BRMerkleBlock.c

//
//  BRMerkleBlock.c
//
//  Created by Aaron Voisine on 8/6/15.
//  Copyright (c) 2015 breadwallet LLC
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in
//  all copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
//  THE SOFTWARE.

#include "BRMerkleBlock.h"
#include "BRCrypto.h"
#include "BRAddress.h"
#include <stdlib.h>
#include <stdint.h>
#include <limits.h>
#include <string.h>
#include <assert.h>

#define MAX_PROOF_OF_WORK 0x1d00ffff    // highest value for difficulty target (higher values are less difficult)
#define TARGET_TIMESPAN   (14*24*60*60) // the targeted timespan between difficulty target adjustments

inline static int _ceil_log2(int x)
{
    int r = (x & (x - 1)) ? 1 : 0;
    
    while ((x >>= 1) != 0) r++;
    return r;
}

// from https://en.bitcoin.it/wiki/Protocol_specification#Merkle_Trees
// Merkle trees are binary trees of hashes. Merkle trees in bitcoin use a double SHA-256, the SHA-256 hash of the
// SHA-256 hash of something. If, when forming a row in the tree (other than the root of the tree), it would have an odd
// number of elements, the final double-hash is duplicated to ensure that the row has an even number of hashes. First
// form the bottom row of the tree with the ordered double-SHA-256 hashes of the byte streams of the transactions in the
// block. Then the row above it consists of half that number of hashes. Each entry is the double-SHA-256 of the 64-byte
// concatenation of the corresponding two hashes below it in the tree. This procedure repeats recursively until we reach
// a row consisting of just a single double-hash. This is the merkle root of the tree.
//
// from https://github.com/bitcoin/bips/blob/master/bip-0037.mediawiki#Partial_Merkle_branch_format
// The encoding works as follows: we traverse the tree in depth-first order, storing a bit for each traversed node,
// signifying whether the node is the parent of at least one matched leaf txid (or a matched txid itself). In case we
// are at the leaf level, or this bit is 0, its merkle node hash is stored, and its children are not explored further.
// Otherwise, no hash is stored, but we recurse into both (or the only) child branch. During decoding, the same
// depth-first traversal is performed, consuming bits and hashes as they were written during encoding.
//
// example tree with three transactions, where only tx2 is matched by the bloom filter:
//
//     merkleRoot
//      /     \
//    m1       m2
//   /  \     /  \
// tx1  tx2 tx3  tx3
//
// flag bits (little endian): 00001011 [merkleRoot = 1, m1 = 1, tx1 = 0, tx2 = 1, m2 = 0, byte padding = 000]
// hashes: [tx1, tx2, m2]
//
// NOTE: this merkle tree design has a security vulnerability (CVE-2012-2459), which can be defended against by
// considering the merkle root invalid if there are duplicate hashes in any rows with an even number of elements

// returns a newly allocated merkle block struct that must be freed by calling BRMerkleBlockFree()
BRMerkleBlock *BRMerkleBlockNew(void)
{
    BRMerkleBlock *block = calloc(1, sizeof(*block));

    assert(block != NULL);
    block->height = BLOCK_UNKNOWN_HEIGHT;
    return block;
}

// buf must contain either a serialized merkleblock or header
// returns a merkle block struct that must be freed by calling BRMerkleBlockFree()
BRMerkleBlock *BRMerkleBlockParse(const uint8_t *buf, size_t bufLen)
{
    BRMerkleBlock *block = (buf && 80 <= bufLen) ? BRMerkleBlockNew() : NULL;
    size_t off = 0, len = 0;
    
    assert(buf != NULL || bufLen == 0);
    
    if (block) {
        block->version = get_u32le(&buf[off]);
        off += sizeof(uint32_t);
        block->prevBlock = get_u256(&buf[off]);
        off += sizeof(UInt256);
        block->merkleRoot = get_u256(&buf[off]);
        off += sizeof(UInt256);
        block->timestamp = get_u32le(&buf[off]);
        off += sizeof(uint32_t);
        block->target = get_u32le(&buf[off]);
        off += sizeof(uint32_t);
        block->nonce = get_u32le(&buf[off]);
        off += sizeof(uint32_t);
        
