Can AES instructions be used independently of RDRAND for random number generation in OpenSSL

AES-CTR PRNG in OpenSSL

OpenSSL provides a way to implement a PRNG using AES in Counter (CTR) mode. This approach involves encrypting a counter with AES to generate pseudorandom data. The CTR mode is particularly suitable because it does not require padding and can produce a continuous stream of pseudorandom bytes.

To implement an AES-CTR PRNG in OpenSSL, you would typically follow these steps:

1. Initialization: Initialize an AES context using `EVP_CIPHER_CTX_new` and set it to use AES in CTR mode. You need to provide a key and an initial counter value.

2. Generating Pseudorandom Data: Use `EVP_EncryptUpdate` to encrypt the counter with AES, incrementing the counter after each call to generate new pseudorandom data. This process can be repeated to produce as much random data as needed.

3. Seeding: While AES itself does not provide randomness, the initial key and counter values must be securely seeded to ensure the generated data is unpredictable. This seeding can be done using a cryptographically secure pseudorandom number generator (CSPRNG), such as the one provided by OpenSSL's `RAND_bytes` function or another trusted entropy source.

Here is an example of how you might implement this in C:

c
#include 
#include 
#include 

void aes_ctr_prng_genrand_uint128_to_buf(aes_ctr_state_t* state, unsigned char* bufpos) {
    int outlen;
    EVP_EncryptUpdate(state->ctx, bufpos, &outlen, bufpos, 16);
    // Increment the counter for the next call
    // This step is crucial but not shown here for brevity
}

int main() {
    EVP_CIPHER_CTX* ctx;
    unsigned char key[16]; // AES-128 key
    unsigned char iv[16];  // Initial counter value

    // Initialize AES context
    ctx = EVP_CIPHER_CTX_new();
    EVP_EncryptInit_ex(ctx, EVP_aes_128_ctr(), NULL, key, iv);

    // Generate random key and IV using OpenSSL's CSPRNG
    RAND_bytes(key, 16);
    RAND_bytes(iv, 16);

    // Use aes_ctr_prng_genrand_uint128_to_buf to generate pseudorandom data
    unsigned char random_data[16];
    aes_ctr_prng_genrand_uint128_to_buf(ctx, random_data);

    EVP_CIPHER_CTX_free(ctx);
    return 0;
}

RDRAND and AES Comparison

- RDRAND: This is a hardware instruction provided by Intel CPUs that generates truly random numbers by leveraging thermal noise. It is designed to be a high-quality entropy source and is often used to seed CSPRNGs. However, it is specific to Intel CPUs and may not be available on all systems.

- AES-CTR PRNG: While AES itself is not a source of randomness, using it in CTR mode with a securely seeded key and counter can produce a stream of pseudorandom data. This approach is software-based and can be implemented on any system supporting AES, making it more versatile than RDRAND.

In summary, AES instructions can be used independently of RDRAND for generating pseudorandom numbers in OpenSSL by implementing an AES-CTR PRNG. However, the quality of the generated randomness depends on the initial seeding, which should ideally come from a trusted entropy source like RDRAND or OpenSSL's CSPRNG.

Citations:
[1] https://dev.to/mochafreddo/a-deep-dive-into-cryptographic-random-number-generation-from-openssl-to-entropy-16e6
[2] https://www.misterpki.com/openssl-rand/
[3] https://news.ycombinator.com/item?id=19848953
[4] https://wiki.openssl.org/index.php/Random_Numbers
[5] https://stackoverflow.com/questions/39635053/how-to-get-a-random-number-using-openssl

How does OpenSSL handle limited entropy in environments like embedded systems

OpenSSL, like many cryptographic libraries, relies on sufficient entropy to generate secure random numbers. In environments with limited entropy, such as embedded systems, ensuring adequate randomness is crucial for maintaining security. Here's how OpenSSL handles limited entropy in such environments:

Understanding Entropy

Entropy refers to the amount of uncertainty or randomness in a system. In cryptographic contexts, entropy is used to generate keys, nonces, and other random data. Embedded systems often have limited sources of entropy due to their constrained hardware and lack of user interaction, which can make it difficult to collect sufficient entropy.

OpenSSL's Approach to Entropy

OpenSSL uses a combination of system-specific entropy sources and algorithms to generate random numbers. Here are some ways OpenSSL handles limited entropy:

1. Entropy Pool: OpenSSL maintains an internal entropy pool, which is a buffer that accumulates entropy from various sources. This pool is used to generate random numbers when needed. In environments with limited entropy, the pool may not fill up quickly, leading to delays or weaker randomness.

