> For the complete documentation index, see [llms.txt](https://www.geralt.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://www.geralt.xyz/constant-time.md).

# Constant time

## Purpose

Whenever you interact with secrets and cryptographic parameters, you should use constant time functions to avoid leaking information via timing to an attacker. Such leaks can completely compromise security.

For example, you should increment counters/nonces, compare tags, compare retyped passwords, and so on using this class.

## Usage

### Equals

Returns whether two spans are equal in length and contain equal data.

```csharp
bool equal = ConstantTime.Equals(ReadOnlySpan<byte> a, ReadOnlySpan<byte> b);
```

#### Exceptions

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`a.Length` is equal to 0.

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`b.Length` is equal to 0.

### Increment

Increments a span counter in little-endian format.

```csharp
ConstantTime.Increment(Span<byte> buffer);
```

#### Exceptions

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`buffer.Length` is equal to 0.

### Add

Fills a span with the sum of two spans in little-endian format.

```csharp
ConstantTime.Add(Span<byte> buffer, ReadOnlySpan<byte> a, ReadOnlySpan<byte> b);
```

#### Exceptions

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`buffer.Length` is equal to 0.

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`a.Length` is equal to 0 or not equal to `buffer.Length`.

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`b.Length` is equal to 0 or not equal to `a.Length`.

### Subtract

Fills a span with the result of subtracting the second span from the first span in little-endian format.

```csharp
ConstantTime.Subtract(Span<byte> buffer, ReadOnlySpan<byte> a, ReadOnlySpan<byte> b);
```

#### Exceptions

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`buffer.Length` is equal to 0.

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`a.Length` is equal to 0 or not equal to `buffer.Length`.

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`b.Length` is equal to 0 or not equal to `a.Length`.

### IsLessThan

Returns whether the contents of the first span is less than the second span in little-endian format.

```csharp
bool lessThan = ConstantTime.IsLessThan(ReadOnlySpan<byte> a, ReadOnlySpan<byte> b);
```

#### Exceptions

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`a.Length` is equal to 0 or not equal to `b.Length`.

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`b.Length` is equal to 0.

### IsGreaterThan

Returns whether the contents of the first span is greater than the second span in little-endian format.

```csharp
bool greaterThan = ConstantTime.IsGreaterThan(ReadOnlySpan<byte> a, ReadOnlySpan<byte> b);
```

#### Exceptions

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`a.Length` is equal to 0 or not equal to `b.Length`.

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`b.Length` is equal to 0.

### IsAllZeros

Returns whether a span only contains zeros.

```csharp
bool allZeros = ConstantTime.IsAllZeros(ReadOnlySpan<byte> buffer);
```

#### Exceptions

[ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception)

`buffer.Length` is equal to 0.

## Notes

{% hint style="danger" %}
[Tags](/message-authentication.md) **MUST** be compared in constant time using `ConstantTime.Equals()` to prevent [forgery](https://crypto.stackexchange.com/a/101635). The `VerifyTag()` and `FinalizeAndVerify()` functions do this for you.
{% endhint %}

{% hint style="success" %}
These constant time functions can also be used for non-secret values. If in doubt, use constant time.
{% endhint %}


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://www.geralt.xyz/constant-time.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.