The Black Chamber

As well as the analysis of letter frequencies, other patterns can also be detected that may help to decipher a piece of ciphertext.

The following text explains some of the clues that can be used to deduce a word or a letter in a piece of ciphertext. If you scroll further down the page, you will see a list of tables that explain letter frequencies and patterns in other languages. This will prove useful for breaking messages not written in English.

• Identify Common Pairs Of Letters: If the ciphertext appears to encode a message in English, but the plaintext does not reveal itself immediately, which is often the case, then focus on pairs of repeated letters. In English the most common repeated letters are ss, ee, tt, ff, ll, mm and oo. If the ciphertext contains any repeated characters, you can assume that they represent one of these.
• Identify The Smallest Words First: If the ciphertext contains spaces between words, then try to identify words containing just one, two or three letters. The only one-letter words in English are a and I. The most common two-letter words are of, to, in, it, is, be, as, at, so, we, he, by, or, on, do, if, me, my, up, an, go, no, us, am. The most common three-letter words are the and and.
• Tailor Made Frequency Tables: If possible, tailor the table of frequencies to the message you are trying to decipher. E.g., military messages tend to omit pronouns and articles, and the loss of words such as I, he, a and they will reduce the frequency of some of the commonest letters. If you know you are tackling a military message, you should use a frequency table generated from other military messages.
• Play The Guessing Game: This can be one of the most useful skills for a cryptanalyst to employ - the ability to identify words, or even entire phrases, based on experience or sheer guesswork. Al-Khalil, an early Arabian cryptanalyst, demonstrated this talent when he cracked a Greek ciphertext. He guessed that the ciphertext began with the greeting 'In the name of God'. Having established that these letters corresponded to a specific section of ciphertext, he could use them as a crowbar to prise open the rest of the ciphertext. This is known as a crib.

Letter and word frequencies have been analysed in a number of different languages. A few of the most commonly used ones are listed below, and may help you to decipher your secret messages...

• ENGLISH
  • Order Of Frequency Of Single Letters
    • E T A O I N S H R D L U
  • Order Of Frequency Of Digraphs
    • th er on an re he in ed nd ha at en es of or nt ea ti to it st io le is ou ar as de rt ve
  • Order Of Frequency Of Trigraphs
    • the and tha ent ion tio for nde has nce edt tis oft sth men
  • Order Of Frequency Of Most Common Doubles
    • ss ee tt ff ll mm oo
  • Order Of Frequency Of Initial Letters
    • T O A W B C D S F M R H I Y E G L N P U J K
  • Order Of Frequency Of Final Letters
    • E S T D N R Y F L O G H A K M P U W
  • One-Letter Words
    • a, I
  • Most Frequent Two-Letter Words
    • of, to, in, it, is, be, as, at, so, we, he, by, or, on, do, if, me, my, up, an, go, no, us, am
  • Most Frequent Three-Letter Words
    • the, and, for, are, but, not, you, all, any, can, had, her, was, one, our, out, day, get, has, him, his, how, man, new, now, old, see, two, way, who, boy, did, its, let, put, say, she, too, use
  • Most Frequent Four-Letter Words
    • that, with, have, this, will, your, from, they, know, want, been, good, much, some, time
• FRENCH
  • Order Of Frequency Of Single Letters
    • E N A S R I U T O L D C M P V F B G X H Q Y Z J K W
  • Most Common Digraphs
    • es en le de on ou nt re ne ed te em se er ar me an it et ie ti el ns ur
  • Most Common Trigraphs
    • ede les lle que ait eme ion eur ell sse est dan del men des tio ese ans ter ons qui ais ous ent
  • Most Frequent Doubles
    • ss ll ee nn tt ff cc rr mm pp
  • One-Letter Words:
    • a, y, o
  • Most Common Two-Letter Words
    • au, ce, ci, de, du, en, et, il, je, la, Ie, ma, me, ne, ni, on, ou, sa, se, si, un au
• GERMAN
  • Order Of Frequency Of Single Letters:
    • E N R I S T U D A H G L O C M B Z F W K V P J Q X Y
  • Most Common Digraphs
    • en er ch de ge ei ie in ne be el te un st di un ue se au re he it ri tz
  • Most Common Trigraphs
    • ein ich den der ten cht sch che die ung gen und nen des ben rch
  • Most Frequent Doubles
    • ee tt ll ss dd mm nn
  • Common Two-Letter Words
    • ab, am, an, da, er, es, ob, so, wo, im, in, um, zu, du, ja ab
• ITALIAN
  • Order Of Frequency Of Single Letters
    • E I A O R L N T S C D P U M G V H Z B F Q J K W X Y
  • Most Common Digraphs
    • er es on re de di ti si el en la al nt ra co ta to le li an in io ar or
  • Most Common Trigraphs
    • che ere zio del que ari ato eco edi ide esi idi ero par nte sta men
  • Most Frequent Doubles
    • ll ss tt ee pp nn bb gg cc
  • One-Letter Words:
    • e, a, i, o
  • Most Common Two-Letter Words
    • di, in, ha, ho
• SPANISH
  • Order Of Frequency Of Single Letters
    • E A O S R I N L D C T U P M Y Q G B H F V J Z K W X
  • Most Common Digraphs
    • es en el de la os ar ue ra re er as on st ad ai or ta co se ac ec ci ia
  • Most Common Trigraphs
    • que est ara ado aqu del cio nte osa ede per ist nei res sde
  • Most Frequent Doubles
    • ee ll rr aa ss cc dd nn
  • One-Letter Words
    • a, e, o, u, y
  • Most Common Two-Letter Words:
    • en, la, de, Io, el, se