EVP_RAND(3ossl)

📦

version 3.5.9 · 8 versions

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3.5.9 · centos-stream 9
3.5.9 · centos-stream 10
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3.5.9 · alpine 3.23, alpine 3.24
3.5.8 · fedora 43
3.5.1 · fedora 43
3.5.5 · fedora 44

✨ `EVP_RAND(3ossl)` — OpenSSL's RAND glow-up, part 1 ✨

okay bestie, welcome to the OpenSSL manual page for the EVP RAND routines 💖 this is the part where we meet the whole cast, no cap.

🎀 NAME 🎀

okay bestie, gather round — this page covers EVP_RAND, EVP_RAND_fetch, EVP_RAND_free, EVP_RAND_up_ref, EVP_RAND_CTX, EVP_RAND_CTX_new, EVP_RAND_CTX_free, EVP_RAND_CTX_up_ref, EVP_RAND_instantiate, EVP_RAND_uninstantiate, EVP_RAND_generate, EVP_RAND_reseed, EVP_RAND_nonce, EVP_RAND_enable_locking, EVP_RAND_verify_zeroization, EVP_RAND_get_strength, EVP_RAND_get_state, EVP_RAND_get0_provider, EVP_RAND_CTX_get0_rand, EVP_RAND_is_a, EVP_RAND_get0_name, EVP_RAND_names_do_all, EVP_RAND_get0_description, EVP_RAND_CTX_get_params, EVP_RAND_CTX_set_params, EVP_RAND_do_all_provided, EVP_RAND_get_params, EVP_RAND_gettable_ctx_params, EVP_RAND_settable_ctx_params, EVP_RAND_CTX_gettable_params, EVP_RAND_CTX_settable_params, EVP_RAND_gettable_params, EVP_RAND_STATE_UNINITIALISED, EVP_RAND_STATE_READY, EVP_RAND_STATE_ERROR — and that whole squad is the EVP RAND routines 💅

🌸 SYNOPSIS 🌸

k so here's the whole API in C, bestie. Don't panic, it's just the prototypes and the little state defines. Copy-paste energy only, fr fr.

        #include <openssl/evp.h>

        typedef struct evp_rand_st EVP_RAND;
        typedef struct evp_rand_ctx_st EVP_RAND_CTX;

        EVP_RAND *EVP_RAND_fetch(OSSL_LIB_CTX *libctx, const char *algorithm,
                               const char *properties);
        int EVP_RAND_up_ref(EVP_RAND *rand);
        void EVP_RAND_free(EVP_RAND *rand);
        EVP_RAND_CTX *EVP_RAND_CTX_new(EVP_RAND *rand, EVP_RAND_CTX *parent);
        void EVP_RAND_CTX_free(EVP_RAND_CTX *ctx);
        int EVP_RAND_CTX_up_ref(EVP_RAND_CTX *ctx);
        EVP_RAND *EVP_RAND_CTX_get0_rand(EVP_RAND_CTX *ctx);
        int EVP_RAND_get_params(EVP_RAND *rand, OSSL_PARAM params[]);
        int EVP_RAND_CTX_get_params(EVP_RAND_CTX *ctx, OSSL_PARAM params[]);
        int EVP_RAND_CTX_set_params(EVP_RAND_CTX *ctx, const OSSL_PARAM params[]);
        const OSSL_PARAM *EVP_RAND_gettable_params(const EVP_RAND *rand);
        const OSSL_PARAM *EVP_RAND_gettable_ctx_params(const EVP_RAND *rand);
        const OSSL_PARAM *EVP_RAND_settable_ctx_params(const EVP_RAND *rand);
        const OSSL_PARAM *EVP_RAND_CTX_gettable_params(EVP_RAND_CTX *ctx);
        const OSSL_PARAM *EVP_RAND_CTX_settable_params(EVP_RAND_CTX *ctx);
        const char *EVP_RAND_get0_name(const EVP_RAND *rand);
        const char *EVP_RAND_get0_description(const EVP_RAND *rand);
        int EVP_RAND_is_a(const EVP_RAND *rand, const char *name);
        const OSSL_PROVIDER *EVP_RAND_get0_provider(const EVP_RAND *rand);
        void EVP_RAND_do_all_provided(OSSL_LIB_CTX *libctx,
                                      void (*fn)(EVP_RAND *rand, void *arg),
                                      void *arg);
        int EVP_RAND_names_do_all(const EVP_RAND *rand,
                                  void (*fn)(const char *name, void *data),
                                  void *data);

