Medieval Cryptography
This page has moved: the most up-to-date version can be found at www.astronomicum.com, my new website for medieval sciences. I'm leaving the original version at stringpage so as not to break anyone's bookmarks, but any updates will only be made at the new site.
This is the outline/handout for a class on medieval cryptography I first taught at Pennsic in 2004. Most of the information here is derived from Kahn 1996 (see references).
For the particularly geeky, I wrote a set of functions for the statistical package R to decode or encode ciphers with a key letter, with a word as a key, or as an autokey. There are examples of all these types of cipher below.
If you are curious, but don't have or use R, no worries. First, find an Rweb server like this one. Then, copy the contents of the appropriate file above and paste them into the Rweb window. Finally, type in the correct command for what you want to do. Press the submit button, and after a bit your answer will be at the bottom of the page, after a bunch of R code.
Here are some hints, along with an example for each of the three functions.
- key letter - either the plaintext or the ciphertext, plus the keyletter
letterkey(plain="plaintext in quotes", keyletter="Q")
or
letterkey(cipher="ciphertext in quotes", keyletter="Q")
letterkey(plain="the east and midrealm are at war", keyletter="B") - key word - either the plaintext or the ciphertext, plus the key word (or phrase)
wordkey(plain="plaintext in quotes", keyword="WORD")
or
wordkey(cipher="ciphertext in quotes", keyword="WORD")
wordkey(plain="the east and midrealm are at war", keyword="ABCDEFGHIJKLMNOPQRSTUVWXYZ")
wordkey(plain="the east and midrealm are at war", keyword="PENNSIC WAR") - autokey - either the plaintext or the ciphertext, plus the initial key letter
autokey(plain="plaintext in quotes", keyletter="Q")
or
autokey(cipher="ciphertext in quotes", keyletter="Q")
autokey(plain="the east and midrealm are at war", keyletter="F")
Basic terminology
- steganography - hide the existence of the message
- cryptography - make the message hard to read
- transposition - rearrange the letters
- substitution - use something different instead
- code - substitute by word or idea
- cipher - substitute by letter
- monoalphabetic - one letter = one symbol
- polyalphabetic - one letter = several possible symbols
- homophonic - some letters have several equivalents
- nomenclator - mix of code and cipher
The earliest beginnings
- Egypt - showoff scribes
- India - highly developed, along with grammar and linguistics; mentioned in Kama Sutra
- Hebrew - three kinds of monoalphabetic ciphers appear in the Old Testament
- Classical - On the Defense of Fortified Places by Aeneas Tacticus is the first surviving written material, describes military uses of cryptography (4th c. BCE) includes steganography with string - punch holes in a disk for the letters, thread string thru to spell out the message
- Polybius square - developed for signalling by Greek historian Polybius, but still used in cryptography (2nd c. BCE)
-
1
2
3
4
5
1
A
B
C
D
E
A = 11
2
F
G
H
I/J
K
B = 12
3
L
M
N
O
P
M = 32
4
Q
R
S
T
U
Z = 55
5
V
W
X
Y
Z
...
-
- Julius Caesar - first claim of actual military use (1st c. BCE) alphabet offset by some number is still called a Caesar alphabet
|
Plaintext |
A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
L |
M |
N |
O |
P |
Q |
R |
S |
T |
U |
V |
W |
X |
Y |
Z |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Cipher |
D |
E |
F |
G |
H |
I |
J |
K |
L |
M |
N |
O |
P |
Q |
R |
S |
T |
U |
V |
W |
X |
Y |
Z |
A |
B |
C |
Medieval examples
- Arab world - highly developed, including first text on cryptanalysis using letter frequencies
doesn't seem to have influenced Western world - Cryptography used to conceal information, especially magic / alchemy
Often used symbols instead of letters - Monastic scribes continue to entertain themselves with ciphers - graffiti and marginalia
- Viking crytography - monoalphabetic substitution ciphers on the Rök runestone, Gotland, Sweden
Runic cryptography was based on the group and place of each rune. The elder futhark had 24 runes in 3 groups, so one part of the symbol used indicated which of the three groups, and the other part which of the eight possible places that rune occupied. - Irish ogham - wide variety of encryptions described by the Book of Ballymote (15th c.) "serpent through the heather", "great speckle", "vexation of a poet's heart"
- Hildegard von Bingen - cipher alphabet given to her in a flash of inspiration (11th c.)
