Andrew Boyson / web

Common stuff for all my devices' web server pages: css, login, log, ipv4, ipv6, firmware update, clock, reset info etc.

Dependents:   oldheating gps motorhome heating

Security

A password has to be set whenever there has been a software reset. Resets following faults or power on do not require a new password as the hash is restored from the RTC GPREG register.

The password is not saved on the device; instead a 32 bit hash of the password is saved. It would take 2^31 attempts to brute force the password: this could be done in under a month if an attempt were possible every millisecond. To prevent this a 200 ms delay is introduced in the reply to the login form, that gives a more reasonable 13 years to brute force the password.

Once the password is accepted a random session id is created. This is 36 bit to give six base 64 characters but without an extra delay. If an attempt could be made every ms then this would still take over a year to brute force.

The most likely attack would to use a dictionary with, say, 10 million entries against the password which would still take 20 days to do.

base/login/web-login-session-id.c

Committer:
andrewboyson
Date:
2019-07-31
Revision:
127:bd6dd135009d
Parent:
126:6b547c86da6e

File content as of revision 127:bd6dd135009d:

#include <stdbool.h>
#include <stdint.h>

#include "random.h"

#define SESSION_ID_BIT_LENGTH 36

static char sessionId[(SESSION_ID_BIT_LENGTH + 5) / 6 + 1]; //Bit lengths not divisible by 6 require an extra space

void WebLoginSessionIdNew()
{
    char acc = 0;
    int srcByteBefore = -1;
    uint8_t randomByte = 0;
    
    for (int i = 0; i < SESSION_ID_BIT_LENGTH; i++)
    {
        int     srcByte = i / 8; 
        int     srcBit  = i - srcByte * 8;
        uint8_t srcMask = 1 << srcBit;
        
        int     dstByte = i / 6;
        int     dstBit  = i - dstByte * 6;
        uint8_t dstMask = 1 << dstBit;
        
        //Reset the accumulator to zero at start of a 6 bit word
        if (!dstBit) acc = 0;
        
        //Fetch a new random byte whenever the src byte changes
        if (srcByte != srcByteBefore) randomByte = RandomGetByte();
        srcByteBefore = srcByte;
        
        //Add the bit to the accumulator
        if (randomByte & srcMask) acc |= dstMask;
        
        //Convert the accumulator to base64 and store in the session Id
        if (dstBit == 5 || i == SESSION_ID_BIT_LENGTH - 1)
        {
            if      (acc < 26) sessionId[dstByte] = acc -  0 + 'A';
            else if (acc < 52) sessionId[dstByte] = acc - 26 + 'a';
            else if (acc < 62) sessionId[dstByte] = acc - 52 + '0';
            else if (acc < 63) sessionId[dstByte] =            '+';
            else               sessionId[dstByte] =            '/';
        }
    }
    sessionId[(SESSION_ID_BIT_LENGTH + 5) / 6] = 0;
}

bool WebLoginSessionIdIsSet()
{
    return sessionId[0];
}
void WebLoginSessionIdReset()
{
    sessionId[0] = 0;
}
char* WebLoginSessionIdGet()
{   
    return sessionId;
}