AEGIS-256
Purpose
AEGIS-256 is an AES-based authenticated encryption with associated data (AEAD) scheme that was a CAESAR competition finalist. It encrypts a plaintext message using a 256-bit key and nonce (number used only once) whilst calculating a 256-bit tag over the plaintext and associated data.
The associated data is useful for authenticating file headers, version numbers, timestamps, counters, and so on. It can be used to prevent confused deputy attacks and replay attacks. It is not encrypted nor part of the ciphertext. It must be reproduceable or stored somewhere for decryption to be possible.
Decryption involves verifying the tag for the given inputs, which detects tampering and incorrect parameters. If verification fails, an error is returned. Otherwise, the plaintext is returned.
For encryption, the nonce MUST NOT be repeated or reused with the same key. You MUST increment or randomly generate the nonce for each plaintext message encrypted using the same key.
Unlike with ChaCha20-Poly1305, it is safe to randomly generate nonces with the same key. Nonces can be public and are typically manually prepended to the ciphertext.
It is not necessary to use the full 256-bit nonce. For example, a 160- or 192-bit random nonce MAY be used, with zero padding up to 256 bits. However, a 256-bit nonce ensures there is no practical limit on the number of messages that can be encrypted using the same key.
Usage
Encrypt
Fills a span with ciphertext and an appended tag computed from a plaintext message, nonce, key, and optional associated data.
AEGIS256.Encrypt(Span<byte> ciphertext, ReadOnlySpan<byte> plaintext, ReadOnlySpan<byte> nonce, ReadOnlySpan<byte> key, ReadOnlySpan<byte> associatedData = default)Exceptions
ciphertext has a length not equal to plaintext.Length + TagSize.
nonce has a length not equal to NonceSize.
key has a length not equal to KeySize.
Encryption failed.
Decrypt
Verifies that the tag appended to the ciphertext is correct for the given inputs. If verification fails, an exception is thrown. Otherwise, it fills a span with the decrypted ciphertext.
AEGIS256.Decrypt(Span<byte> plaintext, ReadOnlySpan<byte> ciphertext, ReadOnlySpan<byte> nonce, ReadOnlySpan<byte> key, ReadOnlySpan<byte> associatedData = default)Exceptions
plaintext has a length not equal to ciphertext.Length - TagSize.
ciphertext has a length less than TagSize.
nonce has a length not equal to NonceSize.
key has a length not equal to KeySize.
Invalid authentication tag for the given inputs.
Constants
These are used for validation and/or save you defining your own constants.
public const int KeySize = 32;
public const int NonceSize = 32;
public const int TagSize = 32;Notes
If you intend to feed multiple, variable-length inputs into the associated data, beware of canonicalization attacks. Please read the Concat page for more information.
The key MUST be uniformly random. It can either be randomly generated or the output of a KDF. Furthermore, it SHOULD be rotated periodically (e.g. a different key per file).
Encrypting data in 16-64 KiB chunks instead of as a single plaintext message is RECOMMENDED to keep memory usage low and detect corrupted chunks early. Unfortunately, it is difficult to get right. You MUST ensure that chunks cannot be:
Truncated
Removed
Reordered
Duplicated
1 and 2 can be accomplished by including the length of all the ciphertext chunks added together in the associated data of the first chunk. Alternatively, you can use the STREAM construction.
3 and 4 can be resolved by using a counter nonce or by including the previous tag in the associated data of the next chunk.
If decryption fails midway through a stream due to tampering or corruption, erase the previous plaintext outputs from memory and/or disk and throw an error.
As a general rule, avoid compression before encryption. It can leak information and has been the cause of several attacks.
To make AEGIS-256 fully committing, you can hash the associated data using a cryptographic hash function with a minimum of 128-bit preimage resistance and use the output as the AEAD associated data parameter.
Current popular AEAD schemes like AES-GCM and ChaCha20-Poly1305 are non-committing. By contrast, AEGIS-256 is believed to be key committing (but not fully committing). This prevents attacks when the adversary can choose the key and/or nonce (but not when they can choose the associated data). For example, it is computationally infeasible to output an AEGIS-256 ciphertext that can be decrypted without an authentication error using a different key.
AEGIS-256 is significantly faster than (X)ChaCha20-Poly1305 and AES-GCM when there is AES hardware support, which is available on most modern x64/ARM64 CPUs.
Without hardware support, (X)ChaCha20-Poly1305 is faster, and the AEGIS-256 implementation may be vulnerable to side-channel attacks.
AEGIS-256 can be used as a MAC by encrypting with the message as the associated data and an empty plaintext, resulting in just a tag. However, it MUST NOT be used as a hash function (e.g. without a secret key or for key derivation).
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