        if (off + sizeof(uint32_t) <= bufLen) {
            block->totalTx = get_u32le(&buf[off]);
            off += sizeof(uint32_t);
            block->hashesCount = (size_t)BRVarInt(&buf[off], (off <= bufLen ? bufLen - off : 0), &len);
            off += len;
            len = block->hashesCount*sizeof(UInt256);
            block->hashes = (off + len <= bufLen) ? malloc(len) : NULL;
            if (block->hashes) memcpy(block->hashes, &buf[off], len);
            off += len;
            block->flagsLen = (size_t)BRVarInt(&buf[off], (off <= bufLen ? bufLen - off : 0), &len);
            off += len;
            len = block->flagsLen;
            block->flags = (off + len <= bufLen) ? malloc(len) : NULL;
            if (block->flags) memcpy(block->flags, &buf[off], len);
        }
        
        BRSHA256_2(&block->blockHash, buf, 80);
    }
    
    return block;
}

// returns number of bytes written to buf, or total len needed if buf is NULL (block->height is not serialized)
size_t BRMerkleBlockSerialize(const BRMerkleBlock *block, uint8_t *buf, size_t bufLen)
{
    size_t off = 0, len = 80;
    
    assert(block != NULL);
    assert(buf != NULL || bufLen == 0);
    
    if (block->totalTx > 0) {
        len += sizeof(uint32_t) + BRVarIntSize(block->hashesCount) + block->hashesCount*sizeof(UInt256) +
               BRVarIntSize(block->flagsLen) + block->flagsLen;
    }
    
    if (buf && len <= bufLen) {
        set_u32le(&buf[off], block->version);
        off += sizeof(uint32_t);
        set_u256(&buf[off], block->prevBlock);
        off += sizeof(UInt256);
        set_u256(&buf[off], block->merkleRoot);
        off += sizeof(UInt256);
        set_u32le(&buf[off], block->timestamp);
        off += sizeof(uint32_t);
        set_u32le(&buf[off], block->target);
        off += sizeof(uint32_t);
        set_u32le(&buf[off], block->nonce);
        off += sizeof(uint32_t);
    
        if (block->totalTx > 0) {
            set_u32le(&buf[off], block->totalTx);
            off += sizeof(uint32_t);
            off += BRVarIntSet(&buf[off], (off <= bufLen ? bufLen - off : 0), block->hashesCount);
            if (block->hashes) memcpy(&buf[off], block->hashes, block->hashesCount*sizeof(UInt256));
            off += block->hashesCount*sizeof(UInt256);
            off += BRVarIntSet(&buf[off], (off <= bufLen ? bufLen - off : 0), block->flagsLen);
            if (block->flags) memcpy(&buf[off], block->flags, block->flagsLen);
            off += block->flagsLen;
        }
    }
    
    return (! buf || len <= bufLen) ? len : 0;
}

static size_t _BRMerkleBlockTxHashesR(const BRMerkleBlock *block, UInt256 *txHashes, size_t hashesCount, size_t *idx,
                                      size_t *hashIdx, size_t *flagIdx, int depth)
{
    uint8_t flag;
    
    if (*flagIdx/8 < block->flagsLen && *hashIdx < block->hashesCount) {
        flag = (block->flags[*flagIdx/8] & (1 << (*flagIdx % 8)));
        (*flagIdx)++;
    
        if (! flag || depth == _ceil_log2(block->totalTx)) {
            if (flag && *idx < hashesCount) {
                if (txHashes) txHashes[*idx] = block->hashes[*hashIdx]; // leaf
                (*idx)++;
            }
        
