> 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/advanced/concat.md). # Concat ## Purpose Sometimes it is necessary to concatenate data together to form an input or output. This is represented via the `||` symbol in pseudocode. For example, an AEAD ciphertext is usually a ciphertext concatenated with a tag. {% hint style="danger" %} Concatenation is often not as simple as it sounds. You **MUST** be careful to avoid [canonicalization attacks](https://soatok.blog/2021/07/30/canonicalization-attacks-against-macs-and-signatures/). This is important for [hashing](/hashing.md), [message authentication](/message-authentication.md) (e.g., [Encrypt-then-MAC](https://samuellucas.com/draft-lucas-generalised-committing-aead/draft-lucas-generalised-committing-aead.html#section-3)), associated data in [AEADs](/authenticated-encryption.md), and info in [key derivation](/key-derivation.md). Two solutions are: 1. Only ever concatenate fixed-length inputs. With two inputs, only one needs to be fixed-length. With three inputs, only two need to be fixed-length. 2. For variable-length inputs, also concatenate the length of each input as a 64-bit unsigned integer ([ulong](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/integral-numeric-types)) [converted to bytes in little-endian](https://docs.microsoft.com/en-us/dotnet/api/system.buffers.binary.binaryprimitives.writeuint64littleendian?). See [this](https://github.com/samuel-lucas6/draft-lucas-generalised-committing-aead/blob/a8feb3e077a89ba6660b3cc21ca0b4f3780aeca8/reference-implementation/cAEAD/cAEAD/ChaCha20BLAKE2b.cs#L88) C# code example. This is done internally in AEADs like [ChaCha20-Poly1305](https://datatracker.ietf.org/doc/html/rfc8439#section-2.8.1). {% endhint %} ## Usage ### Concat Fills a span with the concatenation of two spans. ```csharp Spans.Concat(Span buffer, ReadOnlySpan a, ReadOnlySpan b) ``` #### Exceptions [ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception) `buffer` has a length not equal to `a.Length + b.Length`. [OverflowException](https://learn.microsoft.com/en-us/dotnet/api/system.overflowexception) `a.Length + b.Length` has resulted in an overflow. [ArgumentException](https://learn.microsoft.com/en-us/dotnet/api/system.argumentexception) `buffer` cannot overlap with the spans being copied. ### Concat Fills a span with the concatenation of three spans. ```csharp Spans.Concat(Span buffer, ReadOnlySpan a, ReadOnlySpan b, ReadOnlySpan c) ``` #### Exceptions [ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception) `buffer` has a length not equal to `a.Length + b.Length + c.Length`. [OverflowException](https://learn.microsoft.com/en-us/dotnet/api/system.overflowexception) `a.Length + b.Length + c.Length` has resulted in an overflow. [ArgumentException](https://learn.microsoft.com/en-us/dotnet/api/system.argumentexception) `buffer` cannot overlap with the spans being copied. ### Concat Fills a span with the concatenation of four spans. ```csharp Spans.Concat(Span buffer, ReadOnlySpan a, ReadOnlySpan b, ReadOnlySpan c, ReadOnlySpan d) ``` #### Exceptions [ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception) `buffer` has a length not equal to `a.Length + b.Length + c.Length + d.Length`. [OverflowException](https://learn.microsoft.com/en-us/dotnet/api/system.overflowexception) `a.Length + b.Length + c.Length + d.Length` has resulted in an overflow. [ArgumentException](https://learn.microsoft.com/en-us/dotnet/api/system.argumentexception) `buffer` cannot overlap with the spans being copied. ### Concat Fills a span with the concatenation of five spans. ```csharp Spans.Concat(Span buffer, ReadOnlySpan a, ReadOnlySpan b, ReadOnlySpan c, ReadOnlySpan d, ReadOnlySpan e) ``` #### Exceptions [ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception) `buffer` has a length not equal to `a.Length + b.Length + c.Length + d.Length + e.Length`. [OverflowException](https://learn.microsoft.com/en-us/dotnet/api/system.overflowexception) `a.Length + b.Length + c.Length + d.Length + e.Length` has resulted in an overflow. [ArgumentException](https://learn.microsoft.com/en-us/dotnet/api/system.argumentexception) `buffer` cannot overlap with the spans being copied. ### Concat Fills a span with the concatenation of six spans. ```csharp Spans.Concat(Span buffer, ReadOnlySpan a, ReadOnlySpan b, ReadOnlySpan c, ReadOnlySpan d, ReadOnlySpan e, ReadOnlySpan f) ``` #### Exceptions [ArgumentOutOfRangeException](https://docs.microsoft.com/en-us/dotnet/api/system.argumentoutofrangeexception) `buffer` has a length not equal to `a.Length + b.Length + c.Length + d.Length + e.Length + f.Length`. [OverflowException](https://learn.microsoft.com/en-us/dotnet/api/system.overflowexception) `a.Length + b.Length + c.Length + d.Length + e.Length + f.Length` has resulted in an overflow. [ArgumentException](https://learn.microsoft.com/en-us/dotnet/api/system.argumentexception) `buffer` cannot overlap with the spans being copied. ## Notes {% hint style="info" %} There is unfortunately no `Concat()` function for arrays or spans in .NET, and there is not even a `Span.CopyTo(Span destination, int index)`. This class fills that gap. {% 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. 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