/// <summary>
/// Derives a <see cref="UserKey"/> from the input user secret.
/// </summary>
/// <param name="userSecret">The secret used to derive the key.</param>
/// <param name="keyDerivationSalt">The salt used to derive the key.</param>
/// <param name="securitySettings">Security parameters for the <see cref="SecureArchive"/>.</param>
public static UserKey DeriveFrom(
RawUserSecret userSecret,
ReadOnlySpan <byte> keyDerivationSalt,
SecuritySettings securitySettings)
{
ArgCheck.NotNull(userSecret, nameof(userSecret));
ArgCheck.NotEmpty(keyDerivationSalt, nameof(keyDerivationSalt));
ArgCheck.IsValid(securitySettings, nameof(securitySettings));
var cryptoAlgoProperties = EncryptionAlgoProperties.For(securitySettings.EncryptionAlgo);
// We'll use the user secret to derive the user key as well as the key ID.
// The algorithm is:
// let N be the number of bytes for the key
// let M be the number of bytes for the keyId
// keyMatter := Derive N + M bytes from the secret using the KDF
// key := keyMatter[0..N-1]
// keyId := base64url(keyMatter[N..])
var keyDerivationStrategy = CryptoHelpers.GetKeyDerivationStrategy(securitySettings.KeyDerivationFunction);
var keyMatter = keyDerivationStrategy.DeriveKeyMatter(
cryptoAlgoProperties.KeySizeInBytes + securitySettings.KeyIdSizeInBytes,
userSecret.Key,
keyDerivationSalt,
securitySettings.KeyDerivationWorkFactor);
var key = keyMatter.Slice(0, cryptoAlgoProperties.KeySizeInBytes);
var keyId = Helpers.Base64UrlEncode(keyMatter.Slice(cryptoAlgoProperties.KeySizeInBytes));
return(new UserKey(keyId, key.ToArray()));
}
/// <summary>
/// Creates a new symmetric <see cref="ArchiveKey"/> that can be used with the
/// algorithms described in the given <see cref="SecuritySettings"/>.
/// </summary>
/// <param name="securitySettings">The <see cref="SecuritySettings"/> for the archive.</param>
/// <returns>The new <see cref="ArchiveKey"/>.</returns>
public static ArchiveKey CreateNew(SecuritySettings securitySettings)
{
ArgCheck.IsValid(securitySettings, nameof(securitySettings));
var numKeyBytes = EncryptionAlgoProperties.For(securitySettings.EncryptionAlgo).KeySizeInBytes;
return(new ArchiveKey(CryptoHelpers.GetRandomBytes(numKeyBytes)));
}