            (*hashIdx)++;
        }
        else {
            _BRMerkleBlockTxHashesR(block, txHashes, hashesCount, idx, hashIdx, flagIdx, depth + 1); // left branch
            _BRMerkleBlockTxHashesR(block, txHashes, hashesCount, idx, hashIdx, flagIdx, depth + 1); // right branch
        }
    }

    return *idx;
}

// populates txHashes with the matched tx hashes in the block
// returns number of hashes written, or total number of matched hashes in the block if txHashes is NULL
size_t BRMerkleBlockTxHashes(const BRMerkleBlock *block, UInt256 *txHashes, size_t hashesCount)
{
    size_t idx = 0, hashIdx = 0, flagIdx = 0;

    assert(block != NULL);
    assert(txHashes != NULL || hashesCount == 0);
    return _BRMerkleBlockTxHashesR(block, txHashes, (txHashes) ? hashesCount : SIZE_MAX, &idx, &hashIdx, &flagIdx, 0);
}

// recursively walks the merkle tree to calculate the merkle root
// NOTE: this merkle tree design has a security vulnerability (CVE-2012-2459), which can be defended against by
// considering the merkle root invalid if there are duplicate hashes in any rows with an even number of elements
static UInt256 _BRMerkleBlockRootR(const BRMerkleBlock *block, size_t *hashIdx, size_t *flagIdx, int depth)
{
    uint8_t flag;
    UInt256 hashes[2], md = UINT256_ZERO;

    if (*flagIdx/8 < block->flagsLen && *hashIdx < block->hashesCount) {
        flag = (block->flags[*flagIdx/8] & (1 << (*flagIdx % 8)));
        (*flagIdx)++;

        if (flag && depth != _ceil_log2(block->totalTx)) {
            hashes[0] = _BRMerkleBlockRootR(block, hashIdx, flagIdx, depth + 1); // left branch
            hashes[1] = _BRMerkleBlockRootR(block, hashIdx, flagIdx, depth + 1); // right branch

            if (! UInt256Eq(hashes[0], hashes[1])) {
                if (UInt256IsZero(hashes[1])) hashes[1] = hashes[0]; // if right branch is missing, dup left branch
                BRSHA256_2(&md, hashes, sizeof(hashes));
            }
            else *hashIdx = SIZE_MAX; // defend against (CVE-2012-2459)
        }
        else md = block->hashes[(*hashIdx)++]; // leaf
    }
    
    return md;
}

// true if merkle tree and timestamp are valid, and proof-of-work matches the stated difficulty target
// NOTE: this only checks if the block difficulty matches the difficulty target in the header, it does not check if the
// target is correct for the block's height in the chain - use BRMerkleBlockVerifyDifficulty() for that
int BRMerkleBlockIsValid(const BRMerkleBlock *block, uint32_t currentTime)
{
    assert(block != NULL);
    
    // target is in "compact" format, where the most significant byte is the size of resulting value in bytes, the next
    // bit is the sign, and the remaining 23bits is the value after having been right shifted by (size - 3)*8 bits
    static const uint32_t maxsize = MAX_PROOF_OF_WORK >> 24, maxtarget = MAX_PROOF_OF_WORK & 0x00ffffff;
    const uint32_t size = block->target >> 24, target = block->target & 0x00ffffff;
    size_t hashIdx = 0, flagIdx = 0;
    UInt256 merkleRoot = _BRMerkleBlockRootR(block, &hashIdx, &flagIdx, 0), t = UINT256_ZERO;
    int r = 1;
    
    // check if merkle root is correct
    if (block->totalTx > 0 && ! UInt256Eq(merkleRoot, block->merkleRoot)) r = 0;
    
    // check if timestamp is too far in future
    if (block->timestamp > currentTime + BLOCK_MAX_TIME_DRIFT) r = 0;
    
