- NAME
- SYNOPSIS
- DESCRIPTION
- RETURN VALUES
- NOTES
- SEE ALSO
- HISTORY
- COPYRIGHT

# NAME

EVP_DigestVerifyInit_ex, EVP_DigestVerifyInit, EVP_DigestVerifyUpdate, EVP_DigestVerifyFinal, EVP_DigestVerify - EVP signature verification functions

# SYNOPSIS

     #include <openssl/evp.h>

     int EVP_DigestVerifyInit_ex(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
                                 const char *mdname, const char *props,
                                 EVP_PKEY *pkey, EVP_SIGNATURE *signature);
     int EVP_DigestVerifyInit(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
                              const EVP_MD *type, ENGINE *e, EVP_PKEY *pkey);
     int EVP_DigestVerifyUpdate(EVP_MD_CTX *ctx, const void *d, size_t cnt);
     int EVP_DigestVerifyFinal(EVP_MD_CTX *ctx, const unsigned char *sig,
                               size_t siglen);
     int EVP_DigestVerify(EVP_MD_CTX *ctx, const unsigned char *sigret,
                          size_t siglen, const unsigned char *tbs, size_t tbslen);

# DESCRIPTION

The EVP signature routines are a high level interface to digital signatures. Input data is digested first before the signature verification takes place.

`EVP_DigestVerifyInit_ex()` sets up verification context ctx to use a digest with the name mdname and public key pkey. The signature algorithm signature will be used for the actual signature verification which must be compatible with the public key. The name of the digest to be used is passed to the provider of the signature algorithm in use. How that provider interprets the digest name is provider specific. The provider may implement that digest directly itself or it may (optionally) 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.

The signature parameter may be NULL in which case a suitable signature algorithm implementation will be implicitly fetched based on the type of key in use. See provider(7) for further information about providers and fetching algorithms.

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.

ctx must be created with `EVP_MD_CTX_new()` before calling this function. If pctx is not NULL, the EVP_PKEY_CTX of the verification operation will be written to *pctx: this can be used to set alternative verification options. Note that 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_DigestVerifyInit_ex()` (which means the EVP_PKEY_CTX is created inside `EVP_DigestVerifyInit_ex()` and it will be freed automatically when the EVP_MD_CTX is freed).

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).

Not all digests can be used for all key types. The following combinations apply.

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.

`EVP_DigestVerifyInit()` works in the same way as `EVP_DigestVerifyInit_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 signature verification and digest algorithm implementations. e may be NULL.

`EVP_DigestVerifyUpdate()` hashes cnt bytes of data at d into the verification context ctx. This function can be called several times on the same ctx to include additional data.

`EVP_DigestVerifyFinal()` verifies the data in ctx against the signature in sig of length siglen.

`EVP_DigestVerify()` verifies tbslen bytes at tbs against the signature in sig of length siglen.

# RETURN VALUES

`EVP_DigestVerifyInit()` and `EVP_DigestVerifyUpdate()` return 1 for success and 0 for failure.

`EVP_DigestVerifyFinal()` and `EVP_DigestVerify()` return 1 for success; any other value indicates failure. A return value of zero indicates that the signature did not verify successfully (that is, tbs did not match the original data or the signature had an invalid form), while other values indicate a more serious error (and sometimes also indicate an invalid signature form).

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. This is because the code then becomes transparent to the algorithm used and much more flexible.

`EVP_DigestVerify()` is a one shot operation which verifies a single block of data in one function. For algorithms that support streaming it is equivalent to calling `EVP_DigestVerifyUpdate()` and `EVP_DigestVerifyFinal()`. For algorithms which do not support streaming (e.g. PureEdDSA) it is the only way to verify data.

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.

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_DigestVerifyFinal()` internally finalizes a copy of the digest context. This means that `EVP_VerifyUpdate()` and `EVP_VerifyFinal()` can be called later to digest and verify additional data.

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()` or a memory leak will occur.

# SEE ALSO

EVP_DigestSignInit(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_DigestVerifyInit()`, `EVP_DigestVerifyUpdate()` and `EVP_DigestVerifyFinal()` were added in OpenSSL 1.0.0.

`EVP_DigestVerifyInit_ex()` was added in OpenSSL 3.0.

`EVP_DigestVerifyUpdate()` was converted from a macro to a function in OpenSSL 3.0.

# COPYRIGHT

Copyright 2006-2018 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.