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Crypto Forum Internet-Draft This document describes the AEGIS-128L, AEGIS-256, AEGIS-128X, and AEGIS-256X AES-based authenticated encryption algorithms designed for high-performance applications. The document is a product of the Crypto Forum Research Group (CFRG). It is not an IETF product and is not a standard. Discussion Venues Source for this draft and an issue tracker can be found at .

Introduction This document describes the AEGIS family of Authenticated Encryption with Associated Data (AEAD) algorithms , which were chosen for high-performance applications in the CAESAR (Competition for Authenticated Encryption: Security, Applicability, and Robustness) competition. Among the finalists, AEGIS-128 was chosen as the winner for this category. However, AEGIS-128L, another finalist, offers enhanced performance and a stronger security margin . Additionally, AEGIS-256, which also reached the final round, provides 256-bit security and supports higher usage limits. Therefore, this document specifies the following variants:

• AEGIS-128L, which has a 128-bit key, a 128-bit nonce, a 1024-bit state, a 128- or 256-bit authentication tag, and processes 256-bit input blocks.
• AEGIS-256, which has a 256-bit key, a 256-bit nonce, a 768-bit state, a 128- or 256-bit authentication tag, and processes 128-bit input blocks.
• AEGIS-128X, which is a mode based on AEGIS-128L, specialized for CPUs with large vector registers and vector AES instructions.
• AEGIS-256X, which is a mode based on AEGIS-256, specialized for CPUs with large vector registers and vector AES instructions.

All variants are inverse-free and constructed from the AES encryption round function . The AEGIS cipher family offers performance that significantly exceeds AES-GCM on CPUs with AES instructions. Similarly, software implementations not using AES instructions can also be faster, although to a lesser extent. Unlike with AES-GCM, nonces can be safely chosen at random with no practical limit when using AEGIS-256 and AEGIS-256X. AEGIS-128L and AEGIS-128X also allow for more messages to be safely encrypted when using random nonces. With some existing AEAD schemes, such as AES-GCM, an attacker can generate a ciphertext that successfully decrypts under multiple different keys (a partitioning oracle attack) . This ability to craft a (ciphertext, authentication tag) pair that verifies under multiple keys significantly reduces the number of required interactions with the oracle to perform an exhaustive search, making it practical if the key space is small. For example, with password-based encryption, an attacker can guess a large number of passwords at a time by recursively submitting such a ciphertext to an oracle, which speeds up a password search by reducing it to a binary search. With AEGIS, finding distinct (key, nonce) pairs that successfully decrypt a given (associated data, ciphertext, authentication tag) tuple is believed to have a complexity that depends on the tag size. A 128-bit tag provides 64-bit committing security, which is generally acceptable for interactive protocols. With a 256-bit tag, finding a collision becomes impractical. Unlike most other AES-based AEAD constructions, leaking a state does not leak the key or previous states. Finally, an AEGIS key is not required after the initialization function, and there is no key schedule. Thus, ephemeral keys can be erased from memory before any data has been encrypted or decrypted, mitigating cold boot attacks. Note that an earlier version of Hongjun Wu and Bart Preneel’s paper introducing AEGIS specified AEGIS-128L and AEGIS-256 with a different Finalize function. We follow the specification of , which can be found in the References section of this document.

Conventions and Definitions The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. Throughout this document, “byte” is used interchangeably with “octet” and refers to an 8-bit sequence. Primitives:

• {}: an empty bit array.
• |x|: the length of x in bits.
• a ^ b: the bitwise exclusive OR operation between a and b.
• a & b: the bitwise AND operation between a and b.
• a || b: the concatenation of a and b.
• a mod b: the remainder of the Euclidean division between a as the dividend and b as the divisor.
• LE64(x): returns the little-endian encoding of unsigned 64-bit integer x.
• ZeroPad(x, n): returns x after appending zeros until its length is a multiple of n bits. No padding is added if the length of x is already a multiple of n, including when x is empty.
• Truncate(x, n): returns the first n bits of x.
• Split(x, n): returns x split into n-bit blocks, ignoring partial blocks.
• Tail(x, n): returns the last n bits of x.
• AESRound(in, rk): a single round of the AES encryption round function, which is the composition of the SubBytes, ShiftRows, MixColums, and AddRoundKey transformations, as defined in Section 5 of . Here, in is the 128-bit AES input state, and rk is the 128-bit round key.
• Repeat(n, F): n sequential evaluations of the function F.
• CtEq(a, b): compares a and b in constant-time, returning True for an exact match and False otherwise.

