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When we talk about nonce in Initialization Vector(IV), is it required to used numbers only? not any letters or special characters?

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An IV or nonce is a sequence of bytes, the concept of letters doesn't make much sense here. So what's your question? Are you talking about the ASCII representation of those bytes? Or the hex representation? –  CodesInChaos Jul 10 '12 at 11:30
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It's just a string of bits, how you wish to interpret it is entirely up to you and irrelevant to the actual underlying cryptographic operations being performed (personally I like to use hex. representation for visual inspection as it's easy to compare bitwise, great for debugging) –  Thomas Jul 10 '12 at 11:40

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No. In the context of the Initialization Vector of a cipher operated in some mode, the most significant property of a cryptographic nonce is that it is not reused. It often matters that it is random and unpredictable (e.g. with CBC). It is immaterial that it represents a number or anything else under some particular encoding.

If an IV is chosen at random (which is standard, and best), it would be bad to restrict it to represent a number (say, restricting each byte to be a digit according to ASCII), in particular because it would dramatically increases the odds of reusing the same IV.

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