2. Entropy Sources: OpenSSL can gather entropy from various system sources, such as:
- `/dev/random` and `/dev/urandom`: On Unix-like systems, these special files provide entropy collected from system events like keyboard presses and disk I/O.
- `CryptGenRandom`: On Windows, this API is used to gather entropy.
- Hardware Random Number Generators (HRNGs): Some systems have dedicated hardware for generating random numbers, which can be used if available.

3. Pseudorandom Number Generators (PRNGs): When the entropy pool is depleted, OpenSSL uses PRNGs to generate pseudorandom numbers. These PRNGs are seeded with the available entropy and can produce a stream of numbers that appear random. However, the quality of these numbers depends on the initial seeding.

4. Re-seeding: To maintain randomness, OpenSSL periodically re-seeds its PRNGs with fresh entropy. This ensures that even if the initial entropy was limited, the PRNG will eventually incorporate more randomness as it becomes available.

Handling Limited Entropy in Embedded Systems

In embedded systems, where entropy sources are scarce, additional strategies can be employed:

- Use of External Entropy Sources: If possible, connect external entropy sources like hardware random number generators or even environmental sensors (e.g., temperature or light sensors) to increase the system's entropy.

- Software-based Entropy Collection: Implement software mechanisms to collect entropy from available system events, such as network packets, timer interrupts, or other hardware events.

- Custom Entropy Collection Routines: Developers can write custom routines to gather entropy specific to their embedded system's environment. For example, collecting entropy from button presses or other user interactions.

- Pre-seeding: In some cases, pre-seeding the OpenSSL entropy pool with a secure random seed before deployment can help ensure that initial operations have sufficient randomness.

Example of Custom Entropy Collection

Here's a simplified example of how you might collect entropy from a timer interrupt in an embedded system:

c
#include 
#include 

// Global variables
volatile uint32_t timer_counter = 0;

// Interrupt handler for timer
void timer_interrupt_handler(void) {
    timer_counter++;
    // Collect entropy from timer_counter
    unsigned char entropy;
    memcpy(entropy, &timer_counter, 4);
    RAND_add(entropy, 4, 0); // Add entropy to OpenSSL's pool
}

int main() {
    // Initialize timer interrupt
    // ...

    // Use OpenSSL's RAND_bytes to generate random numbers
    unsigned char random_data[16];
    RAND_bytes(random_data, 16);

    return 0;
}

In summary, OpenSSL handles limited entropy by using a combination of system entropy sources, PRNGs, and periodic re-seeding. In embedded systems, custom entropy collection mechanisms can be implemented to supplement the available entropy and ensure secure random number generation.

Can OpenSSL use multiple entropy sources simultaneously

Yes, OpenSSL can use multiple entropy sources simultaneously. This capability is crucial in environments where a single source may not provide sufficient entropy, such as in embedded systems or other constrained environments. Here's how OpenSSL can leverage multiple entropy sources:

Understanding Entropy Sources in OpenSSL

OpenSSL collects entropy from various system-specific sources, which can vary depending on the operating system and hardware. Common entropy sources include:

- `/dev/random` and `/dev/urandom`: On Unix-like systems, these special files collect entropy from system events like keyboard presses, disk I/O, and network packets.
- `CryptGenRandom`: On Windows, this API is used to gather entropy.
- Hardware Random Number Generators (HRNGs): Some systems have dedicated hardware for generating random numbers.

Using Multiple Entropy Sources

OpenSSL's entropy collection mechanism is designed to accumulate entropy from multiple sources. Here are some ways OpenSSL can use multiple sources:

1. System Entropy Pools: OpenSSL can collect entropy from both `/dev/random` and `/dev/urandom` on Unix-like systems. While `/dev/random` blocks until sufficient entropy is available, `/dev/urandom` will not block but may produce less secure randomness if the entropy pool is depleted.

2. Hardware Random Number Generators (HRNGs): If available, HRNGs can be used alongside other system entropy sources. This is particularly useful in environments where software-based entropy collection is insufficient.

3. Custom Entropy Collection Routines: Developers can implement custom routines to collect entropy from additional sources specific to their environment. For example, entropy could be collected from environmental sensors, network packets, or other hardware events.

4. OpenSSL's `RAND_add` Function: This function allows developers to manually add entropy from custom sources to OpenSSL's internal entropy pool. By using `RAND_add`, multiple entropy sources can be combined to enhance the overall randomness.

Example of Using Multiple Entropy Sources

Here's an example of how you might collect entropy from both `/dev/urandom` and a custom hardware source:

```c
#include
#include
#include

// Function to collect entropy from /dev/urandom
void collect_entropy_from_dev_urandom(void) {
int fd = open("/dev/urandom", O_RDONLY);
if (fd >= 0) {
unsigned char entropy[32];
read(fd, entropy, 32);
RAND_add(entropy, 32, 0); // Add entropy to OpenSSL's pool
close(fd);
}
}