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Consider the following scheme:

  1. Perform an (N,N) Reed-Solomon encoding (i.e. N data blocks, N parity blocks)
  2. Drop the N data blocks and keep only the N parity blocks.

Are these N parity blocks an all-or-nothing transform? (meaning missing even one parity block does not permit decoding the original data blocks)

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  • $\begingroup$ does my answer address your question? $\endgroup$ – kodlu May 23 at 22:24
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Reed-Solomon codes are Maximum-Distance-Separable codes and thus have the property that in a $[n,k]$ RS code ---- meaning than there are a total of $n$ blocks of which $k$ are data blocks and $n-k$ are parity block ($n=2N$ and $k=N$ in the OP's notation) ---- any $k$ blocks are sufficient to reconstruct the entire codeword, that is, all the blocks can be found if we know $k$ of them. Note that it does not matter whether the $k$ blocks available are a mixture of data and parity blocks or parity blocks only, just as long as $k$ blocks are available.

So, in the scheme envisioned by the OP, if one has a $[2N,N]$ Reed-Solomon code ($N$ data blocks, $N$ parity blocks, for a total of $2N$ blocks) and keeps only the $N$ parity blocks, the $N$ data blocks can be reconstructed. If fewer than $N$ of the parity blocks, then the data blocks cannot be reconstructed in the sense that the decoder can come up with a list of $M$ possible candidates for the $N$ data blocks but cannot say which of the candidates is the correct one. Here, each of the $M$ candidates on the list is a vector $[D_1, D_2, \ldots, D_N]$ of $N$ data block (each $D_i$ is a data block) and the decoder cannot determine which is the correct vector of $N$ data blocks. What is $M$? Well, $M$ is the number of possible distinct values that each data block might have and thus is either the number of elements in the finite field over which the Reed-Solomon code is defined, or some power $L$ thereof if each data block is a vector of length $L$ over the finite field (and we are using an interleaved Reed-Solomon code). Either way, $M$ is pretty large.

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  • $\begingroup$ Thank you @dilip, the answer is clear. As a side question, imagine that I want to add an extra constraint to my problem : "The $N$ data blocks should be reconstructed exclusively from the $N$ parity blocks, and not from a mixture of data and parity blocks". Would Reed-Solomon (non-systematic?) still be a candidate for this constrained scheme? $\endgroup$ – stefanix May 26 at 7:09
  • $\begingroup$ @stefanix As I said in my answer, it does not matter to the general (erasures-correcting) decoding algorithm whether the $N$ available blocks are parity blocks only or a mixture of data and parity blocks. But it is certainly possible to modify the general algorithm to tell it to refuse to decode a mixture of data and parity blocks, and decode iff $N$ parity blocks are available. Similarly, when $<N$ blocks are available, the standard RS decoder fails to decode (gives error signal only), and it is necessary to re-program it to spit out the list of $M$ possible vectors of data blocks instead. $\endgroup$ – Dilip Sarwate May 27 at 2:37
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A Reed-Solomon code is an MDS code over the symbol alphabet $\mathbb{F}_q,$ with parameters $[n,k,n-k+1]$. Here, the length $n=2N$ the dimension $k=N$ and the minimum distance is $d=N+1.$

Such codes form Orthogonal Arrays of strength $t=n-d+1=N+1.$

And an OA of strength $t$ is also an OA of strength $t'\leq t.$ Take $t'=N.$ Thus, yes, dropping one of the $N$ parity blocks would result in an AONT, leaking no information about the missing block.

Edit: In general any $N$ blocks are enough to reconstruct the missing blocks.

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  • $\begingroup$ Just to make sure: dropping one of the N parity blocks does not leak any information on any of the N data blocks? $\endgroup$ – stefanix May 22 at 15:24
  • $\begingroup$ So how many blocks do you need in total to reconstruct the data? $\endgroup$ – stefanix May 24 at 8:56
  • $\begingroup$ My comment above is in error, I will edit the body of the paper, to stop the confusion. $\endgroup$ – kodlu May 24 at 22:27
  • $\begingroup$ Your last paragraph is incorrect even after the edit. $\endgroup$ – Dilip Sarwate May 25 at 14:53
  • $\begingroup$ OK, much better after the second edit. $\endgroup$ – Dilip Sarwate May 27 at 2:26

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