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Martin Tournoij wrote:

| On Sun, May 25, 2008 at 04:10:01PM -0400, Aryeh M. Friedman wrote:

|> Lets say I have a string of n bits that needs to be filled by repeatedly

|> applying some function on k strings of m bits where |m|<|n|.   Assuming

|> that m(i) is a random string and n needs to also be random what is

the best

|> way to do this.  Note it is possible that k*|m|>|n|.   What I have been

|> thinking so far is:

|>

|> ~    Let m' be the average length of m       // this is pre-computable

|> ~    Let n be all 0's

|>

|> ~    for i=0 --> k

|> ~       pos=m'*i

|>

|> ~       xor string k[i] onto n starting at pos   // wrap around if needed

|>

|> ~    While this is simple I wonder if it can be attacked (determine what

|> k[i]) if k*|m|>|n| (it is trivial to attack if k*|m|<=|n|).   Also is

there

|> some way to make attacking hard if k*|m|<=|n|?

|>

|> ~    Ideas?

|

| So you want us to do your homework...?

|

If this was homework I would just use the above and take the b or what

ever I would get on it but I need this for something else where any

attack is not good.

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