Andrew Boyson
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.
Diff: login/http-login-session-id.c
- Revision:
- 81:4551f2e0e79b
- Parent:
- 80:9ea202546e7f
--- a/login/http-login-session-id.c Mon Mar 25 10:24:29 2019 +0000
+++ b/login/http-login-session-id.c Mon Mar 25 15:32:06 2019 +0000
@@ -3,32 +3,48 @@
#include "random.h"
-#define SESSION_ID_LENGTH 16 //Make sure this is no more than half the available random length
+#define SESSION_ID_BIT_LENGTH 36
-static char sessionIdString[SESSION_ID_LENGTH + 1]; //Initialised to an empty string but will never be NULL.
+static char sessionId[(SESSION_ID_BIT_LENGTH + 5) / 6 + 1]; //Bit lengths not divisible by 6 require an extra space
void HttpLoginSessionIdNew()
{
- char* p = sessionIdString;
+ char acc = 0;
- for (int i = 0; i < SESSION_ID_LENGTH / 2 && i < RANDOM_LENGTH; i++) //Don't exceed either random or sid lengths
+ for (int i = 0; i < SESSION_ID_BIT_LENGTH; i++)
{
- int nibble;
+ 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;
- nibble = RandomBytes[i] & 0x0F; //low nibble
- *p++ = nibble < 10 ? nibble + '0': nibble - 0xA + 'a';
+ //Add the bit to the accumulator
+ if (RandomBytes[srcByte] & srcMask) acc |= dstMask;
- nibble = RandomBytes[i] >> 4; //high nibble
- *p++ = nibble < 10 ? nibble + '0': nibble - 0xA + 'a';
+ //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] = '/';
+ }
}
-
- *p = 0;
+ sessionId[(SESSION_ID_BIT_LENGTH + 5) / 6] = 0;
}
+
bool HttpLoginSessionIdIsSet()
{
- return sessionIdString[0];
+ return sessionId[0];
}
char* HttpLoginSessionIdGet()
{
- return sessionIdString;
+ return sessionId;
}