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comment Could the Enigma algorithm be classified as a Feistel network?
Stream ciphers operate on characters, or arguably even bits in a stream. For Enigma, it's characters. Binary ones like Lorentz or RC4 can be thought to work on bits. Engima takes a character, rattles it around a path and it goes to another character map. (And the wheels ratchet one place.) Computer stream ciphers are a PRNG that is XORed onto the plaintext to yield ciphertext. Block ciphers take a binary block and the key giving another block. That's it. We use chaining modes to encrypt more than one block, but it works on blocks. It is a mapping of input blocks of bits to output blocks.
comment Are there hash algorithms with variable length output?
Actually, the SHA-3 schedule is "by the end of 2012" so it's about where one would expect.
comment What algorithm does PGP use to encrypt email?
I wouldn't point to It's over a decade out of date and not maintained. But great answer. -- Jon
comment Is it fair to assume that SHA1 collisions won't occur on a set of <100k strings
No, no one has found a way to implement Manuel's 2^51 attack. For that matter, no one has answered my question, "2^51 whats?" 2^51 clock cycles is not very long, and 2^51 years is. Yet both have a work factor of 2^51. (By the way, this is why work factor is a slippery concept.)
comment Can you make a hash out of a stream cipher?
Carter-Wegman MACs are okay for making authenticated encryption, but they're not successful for fast, secure hash functions. As Jalaj points out, there's a different set of security requirements.
comment Predicting values from a Linear Congruential Generator
I've added in more text along with a link to an applicable paper.
comment Is it safer to generate your own Diffie-Hellman primes or to use those defined in RFC 3526?
Thank you for the clarification, poncho. I was trying not to get into the details, but the gist of it. But you are correct, I should have been more weasely.