EVP_SIGNINIT(3SSL)

📦
version 4.0.3 · 14 versions
, in ubuntu stonking

💅 EVP_SIGNINIT ✨

okay bestie, this is the OpenSSL page for the EVP signing functions — EVP_SignInit, EVP_SignInit_ex, EVP_SignUpdate, EVP_SignFinal_ex, and EVP_SignFinal. it's an 3SSL manual, so make yourself comfy, we're doing digital signatures 💖

🌸 NAME 🌸

EVP_SignInit, EVP_SignInit_ex, EVP_SignUpdate, EVP_SignFinal_ex, EVP_SignFinal — the EVP signing functions, aka the glow-up squad for signing stuff ✨

🎀 SYNOPSIS 🎀

#include <openssl/evp.h>

int EVP_SignInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl);
int EVP_SignUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt);
int EVP_SignFinal_ex(EVP_MD_CTX *ctx, unsigned char *md, unsigned int *s,
                     EVP_PKEY *pkey, OSSL_LIB_CTX *libctx, const char *propq);
int EVP_SignFinal(EVP_MD_CTX *ctx, unsigned char *sig, unsigned int *s,
                  EVP_PKEY *pkey);

void EVP_SignInit(EVP_MD_CTX *ctx, const EVP_MD *type);

🦋 DESCRIPTION 🦋

The EVP signature routines are your high-level, main-character interface to digital signatures. no cap 👑

  • EVP_SignInit_ex() — sets up the signing context ctx to use digest type. impl must be NULL (like, literally, don't try it). and heads up: ctx has to be created with EVP_MD_CTX_new() before you call this. prep first, then slay.
  • EVP_SignUpdate() — hashes cnt bytes of data at d into the signature context ctx. you can call this several times on the same ctx to feed in extra data — it's giving "snack by snack" energy 🍓
  • EVP_SignFinal_ex() — signs the data in ctx using the private key pkey and drops the signature into sig. the library context libctx and property query propq get used when creating a context to use with the key pkey. sig must be at least EVP_PKEY_get_size(pkey) bytes in size. s is an OUT parameter, not used as an IN parameter — it does not read from you, it only writes back. the number of bytes written (i.e. the length of the signature) goes into the integer at s, and at most EVP_PKEY_get_size(pkey) bytes will be written.
  • EVP_SignFinal() — basically EVP_SignFinal_ex()'s twin, except it uses default values of NULL for the library context libctx and the property query propq.
  • EVP_SignInit() — initializes a signing context ctx to use the default implementation of digest type.

💖 RETURN VALUES 💖

EVP_SignInit_ex(), EVP_SignUpdate(), EVP_SignFinal_ex() and EVP_SignFinal() return 1 for success and 0 for failure. that's it, that's the vibe check.

the error codes can be obtained by ERR_get_error(3).

✨ NOTES ✨

the EVP interface to digital signatures should almost always be used in preference to the low-level interfaces. why? because then your code becomes transparent to the algorithm used and wayyy more flexible. iconic behaviour tbh.

when signing with some private key types, the random number generator must be seeded. if the automatic seeding or reseeding of the OpenSSL CSPRNG fails due to external circumstances (see RAND(7)), the operation will fail 😭

the call to EVP_SignFinal() internally finalizes a copy of the digest context. this means calls to EVP_SignUpdate() and EVP_SignFinal() can be made later to digest and sign additional data. applications may disable this behavior by setting the EVP_MD_CTX_FLAG_FINALISE context flag via EVP_MD_CTX_set_flags(3).

since only a copy of the digest context is ever finalized, the context must be cleaned up after use by calling EVP_MD_CTX_free() — otherwise a memory leak will occur. yes, a real one. clean up your room bestie 🧹

also note: not all providers support continuation. if the selected provider does not allow duplicating contexts, EVP_SignFinal() will finalize the digest context, and attempting to process additional data via EVP_SignUpdate() will result in an error. so like, read the room (and the provider).

💀 heads up: that memory leak thing is not a joke. EVP_MD_CTX_free() or bust.

🙈 BUGS 🙈

older versions of this documentation wrongly stated that calls to EVP_SignUpdate() could not be made after calling EVP_SignFinal(). incorrect, embarrassing, moving on.

since the private key is passed in the call to EVP_SignFinal(), any error relating to the private key (for example an unsuitable key and digest combination) will not be indicated until after potentially large amounts of data have been passed through EVP_SignUpdate(). so you find out late. tragic.

it is not possible to change the signing parameters using these functions.

the previous two bugs are fixed in the newer EVP_DigestSign*() functions. glow-up achieved ⭐

🌈 SEE ALSO 🌈

EVP_PKEY_get_size(3), EVP_PKEY_get_bits(3), EVP_PKEY_get_security_bits(3), EVP_VerifyInit(3), EVP_DigestInit(3), evp(7), HMAC(3), MD2(3), MD5(3), MDC2(3), RIPEMD160(3), SHA1(3), openssl-dgst(1)

⭐ HISTORY ⭐

the function EVP_SignFinal_ex() was added in OpenSSL 3.0.

Copyright 2000-2023 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>.

and that's the whole page, bestie 🫶 this was OpenSSL 4.0.3, dated 2026-09-29, filed under EVP_SIGNINIT(3SSL). go forth and sign responsibly, ily 💜✨

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