Crypto libraries
introduction
Milan Brož
xbroz@fi.muni.cz
PV181, FI MUNI, Brno
Open source
cryptographic libraries
 Linux environment (with OpenSSL3) – up to you:
 ssh to aisa.fi.muni.cz
 Debian / VirtualBox VM (see course materials)
 Your own distro – need to install development env.:
●
libgcrypt: Fedora: libgcrypt-devel; Debian/Ubuntu: libgcrypt20-dev
●
OpenSSL:Fedora: openssl-devel; Debian/Ubuntu: libssl-dev
●
libsodium:Fedora: libsodium-devel; Debian/Ubuntu: libsodium-dev
 All examples in C language
 We will use only free open-source tools and libraries
 2x Home assignments (10 points each)
Lab environment, git and
VirtualBox image (optional)
 Optional VM install
– Unpack zip archive from IS
– Open VirtualBox (click blue icon – config file)
– Login and password is pv181
(same for sudo and root password)
– In pc181 home is a script to clone examples
 Examples on gitlab (always git pull for updates)
git clone https://gitlab.fi.muni.cz/xbroz/pv181.git
make clean; make; ./example
 Check that you can compile and run examples
1_rng_gcrypt, 1_rng_openssl, 1_rng_sodium
Cryptographic libraries
Goals for this lab
 Crypto libraries and API / abstraction
 More practical and implementation view
 Why legacy code, compatibility and standards
 Coding practices – in C language
 Defensive approach: It will fail, be prepared for it :-)
Why not use a modern language with garbage collection and functional
programming and free massages after lunch?
Here’s the answer: Pointers are real. They’re what the hardware understands.
Somebody has to deal with them.
You can’t just place a LISP book on top of an x86 chip and hope that the
hardware learns about lambda calculus by osmosis.
- James Mickens, https://www.usenix.org/system/files/1311_05-08_mickens.pdf
Why implementation matters
 It works, but …
 How many possible bugs do you see?
/* Read a key from Linux RNG */
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
int main(int argc, char *argv[])
{
int fd;
char key[32];
fd = open("/dev/random", O_RDONLY);
read(fd, key, 32);
close(fd);
/* Do something with the key[] */
memset(key, 0, 32);
return 0;
}
Why implementation matters
 How many possible bugs do you see?
– No check for return code, open(), read()
– Posible reading from invalid fd (no random at all)
– Partial read() is not detected
– Failed read() is not detected
(mandatory access control can block reading)
– Magic numbers (one constant on several places)
– Compiler can optimize memset() out
(secret key remains in memory)
– No error exit code, cannot check for failure
Why implementation matters
 Fixes? Let’s see example 0 in git.
 It is better to use a crypto library.
 Usually, maintainers implement it correctly :-)
https://xkcd.com/221/
Secure implementation notes
 C compilers can do many checks
– Use -Wall option and do not ignore warnings
– non-default warnings options
 User opensource static and dynamic code analyzers
– clang scan-build
– gcc -fanalyzer options
– valgrind
– cppcheck
 Fuzzing can be very powerfull
 Code review (it requires some skills)
Practically oriented books
 Jean-Phillipe Aumasson
Serious Cryptography:
A Practical Introduction
to Modern Encryption (2017)
 Ferguson, Schneier, Kohno
Cryptography Engineering:
Design Principles and Practical
Applications (2010)
 David Wong
Real-World Cryptography (2021)
Cryptographic libraries
Introduction
 Open-source / Proprietary
 Static + embedded / dynamically linked
 Low or high level abstractions
 Multiplatform
 Stable API and ABI
 Policy (approved algorithms)
 Security or platform specific features
 Safe memory use, side-channel resistance, …
 HW acceleration support, “secure” HW support
Crypto libraries – algorithms
 Random Number Generator (RNG) access
 Hash, keyed-hash (HMAC, msg authentication)
 Symmetric ciphers and modes
 Asymmetric ciphers
 Certificate support, ASN.1, ...
 Key exchange, key derivation
 Helpers
 secure memory
 safe comparison
 network / sockets
 plugin support (like OpenSSL3 providers)
 ...
Example libs (C and Linux)
abstraction from low to high
 Nettle
 libgcrypt
 OpenSSL / OpenSSL3
 LibreSSL (clone), BoringSSL (Google)
 NSS
 Network Security Services (Mozilla)
 NaCl ("salt")
 more common as libsodium
Examples in gcrypt, OpenSSL / OpenSSL3 and libsodium