ZFEC Forward Error Correction

Added in version 3.0.0.

The ZFEC class provides forward error correction compatible with the zfec library.

Forward error correction takes an input and creates multiple “shares”, such that any K of N shares is sufficient to recover the entire original input.

Note

Specific to the ZFEC format, the first K generated shares are identical to the original input data, followed by N-K shares of error correcting code. This is very different from threshold secret sharing, where having fewer than K shares gives no information about the original input.

Warning

If a corrupted share is provided to the decoding algorithm, the resulting decoding will be invalid. It is recommended to protect shares using a technique such as a MAC or public key signature, if corruption is likely in your application.

ZFEC requires that the input length be exactly divisible by K; if needed define a padding scheme to pad your input to the necessary size.

An example application that adds padding and a hash checksum is available in src/cli/zfec.cpp and invokable using botan fec_encode and botan fec_decode.

class ZFEC

ZFEC(size_t k, size_t n)

Set up for encoding or decoding using parameters k and n. Both must be less than 256, and k must be less than n.

void encode_shares(const std::vector<const uint8_t*> &shares, size_t share_size, std::function<void(size_t, const uint8_t[], size_t)> output_cb) const

Encode K shares in shares each of length share_size into N shares, also each of length share_size. The output_cb function will be called once for each output share (in some unspecified and possibly non-deterministic order).

The parameters to output_cb are: the share being output, the share contents, and the length of the encoded share (which will always be equal to share_size).

void decode_shares(const std::map<size_t, const uint8_t*> &shares, size_t share_size, std::function<void(size_t, const uint8_t[], size_t)> output_cb) const

Decode some set of shares into the original input. Each share is of share_size bytes. The shares are identified by a small integer (between 0 and 255).

The parameters to output_cb are similar to that of encode_shares.