The most difficult part of the material covered in these sections was probably just the concept of inverting a matrix mod n. I've never really considered modulo arithmetic for anything other than integers, so the reading definitely expanded my understanding of the power of modulo arithmetic.
Something in the reading that I found to be quite interesting was the introduction of block ciphers, and the way in which they add complexity to the encryption of a plaintext message. This is evident from the fact that changing one letter in the plaintext will result in a change of n letters in the ciphertext (depending on the size of the blocks of text, n). Since the use of blocks of size greater than 3 is resistant to frequency analysis, this method can be much more powerful then substitution or Vigenere ciphers. It's a pretty simple method in the context of today's methods, but it laid the foundation for powerful ciphers such as DES, AES, and RSA cryptography.
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