EVP_DIGESTINIT(3SSL)

✨ EVP_DIGESTINIT 💅 — the OpenSSL digest girlies' manual

okay so bestie, this is EVP_DIGESTINIT(3SSL) from OpenSSL, and we're about to get so cozy with message digests. strap in 🎀

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🌸 NAME 🌸

EVP_MD_fetch, EVP_MD_up_ref, EVP_MD_free, EVP_MD_get_params, EVP_MD_gettable_params, EVP_MD_CTX_new, EVP_MD_CTX_reset, EVP_MD_CTX_free, EVP_MD_CTX_dup, EVP_MD_CTX_copy, EVP_MD_CTX_copy_ex, EVP_MD_CTX_ctrl, EVP_MD_CTX_set_params, EVP_MD_CTX_get_params, EVP_MD_settable_ctx_params, EVP_MD_gettable_ctx_params, EVP_MD_CTX_settable_params, EVP_MD_CTX_gettable_params, EVP_MD_CTX_set_flags, EVP_MD_CTX_clear_flags, EVP_MD_CTX_test_flags, EVP_Q_digest, EVP_Digest, EVP_DigestInit_ex2, EVP_DigestInit_ex, EVP_DigestInit, EVP_DigestUpdate, EVP_DigestFinal_ex, EVP_DigestFinalXOF, EVP_DigestFinal, EVP_DigestSqueeze, EVP_MD_CTX_serialize, EVP_MD_CTX_deserialize, EVP_MD_is_a, EVP_MD_get0_name, EVP_MD_get0_description, EVP_MD_names_do_all, EVP_MD_get0_provider, EVP_MD_get_type, EVP_MD_get_pkey_type, EVP_MD_get_size, EVP_MD_get_block_size, EVP_MD_get_flags, EVP_MD_CTX_get0_name, EVP_MD_CTX_md, EVP_MD_CTX_get0_md, EVP_MD_CTX_get1_md, EVP_MD_CTX_get_type, EVP_MD_CTX_get_size_ex, EVP_MD_CTX_get_block_size, EVP_MD_CTX_get0_md_data, EVP_md_null, EVP_get_digestbyname, EVP_get_digestbynid, EVP_get_digestbyobj, EVP_MD_CTX_get_pkey_ctx, EVP_MD_CTX_set_pkey_ctx, EVP_MD_do_all_provided, EVP_MD_type, EVP_MD_nid, EVP_MD_name, EVP_MD_pkey_type, EVP_MD_size, EVP_MD_block_size, EVP_MD_flags, EVP_MD_xof, EVP_MD_CTX_size, EVP_MD_CTX_get_size, EVP_MD_CTX_block_size, EVP_MD_CTX_type, EVP_MD_CTX_pkey_ctx, EVP_MD_CTX_md_data — and what are they all for? EVP digest routines, obviously 💁‍♀️

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🦋 SYNOPSIS 🦋

#include <openssl/evp.h>

EVP_MD *EVP_MD_fetch(OSSL_LIB_CTX *ctx, const char *algorithm,
                     const char *properties);
int EVP_MD_up_ref(EVP_MD *md);
void EVP_MD_free(EVP_MD *md);
int EVP_MD_get_params(const EVP_MD *digest, OSSL_PARAM params[]);
const OSSL_PARAM *EVP_MD_gettable_params(const EVP_MD *digest);
EVP_MD_CTX *EVP_MD_CTX_new(void);
int EVP_MD_CTX_reset(EVP_MD_CTX *ctx);
void EVP_MD_CTX_free(EVP_MD_CTX *ctx);
void EVP_MD_CTX_ctrl(EVP_MD_CTX *ctx, int cmd, int p1, void* p2);
int EVP_MD_CTX_get_params(EVP_MD_CTX *ctx, OSSL_PARAM params[]);
int EVP_MD_CTX_set_params(EVP_MD_CTX *ctx, const OSSL_PARAM params[]);
const OSSL_PARAM *EVP_MD_settable_ctx_params(const EVP_MD *md);
const OSSL_PARAM *EVP_MD_gettable_ctx_params(const EVP_MD *md);
const OSSL_PARAM *EVP_MD_CTX_settable_params(EVP_MD_CTX *ctx);
const OSSL_PARAM *EVP_MD_CTX_gettable_params(EVP_MD_CTX *ctx);
void EVP_MD_CTX_set_flags(EVP_MD_CTX *ctx, int flags);
void EVP_MD_CTX_clear_flags(EVP_MD_CTX *ctx, int flags);
int EVP_MD_CTX_test_flags(const EVP_MD_CTX *ctx, int flags);

