Differences between revisions 10 and 12 (spanning 2 versions)
Revision 10 as of 2017-01-31 11:00:57
Size: 3520
Comment: Embedded code copies
Revision 12 as of 2017-01-31 13:01:53
Size: 3769
Comment:
Deletions are marked like this. Additions are marked like this.
Line 38: Line 38:
OpenBSD bundles implementations of arc4random, getentropy etc. in their 'portable' releases of: LibreSSL, OpenNTPd, OpenSMTPd, OpenSSH, signify. We should be consult with them before changing anything. OpenBSD bundles implementations of arc4random, getentropy etc. in their 'portable' releases of: LibreSSL, OpenNTPd, OpenSMTPd, OpenSSH, signify. We should be consult with them before changing anything there.
Line 58: Line 58:
2. Some implementations don't re-seed reliably (or perhaps at all) when a process forks. 2. Some implementations don't re-seed reliably (or perhaps at all) when a process forks. MAP_INHERIT_ZERO is reliable, but only available on OpenBSD. atfork functions are the best thing available on most other platforms. getpid is slowest (requiring a syscall on every iteration), and there are edge cases where it doesn't work:

arc4random(3) is random-number-generating library function, first available on BSD platforms, and also on GNU/Linux with libbsd.

Manual pages

http://man.openbsd.org/?query=arc4random

https://www.freebsd.org/cgi/man.cgi?query=arc4random

https://manpages.debian.org/cgi-bin/man.cgi?query=arc4random

Implementations

arc4random(3) initialises itself with some random bytes from the kernel, but rather than returning those bytes directly, a stream cipher is used to produce longer streams of output. Re-seeding happens occasionally thereafter. The intention was to avoid the overhead of making a system call, on each call to arc4random(3)

The function when originally implemented in OpenBSD, used the RC4 cipher to produce the output stream. OpenBSD now uses a ChaCha20 stream cipher.

If a process forks, it should re-seed itself to avoid each process sharing the same RNG state. OpenBSD uses the mmap(2) INHERIT_ZERO flag to reliably detect forks, and then re-seed.

FreeBSD makes a system call to getpid(2) to detect forking, but that means the overhead of its arc4random(3) may be worse than if the RC4 cipher wasn't being used there at all...

https://lists.freebsd.org/pipermail/freebsd-security/2014-July/007849.html

FreeBSD's libc still uses RC4 libkern/arc4random.c, but discards the first 1024 bytes of the stream cipher's output, to try to mitigate known weaknesses in the cipher.

https://svnweb.freebsd.org/base/head/lib/libc/gen/arc4random.c?view=markup

and is also used within the kernel:

https://svnweb.freebsd.org/base/head/sys/libkern/arc4random.c?view=markup

libbsd provides an arc4random(3), previously using RC4 but this was changed to ChaCha20 in version 0.8.0 (Debian package version 0.8.0-1). GNU libc's atfork can be used to reliably detect forking.

GNU/kFreeBSD provides only the libbsd implementation.

Embedded code copies

OpenBSD bundles implementations of arc4random, getentropy etc. in their 'portable' releases of: LibreSSL, OpenNTPd, OpenSMTPd, OpenSSH, signify. We should be consult with them before changing anything there.

Embedded copies of this code can also be found in:

  • atheme-services - uses ChaCha20, suffers issues 2, 3 described below

  • getdns - uses ChaCha20, suffers issue 2 described below

  • ncrack - uses ChaCha20, suffers issue 2 described below

  • newlib - uses ChaCha20; unclear how it detects forking

  • pureftpd - uses ChaCha20, suffers issues 2, 3 described below

  • unbound - uses ChaCha20, suffers issue 2 described below; FTBFS on kfreebsd, hurd because getentropy implementations are missing

Similar code to arc4random(3) can also be found in libevent.

Issues

1. RC4 is not considered to be a cryptographically strong cipher any more.

1a. Some implementations using RC4, discard the first 1KByte of more of the keystream as a possible countermeasure, but some did not even do this.

2. Some implementations don't re-seed reliably (or perhaps at all) when a process forks. MAP_INHERIT_ZERO is reliable, but only available on OpenBSD. atfork functions are the best thing available on most other platforms. getpid is slowest (requiring a syscall on every iteration), and there are edge cases where it doesn't work:

https://www.agwa.name/blog/post/libressls_prng_is_unsafe_on_linux

3. Some methods of seeding are unsafe, and failures cannot be indicated to the caller. In a chroot/jail environment, perhaps /dev/urandom is missing. Or if all file descriptors are exhausted, nothing can be read from there. OpenBSD and FreeBSD provide system calls that are guaranteed to not fail.