The Ascon family is specified in the following documents:

- NIST submission: Ascon v1.2 (authenticated encryption and hash)
- CAESAR submission: Ascon v1.2 (authenticated encryption)

### Ascon's Permutation

All Ascon family members use the same lightweight permutation.
This permutation iteratively applies an SPN-based round transformation
*a* = 12 times (for *p ^{a}* )
or

*b*∈ {6, 8} times (for

*p*). The round transformation consists of the following three steps which operate on a 320-bit state divided into 5 words

^{b}*x*

_{0},

*x*

_{1},

*x*

_{2},

*x*

_{3},

*x*

_{4}of 64 bits each:

- Addition of Round Constants: xors a round specific 1-byte constant to word
*x*_{2}. - Nonlinear Substitution Layer: applies a 5-bit S-box 64 times in parallel in a bit-sliced fashion (vertically, across words).
- Linear Diffusion Layer: xors different rotated copies of each word (horizontally, within each word).

Ascon's permutation: ⊕ denotes xor, ⊙ denotes and, ⋙ is rotation to the right.

### Ascon's Authenticated Encryption Modes

Ascon uses a duplex-sponge-based mode of operation for authenticated encryption. The recommended key, tag and nonce length is 128 bits. The sponge operates on a state of 320 bits, with message blocks of 64 or 128 bits. The encryption process is split into four phases:

- Initialization: initializes the state with the key
*K*and nonce*N*. - Associated Data Processing: updates the state with associated data blocks
*A*._{i} - Plaintext Processing: injects plaintext blocks
*P*into the state and extracts ciphertext blocks_{i}*C*._{i} - Finalization: injects the key
*K*again and extracts the tag*T*for authentication.

After each injected block (except the last plaintext block), the core
permutation *p ^{b}* is applied to the complete state.
During initialization and finalization, a stronger permutation

*p*with more rounds is used. The numbers of rounds

^{a}*a*and

*b*, as well as the sponge's rate and capacity, depend on the Ascon variant. The recommended parameters are:

Cipher | Bit size of | Rounds | |||||||
---|---|---|---|---|---|---|---|---|---|

key | nonce | tag | rate | capacity | p^{a} | p^{b} | |||

Ascon-128 | 128 | 128 | 128 | 64 | 256 | 12 | 6 | ||

Ascon-128a | 128 | 128 | 128 | 128 | 192 | 12 | 8 |

### Ascon's Hashing Modes

The Ascon family includes the hash functions Ascon-Hash and Ascon-Hasha as well as the extendable output functions Ascon-Xof and Ascon-Xofa with sponge-based modes of operation. Both provide 128-bit security with a hash size of at least 256 bits. The hashing modes use the same lightweight 320-bit permutation as the authenticated encryption modes.

The hashing modes absorb the message *M* in 64-bit blocks *M _{i}*
and finally squeeze the hash value

*H*in 64-bit blocks

*H*. After each absorbed or squeezed block except the last, the

_{i}*b*-round permutation

*p*is applied to the state. The full

^{b}*a*-round permutation

*p*is applied in the initialization and finalization, after the last message block:

^{a}Algorithm | Bit size of | Rounds | |||||
---|---|---|---|---|---|---|---|

hash output | rate | capacity | p^{a} | p^{b} | |||

Ascon-Hash | 256 | 64 | 256 | 12 | 12 | ||

Ascon-Xof | arbitrary | 64 | 256 | 12 | 12 | ||

Ascon-Hasha | 256 | 64 | 256 | 12 | 8 | ||

Ascon-Xofa | arbitrary | 64 | 256 | 12 | 8 |

For details such as the IV and round constant values, padding rules or the (almost identical) decryption mode, please refer to the latest Ascon submission document.