        int EVP_RAND_instantiate(EVP_RAND_CTX *ctx, unsigned int strength,
                                 int prediction_resistance,
                                 const unsigned char *pstr, size_t pstr_len,
                                 const OSSL_PARAM params[]);
        int EVP_RAND_uninstantiate(EVP_RAND_CTX *ctx);
        int EVP_RAND_generate(EVP_RAND_CTX *ctx, unsigned char *out, size_t outlen,
                              unsigned int strength, int prediction_resistance,
                              const unsigned char *addin, size_t addin_len);
        int EVP_RAND_reseed(EVP_RAND_CTX *ctx, int prediction_resistance,
                            const unsigned char *ent, size_t ent_len,
                            const unsigned char *addin, size_t addin_len);
        int EVP_RAND_nonce(EVP_RAND_CTX *ctx, unsigned char *out, size_t outlen);
        int EVP_RAND_enable_locking(EVP_RAND_CTX *ctx);
        int EVP_RAND_verify_zeroization(EVP_RAND_CTX *ctx);
        unsigned int EVP_RAND_get_strength(EVP_RAND_CTX *ctx);
        int EVP_RAND_get_state(EVP_RAND_CTX *ctx);

        #define EVP_RAND_STATE_UNINITIALISED    0
        #define EVP_RAND_STATE_READY            1
        #define EVP_RAND_STATE_ERROR            2

💖 DESCRIPTION 💖

okay so bestie, the EVP RAND routines are a high-level interface to random number generators — both the deterministic kind and the not-so-deterministic kind. 🎲

But real talk: if all you wanna do is generate random bytes, you do not need these functions. Just call RAND_bytes() or RAND_priv_bytes() and go live your life. ✨ If you wanna do more than that, then use these calls instead of the older RAND and RAND_DRBG functions. glow-up, honestly. 💅

After you create an EVP_RAND_CTX for the algorithm you need using EVP_RAND_CTX_new(), you feed inputs to the algorithm either by:

passing them as part of the EVP_RAND_instantiate() call, or
making calls to EVP_RAND_CTX_set_params() before you call EVP_RAND_instantiate().

And finally?? You call EVP_RAND_generate() to produce those cryptographically secure random bytes. 🎀 that's the whole vibe.

🌸 Types 🌸

EVP_RAND — a type that holds the implementation of a RAND. 👑
EVP_RAND_CTX — a context type that holds the algorithm inputs. EVP_RAND_CTX structures are reference counted. 🧮

🦋 Algorithm implementation fetching 🦋

EVP_RAND_fetch() fetches an implementation of a RAND algorithm, given a library context libctx and a set of properties. Peep "ALGORITHM FETCHING" in crypto(7) for more tea. ☕

The returned value must eventually be freed with EVP_RAND_free(3). don't forget, bestie. 🫶

EVP_RAND_up_ref() increments the reference count of an already fetched RAND. 📈

EVP_RAND_free() frees a fetched algorithm. NULL is a totally valid parameter here, and for that this function is a no-op. 😌

💜 Context manipulation functions 💜

EVP_RAND_CTX_new() creates a new context for the RAND implementation rand. If it's not NULL, parent specifies the seed source for this implementation. Not all random number generators need a seed source specified, no cap. If a parent is required, a NULL parent will utilise the operating system entropy sources. And fr fr, it's recommended to minimise the number of random number generators that rely on the operating system for their randomness, because that entropy is often scarce. 🥺

