EVP_DIGESTINIT(3)

💖 EVP_DIGESTINIT(3) — okay bestie, let's talk digests 💖

So this page lives in the Library Functions Manual, and it's serving you the whole EVP digest fam — every single one of them. Buckle up. 🎀

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✨ NAME ✨

EVP_MD_CTX_new, EVP_MD_CTX_reset, EVP_MD_CTX_free, EVP_MD_CTX_init, EVP_MD_CTX_create, EVP_MD_CTX_cleanup, EVP_MD_CTX_destroy, EVP_DigestInit_ex, EVP_DigestUpdate, EVP_DigestFinal_ex, EVP_Digest, EVP_MD_CTX_copy_ex, EVP_DigestInit, EVP_DigestFinal, EVP_MD_CTX_copy, EVP_MD_CTX_md, EVP_md_null, EVP_sha224, EVP_sha256, EVP_sha384, EVP_sha512, EVP_sha512_224, EVP_sha512_256, EVP_ripemd160, EVP_get_digestbyname, EVP_get_digestbynid, EVP_get_digestbyobj — all of these are the EVP digest routines. That's the whole squad, no cap. 👑

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🌸 SYNOPSIS 🌸

Library: “libcrypto”. Include: #include <openssl/evp.h> — literally the first thing, don't forget it. 🫶

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);

int
EVP_MD_CTX_init(EVP_MD_CTX *ctx);

EVP_MD_CTX *
EVP_MD_CTX_create(void);

int
EVP_MD_CTX_cleanup(EVP_MD_CTX *ctx);

void
EVP_MD_CTX_destroy(EVP_MD_CTX *ctx);

int
EVP_DigestInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *engine);

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_Digest(const void *d, size_t cnt, unsigned char *md,
    unsigned int *s, const EVP_MD *type, ENGINE *engine);

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 EVP_MD *
EVP_MD_CTX_md(const EVP_MD_CTX *ctx);

const EVP_MD *
EVP_md_null(void);

const EVP_MD *
EVP_sha224(void);

const EVP_MD *
EVP_sha256(void);

const EVP_MD *
EVP_sha384(void);

const EVP_MD *
EVP_sha512(void);

const EVP_MD *
EVP_sha512_224(void);

const EVP_MD *
EVP_sha512_256(void);

const EVP_MD *
EVP_ripemd160(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);

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🦋 DESCRIPTION 🦋

The EVP digest routines are, like, the high-level interface to message digests, and honestly you should use them instead of the cipher-specific functions. Periodt. The way that low-level stuff keeps you locked in? Not the vibe. ✨

EVP_MD_CTX_new() allocates a brand-new, empty digest context. Fresh out the box, main character energy. 🎀

EVP_MD_CTX_reset() cleans up ctx and resets it back to the state it had right after EVP_MD_CTX_new(), so you can reuse it. Recycling! We love her. 🌈

EVP_MD_CTX_free() cleans up ctx and frees the space allocated to it. Clean up after yourself, bestie. 🧹

EVP_MD_CTX_init() is a deprecated function used to clear a digest context that lives on the stack before use. Do not use it on a digest context returned from EVP_MD_CTX_new(), and do not use it on one that was already used. Like, don't. 🙈

EVP_MD_CTX_create(), EVP_MD_CTX_cleanup(), and EVP_MD_CTX_destroy() are deprecated aliases for EVP_MD_CTX_new(), EVP_MD_CTX_reset(), and EVP_MD_CTX_free(), respectively. Old names, same energy. 💅

EVP_DigestInit_ex() sets up the digest context ctx to use a digest type. That type will typically come from a function like EVP_sha512(). The ENGINE *engine argument is always ignored, and passing NULL is recommended. Literally always. Ngl, she's just there for the aesthetic. 💀

EVP_DigestUpdate() hashes cnt bytes of data at d into the digest context ctx. You can call this several times on the same ctx to hash more data — like adding more messages to the group chat. 📩