AEGIS internal functions:

• Update(M0, M1) or Update(M): the state update function.
• Init(key, nonce): the initialization function.
• Absorb(ai): the input block absorption function.
• Enc(xi): the input block encryption function.
• Dec(ci): the input block decryption function.
• DecPartial(cn): the input block decryption function for the last ciphertext bits when they do not fill an entire block.
• Finalize(ad_len_bits, msg_len_bits): the authentication tag generation function.

Input blocks are 256 bits for AEGIS-128L and 128 bits for AEGIS-256. AES blocks:

• Si: the i-th AES block of the current state.
• S'i: the i-th AES block of the next state.
• {Si, ...Sj}: the vector of the i-th AES block of the current state to the j-th block of the current state.
• C0: an AES block built from the following bytes in hexadecimal format: { 0x00, 0x01, 0x01, 0x02, 0x03, 0x05, 0x08, 0x0d, 0x15, 0x22, 0x37, 0x59, 0x90, 0xe9, 0x79, 0x62 }.
• C1: an AES block built from the following bytes in hexadecimal format: { 0xdb, 0x3d, 0x18, 0x55, 0x6d, 0xc2, 0x2f, 0xf1, 0x20, 0x11, 0x31, 0x42, 0x73, 0xb5, 0x28, 0xdd }.

AES blocks are always 128 bits in length. Input and output values:

• key: the encryption key (128 bits for AEGIS-128L, 256 bits for AEGIS-256).
• nonce: the public nonce (128 bits for AEGIS-128L, 256 bits for AEGIS-256).
• ad: the associated data.
• msg: the plaintext.
• ct: the ciphertext.
• tag: the authentication tag (128 or 256 bits).

The AEGIS-128L Algorithm AEGIS-128L has a 1024-bit state, made of eight 128-bit blocks {S0, ...S7}. The parameters for this algorithm, whose meaning is defined in , are:

• K_LEN (key length) is 16 bytes (128 bits).
• P_MAX (maximum length of the plaintext) is 2⁶¹ - 1 bytes (2⁶⁴ - 8 bits).
• A_MAX (maximum length of the associated data) is 2⁶¹ - 1 bytes (2⁶⁴ - 8 bits).
• N_MIN (minimum nonce length) = N_MAX (maximum nonce length) = 16 bytes (128 bits).
• C_MAX (maximum ciphertext length) = P_MAX + tag length = (2⁶¹ - 1) + 16 or 32 bytes (in bits: (2⁶⁴ - 8) + 128 or 256 bits).

Distinct associated data inputs, as described in , MUST be unambiguously encoded as a single input. It is up to the application to create a structure in the associated data input if needed.

Authenticated Encryption The Encrypt function encrypts a message and returns the ciphertext along with an authentication tag that verifies the authenticity of the message and associated data, if provided. Security:

• For a given key, the nonce MUST NOT be reused under any circumstances; doing so allows an attacker to recover the internal state.
• The key MUST be randomly chosen from a uniform distribution.

Inputs:

• msg: the message to be encrypted (length MUST be less than or equal to P_MAX).
• ad: the associated data to authenticate (length MUST be less than or equal to A_MAX).
• key: the encryption key.
• nonce: the public nonce.

Outputs:

• ct: the ciphertext.
• tag: the authentication tag.

Steps:

Authenticated Decryption The Decrypt function decrypts a ciphertext, verifies that the authentication tag is correct, and returns the message on success or an error if tag verification failed. Security:

• If tag verification fails, the decrypted message and wrong authentication tag MUST NOT be given as output. The decrypted message MUST be overwritten with zeros before the function returns.
• The comparison of the input tag with the expected_tag MUST be done in constant time.

Inputs:

• ct: the ciphertext to decrypt (length MUST be less than or equal to C_MAX).
• tag: the authentication tag.
• ad: the associated data to authenticate (length MUST be less than or equal to A_MAX).
• key: the encryption key.
• nonce: the public nonce.

Outputs:

• Either the decrypted message msg or an error indicating that the authentication tag is invalid for the given inputs.

Steps:

The Update Function The Update function is the core of the AEGIS-128L algorithm. It updates the state {S0, ...S7} using two 128-bit values. Inputs:

• M0: the first 128-bit block to be absorbed.
• M1: the second 128-bit block to be absorbed.

