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+=pod
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+
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+=head1 NAME
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+
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+DERlib - internal OpenSSL DER library
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+
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+=head1 DESCRIPTION
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+
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+OpenSSL contains an internal small DER reading and writing library,
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+as an alternative to the publically 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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+
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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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+
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+ int DER_w_something(WPACKET *pkt, int tag, ...);
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+
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+=begin comment
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+
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+When readers are added, add this:
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+
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+ int DER_r_something(PACKET *pkt, int tag, ...);
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+
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+=end comment
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+
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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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+
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+=head2 DER writers / encoders
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+
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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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+
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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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+
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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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+
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+With the DER library, this is the correspoding code, given two OpenSSL
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+B<BIGNUM>s I<r> and I<s>:
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+
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+ int ok = DER_w_begin_sequence(pkt, -1)
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+ && DER_w_bn(pkg, -1, s)
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+ && DER_w_bn(pkg, -1, r)
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+ && DER_w_end_sequence(pkt, -1);
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+
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+As an example of the use of I<tag>, an ASN.1 element like this:
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+
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+ v [1] INTEGER OPTIONAL
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+
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+Would be encoded like this:
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+
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+ DER_w_bn(pkt, 1, v)
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+
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+=begin comment
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+
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+=head2 DER readers / decoders
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+
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+TBA
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+
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+=end comment
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+
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+=head1 EXAMPLES
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+
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+A more complex example, encoding the AlgorithmIdentifier with
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+RSASSA-PSS values.
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+
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+As a reminder, the AlgorithmIdentifier is specified like this:
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+
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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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+
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+And the RSASSA-PSS OID and parameters are specified like this:
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+
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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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+
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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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+
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+The value we want to encode, written in ASN.1 syntax:
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+
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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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+
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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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+
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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 DER_w_begin_sequence(pkt, tag)
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+ && (DER_w_begin_sequence(pkt, DER_NO_CONTEXT)
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+ && DER_w_ulong(pkt, 2, 20)
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+ && DER_w_precompiled(pkt, 1,
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+ der_mgf1SHA256Identifier,
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+ sizeof(der_mgf1SHA256Identifier))
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+ && DER_w_precompiled(pkt, 0,
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+ der_sha256Identifier,
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+ sizeof(der_sha256Identifier))
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+ && DER_w_end_sequence(pkt, DER_NO_CONTEXT))
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+ && 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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+ && DER_w_end_sequence(pkt, tag);
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+ }
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+
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+=head1 SEE ALSO
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+
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+L<DER_w_bn(3)>, L<DER_w_begin_sequence(3)>, L<DER_w_precompiled(3)>
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+
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+=head1 COPYRIGHT
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+
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+Copyright 2020 The OpenSSL Project Authors. All Rights Reserved.
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+
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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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+
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+=cut
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