linbug / cryptography

Cryptography

This repository is for all the work done on Cryptography as part of the Cambridge Programmer's Study Group.

Classical cryptography

• US Army Field Manual 34-40-2: Basic Cryptanalysis
• Classical Cryptography (Wikipedia)
• Cryptanalysis of ADFGVX cipher
• Practical Cryptography

Modern Cryptography

• Block ciphers modes of operation
• The notes in the Libtomcrypt library contain test vectors, and many practical implementation guidelines. The Libtomcrypt code also seems to be beginner friendly.
• Clear explanation of the Padding Oracle Attack with examples
• Cryptopals challenges
• Thorough speed test of common crypto libraries Note well, this excludes the more recent NaCl and NSS.

Using OpenSSL from C

The OpenSSL library will have all the functionality we might need during the challenges, especially if you're working in C/C++. The documentation is really poor though. Here are a couple of examples to get you started:

• Good example of use in a stackoverflow question. (note the code has an issue, but it's easily fixed.)
• Usage example from the official docs at the bottom of the page. Note that the cipher EVP_aes_128_ecb() is not documented but is available.

Command Line help

Strings and Hexadecimal

Using xxd to convert a string to Hexadecimal. Note how we use echo's -n option to not print a newline at the end.

 ➤ echo -n "Hello" | xxd -p
 48656c6c6f

Converting hex back to string (which will not include a newline):

 ➤ echo -n "48656c6c6f" | xxd -p -r
 Hello

Strings and base 64

Same idea, but for base 64:

➤ echo -n "Hello" | base64
SGVsbG8=
➤ echo -n "SGVsbG8=" | base64 --decode
Hello

Using OPENSSL to test your code on the command line

Let's set up our key and plaintext

➤ PLAINTEXT="YELLOWFIN TUNAS." # Exactly 16 characters (128 bits)
➤ KEY="YELLOW SUBMARINE" # Exactly 16 characters (128 bits)
➤ HEX_KEY=$(echo $KEY | xxd -p)

Then we can encrypt and decrypt using openssl. We'll need to tell openssl to not add a salt and to not pad our plaintext using the -nopad and -nosalt option. We'll use -aes-128-ecb as a simple test.

➤ CIPHERTEXT=$(echo -n ${PLAINTEXT} | openssl enc -aes-128-ecb -nopad -nosalt -K ${HEX_KEY} )
➤ echo -n ${CIPHERTEXT} | openssl enc -d -aes-128-ecb -nopad -nosalt -K ${HEX_KEY}
YELLOWFIN TUNAS.

Using a file is just as easy:

➤  base64 --decode < secret-lyrics.txt | openssl enc -d -aes-128-ecb -nopad -nosalt -K $(echo -n "YELLOW SUBMARINE" | xxd -p)
I'm back and I'm ringin' the bell
[...many more lines...]
Play that funky music