EVP_RAND_CTX_free() frees up the context ctx. If ctx is NULL, nothing happens. 💀

EVP_RAND_CTX_get0_rand() returns the EVP_RAND associated with the context ctx. 🔍

🎀 Random Number Generator Functions 🎀

EVP_RAND_instantiate() processes any parameters in params and then instantiates the RAND ctx with a minimum security strength of <strength> and personalisation string pstr of length <pstr_len>. If prediction_resistance is specified, fresh entropy from a live source will be sought. 👀 This call operates as per NIST SP 800-90A and SP 800-90C. yes, NIST. we love a standards queen. 📜

EVP_RAND_uninstantiate() uninstantiates the RAND ctx as per NIST SP 800-90A and SP 800-90C. Subsequent to this call, the RAND cannot be used to generate bytes. It can only be freed or instantiated again. that's it. that's the rule. 🙅‍♀️

EVP_RAND_generate() produces random bytes from the RAND ctx with the additional input addin of length addin_len. The bytes produced will meet the security strength. ✨ If prediction_resistance is specified, fresh entropy from a live source will be sought. This call operates as per NIST SP 800-90A and SP 800-90C.

EVP_RAND_reseed() reseeds the RAND with new entropy. Entropy ent of length ent_len bytes can be supplied, as can additional input addin of length addin_len bytes. In the FIPS provider, both are treated as additional input as per NIST SP-800-90Ar1, Sections 9.1 and 9.2. Additional seed material is also drawn from the RAND's parent or the operating system. If prediction_resistance is specified, fresh entropy from a live source will be sought. This call operates as per NIST SP 800-90A and SP 800-90C. 🌊

EVP_RAND_nonce() creates a nonce in out of length outlen bytes from the RAND ctx. 🎯

EVP_RAND_enable_locking() enables locking for the RAND ctx and all of its parents. After this, the ctx will operate in a thread safe manner, albeit more slowly. 🐢 Heads up though — this function is not itself thread safe if called with the same ctx from multiple threads. Typically locking should be enabled before a ctx is shared across multiple threads. iykyk. 🧠

EVP_RAND_get_params() retrieves details about the implementation rand. The set of parameters given with params determine exactly what parameters should be retrieved. Note that a parameter that is unknown in the underlying context is simply ignored. 🫥

EVP_RAND_CTX_get_params() retrieves chosen parameters, given the context ctx and its underlying context. The set of parameters given with params determine exactly what parameters should be retrieved. Note that a parameter that is unknown in the underlying context is simply ignored. 🫥

EVP_RAND_CTX_set_params() passes chosen parameters to the underlying context, given a context ctx. The set of parameters given with params determine exactly what parameters are passed down. Note that a parameter that is unknown in the underlying context is simply ignored. Also, what happens when a needed parameter isn't passed down is defined by the implementation. 👈

EVP_RAND_gettable_params() returns an OSSL_PARAM(3) array that describes the retrievable and settable parameters. EVP_RAND_gettable_params() returns parameters that can be used with EVP_RAND_get_params(). 📋

EVP_RAND_gettable_ctx_params() and EVP_RAND_CTX_gettable_params() return constant OSSL_PARAM(3) arrays that describe the retrievable parameters that can be used with EVP_RAND_CTX_get_params(). EVP_RAND_gettable_ctx_params() returns the parameters that can be retrieved from the algorithm, whereas EVP_RAND_CTX_gettable_params() returns the parameters that can be retrieved in the context's current state. 🌀

EVP_RAND_settable_ctx_params() and EVP_RAND_CTX_settable_params() return constant OSSL_PARAM(3) arrays that describe the settable parameters that can be used with EVP_RAND_CTX_set_params(). EVP_RAND_settable_ctx_params() returns the parameters that can be retrieved from the algorithm, whereas EVP_RAND_CTX_settable_params() returns the parameters that can be retrieved in the context's current state. 🌈

⭐ Information functions ⭐

EVP_RAND_get_strength() returns the security strength of the RAND ctx. 💪

EVP_RAND_get_state() returns the current state of the RAND ctx. The states defined by the OpenSSL RNGs are:

EVP_RAND_STATE_UNINITIALISED: this RNG is currently uninitialised. The instantiate call will change this to the ready state. 🌱
EVP_RAND_STATE_READY: this RNG is currently ready to generate output. 🔥
EVP_RAND_STATE_ERROR: this RNG is in an error state. 💀

EVP_RAND_is_a() returns 1 if rand is an implementation of an algorithm that's identifiable with name, otherwise 0. ✅

EVP_RAND_get0_provider() returns the provider that holds the implementation of the given rand. 🏠

EVP_RAND_do_all_provided() traverses all RAND implemented by all activated providers in the given library context libctx, and for each of the implementations, calls the given function fn with the implementation method and the given arg as argument. 🚶‍♀️

EVP_RAND_get0_name() returns the canonical name of rand. 🏷️

EVP_RAND_names_do_all() traverses all names for rand, and calls fn with each name and data. 📛

EVP_RAND_get0_description() returns a description of the rand, meant for display and human consumption. The description is at the discretion of the rand implementation. 🗣️

EVP_RAND_verify_zeroization() confirms if the internal DRBG state is currently zeroed. This is used by the FIPS provider to support the mandatory self tests. 🧪

✨ PARAMETERS ✨

The standard parameter names are:

"state" (OSSL_RAND_PARAM_STATE) <integer> — Returns the state of the random number generator. 🔄
"strength" (OSSL_RAND_PARAM_STRENGTH) <unsigned integer> — Returns the bit strength of the random number generator. 💪
"fips-indicator" (OSSL_RAND_PARAM_FIPS_APPROVED_INDICATOR) <integer> — A getter that returns 1 if the operation is FIPS approved, or 0 otherwise. This option is used by the OpenSSL FIPS provider and is not supported by all EVP_RAND sources. 🚨

For rands that are also deterministic random bit generators (DRBGs), these additional parameters are recognised. Not all parameters are relevant to, or are understood by, all DRBG rands: 🎲

"reseed_requests" (OSSL_DRBG_PARAM_RESEED_REQUESTS) <unsigned integer> — Reads or sets the number of generate requests before reseeding the associated RAND ctx. 🔁
"reseed_time_interval" (OSSL_DRBG_PARAM_RESEED_TIME_INTERVAL) <integer> — Reads or sets the number of elapsed seconds before reseeding the associated RAND ctx. ⏰
"max_request" (OSSL_RAND_PARAM_MAX_REQUEST) <unsigned integer> — Specifies the maximum number of bytes that can be generated in a single call to OSSL_FUNC_rand_generate. 📏
"min_entropylen" (OSSL_DRBG_PARAM_MIN_ENTROPYLEN) <unsigned integer> and "max_entropylen" (OSSL_DRBG_PARAM_MAX_ENTROPYLEN) <unsigned integer> — Specify the minimum and maximum number of bytes of random material that can be used to seed the DRBG. 🌊
"min_noncelen" (OSSL_DRBG_PARAM_MIN_NONCELEN) <unsigned integer> and "max_noncelen" (OSSL_DRBG_PARAM_MAX_NONCELEN) <unsigned integer> — Specify the minimum and maximum number of bytes of nonce that can be used to seed the DRBG. 🎯
"max_perslen" (OSSL_DRBG_PARAM_MAX_PERSLEN) <unsigned integer> and "max_adinlen" (OSSL_DRBG_PARAM_MAX_ADINLEN) <unsigned integer> — Specify the minimum and maximum number of bytes of personalisation string that can be used with the DRBG. 📝
"reseed_counter" (OSSL_DRBG_PARAM_RESEED_COUNTER) <unsigned integer> — Specifies the number of times the DRBG has been seeded or reseeded. 🧮
"properties" (OSSL_RAND_PARAM_PROPERTIES) <UTF8 string>, "mac" (OSSL_RAND_PARAM_MAC) <UTF8 string>, "digest" (OSSL_RAND_PARAM_DIGEST) <UTF8 string>, "cipher" (OSSL_RAND_PARAM_CIPHER) <UTF8 string> — For RAND implementations that use an underlying computation MAC, digest or cipher, these parameters set what the algorithm should be. 🧬

