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136 lines
4.3 KiB
Plaintext
136 lines
4.3 KiB
Plaintext
=pod
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=head1 NAME
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EVP_KEYEXCH-DH
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- DH Key Exchange algorithm support
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=head1 DESCRIPTION
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Key exchange support for the B<DH> and B<DHX> key types.
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Please note that although both key types support the same key exchange
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operations, they cannot be used together in a single key exchange. It
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is not possible to use a private key of the B<DH> type in key exchange
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with the public key of B<DHX> type and vice versa.
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=head2 DH and DHX key exchange parameters
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=over 4
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=item "pad" (B<OSSL_EXCHANGE_PARAM_PAD>) <unsigned integer>
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Sets the padding mode for the associated key exchange ctx.
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Setting a value of 1 will turn padding on.
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Setting a value of 0 will turn padding off.
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If padding is off then the derived shared secret may be smaller than the
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largest possible secret size.
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If padding is on then the derived shared secret will have its first bytes
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filled with zeros where necessary to make the shared secret the same size as
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the largest possible secret size.
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The padding mode parameter is ignored (and padding implicitly enabled) when
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the KDF type is set to "X942KDF-ASN1" (B<OSSL_KDF_NAME_X942KDF_ASN1>).
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=item "kdf-type" (B<OSSL_EXCHANGE_PARAM_KDF_TYPE>) <UTF8 string>
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=item "kdf-digest" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST>) <UTF8 string>
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=item "kdf-digest-props" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS>) <UTF8 string>
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=item "kdf-outlen" (B<OSSL_EXCHANGE_PARAM_KDF_OUTLEN>) <unsigned integer>
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=item "kdf-ukm" (B<OSSL_EXCHANGE_PARAM_KDF_UKM>) <octet string>
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=item "fips-indicator" (B<OSSL_EXCHANGE_PARAM_FIPS_APPROVED_INDICATOR>) <integer>
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=item "key-check" (B<OSSL_EXCHANGE_PARAM_FIPS_KEY_CHECK>) <integer>
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=item "digest-check" (B<OSSL_EXCHANGE_PARAM_FIPS_DIGEST_CHECK>) <integer>
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See L<provider-keyexch(7)/Common Key Exchange parameters>.
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=item "cekalg" (B<OSSL_KDF_PARAM_CEK_ALG>) <octet string ptr>
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See L<provider-kdf(7)/KDF Parameters>.
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=back
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=head1 EXAMPLES
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The examples assume a host and peer both generate keys using the same
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named group (or domain parameters). See L<EVP_PKEY-DH(7)/Examples>.
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Both the host and peer transfer their public key to each other.
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To convert the peer's generated key pair to a public key in DER format in order
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to transfer to the host:
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EVP_PKEY *peer_key; /* It is assumed this contains the peers generated key */
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unsigned char *peer_pub_der = NULL;
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int peer_pub_der_len;
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peer_pub_der_len = i2d_PUBKEY(peer_key, &peer_pub_der);
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...
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OPENSSL_free(peer_pub_der);
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To convert the received peer's public key from DER format on the host:
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const unsigned char *pd = peer_pub_der;
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EVP_PKEY *peer_pub_key = d2i_PUBKEY(NULL, &pd, peer_pub_der_len);
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...
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EVP_PKEY_free(peer_pub_key);
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To derive a shared secret on the host using the host's key and the peer's public
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key:
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/* It is assumed that the host_key and peer_pub_key are set up */
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void derive_secret(EVP_KEY *host_key, EVP_PKEY *peer_pub_key)
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{
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unsigned int pad = 1;
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OSSL_PARAM params[2];
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unsigned char *secret = NULL;
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size_t secret_len = 0;
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EVP_PKEY_CTX *dctx = EVP_PKEY_CTX_new_from_pkey(NULL, host_key, NULL);
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EVP_PKEY_derive_init(dctx);
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/* Optionally set the padding */
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params[0] = OSSL_PARAM_construct_uint(OSSL_EXCHANGE_PARAM_PAD, &pad);
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params[1] = OSSL_PARAM_construct_end();
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EVP_PKEY_CTX_set_params(dctx, params);
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EVP_PKEY_derive_set_peer(dctx, peer_pub_key);
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/* Get the size by passing NULL as the buffer */
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EVP_PKEY_derive(dctx, NULL, &secret_len);
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secret = OPENSSL_zalloc(secret_len);
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EVP_PKEY_derive(dctx, secret, &secret_len);
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...
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OPENSSL_clear_free(secret, secret_len);
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EVP_PKEY_CTX_free(dctx);
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}
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Very similar code can be used by the peer to derive the same shared secret
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using the host's public key and the peer's generated key pair.
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=head1 SEE ALSO
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L<EVP_PKEY-DH(7)>,
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L<EVP_PKEY-FFC(7)>,
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L<EVP_PKEY(3)>,
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L<provider-keyexch(7)>,
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L<provider-keymgmt(7)>,
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L<OSSL_PROVIDER-default(7)>,
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L<OSSL_PROVIDER-FIPS(7)>,
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=head1 COPYRIGHT
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Copyright 2020-2024 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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