jstedfast/MailKit · error · ArgumentException

Invalid nonce length (should be 8 bytes).

Error message

Invalid nonce length (should be 8 bytes).

What it means

The ServerChallenge property setter throws ArgumentException "Invalid nonce length (should be 8 bytes)." when the supplied array is not exactly 8 bytes. The NTLM server challenge is a fixed 64-bit value from the Type2 message.

Solutions

1. Pass exactly 8 bytes; slice the server response at the correct offset (bytes 16..24 of the Type2 message).
2. Check value.Length == 8 before assigning.
3. Prefer new NtlmChallengeMessage(buffer, startIndex) over manual property assignment so offsets are handled for you.

Example fix

// before
challenge.ServerChallenge = Encoding.ASCII.GetBytes("mychallenge"); // 11 bytes
// after
challenge.ServerChallenge = challengeBytes.AsSpan(16, 8).ToArray(); // exactly 8 bytes

if (serverChallenge == null || serverChallenge.Length != 8) throw new ArgumentException("ServerChallenge must be exactly 8 bytes.");
challenge.ServerChallenge = serverChallenge;

static bool IsValidChallenge(byte[]? value) => value != null && value.Length == 8;

try {
	challenge.ServerChallenge = value;
} catch (ArgumentException ex) when (ex.Message.Contains("nonce length")) {
	// re-slice the Type2 response at the correct offset
	challenge.ServerChallenge = rawResponse.AsSpan(16, 8).ToArray();
}

When it happens

Trigger: Assigning ServerChallenge with an array of length other than 8 — e.g. slicing the raw Type2 response with wrong offsets, or a base64 decode producing 12/16 bytes.

Common situations: Manual Type2 message parsing with incorrect offset arithmetic; hardcoded test challenges typed with the wrong length.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Invalid nonce length (should be 8 bytes).
• value
• HashAlgorithm
• No hash value computed.
• negotiate

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/42fe72cc4a0be2dc. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Security/Ntlm/NtlmChallengeMessage.cs:72

			cached = new byte[length];
			Buffer.BlockCopy (message, startIndex, cached, 0, length);
		}

		~NtlmChallengeMessage ()
		{
			if (serverChallenge != null)
				Array.Clear (serverChallenge, 0, serverChallenge.Length);
		}

		public byte[] ServerChallenge {
			get { return serverChallenge; }
			set { 
				if (value == null)
					throw new ArgumentNullException (nameof (value));

				if (value.Length != 8)
					throw new ArgumentException ("Invalid nonce length (should be 8 bytes).", nameof (value));

				Array.Clear (serverChallenge, 0, serverChallenge.Length);
				serverChallenge = value;
			}
		}

		public string? TargetName {
			get; set;
		}

		public NtlmTargetInfo? TargetInfo {
			get; set;
		}

		public byte[]? GetEncodedTargetInfo ()
		{
			return TargetInfo?.Encode ((Flags & NtlmFlags.NegotiateUnicode) != 0);
		}

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