int EVP_Q_digest(OSSL_LIB_CTX *libctx, const char *name, const char *propq,
                 const void *data, size_t datalen,
                 unsigned char *md, size_t *mdlen);
int EVP_Digest(const void *data, size_t count, unsigned char *md,
               unsigned int *size, const EVP_MD *type, ENGINE *impl);
int EVP_DigestInit_ex2(EVP_MD_CTX *ctx, const EVP_MD *type,
                       const OSSL_PARAM params[]);
int EVP_DigestInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl);
int EVP_DigestUpdate(EVP_MD_CTX *ctx, const void *d, size_t cnt);
int EVP_DigestFinal_ex(EVP_MD_CTX *ctx, unsigned char *md, unsigned int *s);
int EVP_DigestFinalXOF(EVP_MD_CTX *ctx, unsigned char *out, size_t outlen);
int EVP_DigestSqueeze(EVP_MD_CTX *ctx, unsigned char *out, size_t outlen);
int EVP_MD_CTX_serialize(EVP_MD_CTX *ctx, unsigned char *out, size_t *outlen);
int EVP_MD_CTX_deserialize(EVP_MD_CTX *ctx, const unsigned char *in, size_t inlen);

EVP_MD_CTX *EVP_MD_CTX_dup(const EVP_MD_CTX *in);
int EVP_MD_CTX_copy_ex(EVP_MD_CTX *out, const EVP_MD_CTX *in);

int EVP_DigestInit(EVP_MD_CTX *ctx, const EVP_MD *type);
int EVP_DigestFinal(EVP_MD_CTX *ctx, unsigned char *md, unsigned int *s);

int EVP_MD_CTX_copy(EVP_MD_CTX *out, EVP_MD_CTX *in);

const char *EVP_MD_get0_name(const EVP_MD *md);
const char *EVP_MD_get0_description(const EVP_MD *md);
int EVP_MD_is_a(const EVP_MD *md, const char *name);
int EVP_MD_names_do_all(const EVP_MD *md,
                        void (*fn)(const char *name, void *data),
                        void *data);
const OSSL_PROVIDER *EVP_MD_get0_provider(const EVP_MD *md);
int EVP_MD_get_type(const EVP_MD *md);
int EVP_MD_get_pkey_type(const EVP_MD *md);
int EVP_MD_get_size(const EVP_MD *md);
int EVP_MD_get_block_size(const EVP_MD *md);
unsigned long EVP_MD_get_flags(const EVP_MD *md);
int EVP_MD_xof(const EVP_MD *md);

const EVP_MD *EVP_MD_CTX_get0_md(const EVP_MD_CTX *ctx);
EVP_MD *EVP_MD_CTX_get1_md(EVP_MD_CTX *ctx);
const char *EVP_MD_CTX_get0_name(const EVP_MD_CTX *ctx);
int EVP_MD_CTX_get_size_ex(const EVP_MD_CTX *ctx);
int EVP_MD_CTX_get_block_size(const EVP_MD_CTX *ctx);
int EVP_MD_CTX_get_type(const EVP_MD_CTX *ctx);

const EVP_MD *EVP_md_null(void);

const EVP_MD *EVP_get_digestbyname(const char *name);
const EVP_MD *EVP_get_digestbynid(int type);
const EVP_MD *EVP_get_digestbyobj(const ASN1_OBJECT *o);

EVP_PKEY_CTX *EVP_MD_CTX_get_pkey_ctx(const EVP_MD_CTX *ctx);
void EVP_MD_CTX_set_pkey_ctx(EVP_MD_CTX *ctx, EVP_PKEY_CTX *pctx);

void EVP_MD_do_all_provided(OSSL_LIB_CTX *libctx,
                            void (*fn)(EVP_MD *mac, void *arg),
                            void *arg);

#define EVP_MD_type EVP_MD_get_type
#define EVP_MD_nid EVP_MD_get_type
#define EVP_MD_name EVP_MD_get0_name
#define EVP_MD_pkey_type EVP_MD_get_pkey_type
#define EVP_MD_size EVP_MD_get_size
#define EVP_MD_block_size EVP_MD_get_block_size
#define EVP_MD_flags EVP_MD_get_flags
#define EVP_MD_CTX_get_size EVP_MD_CTX_get_size_ex
#define EVP_MD_CTX_size EVP_MD_CTX_get_size_ex
#define EVP_MD_CTX_block_size EVP_MD_CTX_get_block_size
#define EVP_MD_CTX_type EVP_MD_CTX_get_type
#define EVP_MD_CTX_pkey_ctx EVP_MD_CTX_get_pkey_ctx

hehe, one more thing 👀 the following functions are deprecated since OpenSSL 3.0, and you can hide them entirely by defining OPENSSL_API_COMPAT with a suitable version value — see openssl_user_macros(7):

const EVP_MD *EVP_MD_CTX_md(const EVP_MD_CTX *ctx);

and THESE ones are deprecated since OpenSSL 4.0 (also hideable the same way with OPENSSL_API_COMPAT, again openssl_user_macros(7)):

void *EVP_MD_CTX_get0_md_data(const EVP_MD_CTX *ctx);

#define EVP_MD_CTX_md_data EVP_MD_CTX_get0_md_data

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💜 DESCRIPTION 💜

The EVP digest routines are like… the high-level, main-character interface to message digests and to Extendable Output Functions (XOF). 💅

The EVP_MD type is a structure for digest method implementation.

