EVP_DIGESTSIGNINIT(3ossl)

📦
version 3.5.9 · 8 versions
, in fedora 43, fedora 44

💖 EVP_DIGESTSIGNINIT 💖

okay bestie, sit down, hydrate, because today we're learning the EVP_DigestSignInit family — the glow-up signing functions from OpenSSL, manual section 3ossl. it's giving high-level crypto interface and honestly? iconic. 🫶

✨ NAME ✨

EVP_DigestSignInit_ex, EVP_DigestSignInit, EVP_DigestSignUpdate, EVP_DigestSignFinal, EVP_DigestSign — the EVP signing functions, aka the whole squad 💅

🎀 SYNOPSIS 🎀

#include <openssl/evp.h>

int EVP_DigestSignInit_ex(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
                          const char *mdname, OSSL_LIB_CTX *libctx,
                          const char *props, EVP_PKEY *pkey,
                          const OSSL_PARAM params[]);
int EVP_DigestSignInit(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
                       const EVP_MD *type, ENGINE *e, EVP_PKEY *pkey);
int EVP_DigestSignUpdate(EVP_MD_CTX *ctx, const void *d, size_t cnt);
int EVP_DigestSignFinal(EVP_MD_CTX *ctx, unsigned char *sig, size_t *siglen);

int EVP_DigestSign(EVP_MD_CTX *ctx, unsigned char *sig,
                   size_t *siglen, const unsigned char *tbs,
                   size_t tbslen);

🌸 DESCRIPTION 🌸

so the EVP signature routines are a high-level interface to digital signatures, periodt. and the vibe is: your input data gets digested first before any signing happens. digest then sign, in that order, no skipping steps bestie 💖

🦋 EVP_DigestSignInit_ex()

this one sets up the signing context ctx to use a digest with the name mdname and private key pkey. the name of the digest gets passed along to the provider of the signature algorithm in use, and how that provider interprets the digest name is, like, provider specific — their house, their rules. the provider may implement that digest directly itself, or it may (optionally, if it's feeling generous) choose to fetch it, which could result in a digest from a different provider being selected. if the provider supports fetching the digest, then it may use the props argument for the properties to be used during the fetch. and finally, the passed parameters params, if not NULL, are set on the context before returning. slay ✨

the pkey algorithm is used to fetch an EVP_SIGNATURE method implicitly, to be used for the actual signing. see "Implicit fetch" in provider(7) for more info about implicit fetches.

The OpenSSL default and legacy providers support fetching digests and can fetch those digests from any available provider. The OpenSSL FIPS provider also supports fetching digests but will only fetch digests that are themselves implemented inside the FIPS provider. (fips stays in its lane, respect 🫡)

ctx must be created with EVP_MD_CTX_new() before calling this function. if pctx is not NULL, the EVP_PKEY_CTX of the signing operation will be written to *pctx — you can use that to set alternative signing options. heads up though: any existing value in *pctx is overwritten. 😭

the EVP_PKEY_CTX value returned must not be freed directly by the application if ctx is not assigned an EVP_PKEY_CTX value before being passed to EVP_DigestSignInit_ex() — which means the EVP_PKEY_CTX is created inside EVP_DigestSignInit_ex() and it will be freed automatically when the EVP_MD_CTX is freed. do NOT double free, bestie, that's not the vibe 💀

if the EVP_PKEY_CTX to be used is created by EVP_DigestSignInit_ex, then it will use the OSSL_LIB_CTX specified in libctx and the property query string specified in props.

the digest mdname may be NULL if the signing algorithm supports it. the props argument can always be NULL. easy.

No EVP_PKEY_CTX will be created by EVP_DigestSignInit_ex() if the passed ctx has already been assigned one via EVP_MD_CTX_set_pkey_ctx(3). see also SM2(7).

👑 which key types tho

Only EVP_PKEY types that support signing can be used with these functions. and yes, that includes MAC algorithms, because MAC generation is considered a form of "signing" here — we're being inclusive, we love that 🌈

Built-in EVP_PKEY types supported by these functions are: CMAC, Poly1305, DSA, ECDSA, HMAC, RSA, SipHash, Ed25519 and Ed448.

