RDRAND (previously known as Bull Mountain) is an instruction for returning random numbers from an Intel on-chip hardware random number generator which has been seeded by an on-chip entropy source. RDRAND is available in Ivy Bridge processors and is part of the Intel 64 and IA-32 instruction set architectures. AMD added support for the instruction in June 2015.
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The random number generator is compliant with security and cryptographic standards such as NIST SP 800-90A, FIPS 140-2, and ANSI X9.82. Intel also requested Cryptography Research Inc. to review the random number generator in 1999 and 2012, which resulted in two published papers: The Intel Random Number Generator in 1999, and Analysis of Intel's Ivy Bridge Digital Random Number Generator in 2012.
RDSEED is similar to RDRAND and provides higher level access to the entropy hardware. The RDSEED generator and processor instruction rdseed are available with Intel Broadwell CPUs and AMD Zen CPUs.
Overview
The CPUID instruction can be used to check whether the central processing unit (CPU) supports the RDRAND instruction on both AMD and Intel CPUs. If supported, bit 30 of the ECX register is set after calling CPUID standard function 01H. AMD processors are checked for the feature using the same test. RDSEED availability can be checked on Intel CPUs in a similar manner. If RDSEED is supported, the bit 18 of the EBX register is set after calling CPUID standard function 07H.
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The opcode for RDRAND is 0x0F 0xC7, followed by a ModRM byte that specifies the destination register and optionally combined with a REX prefix in 64 bit mode.
Intel Secure Key is Intel's name for both the RDRAND instruction and the underlying random number generator (RNG) hardware implementation, which was codenamed "Bull Mountain" during development. Intel calls their RNG a "digital random number generator" or DRNG. The generator takes pairs of 256-bit raw entropy samples generated by the hardware entropy source and applies them to an Advanced Encryption Standard (AES) (in CBC-MAC mode) conditioner which reduces them to a single 256-bit conditioned entropy sample. A deterministic random-bit generator called CTR_DRBG defined in NIST SP 800-90A is seeded by the output from the conditioner, providing cryptographically secure random numbers to applications requesting them via the RDRAND instruction. The hardware will issue a maximum of 511 128-bit samples before changing the seed value. Using the RDSEED operation provides access to the conditioned 256-bit samples from the AES-CBC-MAC.
The RDSEED instruction was added to Intel Secure Key for seeding another pseudorandom number generator, available in Broadwell CPUs. The entropy source for the RDSEED instruction runs asynchronously on a self-timed circuit and uses thermal noise within the silicon to output a random stream of bits at the rate of 3 GHz, slower than the effective 6.4Gbit/s obtainable from RDRAND (both rates are shared between all cores and threads). The RDSEED instruction is intended for seeding a software PRNG of arbitrary width, whereas the RDRAND is intended for applications that merely require high-quality random numbers. If cryptographic security is not required, a software PRNG such as Xorshift is usually faster.
Compilers
GCC 4.6+ and Clang 3.2+ provide intrinsic functions for RdRand when -mrdrnd is specified in the flags, also setting __RDRND__ to allow conditional compilation. Newer versions additionally provide immintrin.h to wrap these built-ins into functions compatible with version 12.1+ of Intel's C Compiler. These functions write random data to the location pointed to by their parameter, and return 1 on success.
Reception
In September 2013, in response to a New York Times article revealing the NSA's effort to weaken encryption, Theodore Ts'o publicly posted concerning the use of RdRand for /dev/random in the Linux kernel:
I am so glad I resisted pressure from Intel engineers to let /dev/random rely only on the RDRAND instruction. To quote from the article below: 'By this year, the Sigint Enabling Project had found ways inside some of the encryption chips that scramble information for businesses and governments, either by working with chipmakers to insert back doors....' Relying solely on the hardware random number generator which is using an implementation sealed inside a chip which is impossible to audit is a BAD idea.
Linus Torvalds dismissed concerns about the use of RdRand in the Linux kernel, and pointed out that it is not used as the only source of entropy for /dev/random, but rather used to improve the entropy by combining the values received from RdRand with other sources of randomness. However, Taylor Hornby of Defuse Security demonstrated that the Linux random number generator could become insecure if a backdoor is introduced into the RdRand instruction that specifically targets the code using it. Taylor's proof-of-concept implementation works on an unmodified Linux kernel prior to version 3.13.
Developers changed the FreeBSD kernel away from using RdRand and VIA PadLock directly with the comment "For [FreeBSD] 10, we are going to backtrack and remove RDRAND and Padlock backends and feed them into Yarrow instead of delivering their output directly to /dev/random. It will still be possible to access hardware random number generators, that is, RDRAND, Padlock etc., directly by inline assembly or by using OpenSSL from userland, if required, but we cannot trust them any more"