Each Message digest algorithm (like SHA256) produces a fixed size output length, which is what you get back when EVP_DigestFinal_ex() is called. Extendable Output Functions (XOF) — think SHAKE256 — have a variable sized output length outlen that you can use with either EVP_DigestFinalXOF() or EVP_DigestSqueeze(). You can also use EVP_DigestFinal_ex() for XOF algorithms, but bestie, for XOF algorithms that don't have a default size, the "xoflen" must be set beforehand (peep "PARAMETERS" below). 📌 And note that EVP_MD_get_size() and EVP_MD_CTX_get_size_ex() behave differently for an XOF. no cap.

🫶 EVP_MD_fetch()

Fetches the digest implementation for the given algorithm from any provider offering it, within the criteria given by the properties. Wanna know more? See "ALGORITHM FETCHING" in crypto(7).

The returned value must eventually be freed with EVP_MD_free(). 🧹

Fetched EVP_MD structures are reference counted.

🫶 EVP_MD_up_ref()

Increments the reference count for an EVP_MD structure. ✨

🫶 EVP_MD_free()

Decrements the reference count for the fetched EVP_MD structure. If the reference count drops to 0 then the structure is freed. If the argument is NULL, nothing is done. (iconic behaviour honestly)

🫶 EVP_MD_CTX_new()

Allocates and returns a digest context. 🎀

🫶 EVP_MD_CTX_reset()

Resets the digest context ctx. This can be used to reuse an already existing context — sustainability queen 🌿

🫶 EVP_MD_CTX_free()

Cleans up digest context ctx and frees up the space allocated to it. If the argument is NULL, nothing is done.

🫶 EVP_MD_CTX_ctrl()

This is a legacy method. EVP_MD_CTX_set_params() and EVP_MD_CTX_get_params() is the mechanism that should be used to set and get parameters that are used by providers.

Performs digest-specific control actions on context ctx. The control command is indicated in cmd and any additional arguments in p1 and p2. EVP_MD_CTX_ctrl() must be called after EVP_DigestInit_ex2(). Other restrictions may apply depending on the control type and digest implementation.

If this function happens to be used with a fetched EVP_MD, it will translate the controls that are known to OpenSSL into OSSL_PARAM(3) parameters with keys defined by OpenSSL and call EVP_MD_CTX_get_params() or EVP_MD_CTX_set_params() as is appropriate for each control command. 🔄

See "CONTROLS" below for more information, including what translations are being done.

🫶 EVP_MD_get_params()

Retrieves the requested list of params from a MD md. See "PARAMETERS" below for more info. 🔍

🫶 EVP_MD_CTX_get_params()

Retrieves the requested list of params from a MD context ctx. See "PARAMETERS" below for more info.

🫶 EVP_MD_CTX_set_params()

Sets the list of params into a MD context ctx. See "PARAMETERS" below for more info.

🫶 EVP_MD_gettable_params()

Gets a constant OSSL_PARAM(3) array that describes the retrievable parameters that can be used with EVP_MD_get_params(). 📋

🫶 EVP_MD_gettable_ctx_params(), EVP_MD_CTX_gettable_params()

Gets a constant OSSL_PARAM(3) array that describes the retrievable parameters that can be used with EVP_MD_CTX_get_params(). EVP_MD_gettable_ctx_params() returns the parameters that can be retrieved from the algorithm, whereas EVP_MD_CTX_gettable_params() returns the parameters that can be retrieved in the context's current state.

🫶 EVP_MD_settable_ctx_params(), EVP_MD_CTX_settable_params()

Gets a constant OSSL_PARAM(3) array that describes the settable parameters that can be used with EVP_MD_CTX_set_params(). EVP_MD_settable_ctx_params() returns the parameters that can be set from the algorithm, whereas EVP_MD_CTX_settable_params() returns the parameters that can be set in the context's current state.

🫶 EVP_MD_CTX_set_flags(), EVP_MD_CTX_clear_flags(), EVP_MD_CTX_test_flags()

Sets, clears and tests ctx flags. See "FLAGS" below for more info. 🚩

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…and bestie, that's only part 1 — there's so much more coming 💖

