EuCrypt Chapter 2: A Source of Randomness

December 14th, 2017 by Diana Coman

EuCrypt uses as source of randomness the Fuckgoats auditable TRNG (True Random Number Generator) from S.NSA (No Such lAbs). The choice here was made very easy by a basic combination of facts: on one hand, EuCrypt needs an actual, auditable source of randomness1 as opposed to anything else, pseudo-random generators included; on the other hand, the Fuckgoats (FG) device is the only currently available auditable TRNG. So problem solved and even rather narrowly solved at that. I'm quite grateful that for once there IS actually something matching the requirements and therefore I don't have to cobble it together myself from bits and pieces.

The above being said, you as the user of EuCrypt are *expected* to make your own decisions. Consequently, there really is nothing stopping you from using whatever you want as your own "source of randomness", be it actually random or pseudo-random or straight non-random or anything in between, why would EuCrypt care? Just change existing knobs in EuCrypt (see below) or directly replace the relevant methods (mainly get_random_octets, discussed below) with your own code and that's what will get used. For my own needs however, I'll use FG and moreover, one that I actually bought (even with some additional cost) and tested myself.

To keep this as clear as possible, let's start with the tiniest part, namely a single lonely knob for EuCrypt, giving the name of the entropy source to use and defined in knobs.h, as part of the smg_rsa component of EuCrypt (eucrypt/smg_rsa/include/knobs.h):


#define ENTROPY_SOURCE "/dev/ttyUSB0"

#endif /*SMG_RSA_KNOBS_H*/

As it is quite clear from above, current version of EuCrypt assumes that an FG is connected simply to an USB port that can be accessed via /dev/ttyUSB0. If the device dev path on your machine is different, change the value of this knob accordingly. If your FG is connected to something other than an USB port (such as a serial port for instance), you'll need to change more than this knob here.

Now for the signatures of the functions that will provide access to the specified source of randomness, have a look at smg_rsa.h:

/* smg_rsa.h
 * S.MG, 2017

#ifndef SMG_RSA_H
#define SMG_RSA_H

#include "mpi.h"
#include "knobs.h"


 * Opens and configures (as per FG requirements) the specified entropy source (e.g. "/dev/ttyUSB0")
 * @param source_name the name of the file to open (e.g. "/dev/ttyUSB0")
 * @return the descriptor of the open file when successful; negative value otherwise
int open_entropy_source(char* source_name);

 * Returns noctets random octets (i.e. 8*noctets bits in total) as obtained from EuCrypt's preferred source.
 * Preferred source is defined in knobs.h as ENTROPY_SOURCE and should be a TRNG (e.g. Fuckgoats).
 * @param nboctets the length of desired random sequence, in octets
 * @param out pointer to allocated memory space for the requested random noctets;
 * NB: this method does NOT allocate space!
 * @return the actual number of octets that were obtained from the currently configured entropy source
 * (this is equal to noctets on successful read of required noctets)
int get_random_octets(int noctets, unsigned char *out);

/* Returns noctets random octets as obtained from the specified "from" source;
 * NB: the "from" source is considered to be the handle of an already opened stream;
 * This method will simply attempt to read from the source as needed!
 * @param noctets the length of desired random sequence, in octets
 * @param out pointer to allocated memory space for the requested random octets;
 * NB: this method does NOT allocate space!
 * @param from handle of an already opened entropy source - this method will just READ from it as needed
 * @return the actual number of octets that were obtained
int get_random_octets_from(int noctets, unsigned char *out, int from);

#endif /*SMG_RSA*/

The difference between the two functions that retrieve a specified number of octets is that one opens and closes the source itself (hence, every time it is called) while the second one simply reads the specified number of bits from an already opened source that is given as argument. Note that the function opening and closing the source itself uses the other one for the actual reading. The reason for providing both functions is simply the fact that opening/closing the source can easily be a significant overhead when reading only a few octets at a time.

To configure and open the source, the function used is open_entropy_source. This function provides a way to obtain a handler of the opened source that can then be passed repeatedly to the function that retrieves octets, as needed.

One important aspect to note above is that the two functions that retrieve random octets do NOT allocate memory for the output. They assume that the caller has allocated enough memory and provided a valid pointer. The reason for this is two-fold: first, as a design principle I prefer to keep allocation and de-allocation of memory in the same function as much as possible without passing responsibility around; second, for RSA purpose, the memory allocation is often done with specific MPI methods and using those (or indeed being aware of them) is outside the scope of this bit of code as it has nothing at all to do with getting random bits.

The actual implementations of the above functions are found in truerandom.c. UPDATED on 4 January 2018: this code has been changed, see Chapter 4 and corresponding .vpatch!

