76 lines
1.8 KiB
C++
76 lines
1.8 KiB
C++
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#include "GCrypt/GPrng.h"
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#include <cassert>
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namespace Leonetienne::GCrypt {
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GPrng::GPrng(const Block& seed) {
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this->seed = seed;
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hasher.DigestBlock(seed);
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}
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GPrng::GPrng() {
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}
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void GPrng::Seed(const Block& seed) {
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hasher = GHash();
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this->seed = seed;
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hasher.DigestBlock(seed);
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return;
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}
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bool GPrng::GetBit() {
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// If we have no more bits to go, create new ones
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if (nextBit >= BLOCK_SIZE) {
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AdvanceBlock();
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}
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// Return the next bit.
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return hasher.GetHashsum()[nextBit++];
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}
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void GPrng::AdvanceBlock() {
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// To prevent an attacker from being able
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// to predict block n, by knowing block n-1, and block n-2,
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// we will advance the hash function by block n-1 XOR seed.
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// This way it is impossible for an attacker to know the
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// state of the hash function, unless the seed is known.
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// Advance the block (Add the current hashsum XOR seed to the hasher)
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hasher.DigestBlock(Block(hasher.GetHashsum() ^ seed));
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// Reset the pointer
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nextBit = 0;
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return;
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}
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Block GPrng::GetBlock() {
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// Getting a block is a bit troublesome.
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// Just fetching 512 bits would be too much of a performance hog.
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// Slurp up the rest of the current block
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std::stringstream ss;
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const std::size_t bitsLeft = BLOCK_SIZE - nextBit;
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ss << hasher.GetHashsum().to_string().substr(nextBit, bitsLeft);
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// Now we have to advance to the next block
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AdvanceBlock();
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// Now, grab the remaining bits
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const std::size_t remainingBits = BLOCK_SIZE - bitsLeft;
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ss << hasher.GetHashsum().to_string().substr(0, remainingBits);
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// Assert that we have the correct number of bits
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assert(ss.str().length() == BLOCK_SIZE);
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// Set out bitpointer
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nextBit = remainingBits;
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// Return our block
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return Block(ss.str());
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}
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}
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