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. After calling EVP_DigestFinal_ex(), no additional calls to EVP_DigestUpdate() can be made — but EVP_DigestInit_ex() can be called to initialize a new digest operation. The door is closed on that one, but you can open a new one. 🚪✨

EVP_Digest() is a simple wrapper function — one-shot, low-effort, iconic — to hash cnt bytes of data at d using the digest type and place the digest value into md, and, unless s is NULL, the length of the digest in bytes into *s. This wrapper uses a temporary digest context and passes its arguments to EVP_DigestInit_ex(), EVP_DigestUpdate(), and EVP_DigestFinal_ex() internally. The ENGINE *engine argument is always ignored and passing NULL is recommended. Again. Yes. Still. 😭

EVP_MD_CTX_copy_ex() can be used to copy the message digest state from in to out. Super useful if large amounts of data are to be hashed which only differ in the last few bytes — like, why re-hash the whole thing when it's 99% the same? Efficiency slay. 🔥

EVP_DigestInit() is a deprecated function behaving like EVP_DigestInit_ex() except that it requires EVP_MD_CTX_reset() before it can be used on a context that was already used. Extra step. Annoying but that's the deal. 🙃

EVP_DigestFinal() is a deprecated function behaving like EVP_DigestFinal_ex() except that the digest context ctx is automatically cleaned up after use by calling EVP_MD_CTX_reset() internally. She tidies up behind herself, at least. 🫶

EVP_MD_CTX_copy() is a deprecated function behaving like EVP_MD_CTX_copy_ex() except that it requires EVP_MD_CTX_reset() before a context that was already used can be passed as out. Same vibe, more homework. 📝

EVP_sha224(), EVP_sha256(), EVP_sha384(), EVP_sha512(), and EVP_ripemd160() return EVP_MD structures for the SHA-224, SHA-256, SHA-384, SHA-512 and RIPEMD-160 digest algorithms respectively. The classic lineup. 👑

EVP_sha512_224() and EVP_sha512_256() return an EVP_MD structure that provides the truncated SHA-512 variants SHA-512/224 and SHA-512/256, respectively. Truncated but make it fashion. ✂️💅

EVP_md_null() is a "null" message digest that does nothing: i.e. the hash it returns is of zero length. She's giving absolutely nothing and she's thriving. 💀✨

EVP_get_digestbyname(), EVP_get_digestbynid(), and EVP_get_digestbyobj() return an EVP_MD structure when passed a digest name, a digest NID, or an ASN1_OBJECT structure respectively. Name, NID, or object — pick your fighter. 🎀

EVP_get_digestbynid() and EVP_get_digestbyobj() are implemented as macros. Surprise! 🎁

The EVP interface to message digests should almost always be used in preference to the low-level interfaces. Why? Because the code then becomes transparent to the digest used and much more flexible. Glow-up via abstraction, honestly. 🦋

The ENGINE *engine argument is always ignored and passing NULL is recommended. (Third time saying it. Commit it to memory fr fr.) 💜

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() because they can efficiently reuse a digest context instead of initializing and cleaning it up on each call. New code, new you. 💅

⚠️ If digest contexts are not cleaned up after use, memory leaks will occur. Not a drill, bestie. Leaks are not cute. Clean up your contexts. 🚨

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👑 RETURN VALUES 👑

• EVP_MD_CTX_new() and EVP_MD_CTX_create() return the new EVP_MD_CTX object or NULL for failure. 🫶
• EVP_MD_CTX_reset(), EVP_MD_CTX_init(), and EVP_MD_CTX_cleanup() always return 1. Always. Consistent queen. 👑
• EVP_DigestInit_ex(), EVP_DigestUpdate(), EVP_DigestFinal_ex(), EVP_Digest(), EVP_MD_CTX_copy_ex(), EVP_DigestInit(), EVP_DigestFinal(), and EVP_MD_CTX_copy() return 1 for success or 0 for failure. Classic. ✨
• EVP_MD_CTX_md() returns the EVP_MD object used by ctx, or NULL if ctx is NULL or does not have any message digest algorithm assigned yet. No algorithm, no answer. 💀
• EVP_md_null(), EVP_sha224(), EVP_sha256(), EVP_sha384(), EVP_sha512(), EVP_sha512_224(), EVP_sha512_256(), and EVP_ripemd160() return pointers to constant static objects owned by the library. Don't go freeing those, they're not yours. 🙈
• EVP_get_digestbyname(), EVP_get_digestbynid(), and EVP_get_digestbyobj() return either an EVP_MD structure or NULL if an error occurs. 🌸

