getdns/src/pubkey-pinning.c

/**
 *
 * /brief functions for Public Key Pinning
 *
 */

/*
 * Copyright (c) 2015, Daniel Kahn Gillmor
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 * * Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * * Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 * * Neither the names of the copyright holders nor the
 *   names of its contributors may be used to endorse or promote products
 *   derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL Verisign, Inc. BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/**
 * getdns Public Key Pinning
 * 
 * a public key pinset is a list of dicts.  each dict should have a
 * "digest" and a "value".
 * 
 * "digest": a string indicating the type of digest. at the moment, we
 *           only support a "digest" of "sha256".
 * 
 * "value": a binary representation of the digest provided.
 * 
 * given a such a pinset, we should be able to validate a chain
 * properly according to section 2.6 of RFC 7469.
 */
#include <getdns/getdns.h>
#include <openssl/evp.h>
#include <openssl/bio.h>
#include <openssl/sha.h>
#include <string.h>

/* we only support sha256 at the moment.  adding support for another
   digest is more complex than just adding another entry here. in
   particular, you'll probably need a match for a particular cert
   against all supported algorithms.  better to wait on doing that
   until it is a better-understood problem (i.e. wait until hpkp is
   updated and follow the guidance in rfc7469bis)
*/

static const getdns_bindata sha256 = {
	.size = sizeof("sha256") - 1,
	.data = (uint8_t*)"sha256"
};
  

#define PIN_PREFIX "pin-sha256=\""
#define PIN_PREFIX_LENGTH (sizeof(PIN_PREFIX) - 1)
/* b64 turns every 3 octets (or fraction thereof) into 4 octets */
#define B64_ENCODED_SHA256_LENGTH (((SHA256_DIGEST_LENGTH + 2)/3)  * 4)

/* convert an HPKP-style pin description to an appropriate getdns data
   structure.  An example string is: (with the quotes, without any
   leading or trailing whitespace):

      pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="

   getdns_build_pin_from_string returns a dict created from ctx, or
   NULL if the string did not match.  If ctx is NULL, the dict is
   created via getdns_dict_create().

   It is the caller's responsibility to call getdns_dict_destroy when
   it is no longer needed.
 */
getdns_dict* getdns_pubkey_pin_create_from_string(
	getdns_context* context,
	const char* str)
{
	BIO *bio = NULL;
	int i;
	uint8_t buf[SHA256_DIGEST_LENGTH];
	char inbuf[B64_ENCODED_SHA256_LENGTH + 1];
	getdns_bindata value = { .size = SHA256_DIGEST_LENGTH, .data = buf };
	getdns_dict* out = NULL;
	
	/* we only do sha256 right now, make sure this is well-formed */
	if (strncmp(PIN_PREFIX, str, PIN_PREFIX_LENGTH))
		return NULL;
	for (i = PIN_PREFIX_LENGTH; i < PIN_PREFIX_LENGTH + B64_ENCODED_SHA256_LENGTH - 1; i++)
		if (!((str[i] >= 'a' && str[i] <= 'z') ||
		      (str[i] >= 'A' && str[i] <= 'Z') ||
		      (str[i] >= '0' && str[i] <= '9') ||
		      (str[i] == '+') || (str[i] == '/')))
			return NULL;
	if (str[i++] != '=')
		return NULL;
	if (str[i++] != '"')
		return NULL;
	if (str[i++] != '\0')
		return NULL;

	/* openssl needs a trailing newline to base64 decode */
	memcpy(inbuf, str + PIN_PREFIX_LENGTH, B64_ENCODED_SHA256_LENGTH);
	inbuf[B64_ENCODED_SHA256_LENGTH] = '\n';
	
	bio = BIO_push(BIO_new(BIO_f_base64()),
		       BIO_new_mem_buf(inbuf, sizeof(inbuf)));
	if (BIO_read(bio, buf, sizeof(buf)) != sizeof(buf))
		goto fail;
	
	if (context)
		out = getdns_dict_create_with_context(context);
	else
		out = getdns_dict_create();
	if (out == NULL)
		goto fail;
	if (getdns_dict_set_bindata(out, "digest", &sha256))
		goto fail;
	if (getdns_dict_set_bindata(out, "value", &value))
		goto fail;
	return out;

 fail:
	BIO_free_all(bio);
	getdns_dict_destroy(out);
	return NULL;
}


/* Test whether a given pinset is reasonable, including:

 * is it well-formed?
 * are there at least two pins?
 * are the digests used sane?

   if errorlist is NULL, the sanity check just returns success or
   failure.

   if errorlist is not NULL, we append human-readable strings to
   report the errors.
*/

#define PKP_SC_ERR(e) { \
       err.size = sizeof(e); \
       err.data = (uint8_t*)e; \
       if (errorlist) \
	       getdns_list_set_bindata(errorlist, \
				       preverrs + errorcount, &err); \
       errorcount++; \
	}
#define PKP_SC_HARDERR(e, val) { \
		PKP_SC_ERR(e); return val; \
	}
getdns_return_t getdns_pubkey_pinset_sanity_check(
	const getdns_list* pinset,
	getdns_list* errorlist)
{
	size_t errorcount = 0, preverrs = 0, pins = 0, i;
	getdns_bindata err;
	getdns_dict * pin;
	getdns_bindata * data;

	if (errorlist)
		if (getdns_list_get_length(errorlist, &preverrs))
			return GETDNS_RETURN_INVALID_PARAMETER;
	
	if (getdns_list_get_length(pinset, &pins))
		PKP_SC_HARDERR("Can't get length of pinset",
			       GETDNS_RETURN_INVALID_PARAMETER);
	if (pins < 2)
		PKP_SC_ERR("This pinset has fewer than 2 pins");
	for (i = 0; i < pins; i++)
	{
		/* is it a dict? */
		if (getdns_list_get_dict(pinset, i, &pin)) {
			PKP_SC_ERR("Could not retrieve a pin");
		} else {
		/* does the pin have the right digest type? */
			if (getdns_dict_get_bindata(pin, "digest", &data)) {
				PKP_SC_ERR("Pin has no 'digest' entry");
			} else {
				if (data->size != sha256.size ||
				    memcmp(data->data, sha256.data, sha256.size))
					PKP_SC_ERR("Pin has 'digest' other than sha256");
			}
			/* if it does, is the value the right length? */
			if (getdns_dict_get_bindata(pin, "value", &data)) {
				PKP_SC_ERR("Pin has no 'value' entry");
			} else {
				if (data->size != SHA256_DIGEST_LENGTH)
					PKP_SC_ERR("Pin has the wrong size 'value' (should be 32 octets for sha256)");
			}
			
		/* should we choke if it has some other key? for
		 * extensibility, we will not treat this as an
		 * error.*/
		}
	}
	
	if (errorcount > 0)
		return GETDNS_RETURN_GENERIC_ERROR;
	return GETDNS_RETURN_GOOD;
}
/* pubkey-pinning.c */