MimeKit.Cryptography.TemporarySecureMimeContext.GetCmsSigner C# (CSharp) Method

		protected override CmsSigner GetCmsSigner (MailboxAddress mailbox, DigestAlgorithm digestAlgo)
		{
			var now = DateTime.UtcNow;

			foreach (var certificate in certificates) {
				AsymmetricKeyParameter key;

				if (certificate.NotBefore > now || certificate.NotAfter < now)
					continue;

				var keyUsage = certificate.GetKeyUsageFlags ();
				if (keyUsage != 0 && (keyUsage & SecureMimeContext.DigitalSignatureKeyUsageFlags) == 0)
					continue;

				if (!keys.TryGetValue (certificate, out key))
					continue;

				var address = certificate.GetSubjectEmailAddress ();

				if (address.Equals (mailbox.Address, StringComparison.OrdinalIgnoreCase)) {
					var signer = new CmsSigner (certificate, key);
					signer.DigestAlgorithm = digestAlgo;
					return signer;
				}
			}

			throw new CertificateNotFoundException (mailbox, "A valid signing certificate could not be found.");
		}