syswraith's workbench

Breaking a Substitution Cipher with Cryptanalysis

3 min read

A cipher is an algorithm for encoding or decoding text in such a way that only the intended recipient can understand it.

At a high level, all cryptographic algorithms work as follows:

  1. The unencrypted message is called plaintext.
  2. Plaintext is passed to an encryption function, which is a set of rules for transforming the message.
  3. The function also takes a key, known only to the sender and receiver.
  4. Using the key, the function produces the ciphertext (encrypted message).

It is crucial that the key and encryption method remain secret; otherwise, anyone can decrypt the message.

The Ciphertext

We are given the following ciphertext

psili nli cg bgli onllailm pg flgmm. nkk a snvi ac fgbbgc waps psi qcfgcplgkknoki ncr psi acmnci, psi vafagqm ncr psi ivak, nkk psi bnysib a snvi fnqmir ncr by qppil acraeeilicfi pgwnlr ap a snvi cgw mqlhnmmir. by hnac am fgcmpncp ncr msnlh ncr a rg cgp sghi egl n oippil wglkr egl ncygci, ac enfp a wncp by hnac pg oi acekafpir gc gpsilm. a wncp cg gci pg imfnhi, oqp ivic nepil nrbappacd psam psili am cg fnpsnlmam, by hqcamsbicp fgcpacqim pg ikqri bi ncr a dnac cg riihil ucgwkirdi ge bymike; cg ciw ucgwkirdi fnc oi ixplnfpir elgb by pikkacd. psam fgceimmagc snm bincp cgpsacd.

Uppercase letters denote letters that are decrypted and lowercase letters denote letters that are not.

Step 1: Initial Guesses

Observing the text, we notice frequent repetitions such as psi and ncr . We can guess 2 common 3-letter words:

CipherLikely Plaintext
psiTHE
ncrAND

Let’s assign them:

p => T
s => H
i => E
n => A
c => N
r => D

In the first word, if we change l ⇒ R, we get the word THERE.
In the second word, if we change n ⇒ A, we get the word ARE.

l => R
n => A

Step 2: Single-character Words

The character a comes by itself. There are two possibilities — A and I. Since A is already used, we replace it with:

a => I

Step 3: Recognizing Common Words

The phrase AND appears in context; we assign:

v => V
w => W
c => N
r => D

We can assume that since since v, w have their usual meanings, the letters coming after them follow this property as well.

x => X
y => Y
z => Z

Step 4: Repeating Letters

  • Repeated ciphertext kk likely is likely ll because ALL fits more in this context than ASS.
  • m → S to form the word INSANE.

Step 5: Completing the Key

Based on context, we fill remaining letters:

g => O
b => M
o => B
q => U
h => P
d => G
f => C
e => F

Final Key

Letterabcdefghijklmnopqrstuvwxyz
MappingIMNGFCOPE__RSABTUDH__VWXYZ

Step 6: Frequency Analysis & Observations

  • Observing repeating letters and common patterns (THE, ARE, AND) helped identify substitutions.
  • Double letters (ll, ss) give clues about word structure.
  • Context is key: guessing likely words makes solving faster.

Tips for Solving Substitution Ciphers

  1. Start with short, common words (the, and, are, is).
  2. Look for repeating patterns — double letters, common suffixes (-ing, -ed).
  3. Track mappings in a table. Sometimes one may be able to bruteforce the key used to encrypt the message, and then break the cipher that way.
  4. Analyze the frequency of letters. For example, letters like E, T, A appear often in English.
  5. A good resource for more information on cryptography is The Code Book by Simon Singh.

Graph View

Backlinks