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This would usually I think be more suited as a comment, but you were looking for guidance instead of a complete answer as it were, and I don't have enough reputation to comment. I'd highly recommend Claude Shannon's introduction (the literal introduction, he developed the concept) to the question you pose. Here’s the paper (PDF) and the section is Part 10 ...
I guess because because foo.txt has a lot of redundancy and thus can be quite significantly compressed. Consequently, the compressed file foo.txt.zip is much smaller (you should take a look at data compression).
To answer the question "why does a block cipher use a Mode of Operation", we need to first examine the question "what is a block cipher?" A block cipher is a keyed operation that converts a string of N bits to a string of N bits (where N is usually fixed by the block cipher; for AES, N=128), in a way that, without the key, looks like a random permutation, ...
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