#include < stdio.h>
#include < stdlib.h>
#include < string.h>

#include < fcntl.h>
#include < unistd.h>
#include < termios.h>
#include < errno.h>

#include "smg_rsa.h"

int set_usb_attribs(int fd, int speed) {
	struct termios tty;
	if (tcgetattr(fd, &tty) < 0) {
		return -1;

	//input and output speeds
	cfsetospeed(&tty, (speed_t)speed);
	cfsetispeed(&tty, (speed_t)speed);

	tty.c_cflag |= (CLOCAL | CREAD);	//ignore modem controls
	tty.c_cflag &= ~CSIZE;
	tty.c_cflag |= CS8;			//8 bit characters
	tty.c_cflag &= ~PARENB;	//no parity bit
	tty.c_cflag &= ~CSTOPB;	//only need 1 stop bit
	tty.c_cflag &= ~CRTSCTS;	//no hardware flow control

	//non-canonical mode
	tty.c_cflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
	tty.c_cflag &= ~OPOST;

	//read at least one octet at a time; timeout 1 tenth of second between octets read
	tty.c_cc[VMIN] = 1;
	tty.c_cc[VTIME] = 1;

	if (tcsetattr(fd, TCSANOW, &tty) != 0)
		return -1;

	return 0;

int open_entropy_source(char* source_name) {
	int in, err;

	in = open(source_name, O_RDONLY | O_NOCTTY | O_NDELAY);
	if (in == -1) {
		printf("ERROR: failure to open entropy source %s: %sn", source_name, strerror(errno));
		return in;	//failed to access entropy source

	fcntl(in, F_SETFL, 0);

	err = set_usb_attribs(in, B115200);
	if (err==-1) {
		printf("Error setting attributes on %s: %sn", source_name, strerror(errno));
		return err;

	return in;	//source opened, return its descriptor

int get_random_octets_from(int noctets, unsigned char *out, int from) {

	int nread;
	int total = 0;

	while (total < noctets) {
           nread = read(from, out+total, noctets-total);
           //on interrupt received just try again
           if (nread == -1 && errno == EINTR)
           //on error condition abort
           if (nread == -1 || nread == 0) {
             printf("Error reading from entropy source %s: %sn", ENTROPY_SOURCE, strerror(errno));
             return total; //total read so far
           if (nread > 0)
		total = total + nread;
	return total;	//return number of octets read

int get_random_octets(int noctets, unsigned char *out) {
	int in;
	int nread = 0;

	in = open_entropy_source(ENTROPY_SOURCE);
	if (in > 0) {
		nread = get_random_octets_from(noctets, out, in);
	return nread;

As it can be seen above, a significant part of the code is simply for configuring the device. Most importantly, the configuration aims to turn OFF all flow control and to set the baud rate as required by FG. While this should work under most versions of Linux, be aware of the known pl2303 vs pl2303x issue with some connectors on older systems.

Note that an incorrectly configured device will simply block and since the functions above are written to always wait for the full number of bits required, they will *also* block in this case.

Finally, a basic test in tests/test.c:

#include "smg_rsa.h"

#include < stdlib.h>
#include < time.h>

void err(char *msg)
  fprintf(stderr, "%sn", msg);

void time_entropy_source(int nruns, int noctets) {
	unsigned char buffer[noctets];
	int read, i;
	struct timespec tstart, tend;
	long int diff;

	clock_gettime(CLOCK_MONOTONIC, &tstart);
	for (i=0; i < nruns; i++) {
		read = get_random_octets(noctets,buffer);
		if (read != noctets)
			err("Failed reading from entropy source!");
	clock_gettime(CLOCK_MONOTONIC, &tend);

	diff = tend.tv_sec-tstart.tv_sec;
	double kbps = (nruns*noctets) / (diff*1000.0);
	printf("ENTROPY source timing: %d kB in %ld seconds, at an average speed of %f kB/s over %d runs of %d octets eachn", nruns*noctets, diff, kbps, nruns, noctets);

int main(int ac, char **av)
	int nruns;

	if (ac<2) {
		printf("Usage: %s number_of_runsn", av[0]);
		return -1;
	nruns = atoi(av[1]);

	printf("Timing entropy source...n");

  return 0;

For testing, simply plug into an USB port your (previously audited, hopefully) FG, compile everything and then run the test with as many runs as you want. When it's done (so after a while, depending on how many runs you asked for), it should print on screen the speed at which it obtained the random bits from FG.

Following the sad realisation that I can't currently safely alter folder structure under V, I created a genesis patch for EuCrypt containing the intended structure (more like: writing in stone the intended structure) and it's on top of this that each chapter of EuCrypt adds new content by means of vpatches. Here you have everything you need so far:

Note that the patch in Chapter 1 is NOT needed anymore directly for EuCrypt (it still is valid though in itself and as a further snip on the standalone mpi so I will keep it where it is). The changes that patch makes are already included in the version of mpi that ch1_mpi.vpatch simply bring into EuCrypt.

In the next chapter, since we have already an MPI implementation as well as a way to access true randomness, we can get ever so closer to the actual RSA itself, so stay tuned!

  1. I'll likely expand on this as soon as I get to the actual implementation of RSA so in the next few chapters. 

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3 Responses to “EuCrypt Chapter 2: A Source of Randomness”

  1. On my boxes, I do the port init outside (in init script, at boot time) but can't think of any particular reason not to do it in the proggy like this.

    You might want to at some point write a GB of entropy read via this proggy to disk, and do the traditional audits on it.

  2. Diana Coman says:

    The outside init script makes sense from a system / user perspective for sure, good point. However, from the point of view of eucrypt itself, there is a case for init in the code too (at the very least a matter of init before open before read so basically part of the code's responsibility). No clash though: can happily have both of course.

    Such an audit is on the ever-growing list of things to do, yes.

  3. […] is a sad but necessary interruption in the EuCrypt series itself: although coming immediately after chapter 2, this is not chapter 3 at all, I’m sorry to say. Instead of adding another useful part of […]

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