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In block cipher modes the nonce / iv and counters are introduced ( http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation). Should they also be private as a key and why?

good_evening
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    Questions about security or cryptography that do not include a programming problem are off-topic for Stack Overflow. I have voted to close. – Duncan Jones Oct 07 '13 at 07:31
  • They're not secret, but you need to generate a new one for each message. For CBC mode they need to be unpredictably random. Standard practice is to generate a new one using a secure PRNG (e.g. from `/dev/urandom` or `CryptGenRandom`) prepending it to the message so it's available to the recipient. – CodesInChaos Oct 07 '13 at 07:50

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No, the nonce/IV are just there to provide distinct outputs provided the same input plaintext.

There's no requirements to keep them confidential (but they shouldn't be guessable or re-used usually), and in most protocols, e.g. TLS >= 1.1, it's something sent in clear at the beginning of every encrypted message.

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