Modifies:

• {S0, ...S7}: the state.

Steps:

The Init Function The Init function constructs the initial state {S0, ...S7} using the given key and nonce. Inputs:

• key: the encryption key.
• nonce: the public nonce.

Defines:

• {S0, ...S7}: the initial state.

Steps:

The Absorb Function The Absorb function absorbs a 256-bit input block ai into the state {S0, ...S7}. Inputs:

• ai: the 256-bit input block.

Steps:

The Enc Function The Enc function encrypts a 256-bit input block xi using the state {S0, ...S7}. Inputs:

• xi: the 256-bit input block.

Outputs:

• ci: the 256-bit encrypted block.

Steps:

The Dec Function The Dec function decrypts a 256-bit input block ci using the state {S0, ...S7}. Inputs:

• ci: the 256-bit encrypted block.

Outputs:

• xi: the 256-bit decrypted block.

Steps:

The DecPartial Function The DecPartial function decrypts the last ciphertext bits cn using the state {S0, ...S7} when they do not fill an entire block. Inputs:

• cn: the encrypted input.

Outputs:

• xn: the decryption of cn.

Steps:

The Finalize Function The Finalize function computes a 128- or 256-bit tag that authenticates the message and associated data. Inputs:

• ad_len_bits: the length of the associated data in bits.
• msg_len_bits: the length of the message in bits.

Outputs:

• tag: the authentication tag.

Steps:

The AEGIS-256 Algorithm AEGIS-256 has a 768-bit state, made of six 128-bit blocks {S0, ...S5}. The parameters for this algorithm, whose meaning is defined in , are:

• K_LEN (key length) is 32 bytes (256 bits).
• P_MAX (maximum length of the plaintext) is 2⁶¹ - 1 bytes (2⁶⁴ - 8 bits).
• A_MAX (maximum length of the associated data) is 2⁶¹ - 1 bytes (2⁶⁴ - 8 bits).
• N_MIN (minimum nonce length) = N_MAX (maximum nonce length) = 32 bytes (256 bits).
• C_MAX (maximum ciphertext length) = P_MAX + tag length = (2⁶¹ - 1) + 16 or 32 bytes (in bits: (2⁶⁴ - 8) + 128 or 256 bits).

Distinct associated data inputs, as described in , MUST be unambiguously encoded as a single input. It is up to the application to create a structure in the associated data input if needed.

Authenticated Encryption The Encrypt function encrypts a message and returns the ciphertext along with an authentication tag that verifies the authenticity of the message and associated data, if provided. Security:

• For a given key, the nonce MUST NOT be reused under any circumstances; doing so allows an attacker to recover the internal state.
• The key MUST be randomly chosen from a uniform distribution.

Inputs:

• msg: the message to be encrypted (length MUST be less than or equal to P_MAX).
• ad: the associated data to authenticate (length MUST be less than or equal to A_MAX).
• key: the encryption key.
• nonce: the public nonce.

Outputs:

• ct: the ciphertext.
• tag: the authentication tag.

Steps:

Authenticated Decryption The Decrypt function decrypts a ciphertext, verifies that the authentication tag is correct, and returns the message on success or an error if tag verification failed. Security:

• If tag verification fails, the decrypted message and wrong authentication tag MUST NOT be given as output. The decrypted message MUST be overwritten with zeros before the function returns.
• The comparison of the input tag with the expected_tag MUST be done in constant time.

Inputs:

• ct: the ciphertext to decrypt (length MUST be less than or equal to C_MAX).
• tag: the authentication tag.
• ad: the associated data to authenticate (length MUST be less than or equal to A_MAX).
• key: the encryption key.
• nonce: the public nonce.

Outputs:

• Either the decrypted message msg or an error indicating that the authentication tag is invalid for the given inputs.

Steps:

The Update Function The Update function is the core of the AEGIS-256 algorithm. It updates the state {S0, ...S5} using a 128-bit value. Inputs:

• msg: the 128-bit block to be absorbed.

Modifies:

• {S0, ...S5}: the state.

Steps:

The Init Function The Init function constructs the initial state {S0, ...S5} using the given key and nonce. Inputs:

• key: the encryption key.
• nonce: the public nonce.

Defines:

• {S0, ...S5}: the initial state.

