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//===----------------------------------------------------------------------===//
//
// This source file is part of the SwiftASN1 open source project
//
// Copyright (c) 2019-2020 Apple Inc. and the SwiftASN1 project authors
// Licensed under Apache License v2.0
//
// See LICENSE.txt for license information
// See CONTRIBUTORS.txt for the list of SwiftASN1 project authors
//
// SPDX-License-Identifier: Apache-2.0
//
//===----------------------------------------------------------------------===//
/// A bitstring is a representation of a sequence of bits.
///
/// In ASN.1, bitstrings serve two different use-cases. The first is as arbitrary-length sequences of bits.
/// These are typically used to encode cryptographic keys, such as RSA keys. The second is as a form of bitset.
///
/// In the case of a bitset, DER has additional requirements as to how to represent the object. This type does not
/// enforce those additional rules: users are expected to implement that validation themselves.
public struct ASN1BitString: DERImplicitlyTaggable {
/// The default identifier for this type.
///
/// Evaluates to ``ASN1Identifier/bitString``.
@inlinable
public static var defaultIdentifier: ASN1Identifier {
.bitString
}
/// The raw bytes that make up this bitstring.
///
/// The last ``paddingBits`` number of bits in the final octet of this byte sequence must be zero.
public var bytes: ArraySlice<UInt8> {
didSet {
try! self._validate()
}
}
/// The number of bits in the last octet of ``bytes`` that are not part of this bitstring.
///
/// The excluded bits are the least significant bits.
///
/// If ``bytes`` is empty then this value must be 0.
public var paddingBits: Int {
didSet {
precondition((0..<8).contains(self.paddingBits))
try! self._validate()
}
}
@inlinable
public init(derEncoded node: ASN1Node, withIdentifier identifier: ASN1Identifier) throws {
guard node.identifier == identifier else {
throw ASN1Error.unexpectedFieldType(node.identifier)
}
guard case .primitive(let content) = node.content else {
preconditionFailure("ASN.1 parser generated primitive node with constructed content")
}
// The initial octet explains how many of the bits in the _final_ octet are not part of the bitstring.
guard let paddingBits = content.first, (0..<8).contains(paddingBits) else {
throw ASN1Error.invalidASN1Object(
reason: "Unable to determine a valid number of padding bits for ASN1BitString"
)
}
self.paddingBits = Int(paddingBits)
self.bytes = content.dropFirst()
try self._validate()
}
/// Construct an ``ASN1BitString`` from raw components.
///
/// - parameters:
/// - bytes: The bytes to represent this bitstring
/// - paddingBits: The number of bits in the trailing byte that are not actually part of this bitstring.
@inlinable
public init(bytes: ArraySlice<UInt8>, paddingBits: Int = 0) {
self.bytes = bytes
self.paddingBits = paddingBits
try! self._validate()
}
@inlinable
public func serialize(into coder: inout DER.Serializer, withIdentifier identifier: ASN1Identifier) throws {
coder.appendPrimitiveNode(identifier: identifier) { bytes in
bytes.append(UInt8(truncatingIfNeeded: self.paddingBits))
bytes.append(contentsOf: self.bytes)
}
}
@inlinable
internal func _validate() throws {
guard let finalByte = self.bytes.last else {
if self.paddingBits != 0 {
// If there are no bytes, there must be no padding bits.
throw ASN1Error.invalidASN1Object(
reason: "Invalid number of padding bits for ASN1BitString: \(self.paddingBits)"
)
}
return
}
// Oooh, bit twiddling.
//
// All joking aside, this sets the bottom `self.paddingBits` to 1.
let mask = ~(UInt8.max << self.paddingBits)
if (finalByte & mask) != 0 {
throw ASN1Error.invalidASN1Object(
reason: "Invalid padding bits in ASN1BitString: \(self.paddingBits) of padding, \(finalByte) final byte"
)
}
}
}
extension ASN1BitString: Hashable {}
extension ASN1BitString: Sendable {}
extension ASN1BitString {
@inlinable
public func withUnsafeBytes<R>(_ body: (UnsafeRawBufferPointer) throws -> R) rethrows -> R {
return try self.bytes.withUnsafeBytes(body)
}
}
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