The value is always the name of the intended algorithm, or the properties in the case of OSSL_RAND_PARAM_PROPERTIES. 🏷️

📝 NOTES 📝

okay bestie, listen up, this part is important 💖

Setting that prediction_resistance argument to a nonzero value in EVP_RAND_instantiate(), EVP_RAND_generate() or EVP_RAND_reseed()? Yeah, use that sparingly. Like, it's a treat, not a snack. In the default setup, it makes all the public and private DRBGs get reseeded on their next use. And since, by default, public and private DRBGs are allocated on a per thread basis, that can mean significant overhead for highly multi-threaded apps 😭 like the performance just walks out the door.

For normal use-cases though? The default "reseed_requests" and "reseed_time_interval" thresholds already keep you at sufficient prediction resistance over time, and if you think they're too high you can just lower those values, easy ✨. Explicitly asking for prediction resistance is meant for the special use-cases, the main-character scenarios, like generating long-term secrets 👑

Also! An EVP_RAND_CTX needs to have locking enabled if it's acting as the parent of more than one child and those children can be accessed concurrently. And you have to do that by explicitly calling EVP_RAND_enable_locking(). No vibes-based locking, bestie, you gotta ask for it 💅

The RAND life-cycle is described over in life_cycle-rand(7). And heads up: in the future, the transitions described there will be enforced. When that happens, it will not be considered a breaking change to the API. So like, don't come for them later, they told us 🫶

🌈 RETURN VALUES 🌈

okay so here's what everything hands back to you:

EVP_RAND_fetch() returns a pointer to a newly fetched EVP_RAND, or NULL if allocation failed.
EVP_RAND_get0_provider() returns a pointer to the provider for the RAND, or NULL on error.
EVP_RAND_CTX_get0_rand() returns a pointer to the EVP_RAND associated with the context.
EVP_RAND_get0_name() returns the name of the random number generation algorithm.
EVP_RAND_up_ref() returns 1 on success, 0 on error.
EVP_RAND_names_do_all() returns 1 if the callback was called for all names. A return value of 0 means the callback was not called for any names.
EVP_RAND_CTX_new() returns either the newly allocated EVP_RAND_CTX structure or NULL if an error occurred.
EVP_RAND_CTX_free() does not return a value. it's giving silence, and that's fine 💀
EVP_RAND_CTX_up_ref() returns 1 on success, 0 on error.
EVP_RAND_nonce() returns 1 on success, 0 on error.
EVP_RAND_get_strength() returns the strength of the random number generator in bits.
EVP_RAND_gettable_params(), EVP_RAND_gettable_ctx_params() and EVP_RAND_settable_ctx_params() return an array of OSSL_PARAMs.
EVP_RAND_verify_zeroization() returns 1 if the internal DRBG state is currently zeroed, and 0 if not.
The remaining functions return 1 for success and 0 or a negative value for failure.

👀 SEE ALSO 👀

go read these next, they're the whole friend group:

🕰️ HISTORY 🕰️

EVP_RAND_CTX_up_ref() was added in OpenSSL 3.1.
The remaining functions were added in OpenSSL 3.0.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

and that's a wrap on the whole page bestie, ty for reading all three parts, you ate 💅✨

this lil manual page is from OpenSSL 3.5.9, dated 2026-09-29, and it's EVP_RAND(3ossl) — periodt 🎀🦋

📦 where this page comes from

fedora 43: openssl-devel 1:3.5.9-1.fc43 · Apache-2.0
fedora 43: openssl-devel 1:3.5.9-1.fc43 · Apache-2.0
fedora 44: openssl-devel 1:3.5.9-1.fc44 · Apache-2.0
fedora 44: openssl-devel 1:3.5.9-1.fc44 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-14: a language model's rewrite of the original, under its terms