.\" $OpenBSD: EVP_PKEY_size.3,v 1.2 2023/09/12 15:29:03 schwarze Exp $ .\" full merge up to: OpenSSL eed9d03b Jan 8 11:04:15 2020 +0100 .\" .\" This file is a derived work. .\" The changes are covered by the following Copyright and license: .\" .\" Copyright (c) 2022, 2023 Ingo Schwarze .\" .\" Permission to use, copy, modify, and distribute this software for any .\" purpose with or without fee is hereby granted, provided that the above .\" copyright notice and this permission notice appear in all copies. .\" .\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS. 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IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: September 12 2023 $ .Dt EVP_PKEY_SIZE 3 .Os .Sh NAME .Nm EVP_PKEY_size , .Nm EVP_PKEY_bits , .Nm EVP_PKEY_security_bits .Nd EVP_PKEY information functions .Sh SYNOPSIS .In openssl/evp.h .Ft int .Fo EVP_PKEY_size .Fa "const EVP_PKEY *pkey" .Fc .Ft int .Fo EVP_PKEY_bits .Fa "const EVP_PKEY *pkey" .Fc .Ft int .Fo EVP_PKEY_security_bits .Fa "const EVP_PKEY *pkey" .Fc .Sh DESCRIPTION .Fn EVP_PKEY_size returns the maximum size in bytes needed for the output buffer for almost any operation that can be done with .Fa pkey . The primary use is with .Xr EVP_SignFinal 3 and .Xr EVP_SealInit 3 . The returned size is also large enough for the output buffer of .Xr EVP_PKEY_sign 3 , .Xr EVP_PKEY_encrypt 3 , .Xr EVP_PKEY_decrypt 3 , and .Xr EVP_PKEY_derive 3 . .Pp Unless the documentation for the operation says otherwise, the size returned by .Fn EVP_PKEY_size is only an upper limit and the final content of the target buffer may be smaller. It is therefore crucial to take note of the size given back by the function that performs the operation. For example, .Xr EVP_PKEY_sign 3 returns that length in the .Pf * Fa siglen argument. .Pp Using .Fn EVP_PKEY_size is discouraged with .Xr EVP_DigestSignFinal 3 . .Pp Most functions using an output buffer support passing .Dv NULL for the buffer and a pointer to an integer to get the exact size that this function call delivers in the context that it is called in. This allows those functions to be called twice, once to find out the exact buffer size, then allocate the buffer in between, and call that function again to actually output the data. For those functions, it isn't strictly necessary to call .Fn EVP_PKEY_size to find out the buffer size, but it may still be useful in cases where it's desirable to know the upper limit in advance. .Pp By default, .Fn EVP_PKEY_size is supported for the following algorithms: .Bl -column ED25519 "EVP_MAX_BLOCK_LENGTH = 32" .It Ta same result as from: .It CMAC Ta Dv EVP_MAX_BLOCK_LENGTH No = 32 .It DH Ta Xr DH_size 3 .It DSA Ta Xr DSA_size 3 .It EC Ta Xr ECDSA_size 3 .It ED25519 Ta 64, but see below .It GOST01 Ta 64 or 128 .It HMAC Ta Dv EVP_MAX_MD_SIZE No = 64 .It RSA Ta Xr RSA_size 3 .It X25519 Ta Dv X25519_KEYLEN No = 32 .El .Pp For .Dv EVP_PKEY_ED25519 , the situation is special: while the key size is .Dv ED25519_KEYLEN No = 32 bytes , .Fn EVP_PKEY_size returns 64 because the signature is longer than the keys. .Pp The application program can support additional algorithms by calling .Xr EVP_PKEY_asn1_set_public 3 . .Pp .Fn EVP_PKEY_bits returns the cryptographic length of the cryptosystem to which the key in .Fa pkey belongs, in bits. The definition of cryptographic length is specific to the key cryptosystem. By default, the following algorithms are supported: .Bl -column ED25519 "the public domain parameter p" DSA_bits(3) .It Ta cryptographic length = Ta same result as from: .It Ta significant bits in ... Ta .It DH Ta the public domain parameter Fa p Ta Xr DH_bits 3 .It DSA Ta the public domain parameter Fa p Ta Xr DSA_bits 3 .It EC Ta the order of the group Ta Xr EC_GROUP_order_bits 3 .It ED25519 Ta 253 Ta \(em .It GOST01 Ta 256 or 512 Ta \(em .It RSA Ta the public modulus Ta Xr RSA_bits 3 .It X25519 Ta 253 Ta \(em .El .Pp The application program can support additional algorithms by calling .Xr EVP_PKEY_asn1_set_public 3 . .Pp .Fn EVP_PKEY_security_bits returns the security strength measured in bits of the given .Fa pkey as defined in NIST SP800-57. By default, the following algorithms are supported: .Bl -column ED25519 DSA_security_bits(3) .It Ta same result as from: .It DH Ta Xr DH_security_bits 3 .It DSA Ta Xr DSA_security_bits 3 .It EC Ta Xr EC_GROUP_order_bits 3 divided by 2 .It ED25519 Ta 128 .It GOST01 Ta not supported, return value is \-2 .It RSA Ta Xr RSA_security_bits 3 .It X25519 Ta 128 .El .Pp For EC keys, if the result is greater than 80, it is rounded down to 256, 192, 128, 112, or 80. .Pp The application program can support additional algorithms by calling .Xr EVP_PKEY_asn1_set_security_bits 3 . .Sh RETURN VALUES .Fn EVP_PKEY_size and .Fn EVP_PKEY_bits return a positive number or 0 if this size isn't available. .Pp .Fn EVP_PKEY_security_bits returns a number in the range from 0 to 256 inclusive or \-2 if this function is unsupported for the algorithm used by .Fa pkey . It returns 0 if .Fa pkey is .Dv NULL . .Sh SEE ALSO .Xr EVP_PKEY_decrypt 3 , .Xr EVP_PKEY_derive 3 , .Xr EVP_PKEY_encrypt 3 , .Xr EVP_PKEY_new 3 , .Xr EVP_PKEY_sign 3 , .Xr EVP_SealInit 3 , .Xr EVP_SignFinal 3 .Sh HISTORY .Fn EVP_PKEY_size first appeared in SSLeay 0.6.0 and .Fn EVP_PKEY_bits in SSLeay 0.9.0. Both functions have been available since .Ox 2.4 . .Pp .Fn EVP_PKEY_security_bits first appeared in OpenSSL 1.1.0 and has been available since .Ox 7.2 .