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.
Concatenation is often not as simple as it sounds. You MUST be careful to avoid canonicalization attacks.
This is important for hashing, message authentication (e.g., Encrypt-then-MAC), associated data in AEADs, and info in key derivation.
Two solutions are:
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.
For variable-length inputs, also concatenate the length of each input as a 64-bit unsigned integer (ulong) converted to bytes in little-endian. See this C# code example. This is done internally in AEADs like ChaCha20-Poly1305.
Usage
Concat
Fills a span with the concatenation of two spans.
Spans.Concat(Span<byte> buffer, ReadOnlySpan<byte> a, ReadOnlySpan<byte> b)Exceptions
buffer has a length not equal to a.Length + b.Length.
a.Length + b.Length has resulted in an overflow.
Concat
Fills a span with the concatenation of three spans.
Exceptions
buffer has a length not equal to a.Length + b.Length + c.Length.
a.Length + b.Length + c.Length has resulted in an overflow.
Concat
Fills a span with the concatenation of four spans.
Exceptions
buffer has a length not equal to a.Length + b.Length + c.Length + d.Length.
a.Length + b.Length + c.Length + d.Length has resulted in an overflow.
Concat
Fills a span with the concatenation of five spans.
Exceptions
buffer has a length not equal to a.Length + b.Length + c.Length + d.Length + e.Length.
a.Length + b.Length + c.Length + d.Length + e.Length has resulted in an overflow.
Concat
Fills a span with the concatenation of six spans.
Exceptions
buffer has a length not equal to a.Length + b.Length + c.Length + d.Length + e.Length + f.Length.
a.Length + b.Length + c.Length + d.Length + e.Length + f.Length has resulted in an overflow.
Notes
There is unfortunately no Concat() function for arrays or spans in .NET, and there is not even a Span<T>.CopyTo(Span<T> destination, int index). This class fills that gap.
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