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// Copyright 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "base/compiler_specific.h"
#include "base/time/time.h"
#include "net/base/net_export.h"
#include "net/der/input.h"
#include "net/der/tag.h"
namespace net {
namespace der {
// Parses a DER-encoded ASN.1 structure. DER (distinguished encoding rules)
// encodes each data value with a tag, length, and value (TLV). The tag
// indicates the type of the ASN.1 value. Depending on the type of the value,
// it could contain arbitrary bytes, so the length of the value is encoded
// after the tag and before the value to indicate how many bytes of value
// follow. DER also defines how the values are encoded for particular types.
// This Parser places a few restrictions on the DER encoding it can parse. The
// largest restriction is that it only supports tags which have a tag number
// no greater than 30 - these are the tags that fit in a single octet. The
// second restriction is that the maximum length for a value that can be parsed
// is 4GB. Both of these restrictions should be fine for any reasonable input.
// The Parser class is mainly focused on parsing the TLV structure of DER
// encoding, and does not directly handle parsing primitive values (other
// functions in the net::der namespace are provided for this.) When a Parser
// is created, it is passed in a reference to the encoded data. Because the
// encoded data is not owned by the Parser, the data cannot change during the
// lifespan of the Parser. The Parser functions by keeping a pointer to the
// current TLV which starts at the beginning of the input and advancing through
// the input as each TLV is read. As such, a Parser instance is thread-unsafe.
// Most methods for using the Parser write the current tag and/or value to
// the output parameters provided and then advance the input to the next TLV.
// None of the methods explicitly expose the length because it is part of the
// value. All methods return a boolean indicating whether there was a parsing
// error with the current TLV.
// Some methods are provided in the Parser class as convenience to both read
// the current TLV from the input and also parse the DER encoded value,
// converting it to a corresponding C++ type. These methods simply combine
// ReadTag() with the appropriate ParseType() free function.
// The design of DER encoding allows for nested data structures with
// constructed values, where the value is a series of TLVs. The Parser class
// is not designed to traverse through a nested encoding from a single object,
// but it does facilitate parsing nested data structures through the
// convenience methods ReadSequence() and the more general ReadConstructed(),
// which provide the user with another Parser object to traverse the next
// level of TLVs.
// For a brief example of how to use the Parser, suppose we have the following
// ASN.1 type definition:
// Foo ::= SEQUENCE {
// quux OCTET STRING }
// If we have a DER-encoded Foo in an Input |encoded_value|, the
// following code shows an example of how to parse the quux field from the
// encoded data.
// bool ReadQuux(const Input& encoded_value, Input* quux_out) {
// Parser parser(encoded_value);
// Parser foo_parser;
// if (!parser.ReadSequence(&foo_parser))
// return false;
// if (!foo_parser->SkipOptionalTag(kOctetString))
// return false;
// if (!foo_parser->ReadTag(kOctetString, quux_out))
// return false;
// return true;
// }
class NET_EXPORT Parser {
// Default constructor; equivalent to calling Parser(Input()). This only
// exists so that a Parser can be stack allocated and passed in to
// ReadConstructed() and similar methods.
// Creates a parser to parse over the data represented by input. This class
// assumes that the underlying data will not change over the lifetime of
// the Parser object.
explicit Parser(const Input& input);
// Returns whether there is any more data left in the input to parse. This
// does not guarantee that the data is parseable.
bool HasMore();
// Reads the current TLV from the input and advances. If the tag or length
// encoding for the current value is invalid, this method returns false and
// does not advance the input. Otherwise, it returns true, putting the
// read tag in |tag| and the value in |out|.
bool ReadTagAndValue(Tag* tag, Input* out) WARN_UNUSED_RESULT;
// Reads the current TLV from the input and advances. Unlike ReadTagAndValue
// where only the value is put in |out|, this puts the raw bytes from the
// tag, length, and value in |out|.
bool ReadRawTLV(Input* out) WARN_UNUSED_RESULT;
// Basic methods for reading or skipping the current TLV, with an
// expectation of what the current tag should be. It should be possible
// to parse any structure with these 4 methods; convenience methods are also
// provided to make some cases easier.
// If the current tag in the input is |tag|, it puts the corresponding value
// in |out|, sets |was_present| to true, and advances the input to the next
// TLV. If the current tag is something else, then |was_present| is set to
// false and the input is not advanced. Like ReadTagAndValue, it returns
// false if the encoding is invalid and does not advance the input.
bool ReadOptionalTag(Tag tag,
Input* out,
bool* was_present) WARN_UNUSED_RESULT;
// Like ReadOptionalTag, but the value is discarded.
bool SkipOptionalTag(Tag tag, bool* was_present) WARN_UNUSED_RESULT;
// If the current tag matches |tag|, it puts the current value in |out|,
// advances the input, and returns true. Otherwise, it returns false.
bool ReadTag(Tag tag, Input* out) WARN_UNUSED_RESULT;
// Advances the input and returns true if the current tag matches |tag|;
// otherwise it returns false.
bool SkipTag(Tag tag) WARN_UNUSED_RESULT;
// Convenience methods to combine parsing the TLV with parsing the DER
// encoding for a specific type.
// Reads the current TLV from the input, checks that the tag matches |tag|
// and is a constructed tag, and creates a new Parser from the value.
bool ReadConstructed(Tag tag, Parser* out) WARN_UNUSED_RESULT;
// A more specific form of ReadConstructed that expects the current tag
// to be 0x30 (SEQUENCE).
bool ReadSequence(Parser* out) WARN_UNUSED_RESULT;
// Expects the current tag to be kInteger, and calls ParseUint64 on the
// current value. Note that DER-encoded integers are arbitrary precision,
// so this method will fail for valid input that represents an integer
// outside the range of an int64.
bool ReadUint64(uint64_t* out) WARN_UNUSED_RESULT;
// Lower level methods. The previous methods couple reading data from the
// input with advancing the Parser's internal pointer to the next TLV; these
// lower level methods decouple those two steps into methods that read from
// the current TLV and a method that advances the internal pointer to the
// next TLV.
// Reads the current TLV from the input, putting the tag in |tag| and the raw
// value in |out|, but does not advance the input. Returns true if the tag
// and length are successfully read and the output exists.
bool PeekTagAndValue(Tag* tag, Input* out) WARN_UNUSED_RESULT;
// Advances the input to the next TLV. This method only needs to be called
// after PeekTagAndValue; all other methods will advance the input if they
// read something.
bool Advance();
ByteReader input_;
Mark advance_mark_;
} // namespace der
} // namespace net
#endif // NET_DER_PARSER_H_