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Substitution ciphers are not safe enough to use for important messages. Substitution ciphers can be broken by an idea called ''frequency analysis''. Some letters are more common than others in English sentences: ''E'' is the most common, then ''T'', then ''A'', and so on. A message that has been changed by a substitution cipher will have different common letters, but this gives a hint about the rule. The most common letters in the changed message are likely to be the most common letters in English. Breaking ''cryptograms'' (messages hidden with a substitution cipher) is a common [[puzzle]] often found in newspapers.
Substitution ciphers are not safe enough to use for important messages. Substitution ciphers can be broken by an idea called ''frequency analysis''. Some letters are more common than others in English sentences: ''E'' is the most common, then ''T'', then ''A'', and so on. A message that has been changed by a substitution cipher will have different common letters, but this gives a hint about the rule. The most common letters in the changed message are likely to be the most common letters in English. Breaking ''cryptograms'' (messages hidden with a substitution cipher) is a common [[puzzle]] often found in newspapers.
In past centuries substitution ciphers were sometimes strengthened by combining them in superencryption with transposition ciphers. Improvements in [[cryptanalysis]] caused this method to be abandoned in the early 20th century.
== Example ==
== Example ==
<!-- decryption alphabet
<!-- decryption alphabet
ABCDEFGHIJKLMNOPQRSTUVWXYZ
ABCDEFGHIJKLMNOPQRSTUVWXYZ
[[Category:Cryptography]]
[[Category:Cryptography]]
== Substitution Cipher Media ==
<gallery widths='160px' heights='100%' mode='traditional' caption=''>
File:ROT13.png|[[ROT13]] is a [[Caesar cipher]], a type of substitution cipher. In ROT13, the alphabet is shifted 13 steps.
File:A-pigpen-message.svg|The SVG version of a sample en:pigpen cipher message. Original diagram for Wikipedia created in en:Dia.
File:Babington postscript.jpg|The forged nomenclator message used in the [[Babington Plot]]
File:Nomenclator-French 17th cent.jpg|A French nomenclator code table
File:HGM Enigma.jpg|[[Enigma cipher]] machine as used by the German military in World War II
</gallery>