a t a t

6
y must be i

• cop raqv obl eqmp qipa cop kppr pmk
• the ha e e the ee e
• fp hxlc obip b hxcymw kxayme gzbll
• e t ha e a ti i a
• ptpgxcp bc obzv rblc vqxa lbw
• e e te at ha a t a
• bahbkyzzq cq gbzz yc qvv
• a a i t a it

7
Is l s?

• cop raqv obl eqmp qipa cop kppr pmk
• the has e e the ee e
• fp hxlc obip b hxcymw kxayme gzbll
• e st ha e a ti i ass
• ptpgxcp bc obzv rblc vqxa lbw
• e e te at ha ast sa
• bahbkyzzq cq gbzz yc qvv
• a a i t a it

8
q must be o

• cop raqv obl eqmp qipa cop kppr pmk
• the o has o e o e the ee e
• fp hxlc obip b hxcymw kxayme gzbll
• e st ha e a ti i ass
• ptpgxcp bc obzv rblc vqxa lbw
• e e te at ha ast o sa
• bahbkyzzq cq gbzz yc qvv
• a a i o to a it o

9
Double letters likely l

• cop raqv obl eqmp qipa cop kppr pmk
• the o has o e o e the ee e
• fp hxlc obip b hxcymw kxayme gzbll
• e st ha e a ti i lass
• ptpgxcp bc obzv rblc vqxa lbw
• e e te at hal ast o sa
• bahbkyzzq cq gbzz yc qvv
• a a illo to all it o

10
g into f?

• cop raqv obl eqmp qipa cop kppr pmk
• the o has o e o e the ee e
• fp hxlc obip b hxcymw kxayme gzbll
• e st ha e a ti i flass?
• ptpgxcp bc obzv rblc vqxa lbw
• e ef te at hal ast o sa
• bahbkyzzq cq gbzz yc qvv
• a a illo to fall it o

11
c is more likely

• cop raqv obl eqmp qipa cop kppr pmk
• the o has o e o e the ee e
• fp hxlc obip b hxcymw kxayme gzbll
• e st ha e a ti i class
• ptpgxcp bc obzv rblc vqxa lbw
• e ec te at hal ast o sa
• bahbkyzzq cq gbzz yc qvv
• a a illo to call it o

12
v possibilities

• cop raqv obl eqmp qipa cop kppr pmk
• the od has o e o e the ee e
• fp hxlc obip b hxcymw kxayme gzbll
• e st ha e a ti i class
• ptpgxcp bc obzv rblc vqxa lbw
• e ec te at hald? ast o sa
• bahbkyzzq cq gbzz yc qvv
• a a illo to call it odd

13
f works better

• cop raqv obl eqmp qipa cop kppr pmk
• the of has o e o e the ee e
• fp hxlc obip b hxcymw kxayme gzbll
• e st ha e a ti i class
• ptpgxcp bc obzv rblc vqxa lbw
• e ec te at half ast fo sa
• bahbkyzzq cq gbzz yc qvv
• a a illo to call it off

14
half mast makes sense

• cop raqv obl eqmp qipa cop kppr pmk
• the m of has o e o e the eem e
• fp hxlc obip b hxcymw kxayme gzbll
• e st ha e a ti i class
• ptpgxcp bc obzv rblc vqxa lbw
• e ec te at half mast? fo sa
• bahbkyzzq cq gbzz yc qvv
• a a illo to call it off

15
But half past works better

• cop raqv obl eqmp qipa cop kppr pmk
• the p of has o e o e the eep e
• fp hxlc obip b hxcymw kxayme gzbll
• e st ha e a ti i class
• ptpgxcp bc obzv rblc vqxa lbw
• e ec te at half past fo sa
• bahbkyzzq cq gbzz yc qvv
• a a illo to call it off

16
That must be prof!

• cop raqv obl eqmp qipa cop kppr pmk
• the prof has o e o er the eep e
• fp hxlc obip b hxcymw kxayme gzbll
• e st ha e a ti ri class
• ptpgxcp bc obzv rblc vqxa lbw
• e ec te at half past fo r sa
• bahbkyzzq cq gbzz yc qvv
• ar a illo to call it off

17
i as v

• cop raqv obl eqmp qipa cop kppr pmk
• the prof has o e over the eep e
• fp hxlc obip b hxcymw kxayme gzbll
• e st have a ti ri class
• ptpgxcp bc obzv rblc vqxa lbw
• e ec te at half past fo r sa
• bahbkyzzq cq gbzz yc qvv
• ar a illo to call it off

18
Only d works here

• cop raqv obl eqmp qipa cop kppr pmk
• the prof has o e over the deep e d
• fp hxlc obip b hxcymw kxayme gzbll
• e st have a ti d ri class
• ptpgxcp bc obzv rblc vqxa lbw
• e ec te at half past fo r sa
• bahbkyzzq cq gbzz yc qvv
• ar adillo to call it off

19
First line solved!

• cop raqv obl eqmp qipa cop kppr pmk
• the prof has gone over the deep end
• fp hxlc obip b hxcymw kxayme gzbll
• e m st have a m tin d ring class
• ptpgxcp bc obzv rblc vqxa lbw
• e ec te at half past fo r sa
• bahbkyzzq cq gbzz yc qvv
• armadillo to call it off

20
Found the last vowel

• cop raqv obl eqmp qipa cop kppr pmk
• the prof has gone over the deep end
• fp hxlc obip b hxcymw kxayme gzbll
• e must have a mutin during class
• ptpgxcp bc obzv rblc vqxa lbw
• e ecute at half past four sa
• bahbkyzzq cq gbzz yc qvv
• armadillo to call it off

21
Finished!

• cop raqv obl eqmp qipa cop kppr pmk
• the prof has gone over the deep end
• fp hxlc obip b hxcymw kxayme gzbll
• we must have a mutiny during class
• ptpgxcp bc obzv rblc vqxa lbw
• execute at half past four say
• bahbkyzzq cq gbzz yc qvv
• armadillo to call it off