• EVP_Q_digest() — bestie, this is the quick one-shot digest function 💅. It hashes datalen bytes of data at data using the digest algorithm name, which is fetched using the optional libctx and propq parameters. The digest value is placed in md and its length is written at mdlen if the pointer is not NULL. At most EVP_MAX_MD_SIZE bytes will be written. No cap. ✨
• EVP_Digest() — a wrapper around the EVP_DigestInit_ex(), EVP_DigestUpdate() and EVP_DigestFinal_ex() functions. It hashes count bytes of data at data. The digest value is placed in md and its length is written at size if the pointer is not NULL. At most EVP_MAX_MD_SIZE bytes will be written. impl must be NULL and the default implementation of digest type is used. Periodt. 💖
• EVP_DigestInit_ex2() — sets up digest context ctx to use a digest type. type is typically supplied by a function such as EVP_sha1(), or a value explicitly fetched with EVP_MD_fetch(). ctx MUST NOT be NULL. 🚨 The parameters params are set on the context after initialisation. The type parameter can be NULL if ctx has been already initialized with another EVP_DigestInit_ex() call and has not been reset with EVP_MD_CTX_reset(). Got it? 😌
• EVP_DigestInit_ex() — sets up digest context ctx to use a digest type. type is typically supplied by a function such as EVP_sha1(), or a value explicitly fetched with EVP_MD_fetch(). If impl is non-NULL, its implementation of the digest type is used if there is one, and if not, the default implementation is used. The type parameter can be NULL if ctx has been already initialized with another EVP_DigestInit_ex() call and has not been reset with EVP_MD_CTX_reset(). Lowkey the same vibe as ex2 but with impl. 🦋
• EVP_DigestUpdate() — hashes cnt bytes of data at d into the digest context ctx. This function can be called several times on the same ctx to hash additional data. So you can keep feeding it, bestie. 🍓
• EVP_DigestFinal_ex() — retrieves the digest value from ctx and places it in md. If the s parameter is not NULL then the number of bytes of data written (i.e. the length of the digest) will be written to the integer at s, at most EVP_MAX_MD_SIZE bytes will be written unless the digest implementation allows changing the digest size and it is set to a larger value by the application. After calling EVP_DigestFinal_ex() no additional calls to EVP_DigestUpdate() can be made, but EVP_DigestInit_ex2() can be called to initialize a new digest operation. ctx MUST NOT be NULL. 💀 Don't try to update after finalizing, it's a wrap.
• EVP_DigestFinalXOF() — interfaces to extendable-output functions, XOFs, such as SHAKE128 and SHAKE256. It retrieves the digest value from ctx and places it in outlen-sized out. After calling this function no additional calls to EVP_DigestUpdate() can be made, but EVP_DigestInit_ex2() can be called to initialize a new operation. EVP_DigestFinalXOF() may only be called once. One and done, bestie. 🎀
• EVP_DigestSqueeze() — similar to EVP_DigestFinalXOF() but allows multiple calls to be made to squeeze variable length output data. EVP_DigestFinalXOF() should not be called after this. Squeeze it like a glow-up 💅.
• EVP_MD_CTX_serialize() and EVP_MD_CTX_deserialize() — EVP_MD_CTX_serialize() serializes the state of the digest context ctx and stores it in out, unless out is NULL. Otherwise the maximum necessary size of the output buffer is written to the outlen parameter. If out is not NULL then before the call the outlen parameter must contain the length of the out buffer. If the call is successful the data is written to out and the amount of data written to outlen. EVP_MD_CTX_deserialize() deserializes the data from in of size inlen into the digest context ctx. These functions do not guarantee importability of state exported by a different OpenSSL version and does not guarantee interoperability between different providers. Some providers may not allow export/import across process boundaries.

🚨 NOTE: Applications must guarantee that only trusted data is used during deserialization. The deserialization operation is not built to address adversarial data compromise, and only basic checks to protect from simple mistakes are implemented. So don't be messy with untrusted data, bestie! 🙅‍♀️

