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//===----------------------------------------------------------------------===//
//
// This source file is part of the SwiftCrypto open source project
//
// Copyright (c) 2019-2020 Apple Inc. and the SwiftCrypto project authors
// Licensed under Apache License v2.0
//
// See LICENSE.txt for license information
// See CONTRIBUTORS.md for the list of SwiftCrypto project authors
//
// SPDX-License-Identifier: Apache-2.0
//
//===----------------------------------------------------------------------===//
#if CRYPTO_IN_SWIFTPM && !CRYPTO_IN_SWIFTPM_FORCE_BUILD_API
@_exported import CryptoKit
#else
import Foundation
/// A Diffie-Hellman Key Agreement Key
public protocol DiffieHellmanKeyAgreement {
/// The public key share type to perform the DH Key Agreement
associatedtype PublicKey
var publicKey: PublicKey { get }
/// Performs a Diffie-Hellman Key Agreement.
///
/// - Parameters:
/// - publicKeyShare: The public key share.
/// - Returns: The resulting key agreement result.
func sharedSecretFromKeyAgreement(with publicKeyShare: PublicKey) throws -> SharedSecret
}
/// A key agreement result from which you can derive a symmetric cryptographic
/// key.
///
/// Generate a shared secret by calling your private key’s
/// `sharedSecretFromKeyAgreement(publicKeyShare:)` method with the public key
/// from another party. The other party computes the same secret by passing your
/// public key to the equivalent method on their own private key.
///
/// The shared secret isn’t suitable as a symmetric cryptographic key
/// (``SymmetricKey``) by itself. However, you use it to generate a key by
/// calling either the
/// ``hkdfDerivedSymmetricKey(using:salt:sharedInfo:outputByteCount:)`` or
/// ``x963DerivedSymmetricKey(using:sharedInfo:outputByteCount:)`` method of the
/// shared secret. After the other party does the same, then you both share a
/// symmetric key suitable for creating a message authentication code like
/// ``HMAC``, or for opening and closing a sealed box with a cipher like
/// ``ChaChaPoly`` or ``AES``.
public struct SharedSecret: ContiguousBytes {
var ss: SecureBytes
/// Invokes the given closure with a buffer pointer covering the raw bytes
/// of the shared secret.
///
/// - Parameters:
/// - body: A closure that takes a raw buffer pointer to the bytes of the
/// shared secret and returns the shared secret.
///
/// - Returns: The shared secret, as returned from the body closure.
public func withUnsafeBytes<R>(_ body: (UnsafeRawBufferPointer) throws -> R) rethrows -> R {
return try ss.withUnsafeBytes(body)
}
/// Derives a symmetric encryption key from the secret using x9.63 key
/// derivation.
///
/// - Parameters:
/// - hashFunction: The hash function to use for key derivation.
/// - sharedInfo: The shared information to use for key derivation.
/// - outputByteCount: The length in bytes of resulting symmetric key.
///
/// - Returns: The derived symmetric key.
public func x963DerivedSymmetricKey<H: HashFunction, SI: DataProtocol>(using hashFunction: H.Type, sharedInfo: SI, outputByteCount: Int) -> SymmetricKey {
// SEC1 defines 3 inputs to the KDF:
//
// 1. An octet string Z which is the shared secret value. That's `self` here.
// 2. An integer `keydatalen` which is the length in octets of the keying data to be generated. Here that's `outputByteCount`.
// 3. An optional octet string `SharedInfo` which consists of other shared data. Here, that's `sharedInfo`.
//
// We then need to perform the following steps:
//
// 1. Check that keydatalen < hashlen × (2³² − 1). If keydatalen ≥ hashlen × (2³² − 1), fail.
// 2. Initiate a 4 octet, big-endian octet string Counter as 0x00000001.
// 3. For i = 1 to ⌈keydatalen/hashlen⌉, do the following:
// 1. Compute: Ki = Hash(Z || Counter || [SharedInfo]).