    // check if proof-of-work target is out of range
    if (target == 0 || target & 0x00800000 || size > maxsize || (size == maxsize && target > maxtarget)) r = 0;
    
    if (size > 3) set_u32le(&t.u8[size - 3], target);
    else set_u32le(t.u8, target >> (3 - size)*8);
    
    for (int i = sizeof(t) - 1; r && i >= 0; i--) { // check proof-of-work
        if (block->blockHash.u8[i] < t.u8[i]) break;
        if (block->blockHash.u8[i] > t.u8[i]) r = 0;
    }
    
    return r;
}

// true if the given tx hash is known to be included in the block
int BRMerkleBlockContainsTxHash(const BRMerkleBlock *block, UInt256 txHash)
{
    int r = 0;
    
    assert(block != NULL);
    assert(! UInt256IsZero(txHash));
    
    for (size_t i = 0; ! r && i < block->hashesCount; i++) {
        if (UInt256Eq(block->hashes[i], txHash)) r = 1;
    }
    
    return r;
}

// verifies the block difficulty target is correct for the block's position in the chain
// transitionTime is the timestamp of the block at the previous difficulty transition
// transitionTime may be 0 if height is not a multiple of BLOCK_DIFFICULTY_INTERVAL
//
// The difficulty target algorithm works as follows:
// The target must be the same as in the previous block unless the block's height is a multiple of 2016. Every 2016
// blocks there is a difficulty transition where a new difficulty is calculated. The new target is the previous target
// multiplied by the time between the last transition block's timestamp and this one (in seconds), divided by the
// targeted time between transitions (14*24*60*60 seconds). If the new difficulty is more than 4x or less than 1/4 of
// the previous difficulty, the change is limited to either 4x or 1/4. There is also a minimum difficulty value
// intuitively named MAX_PROOF_OF_WORK... since larger values are less difficult.
int BRMerkleBlockVerifyDifficulty(const BRMerkleBlock *block, const BRMerkleBlock *previous, uint32_t transitionTime)
{
    int r = 1;
    
    assert(block != NULL);
    assert(previous != NULL);
    if (! previous || !UInt256Eq(block->prevBlock, previous->blockHash) || block->height != previous->height + 1) r = 0;
    if (r && (block->height % BLOCK_DIFFICULTY_INTERVAL) == 0 && transitionTime == 0) r = 0;
    
#if BITCOIN_TESTNET
    // TODO: implement testnet difficulty rule check
    return r; // don't worry about difficulty on testnet for now
#endif
    
    if (r && (block->height % BLOCK_DIFFICULTY_INTERVAL) == 0) {
        // target is in "compact" format, where the most significant byte is the size of resulting value in bytes, next
        // bit is the sign, and the remaining 23bits is the value after having been right shifted by (size - 3)*8 bits
        static const uint32_t maxsize = MAX_PROOF_OF_WORK >> 24, maxtarget = MAX_PROOF_OF_WORK & 0x00ffffff;
        int timespan = (int)((int64_t)previous->timestamp - (int64_t)transitionTime), size = previous->target >> 24;
        uint64_t target = previous->target & 0x00ffffff;
    
        // limit difficulty transition to -75% or +400%
        if (timespan < TARGET_TIMESPAN/4) timespan = TARGET_TIMESPAN/4;
        if (timespan > TARGET_TIMESPAN*4) timespan = TARGET_TIMESPAN*4;
    
        // TARGET_TIMESPAN happens to be a multiple of 256, and since timespan is at least TARGET_TIMESPAN/4, we don't
        // lose precision when target is multiplied by timespan and then divided by TARGET_TIMESPAN/256
        target *= timespan;
        target /= TARGET_TIMESPAN >> 8;
        size--; // decrement size since we only divided by TARGET_TIMESPAN/256
    
        while (size < 1 || target > 0x007fffff) target >>= 8, size++; // normalize target for "compact" format
    
        // limit to MAX_PROOF_OF_WORK
        if (size > maxsize || (size == maxsize && target > maxtarget)) target = maxtarget, size = maxsize;
    
        if (block->target != ((uint32_t)target | size << 24)) r = 0;
    }
    else if (r && block->target != previous->target) r = 0;
    
    return r;
}

// frees memory allocated by BRMerkleBlockParse
void BRMerkleBlockFree(BRMerkleBlock *block)
{
    assert(block != NULL);
    if (block->hashes) free(block->hashes);
    if (block->flags) free(block->flags);
    free(block);
}