• EVP_MD_CTX_dup() — can be used to duplicate the message digest state from in. This is useful to avoid multiple EVP_MD_fetch() calls or if large amounts of data are to be hashed which only differ in the last few bytes. Clone it, own it. 👑
• EVP_MD_CTX_copy_ex() — can be used to copy the message digest state from in to out. This is useful if large amounts of data are to be hashed which only differ in the last few bytes. Copy-paste but make it secure. 📋
• EVP_DigestInit() — behaves in the same way as EVP_DigestInit_ex2() except it doesn't set any parameters and calls EVP_MD_CTX_reset() so it cannot be used with an type of NULL. So no null vibes here. 🚫
• EVP_DigestFinal() — similar to EVP_DigestFinal_ex() except after computing the digest the digest context ctx is automatically cleaned up with EVP_MD_CTX_reset(). It cleans up after itself, iconic. ✨
• EVP_MD_CTX_copy() — similar to EVP_MD_CTX_copy_ex() except the destination out does not have to be initialized. Easy peasy. 🍋
• EVP_MD_is_a() — returns 1 if md is an implementation of an algorithm that's identifiable with name, otherwise 0. If md is a legacy digest (it's the return value from the likes of EVP_sha256() rather than the result of an EVP_MD_fetch()), only cipher names registered with the default library context (see OSSL_LIB_CTX(3)) will be considered. Legacy tea only. ☕
• EVP_MD_xof() — returns 1 if md is an Extendable-output Function (XOF) otherwise it returns 0. SHAKE128, SHAKE256, CSHAKE128 and CSHAKE256 are XOF functions. It returns 0 for BLAKE2B algorithms. So BLAKE2B is not a XOF, noted. 📝
• EVP_MD_get0_name(), EVP_MD_CTX_get0_name() — return the name of the given message digest. For fetched message digests with multiple names, only one of them is returned; it's recommended to use EVP_MD_names_do_all() instead. One name at a time, but there's a better way. 💁‍♀️
• EVP_MD_names_do_all() — traverses all names for the md, and calls fn with each name and data. This is only useful with fetched EVP_MDs. Fetched only, bestie. 🛍️
• EVP_MD_get0_description() — returns a description of the digest, meant for display and human consumption. The description is at the discretion of the digest implementation. It's giving human-readable. 👀
• EVP_MD_get0_provider() — returns an OSSL_PROVIDER pointer to the provider that implements the given EVP_MD. Who's your provider? 💳
• EVP_MD_get_size() — return the size of the message digest when passed an EVP_MD, i.e. the size of the hash. A negative value or 0 can occur for invalid size. For an XOF with no default size this returns 0. Size matters, but sometimes it's zero. 😭
• EVP_MD_CTX_get_size_ex(), EVP_MD_CTX_get_size() — for a normal digest this is the same as EVP_MD_get_size(). For an XOF this returns the "xoflen" if it has been set, otherwise it returns 0. XOF gets special treatment. 🌈
• EVP_MD_get_block_size(), EVP_MD_CTX_get_block_size() — return the block size of the message digest when passed an EVP_MD or an EVP_MD_CTX structure. Block size, not to be confused with hash size. 🧱
• EVP_MD_get_type(), EVP_MD_CTX_get_type() — return the NID of the OBJECT IDENTIFIER representing the given message digest when passed an EVP_MD structure. For example, "EVP_MD_get_type(EVP_sha1())" returns NID_sha1. This function is normally used when setting ASN1 OIDs. OID vibes. 🔢
• EVP_MD_CTX_get0_md_data() — this function is deprecated and exists for legacy backward compatibility reasons and always returns NULL. Deprecated, don't use it, bestie. 🚫
• EVP_MD_CTX_get0_md(), EVP_MD_CTX_get1_md() — EVP_MD_CTX_get0_md() returns the EVP_MD structure corresponding to the passed EVP_MD_CTX. This will be the same EVP_MD object originally passed to EVP_DigestInit_ex2() (or other similar function) when the EVP_MD_CTX was first initialised. Note that where explicit fetch is in use (see EVP_MD_fetch(3)) the value returned from this function will not have its reference count incremented and therefore it should not be used after the EVP_MD_CTX is freed. EVP_MD_CTX_get1_md() is the same except the ownership is passed to the caller and is from the passed EVP_MD_CTX. Get0 vs get1, know the difference. 🧐
• EVP_MD_get_flags() — returns the md flags. Note that these are different from the EVP_MD_CTX ones. See the "flags" parameter as documented in EVP_MD-common(7) for a description of the available flags. Flags, but not the context flags. 🚩
• EVP_MD_get_pkey_type() — returns the NID of the public key signing algorithm associated with this digest. For example EVP_sha1() is associated with RSA so this will return NID_sha1WithRSAEncryption. Since digests and signature algorithms are no longer linked this function is only retained for compatibility reasons. Old school link, kept for the vibes. 🔗
• EVP_md_null() — a "null" message digest that does nothing: i.e. the hash it returns is of zero length. Doing nothing but slaying. 💅
• EVP_get_digestbyname(), EVP_get_digestbynid(), EVP_get_digestbyobj() — returns an EVP_MD structure when passed a digest name, a digest NID or an ASN1_OBJECT structure respectively. The EVP_get_digestbyname() function is present for backwards compatibility with OpenSSL prior to version 3 and is different to the EVP_MD_fetch() function since it does not attempt to "fetch" an implementation of the cipher. Additionally, it only knows about digests that are built-in to OpenSSL and have an associated NID. Similarly EVP_get_digestbynid() and EVP_get_digestbyobj() also return objects without an associated implementation. When the digest objects returned by these functions are used (such as in a call to EVP_DigestInit_ex()) an implementation of the digest will be implicitly fetched from the loaded providers. This fetch could fail if no suitable implementation is available. Use EVP_MD_fetch() instead to explicitly fetch the algorithm and an associated implementation from a provider. See "ALGORITHM FETCHING" in crypto(7) for more information about fetching. The digest objects returned from these functions do not need to be freed with EVP_MD_free(). Old school, but still useful. 📚
• EVP_MD_CTX_get_pkey_ctx() — returns the EVP_PKEY_CTX assigned to ctx. The returned pointer should not be freed by the caller. Don't free it, it's not yours. 🙅‍♀️
• EVP_MD_CTX_set_pkey_ctx() — assigns an EVP_PKEY_CTX to EVP_MD_CTX. This is usually used to provide a customized EVP_PKEY_CTX to EVP_DigestSignInit(3) or EVP_DigestVerifyInit(3). The pctx passed to this function should be freed by the caller. A NULL pctx pointer is also allowed to clear the EVP_PKEY_CTX assigned to ctx. In such case, freeing the cleared EVP_PKEY_CTX or not depends on how the EVP_PKEY_CTX is created. Set it, clear it, just know who frees it. 🔑
• EVP_MD_do_all_provided() — literally takes a lil walk through every message digest that all the activated providers have in the library context libctx 👑 and for each one it's like "hey fn, here's the implementation method, and also here's arg" — calling fn with the implementation method and the given arg as arguments. A whole tour, bestie.

