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comment One time pad: why is it useless in practice?
Since the pad is at least the same size as the message, then if you have a secure channel to transmit the pad, you can often use the same channel to transmit the message itself, and not bother with the pad at all. OTPs are sometimes used when the pad transmission is simple (hand a memory stick to your spy) but the later message transmission is difficult (transmit the secret enemy plans).
comment Positioning of keys in encrypted text
Are you sure that you are not confusing the IV (Initialization Vector) with the key? It is common to prepend the IV to the cyphertext. It is an obvious error to prepend the key to the cyphertext.
comment detecting ROT13/base64 encryption
@fgrieu: technically, ROT13 is a cypher, albeit a very weak one. It is a Caesar cypher with a key (=shift) of 13. The problem being that the key is not really a secret.
comment “Padless” One-time-Pad encryption
And if the "secret algorithm" is shorter than the message, then you no longer have a One Time Pad. If the "secret algorithm" is longer than the message, then why not use a plain OTP?
comment Crack SHA1 hash code
Brute force will do it. How long it takes depends on how many bits there are in the constant value.
comment How to encrypt a short string and keep the length secret
@Erik: I was giving the standard definition of bit padding, which is defined in terms of bits. My last sentence in that section, "In byte terms..." gives the byte version.
comment Removing Padded Value in Decrypted Message
PKCS#7 padding can always be removed because the last byte of the padded message tells you how much padding there is. If your original message is 02 02 02 ... 02 02, and the padded message is 02 02 02 ... 02 02 02 02, then you know that the last two bytes, and only the last two bytes, are padding. The other bytes are the actual message.
comment How to generate a list of unique random strings?
Agreed, but for most uses pseudo-random is going to be sufficient. With a requirement for non-repetition, each successive use is 'less random' since it is being picked from a smaller pool. The last number picked has no randomness at all - it is fully determined by the preceding numbers.
comment How can I make my cipher show the avalanche effect?
Cypher/cipher is a UK/US thing.