- Roger Bacon - wrote about cryptography in Secret Works of Art and the Nobility of Magic (13th c.) "A man is crazy who writes a secret in any other way than one which will conceal it from the vulgar."
- Geoffrey Chaucer (maybe) - monoalphabetic substitution cipher to conceal important information within The Equatorie of the Planetis
Renaissance achievements
- Cryptography widely used for diplomatic purposes, especially among the Italian city-states
full-time cipher secretaries for making and breaking codes and ciphers
used nulls, concealed word length, other ways of making things more difficult - nomenclators used from 15th - 19th centuries
code words for common/important terms, cipher for the rest - Leon Batista Alberti - first European publication on frequency analysis (15th c.)
developed the idea of using multiple cipher alphabets (switching every few words)
developed the cipher diskPlaintext: The East and Midrealm are at war. Alphabet: BBB BBBB CCC CCCCCCCC DDD DD DDDD Cipher: UIF FBTU CPF OKFTGBNO DUH DW ZDU
- Johannes Trithemius - idea of changing alphabets with each letter (but in order)
first printed book on cryptology, Steganographia (1518)Plaintext: The East and Midrealm are at war. Alphabet: ABC DEFG HIJ KLMNOPQR STU VW XYZ Cipher: TIG HEXZ HVM WTPESPBD SKY VP TYQ
- Giovan Batista Belaso - using a passphrase for a polyalphabetic cipher
called Vigenère, but Vigenère's true contribution was much coolerPlaintext: The East and Midrealm are at war. Phrase: PEN NSIC WAR PENNSICW ARP EN NSI Cipher: ILR RSAV WNU BMQEWINI AIT EG JSZ
- Blaise de Vigenère - autokey ciphers, using the text itself as the passphrase (1585)
forgotten, reinvented in the 19th c. (need to already know the first letter, F in this example)Plaintext: The East and Midrealm are at war. Autokey: FTH EEAS TAN DMIDREAL MAR EA TWA Cipher: YAL IESL TNQ PULUVELX MRV ET PWR
- Mary Queen of Scots - executed after messages plotting against Elizabeth were deciphered (1587)
History of cryptography
- Beutelspacher, Albrecht. 1994. Cryptology. The Mathematics Association of America.
Easy-to-read introduction to cryptology, but history often contradicts Kahn, so probably wrong - Blatt, Heather. XLH TZHQH MX GHDG! Secret Writing in the Middle Ages and Renaissance. Accessed 12 August 2004.
has an illustration of Chaucer's cipher - Goebel, Greg. 2004. Codes, Ciphers, and Codebreaking v2.2.0 Accessed 12 August 2004.
- Kahn, David. 1996. The Codebreakers: the Story of Secret Writing. Second edition. Scribener, NY.
best book on the history of cryptography, and source of most of the material here very large, and mostly about World War II - Cryptography Timeline Accessed 12 August 2004.
- History of cryptography Accessed 12 August 2004.
Other material
- Brown, Dan. 2003. The da Vinci Code. Doubleday.
fiction, but the code parts are correct to the best of my knowledge - Mongé, Alf and O.G. Landverk. 1967. Norse medieval cryptography in runic carvings. Norseman Press, Glendale, CA.
Amusing nonsense "proving" the authenticity of the Kensington Runestone - Rok Runestone pictures
- Runes and Runic Cryptography Accessed 12 August 2004.
- The Voynich Manuscript Accessed 12 August 2004.
more information and pictures on a fascinating hoax (?) - Rugg, Gordon. July 2004. The Mystery of the Voynich Manuscript. Scientific American.
- Rugg, Gordon. Replicating the Voynich Manuscript. Accessed 12 August 2004.