💖 PARAMETERS 💖

So OSSL_PARAM(3) is where you go to learn about the whole passing-parameters situation. Go peek at it, iykyk.

EVP_MD_CTX_set_params() and EVP_MD_CTX_get_params() can be used with these OSSL_PARAM keys:

• "xoflen" (OSSL_DIGEST_PARAM_XOFLEN) <unsigned integer> — Sets or gets the digest length for extendable output functions. The value should not exceed what can be given using a size_t. It may be used by SHAKE-128, CSHAKE-128, SHAKE-256 and CSHAKE-256 to set the output length used by EVP_DigestFinal_ex() and EVP_DigestFinal().
• "size" (OSSL_DIGEST_PARAM_SIZE) <unsigned integer> — Sets or gets a fixed digest length. The value should not exceed what can be given using a size_t. It may be used by BLAKE2B-512 to set the output length used by EVP_DigestFinal_ex() and EVP_DigestFinal().

And EVP_MD_CTX_set_params() — just the setter bestie, no getting — can be used with these OSSL_PARAM keys:

• "pad-type" (OSSL_DIGEST_PARAM_PAD_TYPE) <unsigned integer> — Sets the padding type. It is used by the MDC2 algorithm.
• "function-name" (OSSL_DIGEST_PARAM_FUNCTION_NAME) <UTF8 string> — Sets the function name string. It is used by EVP_MD-CSHAKE(7).
• "customization" (OSSL_DIGEST_PARAM_CUSTOMIZATION) <UTF8 string> — Sets a customisation string. It is used by EVP_MD-CSHAKE(7).
• "properties" (OSSL_DIGEST_PARAM_PROPERTIES) <UTF8 string> — Sets properties to be used when fetching algorithm implementations.

And EVP_MD_CTX_get_params() — getter era only — can be used with this OSSL_PARAM key:

• "micalg" (OSSL_DIGEST_PARAM_MICALG) <UTF8 string> — Gets the digest Message Integrity Check algorithm string. This is used when creating S/MIME multipart/signed messages, as specified in RFC 3851. It may be used by external providers.

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🎛️ CONTROLS 🎛️

EVP_MD_CTX_ctrl() is your little remote control — you can use it to send these standard controls:

• EVP_MD_CTRL_MICALG — Gets the digest Message Integrity Check algorithm string. This is used when creating S/MIME multipart/signed messages, as specified in RFC 3851. The string value is written to p2.

When used with a fetched EVP_MD, EVP_MD_CTX_get_params() gets called with an OSSL_PARAM(3) item with the key "micalg" (OSSL_DIGEST_PARAM_MICALG).

• EVP_MD_CTRL_XOF_LEN — This control sets the digest length for extendable output functions to p1. Sending this control directly should not be necessary, the use of EVP_DigestFinalXOF() is preferred. Currently used by SHAKE algorithms.

When used with a fetched EVP_MD, EVP_MD_CTX_get_params() gets called with an OSSL_PARAM(3) item with the key "xoflen" (OSSL_DIGEST_PARAM_XOFLEN).

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🚩 FLAGS 🚩

EVP_MD_CTX_set_flags(), EVP_MD_CTX_clear_flags() and EVP_MD_CTX_test_flags() — the trio — can be used to manipulate and test these EVP_MD_CTX flags, bestie:

• EVP_MD_CTX_FLAG_ONESHOT — This flag instructs the digest to optimize for one update only, if possible. One and done energy ✨
• EVP_MD_CTX_FLAG_CLEANED — This flag is for internal use only and must not be used in user code. Hands off, that's not for us 🙈
• EVP_MD_CTX_FLAG_REUSE — This flag is for internal use only and must not be used in user code. Same deal, do not touch 💀
• EVP_MD_CTX_FLAG_NO_INIT — This flag instructs EVP_DigestInit() and similar not to initialise the implementation specific data.
• EVP_MD_CTX_FLAG_FINALISE — Some functions such as EVP_DigestSign only finalise copies of internal contexts so additional data can be included after the finalisation call. This is inefficient if this functionality is not required, and can be disabled with this flag.