// 2. Increment Counter.
// 3. Increment i.
// 4. Set K to be the leftmost keydatalen octets of: K1 || K2 || . . . || K⌈keydatalen/hashlen⌉.
// 5. Output K.
//
// The loop in step 3 is not very Swifty, so instead we generate the counter directly.
// Step 1: Check that keydatalen < hashlen × (2³² − 1).
// We do this math in UInt64-space, because we'll overflow 32-bit integers.
guard UInt64(outputByteCount) < (UInt64(H.Digest.byteCount) * UInt64(UInt32.max)) else {
fatalError("Invalid parameter size")
}
var key = SecureBytes()
key.reserveCapacity(outputByteCount)
var remainingBytes = outputByteCount
var counter = UInt32(1)
while remainingBytes > 0 {
// 1. Compute: Ki = Hash(Z || Counter || [SharedInfo]).
var hasher = H()
hasher.update(self)
hasher.update(counter.bigEndian)
hasher.update(data: sharedInfo)
let digest = hasher.finalize()
// 2. Increment Counter.
counter += 1
// Append the bytes of the digest. We don't want to append more than the remaining number of bytes.
let bytesToAppend = min(remainingBytes, H.Digest.byteCount)
digest.withUnsafeBytes { digestPtr in
key.append(digestPtr.prefix(bytesToAppend))
}
remainingBytes -= bytesToAppend
}
precondition(key.count == outputByteCount)
return SymmetricKey(data: key)
}
/// Derives a symmetric encryption key from the secret using HKDF key
/// derivation.
///
/// - Parameters:
/// - hashFunction: The hash function to use for key derivation.
/// - salt: The salt to use for key derivation.
/// - sharedInfo: The shared information to use for key derivation.
/// - outputByteCount: The length in bytes of resulting symmetric key.
///
/// - Returns: The derived symmetric key.
public func hkdfDerivedSymmetricKey<H: HashFunction, Salt: DataProtocol, SI: DataProtocol>(using hashFunction: H.Type, salt: Salt, sharedInfo: SI, outputByteCount: Int) -> SymmetricKey {
#if os(iOS) && (arch(arm) || arch(i386))
fatalError("Unsupported architecture")
#else
return HKDF<H>.deriveKey(inputKeyMaterial: SymmetricKey(data: ss), salt: salt, info: sharedInfo, outputByteCount: outputByteCount)
#endif
}
}
extension SharedSecret: Hashable {
public func hash(into hasher: inout Hasher) {
ss.withUnsafeBytes { hasher.combine(bytes: $0) }
}
}
// We want to implement constant-time comparison for digests.
extension SharedSecret: CustomStringConvertible, Equatable {
public static func == (lhs: Self, rhs: Self) -> Bool {
return safeCompare(lhs, rhs)
}
/// Determines whether a shared secret is equivalent to a collection of
/// contiguous bytes.
///
/// - Parameters:
/// - lhs: The shared secret to compare.
/// - rhs: A collection of contiguous bytes to compare.
///
/// - Returns: A Boolean value that’s `true` if the shared secret and the
/// collection of binary data are equivalent.
public static func == <D: DataProtocol>(lhs: Self, rhs: D) -> Bool {
if rhs.regions.count != 1 {
let rhsContiguous = Data(rhs)
return safeCompare(lhs, rhsContiguous)
} else {
return safeCompare(lhs, rhs.regions.first!)
}
}
public var description: String {
return "\(Self.self): \(ss.hexString)"
}
}
extension HashFunction {
// A wrapper function to keep the unsafe code in one place.
mutating func update(_ secret: SharedSecret) {
secret.withUnsafeBytes {
self.update(bufferPointer: $0)
}
}
mutating func update(_ counter: UInt32) {
withUnsafeBytes(of: counter) {
self.update(bufferPointer: $0)
}
}
}
#endif // Linux or !SwiftPM
|
