mirror of https://github.com/openssl/openssl.git
150 lines
4.9 KiB
Plaintext
150 lines
4.9 KiB
Plaintext
=pod
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=head1 NAME
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DERlib - internal OpenSSL DER library
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=head1 DESCRIPTION
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OpenSSL contains an internal small DER reading and writing library,
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as an alternative to the publicly known i2d and d2i functions. It's
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solely constituted of functions that work as building blocks to create
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more similar functions to encode and decode larger structures.
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All these functions have similar function signatures (C<something>
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will vary depending on what the function will encode):
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int DER_w_something(WPACKET *pkt, int tag, ...);
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=begin comment
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When readers are added, add this:
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int DER_r_something(PACKET *pkt, int tag, ...);
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=end comment
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I<pkt> is the packet context used, and I<tag> should be the
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context-specific tag value of the element being handled, or -1 if there
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is no tag number for that element (you may use the convenience macro
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B<DER_NO_CONTEXT> instead of -1). Any argument following is the C
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variable that's being encoded or decoded.
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=head2 DER writers / encoders
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DER writers are based in L<WPACKET(3)>, a generic packet writing
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library, so before using any of them, I<pkt> must be initialized
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using L<WPACKET_init_der(3)> or L<WPACKET_init_null_der(3)>
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DER writers must be used in reverse order, except for the wrapping
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functions that implement a constructed element. The latter are easily
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recognised by their function name including the words C<begin> and
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C<end>. As an example, we can look at the DSA signature structure,
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which is defined like this in ASN.1 terms:
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-- Copied from RFC 3279, section 2.2.2
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Dss-Sig-Value ::= SEQUENCE {
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r INTEGER,
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s INTEGER }
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With the DER library, this is the corresponding code, given two OpenSSL
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B<BIGNUM>s I<r> and I<s>:
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int ok = ossl_DER_w_begin_sequence(pkt, -1)
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&& ossl_DER_w_bn(pkg, -1, s)
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&& ossl_DER_w_bn(pkg, -1, r)
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&& ossl_DER_w_end_sequence(pkt, -1);
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As an example of the use of I<tag>, an ASN.1 element like this:
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v [1] INTEGER OPTIONAL
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Would be encoded like this:
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ossl_DER_w_bn(pkt, 1, v)
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=begin comment
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=head2 DER readers / decoders
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TBA
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=end comment
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=head1 EXAMPLES
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A more complex example, encoding the AlgorithmIdentifier with
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RSASSA-PSS values.
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As a reminder, the AlgorithmIdentifier is specified like this:
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-- From RFC 3280, section 4.1.1.2
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AlgorithmIdentifier ::= SEQUENCE {
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algorithm OBJECT IDENTIFIER,
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parameters ANY DEFINED BY algorithm OPTIONAL }
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And the RSASSA-PSS OID and parameters are specified like this:
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-- From RFC 3279, section 3.1
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id-RSASSA-PSS OBJECT IDENTIFIER ::= { pkcs-1 10 }
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RSASSA-PSS-params ::= SEQUENCE {
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hashAlgorithm [0] HashAlgorithm DEFAULT
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sha1Identifier,
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maskGenAlgorithm [1] MaskGenAlgorithm DEFAULT
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mgf1SHA1Identifier,
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saltLength [2] INTEGER DEFAULT 20,
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trailerField [3] INTEGER DEFAULT 1 }
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The value we want to encode, written in ASN.1 syntax:
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{
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algorithm id-RSASSA-PSS,
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parameters {
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hashAlgorithm sha256Identifier,
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maskGenAlgorithm mgf1SHA256Identifier,
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saltLength 20 -- unnecessarily explicit
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}
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}
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Assuming that we have precompiled constants for C<id-RSASSA-PSS>,
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C<sha256Identifier> and C<mgf1SHA256Identifier>, the DER writing code
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looks as follows. This is a complete function to write that specific
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value:
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int DER_w_AlgorithmIdentifier_RSASSA_PSS_special(WPACKET *pkt,
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int tag,
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RSA *rsa)
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{
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return ossl_DER_w_begin_sequence(pkt, tag)
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&& (ossl_DER_w_begin_sequence(pkt, DER_NO_CONTEXT)
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&& ossl_DER_w_ulong(pkt, 2, 20)
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&& ossl_DER_w_precompiled(pkt, 1,
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der_mgf1SHA256Identifier,
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sizeof(der_mgf1SHA256Identifier))
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&& ossl_DER_w_precompiled(pkt, 0,
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der_sha256Identifier,
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sizeof(der_sha256Identifier))
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&& ossl_DER_w_end_sequence(pkt, DER_NO_CONTEXT))
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&& ossl_DER_w_precompiled(pkt, DER_NO_CONTEXT,
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der_id_RSASSA_PSS,
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sizeof(der_id_RSASSA_PSS))
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&& ossl_DER_w_end_sequence(pkt, tag);
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}
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=head1 SEE ALSO
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L<ossl_DER_w_bn(3)>, L<ossl_DER_w_begin_sequence(3)>,
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L<ossl_DER_w_precompiled(3)>
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=head1 COPYRIGHT
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Copyright 2020 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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