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ECB and CBC require padding or a scheme called ciphertext stealing if the size of the messages is not a multiple of the block size. Commonly a scheme such as PKCS#7 padding is applied, quite often by default (i.e. without you explicitly specifying it, e.g. OpenSSL defaults to PKCS#7 compatible padding). Such padding schemes are created to apply for messages ...


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If you reuse key&IV(nonce) in CTR mode you have almost no security. You get a two time pad. Xoring to messages, will eliminate the key stream and give you the XOR of the plain texts which usually gives you plenty of information. With multiple such cipher text it gets even easier. If you reuse IV and key in CBC mode, it isn't good, but it isn't anywhere ...


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