/// <summary>
/// Verify the multipart/signed part.
/// </summary>
/// <remarks>
/// Verifies the multipart/signed part using the supplied cryptography context.
/// </remarks>
/// <returns>A signer info collection.</returns>
/// <param name="ctx">The cryptography context to use for verifying the signature.</param>
/// <param name="cancellationToken">The cancellation token.</param>
/// <exception cref="System.ArgumentNullException">
/// <paramref name="ctx"/> is <c>null</c>.
/// </exception>
/// <exception cref="System.FormatException">
/// The multipart is malformed in some way.
/// </exception>
/// <exception cref="System.NotSupportedException">
/// <paramref name="ctx"/> does not support verifying the signature part.
/// </exception>
/// <exception cref="System.OperationCanceledException">
/// The operation was cancelled via the cancellation token.
/// </exception>
/// <exception cref="Org.BouncyCastle.Cms.CmsException">
/// An error occurred in the cryptographic message syntax subsystem.
/// </exception>
public async Task <DigitalSignatureCollection> VerifyAsync(CryptographyContext ctx, CancellationToken cancellationToken = default(CancellationToken))
{
if (ctx == null)
{
throw new ArgumentNullException(nameof(ctx));
}
var protocol = ContentType.Parameters["protocol"]?.Trim();
if (string.IsNullOrEmpty(protocol))
{
throw new FormatException("The multipart/signed part did not specify a protocol.");
}
if (!ctx.Supports(protocol))
{
throw new NotSupportedException("The specified cryptography context does not support the signature protocol.");
}
if (Count < 2)
{
throw new FormatException("The multipart/signed part did not contain the expected children.");
}
var signature = this[1] as MimePart;
if (signature == null || signature.Content == null)
{
throw new FormatException("The signature part could not be found.");
}
var ctype = signature.ContentType;
var value = string.Format("{0}/{1}", ctype.MediaType, ctype.MediaSubtype);
if (!ctx.Supports(value))
{
throw new NotSupportedException(string.Format("The specified cryptography context does not support '{0}'.", value));
}
using (var signatureData = new MemoryBlockStream()) {
await signature.Content.DecodeToAsync(signatureData, cancellationToken).ConfigureAwait(false);
signatureData.Position = 0;
using (var cleartext = new MemoryBlockStream()) {
// Note: see rfc2015 or rfc3156, section 5.1
var options = FormatOptions.CloneDefault();
options.NewLineFormat = NewLineFormat.Dos;
options.VerifyingSignature = true;
await this[0].WriteToAsync(options, cleartext, cancellationToken);
cleartext.Position = 0;
return(await ctx.VerifyAsync(cleartext, signatureData, cancellationToken).ConfigureAwait(false));
}
}
}
