Use arc4random(3) as system rng on OpenBSD.

OpenBSD provides the arc4random(3) function in libc for user land
programs that need good random data.  Use this to implement the
Botan system random number generator.  It has the advantage over
/dev/urandom that it works without file descriptors and in chroot(2)
environment.  Internally libc is currently using a ChaCha20 cipher
as PRNG and getentropy(2) to reseed itself automatically.

2 files changed, 16 insertions, 0 deletions

diff --git a/src/build-data/os/openbsd.txt b/src/build-data/os/openbsd.txt
index 89d291ddc..632879381 100644
--- a/src/build-data/os/openbsd.txt
+++ b/src/build-data/os/openbsd.txt
@@ -5,6 +5,7 @@ soname_pattern_abi   "libbotan-{version_major}.so.{abi_rev}"
 soname_pattern_patch "libbotan-{version_major}.so.{abi_rev}.{version_minor}"
 
 <target_features>
+arc4random
 clock_gettime
 gettimeofday
 posix_mlock
diff --git a/src/lib/rng/system_rng/system_rng.cpp b/src/lib/rng/system_rng/system_rng.cpp
index 12b087661..ed0cc3d2e 100644
--- a/src/lib/rng/system_rng/system_rng.cpp
+++ b/src/lib/rng/system_rng/system_rng.cpp
@@ -6,6 +6,7 @@
 */
 
 #include <botan/system_rng.h>
+#include <botan/build.h>
 
 #if defined(BOTAN_TARGET_OS_HAS_CRYPTGENRANDOM)
 
@@ -13,6 +14,10 @@
 #define NOMINMAX 1
 #include <wincrypt.h>
 
+#elif defined(BOTAN_TARGET_OS_HAS_ARC4RANDOM)
+
+#include <stdlib.h>
+
 #else
 
 #include <sys/types.h>
@@ -55,6 +60,8 @@ std::string System_RNG_Impl::name() const
    {
 #if defined(BOTAN_TARGET_OS_HAS_CRYPTGENRANDOM)
    return "cryptoapi";
+#elif defined(BOTAN_TARGET_OS_HAS_ARC4RANDOM)
+   return "arc4random";
 #else
    return BOTAN_SYSTEM_RNG_DEVICE;
 #endif
@@ -67,6 +74,8 @@ System_RNG_Impl::System_RNG_Impl()
    if(!CryptAcquireContext(&m_prov, 0, 0, BOTAN_SYSTEM_RNG_CRYPTOAPI_PROV_TYPE, CRYPT_VERIFYCONTEXT))
       throw Exception("System_RNG failed to acquire crypto provider");
 
+#elif defined(BOTAN_TARGET_OS_HAS_ARC4RANDOM)
+   // Nothing to do, arc4random(3) works from the beginning.
 #else
 
 #ifndef O_NOCTTY
@@ -89,6 +98,8 @@ System_RNG_Impl::~System_RNG_Impl()
    {
 #if defined(BOTAN_TARGET_OS_HAS_CRYPTGENRANDOM)
    ::CryptReleaseContext(m_prov, 0);
+#elif defined(BOTAN_TARGET_OS_HAS_ARC4RANDOM)
+   // Nothing to do.
 #else
    ::close(m_fd);
    m_fd = -1;
@@ -117,6 +128,8 @@ void System_RNG_Impl::add_entropy(const uint8_t input[], size_t len)
       secure_vector<uint8_t> buf(input, input + len);
       ::CryptGenRandom(m_prov, static_cast<DWORD>(buf.size()), buf.data());
       }
+#elif defined(BOTAN_TARGET_OS_HAS_ARC4RANDOM)
+   // arc4random(3) reseeds itself from the OpenBSD kernel, not from user land.
 #else
    while(len)
       {
@@ -156,6 +169,8 @@ void System_RNG_Impl::randomize(uint8_t buf[], size_t len)
    {
 #if defined(BOTAN_TARGET_OS_HAS_CRYPTGENRANDOM)
    ::CryptGenRandom(m_prov, static_cast<DWORD>(len), buf);
+#elif defined(BOTAN_TARGET_OS_HAS_ARC4RANDOM)
+   ::arc4random_buf(buf, len);
 #else
    while(len)
       {