Wrong VARINT encoding for Transaction Count field in Blocktemplate

[DeckerSU]

We have wrong varint encoding here https://github.com/z-classic/node-stratum-pool/blob/master/lib/blockTemplate.js#L91 for transaction count in block. Let's see results of default implementation:

1. this.txCount = 15, varInt = 0f
2. this.txCount = 127, varInt = 7f
3. this.txCount = 128, varInt = fd80
4. this.txCount = 256, varInt = fd0100
5. this.txCount = 777, varInt = fd0309

This is totally wrong for cases 3-5 and block with this encoding in Transaction Count field will be rejected with block decode failed error. Should be:

1. this.txCount = 15, varInt = 0f
2. this.txCount = 127, varInt = 7f
3. this.txCount = 128, varInt = fd8000
4. this.txCount = 256, varInt = fd0001
5. this.txCount = 777, varInt = fd0903

Here is a fix:

if (this.txCount <= 0x7f){
    var varInt = new Buffer(txCount, 'hex');
}
else if (this.txCount <= 0x7fff){
    if (txCount.length == 2) txCount = "00" + txCount;
    var varInt = new Buffer.concat([Buffer('FD', 'hex'), util.reverseBuffer(new Buffer(txCount, 'hex'))]);
}

[Alrighttt]

This should be considered a security related bug. One can prevent a pool from mining blocks by making the mempool size between 129-255

[DeckerSU]

Updated fix:

        /* https://en.bitcoin.it/wiki/Protocol_documentation#Variable_length_integer */
        if (this.txCount < 0xfd){
            var varInt = new Buffer(txCount, 'hex');
        }
        else if (this.txCount <= 0xffff){
            var varInt = new Buffer.concat([Buffer('FD', 'hex'), util.reverseBuffer(new Buffer(txCount, 'hex'))]);
        }

Example:

this.txCount = 183, varInt = b7 
this.txCount = 1344, varInt = fd4005

Format of VARINT in Bitcoin explained here - https://en.bitcoin.it/wiki/Protocol_documentation#Variable_length_integer .