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🔙 RETURN VALUES 🔙

Here's the whole receipts list, no skipping:

• EVP_MD_fetch() — Returns a pointer to an EVP_MD for success or NULL for failure.
• EVP_MD_up_ref() — Returns 1 for success or 0 for failure.
• EVP_Q_digest(), EVP_Digest(), EVP_DigestInit_ex2(), EVP_DigestInit_ex(), EVP_DigestInit(), EVP_DigestUpdate(), EVP_DigestFinal_ex(), EVP_DigestFinalXOF(), EVP_MD_CTX_serialize(), EVP_MD_CTX_deserialize(), and EVP_DigestFinal() — return 1 for success and 0 for failure.
• EVP_MD_CTX_ctrl() — Returns 1 if successful or 0 for failure.
• EVP_MD_CTX_set_params(), EVP_MD_CTX_get_params() — Returns 1 if successful or 0 for failure.
• EVP_MD_CTX_settable_params(), EVP_MD_CTX_gettable_params() — Return an array of constant OSSL_PARAM(3)s, or NULL if there is none to get.
• EVP_MD_CTX_dup() — Returns a new EVP_MD_CTX if successful or NULL on failure.
• EVP_MD_CTX_copy_ex() — Returns 1 if successful or 0 for failure.
• EVP_MD_get_type(), EVP_MD_get_pkey_type() — Returns the NID of the corresponding OBJECT IDENTIFIER or NID_undef if none exists.
• EVP_MD_get_size(), EVP_MD_get_block_size(), EVP_MD_CTX_get_size(), EVP_MD_CTX_get_block_size() — Returns the digest or block size in bytes or -1 for failure.
• EVP_md_null() — Returns a pointer to the EVP_MD structure of the "null" message digest.
• EVP_get_digestbyname(), EVP_get_digestbynid(), EVP_get_digestbyobj() — Returns either an EVP_MD structure or NULL if an error occurs.
• EVP_MD_CTX_set_pkey_ctx() — This function has no return value. Nothing. Void. Silencio 🤫
• EVP_MD_names_do_all() — Returns 1 if the callback was called for all names. A return value of 0 means that the callback was not called for any names.

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📝 NOTES 📝

Okay so the whole tea: the EVP interface to message digests should almost always be used in preference to the low-level interfaces. Why? Because then your code becomes transparent to the digest used and much more flexible. Main character energy for your codebase, fr fr 💅

New applications should use the SHA-2 (such as EVP_sha256(3)) or the SHA-3 digest algorithms (such as EVP_sha3_512(3)). The other digest algorithms are still in common use though, no shade.

For most applications the impl parameter to EVP_DigestInit_ex() will be set to NULL to use the default digest implementation.

⚠️ Bestie, listen. Ignoring failure returns of EVP_DigestInit_ex(), EVP_DigestInit_ex2(), or EVP_DigestInit() can lead to undefined behavior on subsequent calls updating or finalizing the EVP_MD_CTX — like the EVP_DigestUpdate() or EVP_DigestFinal() functions. The only valid calls on the EVP_MD_CTX when initialization fails are calls that attempt another initialization of the context or release the context. That's it. That's the whole list. Do not pass go 🚫

The functions EVP_DigestInit(), EVP_DigestFinal() and EVP_MD_CTX_copy() are obsolete but are retained to maintain compatibility with existing code. New applications should use EVP_DigestInit_ex(), EVP_DigestFinal_ex() and EVP_MD_CTX_copy_ex() instead — because they can efficiently reuse a digest context instead of initializing and cleaning it up on each call, and they allow non default implementations of digests to be specified. Glow up 🌟

🫶 Also: if digest contexts are not cleaned up after use, memory leaks will occur. Clean up after yourself like the icon you are.

Oh and heads up — EVP_MD_CTX_get0_name(), EVP_MD_CTX_get_size(), EVP_MD_CTX_get_block_size(), EVP_MD_CTX_get_type(), EVP_get_digestbynid() and EVP_get_digestbyobj() are defined as macros. Surprise! ✨

EVP_MD_CTX_ctrl() sends commands to message digests for additional configuration or control. It's giving manager.

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🌈 EXAMPLES 🌈

Okay here's a whole lil program, she's cute. This example digests the data "Test Message\n" and "Hello World\n", using the digest name passed on the command line:

#include <stdio.h>
#include <string.h>
#include <openssl/evp.h>

int main(int argc, char *argv[])
{
    EVP_MD_CTX *mdctx;
    const EVP_MD *md;
    char mess1[] = "Test Message\n";
    char mess2[] = "Hello World\n";
    unsigned char md_value[EVP_MAX_MD_SIZE];
    unsigned int md_len, i;

    if (argv[1] == NULL) {
        printf("Usage: mdtest digestname\n");
        exit(1);
    }

    md = EVP_get_digestbyname(argv[1]);
    if (md == NULL) {
        printf("Unknown message digest %s\n", argv[1]);
        exit(1);
    }

