Poly1305

Purpose

Poly1305 is a fast one-time message authentication code (MAC). It takes a 256-bit key that can only be used once and produces a 128-bit tag.

You almost definitely want BLAKE2b instead. Poly1305 is easy to misuse and less secure due to the short tag length.

Usage

ComputeTag

Fills a span with a tag computed from a message and a one-time key.

Poly1305.ComputeTag(Span<byte> tag, ReadOnlySpan<byte> message, ReadOnlySpan<byte> oneTimeKey)

Exceptions

ArgumentOutOfRangeException

tag has a length not equal to TagSize.

ArgumentOutOfRangeException

oneTimeKey has a length not equal to KeySize.

CryptographicException

The tag could not be computed.

VerifyTag

Verifies that a tag is correct in constant time for a given message and one-time key. It returns true if the tag is valid and false otherwise.

Poly1305.VerifyTag(ReadOnlySpan<byte> tag, ReadOnlySpan<byte> message, ReadOnlySpan<byte> oneTimeKey)

Exceptions

ArgumentOutOfRangeException

tag has a length not equal to TagSize.

ArgumentOutOfRangeException

oneTimeKey has a length not equal to KeySize.

IncrementalPoly1305

Provides support for computing a tag from several messages and a one-time key.

using var poly1305 = new IncrementalPoly1305(ReadOnlySpan<byte> oneTimeKey);
poly1305.Update(ReadOnlySpan<byte> message1);
poly1305.Update(ReadOnlySpan<byte> message2);
// Compute
poly1305.Finalize(Span<byte> tag);
// Or verify
bool valid = poly1305.FinalizeAndVerify(ReadOnlySpan<byte> tag);

// Avoid another using statement
// WARNING: Do NOT reuse the same key
poly1305.Reinitialize(ReadOnlySpan<byte> differentOneTimeKey);
poly1305.Update(ReadOnlySpan<byte> message3);
poly1305.Finalize(Span<byte> tag);

Exceptions

ArgumentOutOfRangeException

oneTimeKey has a length not equal to KeySize.

ArgumentOutOfRangeException

tag has a length not equal to TagSize.

CryptographicException

The tag could not be computed.

InvalidOperationException

Cannot update after finalizing or finalize twice (without reinitializing).

ObjectDisposedException

The object has been disposed.

Constants

These are used for validation and/or save you defining your own constants.

public const int KeySize = 32;
public const int TagSize = 16;

Notes

Each key MUST be uniformly random, unpredictable, and unique. You MUST NOT reuse a key or use the same key for multiple purposes (e.g. encryption and Poly1305).

Do NOT use Poly1305 as a hash function or key derivation function (KDF). Use BLAKE2b.

Tags MUST be compared in constant time to avoid leaking information, so use the VerifyTag() or FinalizeAndVerify() function.​

Tags MUST NOT be truncated to minimise the opportunity for forgery.

BLAKE2b is strongly recommended over Poly1305 as a MAC because it has better security guarantees. Due to the 128-bit tag length, Poly1305 should only ever be used for online protocols and small messages.