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🍓 EXAMPLES 🍓

Okay so this example digests the data "Test Message\n" and "Hello World\n", using the digest name passed on the command line. Chef's kiss, honestly. 😭🔥

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

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

        if (argc <= 1) {
                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();
        EVP_DigestInit_ex(mdctx, md, NULL);
        EVP_DigestUpdate(mdctx, mess1, strlen(mess1));
        EVP_DigestUpdate(mdctx, mess2, strlen(mess2));
        EVP_DigestFinal_ex(mdctx, md_value, &md_len);
        EVP_MD_CTX_free(mdctx);

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

        return 0;
}

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🌈 SEE ALSO 🌈

BIO_f_md(3), CMAC_Init(3), evp(3), EVP_BytesToKey(3), EVP_DigestSignInit(3), EVP_DigestVerifyInit(3), EVP_MD_CTX_ctrl(3), EVP_MD_nid(3), EVP_PKEY_CTX_set_signature_md(3), EVP_sha1(3), EVP_sha3_224(3), EVP_SignInit(3), EVP_sm3(3), EVP_VerifyInit(3), HMAC(3), OCSP_basic_sign(3), OCSP_request_sign(3), PKCS5_PBKDF2_HMAC(3), PKCS7_sign_add_signer(3), X509_ALGOR_set0(3), X509_digest(3), X509_sign(3) — go read those next, bestie, the lore is deep. 📚✨

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⭐ HISTORY ⭐

EVP_DigestInit(), EVP_DigestUpdate(), and EVP_DigestFinal() first appeared in SSLeay 0.5.1. EVP_md_null() and EVP_get_digestbyname() first appeared in SSLeay 0.8.0. EVP_get_digestbynid() and EVP_get_digestbyobj() first appeared in SSLeay 0.8.1. EVP_ripemd160() first appeared in SSLeay 0.9.0. All these functions have been available since OpenBSD 2.4. Ancient. Iconic. Still here. 🏛️

EVP_MD_CTX_copy() first appeared in OpenSSL 0.9.2b and has been available since OpenBSD 2.6. 💫

EVP_MD_CTX_md() first appeared in OpenSSL 0.9.5 and has been available since OpenBSD 2.7. 🦋

EVP_MD_CTX_init(), EVP_MD_CTX_create(), EVP_MD_CTX_cleanup(), EVP_MD_CTX_destroy(), EVP_DigestInit_ex(), EVP_DigestFinal_ex(), EVP_Digest(), and EVP_MD_CTX_copy_ex() first appeared in OpenSSL 0.9.7 and have been available since OpenBSD 3.2. ✨

EVP_sha224(), EVP_sha256(), EVP_sha384(), and EVP_sha512() first appeared in OpenSSL 0.9.7h and 0.9.8a and have been available since OpenBSD 4.0. 🌸

EVP_MD_CTX_new(), EVP_MD_CTX_reset(), and EVP_MD_CTX_free() first appeared in OpenSSL 1.1.0 and have been available since OpenBSD 6.3. 🎀

EVP_sha512_224() and EVP_sha512_256() first appeared in OpenSSL 1.1.1 and have been available since OpenBSD 7.4. The newest additions to the fam. 💖

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💌 Okay that's the whole page, bestie! This one came from Debian, dated June 8, 2025, and it's EVP_DIGESTINIT(3) if you need to cite her. Go hash something responsibly, and remember: free your contexts. 🫶✨💅