    mdctx = EVP_MD_CTX_new();
    if (mdctx == NULL) {
       printf("Message digest create failed.\n");
       exit(1);
    }
    if (!EVP_DigestInit_ex2(mdctx, md, NULL)) {
        printf("Message digest initialization failed.\n");
        EVP_MD_CTX_free(mdctx);
        exit(1);
    }
    if (!EVP_DigestUpdate(mdctx, mess1, strlen(mess1))) {
        printf("Message digest update failed.\n");
        EVP_MD_CTX_free(mdctx);
        exit(1);
    }
    if (!EVP_DigestUpdate(mdctx, mess2, strlen(mess2))) {
        printf("Message digest update failed.\n");
        EVP_MD_CTX_free(mdctx);
        exit(1);
    }
    if (!EVP_DigestFinal_ex(mdctx, md_value, &md_len)) {
        printf("Message digest finalization failed.\n");
        EVP_MD_CTX_free(mdctx);
        exit(1);
    }
    EVP_MD_CTX_free(mdctx);

    printf("Digest is: ");
    for (i = 0; i < md_len; i++)
        printf("%02x", md_value[i]);
    printf("\n");

    exit(0);
}

Look at her. Checks every single return value. That's the standard, periodt 🎀

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👀 SEE ALSO 👀

openssl-dgst(1), evp(7), OSSL_PROVIDER(3), OSSL_PARAM(3), property(7), "ALGORITHM FETCHING" in crypto(7), provider-digest(7), life_cycle-digest(7)

The full list of digest algorithms are provided below. It's a whole roster bestie 📋

EVP_blake2b512(3), EVP_md2(3), EVP_md4(3), EVP_md5(3), EVP_mdc2(3), EVP_ripemd160(3), EVP_sha1(3), EVP_sha224(3), EVP_sha3_224(3), EVP_sm3(3), EVP_whirlpool(3)

🦋 HISTORY 🦋

okay bestie, storytime 📖✨ here's the lore on how we ended up with all these functions, in order, no cap:

• EVP_MD_CTX_create() and EVP_MD_CTX_destroy() were renamed to EVP_MD_CTX_new() and EVP_MD_CTX_free() in OpenSSL 1.1.0, respectively 💅 (one-to-one, in that exact order).
• The link between digests and signing algorithms was fixed in OpenSSL 1.0 and later, so now EVP_sha1() can be used with RSA and DSA 🎀 it just works, your honour.
• EVP_dss1() was removed in OpenSSL 1.1.0 💀 gone gone.
• EVP_MD_CTX_set_pkey_ctx() was added in OpenSSL 1.1.1 ✨
• Then OpenSSL 3.0 pulled up and added all of these: EVP_Q_digest(), EVP_DigestInit_ex2(), EVP_MD_fetch(), EVP_MD_free(), EVP_MD_up_ref(), EVP_MD_get_params(), EVP_MD_CTX_set_params(), EVP_MD_CTX_get_params(), EVP_MD_gettable_params(), EVP_MD_gettable_ctx_params(), EVP_MD_settable_ctx_params(), EVP_MD_CTX_settable_params() and EVP_MD_CTX_gettable_params() 🔥 that's a whole roster.
• Also in OpenSSL 3.0, these got renamed to include "get" or "get0" in their names, respectively: EVP_MD_type(), EVP_MD_nid(), EVP_MD_name(), EVP_MD_pkey_type(), EVP_MD_size(), EVP_MD_block_size(), EVP_MD_flags(), EVP_MD_CTX_size(), EVP_MD_CTX_block_size(), EVP_MD_CTX_type(), and EVP_MD_CTX_md_data(). And before you panic — the old names are kept as non-deprecated alias macros 🫶 so your existing code is still safely eating.
• EVP_MD_CTX_md() was deprecated in OpenSSL 3.0; use EVP_MD_CTX_get0_md() instead. Also EVP_MD_CTX_update_fn() and EVP_MD_CTX_set_update_fn() were deprecated in OpenSSL 3.0 🙈 (rest in peace besties).
• EVP_MD_CTX_dup() was added in OpenSSL 3.1 🌸
• EVP_DigestSqueeze() was added in OpenSSL 3.3 🍓
• EVP_MD_CTX_get_size_ex() and EVP_xof() were added in OpenSSL 3.4 ✨ And okay plot twist time: the macros EVP_MD_CTX_get_size() and EVP_MD_CTX_size were changed in OpenSSL 3.4 to be aliases for EVP_MD_CTX_get_size_ex(), whereas previously they were aliases for EVP_MD_get_size, which returned a constant value. Why, you ask? Because this is required for XOF digests since they do not have a fixed size 👑 (real ones know).
• EVP_MD_CTX_serialize() and EVP_MD_CTX_deserialize() were added in OpenSSL 4.0 💜

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💖 COPYRIGHT 💖

Copyright 2000-2026 The OpenSSL Project Authors. All Rights Reserved. 🌈

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>. 🎀

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and that's a wrap on the whole page, bestie 🫶 this was OpenSSL 4.0.3, dated 2026-09-29, and you've been reading the EVP_DIGESTINIT(3SSL) manual page. go forth and digest, you absolute icon 💅✨