Not all digests can be used for all key types. The following combinations apply. (this is the compatibility chart, memorize it for the group chat quiz 📝)

  • DSA — Supports SHA1, SHA224, SHA256, SHA384 and SHA512
  • ECDSA — Supports SHA1, SHA224, SHA256, SHA384, SHA512 and SM3
  • RSA with no padding — Supports no digests (the digest type must be NULL)
  • RSA with X931 padding — Supports SHA1, SHA256, SHA384 and SHA512
  • All other RSA padding types — Support SHA1, SHA224, SHA256, SHA384, SHA512, MD5, MD5_SHA1, MD2, MD4, MDC2, SHA3-224, SHA3-256, SHA3-384, SHA3-512
  • Ed25519 and Ed448 — Support no digests (the digest type must be NULL)
  • HMAC — Supports any digest
  • CMAC, Poly1305 and SipHash — Will ignore any digest provided

If RSA-PSS is used and restrictions apply then the digest must match. fr fr, no mismatching 🙈

🧚 EVP_DigestSignInit()

works in the same way as EVP_DigestSignInit_ex() except that the mdname parameter will be inferred from the supplied digest type, and props will be NULL. where supplied, the ENGINE e will be used for the signing and digest algorithm implementations. e may be NULL. (the classic version, still cute 💜)

🌟 EVP_DigestSignUpdate()

hashes cnt bytes of data at d into the signature context ctx. this function can be called several times on the same ctx to include additional data — stack it up, babe 🍓

ctx MUST NOT be NULL.

⭐ EVP_DigestSignFinal()

unless sig is NULL, this signs the data in ctx and places the signature in sig. otherwise, the maximum necessary size of the output buffer is written to the siglen parameter. if sig is not NULL, then before the call the siglen parameter should contain the length of the sig buffer. if the call is successful, the signature is written to sig and the amount of data written goes to siglen. two-step dance, very choreographed 💅

🌈 EVP_DigestSign()

similar to a single call to EVP_DigestSignUpdate() and EVP_DigestSignFinal(). unless sig is NULL, EVP_DigestSign() signs the data tbs of length tbslen bytes and places the signature in a buffer sig of size siglen. if sig is NULL, the maximum necessary size of the signature buffer is written to the siglen parameter. one shot, one vibe, no cap 🔥

💀 RETURN VALUES 💀

EVP_DigestSignInit(), EVP_DigestSignUpdate(), EVP_DigestSignFinal() and EVP_DigestSign() return 1 for success and 0 for failure. that's it, that's the whole mood.

The error codes can be obtained from 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 the code then becomes transparent to the algorithm used and much more flexible. glow-up unlocked ✨

EVP_DigestSign() is a one shot operation which signs a single block of data in one function. for algorithms that support streaming, it is equivalent to calling EVP_DigestSignUpdate() and EVP_DigestSignFinal(). for algorithms which do not support streaming (e.g. PureEdDSA), it is the only way to sign data. so if your algo won't stream, this is your main character moment 👑

In previous versions of OpenSSL there was a link between message digest types and public key algorithms. this meant that "clone" digests such as EVP_dss1() needed to be used to sign using SHA1 and DSA. this is no longer necessary and the use of clone digest is now discouraged. we don't do that anymore, bestie, that's so 1.x 🚫

For some key types and parameters 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_DigestSignFinal() internally finalizes a copy of the digest context. this means that calls to EVP_DigestSignUpdate() and EVP_DigestSignFinal() can be called 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).

Note that not all providers support continuation. in case the selected provider does not allow duplicating contexts, EVP_DigestSignFinal() will finalize the digest context and attempting to process additional data via EVP_DigestSignUpdate() will result in an error. rude but okay 🙈

EVP_DigestSignInit() and EVP_DigestSignInit_ex() functions can be called multiple times on a context and the parameters set by previous calls should be preserved if the pkey parameter is NULL. the call then just resets the state of the ctx. a lil reset, a lil refresh 🌸

EVP_DigestSign() can not be called again, once a signature is generated (by passing sig as non NULL), unless the EVP_MD_CTX is reinitialised by calling EVP_DigestSignInit_ex().

Ignoring failure returns of EVP_DigestSignInit() and EVP_DigestSignInit_ex() functions can lead to subsequent undefined behavior when calling EVP_DigestSignUpdate(), EVP_DigestSignFinal(), or EVP_DigestSign(). check your return values, bestie, i am begging 😭

The use of EVP_PKEY_get_size() with these functions is discouraged because some signature operations may have a signature length which depends on the parameters set. as a result EVP_PKEY_get_size() would have to return a value which indicates the maximum possible signature for any set of parameters.

🫶 SEE ALSO 🫶

EVP_DigestVerifyInit(3), EVP_DigestInit(3), evp(7), HMAC(3), MD2(3), MD5(3), MDC2(3), RIPEMD160(3), SHA1(3), openssl-dgst(1), RAND(7)

📜 HISTORY 📜

  • EVP_DigestSignInit(), EVP_DigestSignUpdate() and EVP_DigestSignFinal() were added in OpenSSL 1.0.0.
  • EVP_DigestSignInit_ex() was added in OpenSSL 3.0.
  • EVP_DigestSignUpdate() was converted from a macro to a function in OpenSSL 3.0. glow-up arc complete 🦋

Copyright 2006-2024 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 3.5.9, dated 2026-09-29, from manual EVP_DIGESTSIGNINIT(3ossl). go sign something responsibly, drink water, and remember: check your return values 💅✨🎀

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