diff --git a/README.md b/README.md
index 023676e..246261f 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,7 @@ The Solitaire cipher is included for completeness, though it is perhaps not stri
 * [Caesar]
 * [Monoalphabetic substitution]
 * [Vigenère]
+* [Portas]
 * [Solitaire]
 
 ## Gotchas
@@ -131,6 +132,28 @@ It returns (key, decrypted text).
 If the keylength is known, specifying it as `crack_vigenere(str, keylength=6)`
 may aid decryption.
 
+### Portas cipher
+
+Encrypt the text "Hello, World!" with a Portas cipher of key "ab":
+
+```julia
+encrypt_portas("Hello, World!", "ab")
+# outputs "URYYB, JBEYQ!"
+```
+
+Note that the input has been made uppercase, but symbols have been preserved.
+The key is expected to be letters only; it is converted to uppercase and symbols
+are stripped out before use.
+
+Decrypt the same text:
+
+```julia
+decrypt_portas("URYYB, JBEYQ!", "ab") 
+# outputs "hello, world!"
+```
+
+Notice that the input has been made lowercase.
+
 ### Solitaire cipher
 
 Encrypt the text "Hello, World!" with the Solitaire cipher, key "crypto":
@@ -147,8 +170,8 @@ decrypt_solitaire("EXKYI ZSGEH UNTIQ", collect(1:54))
 # outputs "aaaaaaaaaaaaaaa", as per https://www.schneier.com/code/sol-test.txt
 ```
 
-
 [Caesar]: https://en.wikipedia.org/wiki/Caesar_cipher
 [Vigenère]: https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher
 [Monoalphabetic substitution]: https://en.wikipedia.org/wiki/Substitution_cipher
 [Solitaire]: https://en.wikipedia.org/wiki/Solitaire_(cipher)
+[Portas]: http://practicalcryptography.com/ciphers/porta-cipher/
diff --git a/src/ClassicalCiphers.jl b/src/ClassicalCiphers.jl
index c96ec40..afdafc8 100644
--- a/src/ClassicalCiphers.jl
+++ b/src/ClassicalCiphers.jl
@@ -7,10 +7,12 @@ include("monoalphabetic.jl")
 include("caesar.jl")
 include("vigenere.jl")
 include("solitaire.jl")
+include("portas.jl")
 
 export encrypt_monoalphabetic, decrypt_monoalphabetic, crack_monoalphabetic,
        encrypt_caesar, decrypt_caesar, crack_caesar,
        encrypt_vigenere, decrypt_vigenere, crack_vigenere,
+       encrypt_portas, decrypt_portas,
        encrypt_solitaire, decrypt_solitaire,
        string_fitness, index_of_coincidence
 
diff --git a/src/portas.jl b/src/portas.jl
new file mode 100644
index 0000000..051481d
--- /dev/null
+++ b/src/portas.jl
@@ -0,0 +1,47 @@
+"""
+Encrypts the given plaintext with the Portas cipher.
+
+The key must be given as a string, whose characters are letters.
+
+Converts the text to uppercase.
+"""
+function encrypt_portas(plaintext, key_in::AbstractString)
+  key = uppercase(letters_only(key_in))
+  plaintext = uppercase(plaintext)
+  keyarr = [div(Int(ch) - 65, 2) for ch in key]
+
+  keycounter = 1
+  ans = IOBuffer()
+
+  for i in 1:length(plaintext)
+  	if ('A' <= plaintext[i] <= 'Z')
+      plainch = Int(plaintext[i]) # 68
+      keych = keyarr[keycounter] # 4
+      if 'Z' >= plaintext[i] >= 'M'
+      	print(ans, Char(((plainch - 65 - keych + 13) % 13) + 65))
+      else
+      	print(ans, Char(((plainch - 65 + keych) % 13) + 65+13))
+      end
+
+      keycounter += 1
+      if keycounter == length(key) + 1
+      	keycounter = 1
+      end
+    else
+      print(ans, plaintext[i])
+    end
+  end
+
+  takebuf_string(ans)
+end
+
+"""
+Decrypts the given ciphertext with the Portas cipher.
+
+The key must be given as a string, whose characters are letters.
+
+Converts the text to lowercase.
+"""
+function decrypt_portas(ciphertext, key::AbstractString)
+	lowercase(encrypt_portas(ciphertext, key))
+end
\ No newline at end of file
diff --git a/test/portas.jl b/test/portas.jl
new file mode 100644
index 0000000..4a60897
--- /dev/null
+++ b/test/portas.jl
@@ -0,0 +1,12 @@
+using ClassicalCiphers
+using Base.Test
+
+@test encrypt_portas("DEFENDTHEEASTWALLOFTHECASTLE", "FORTIFICATION") == uppercase("synnjscvrnrlahutukucvryrlany")
+@test decrypt_portas("synnjscvrnrlahutukucvryrlany", "FORTIFICATION") == lowercase("DEFENDTHEEASTWALLOFTHECASTLE")
+
+@test decrypt_portas("synnjs cvr nrla hutu ku cvr yrlany!", "FORTIFICATION") == lowercase("DEFEND THE EAST WALL OF THE CASTLE!")
+
+# doc tests
+
+@test decrypt_portas("URYYB, JBEYQ!", "ab") == "hello, world!"
+@test encrypt_portas("Hello, World!", "ab") == "URYYB, JBEYQ!"
\ No newline at end of file
diff --git a/test/runtests.jl b/test/runtests.jl
index fb789da..4c3879d 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -1,6 +1,6 @@
 using ClassicalCiphers
 
-tests = ["vigenere", "monoalphabetic", "solitaire", "caesar"]
+tests = ["vigenere", "monoalphabetic", "solitaire", "caesar", "portas"]
 
 println("Running tests:")