Steps:

The Absorb Function The Absorb function absorbs a 128-bit input block ai into the state {S0, ...S5}. Inputs:

• ai: the 128-bit input block.

Steps:

The Enc Function The Enc function encrypts a 128-bit input block xi using the state {S0, ...S5}. Inputs:

• xi: the 128-bit input block.

Outputs:

• ci: the 128-bit encrypted block.

Steps:

The Dec Function The Dec function decrypts a 128-bit input block ci using the state {S0, ...S5}. Inputs:

• ci: the 128-bit encrypted block.

Outputs:

• xi: the 128-bit decrypted block.

Steps:

The DecPartial Function The DecPartial function decrypts the last ciphertext bits cn using the state {S0, ...S5} when they do not fill an entire block. Inputs:

• cn: the encrypted input.

Outputs:

• xn: the decryption of cn.

Steps:

The Finalize Function The Finalize function computes a 128- or 256-bit tag that authenticates the message and associated data. Inputs:

• ad_len_bits: the length of the associated data in bits.
• msg_len_bits: the length of the message in bits.

Outputs:

• tag: the authentication tag.

Steps:

Parallel Modes Some CPUs, such as Intel and Intel-compatible CPUs with the VAES extensions, include instructions to efficiently apply the AES round function to a vector of AES blocks. AEGIS-128X and AEGIS-256X are optional, specialized modes designed to take advantage of these instructions. They share the same properties as the ciphers they are based on but can be significantly faster on these platforms, even for short messages. AEGIS-128X and AEGIS-256X are parallel evaluations of multiple AEGIS-128L and AEGIS-256 instances, respectively, with distinct initial states. On CPUs with wide vector registers, different states can be stored in different 128-bit lanes of the same vector register, allowing parallel updates using vector instructions. The modes are parameterized by the parallelism degree. With 256-bit registers, 2 parallel operations can be applied to 128-bit AES blocks. With 512-bit registers, the number of instances can be raised to 4. The state of a parallel mode is represented as a vector of AEGIS-128L or AEGIS-256 states.

Additional Conventions and Definitions

• D: the degree of parallelism.
• R: the absorption and output rate of the mode. With AEGIS-128X, the rate is 256 * D bits. With AEGIS-256X, the rate is 128 * D bits.
• V[j,i]: the j-th AES block of the i-th state. i is in the [0..D) range. For AEGIS-128X, j is in the [0..8) range, while for AEGIS-256X, j is in the [0..6) range.
• V'[j,i]: the j-th AES block of the next i-th state.
• ctx[i]: the i-th context separator. This is a 128-bit mask made of a byte representing the state index, followed by a byte representing the highest index and 112 all-zero bits.
• Byte(x): the value x encoded as 8 bits.

Authenticated Encryption The Encrypt function of AEGIS-128X resembles that of AEGIS-128L, and similarly, the Encrypt function of AEGIS-256X mirrors that of AEGIS-256, but processes R-bit input blocks per update. Steps:

Authenticated Decryption The Decrypt function of AEGIS-128X resembles that of AEGIS-128L, and similarly, the Decrypt function of AEGIS-256X mirrors that of AEGIS-256, but processes R-bit input blocks per update. Steps:

AEGIS-128X

The Update Function The AEGIS-128X Update function is similar to the AEGIS-128L Update function but absorbs R (= 256 * D) bits at once. M0 and M1 are 128 * D bits instead of 128 bits but are split into 128-bit blocks, each of them updating a different AEGIS-128L state. Steps:

The Init Function The Init function initializes a vector of D AEGIS-128L states with the same key and nonce but a different context ctx[i]. The context is added to the state before every update. Steps:

The Absorb Function The Absorb function is similar to the AEGIS-128L Absorb function but absorbs R bits instead of 256 bits. Steps:

The Enc Function The Enc function is similar to the AEGIS-128L Enc function but encrypts R bits instead of 256 bits. Steps:

The Dec Function The Dec function is similar to the AEGIS-128L Dec function but decrypts R bits instead of 256 bits. Steps:

The DecPartial Function The DecPartial function is similar to the AEGIS-128L DecPartial function but decrypts up to R bits instead of 256 bits. Steps:

The Finalize Function The Finalize function finalizes every AEGIS-128L instance and combines the resulting authentication tags using the bitwise exclusive OR operation. Steps:

AEGIS-256X

The Update Function The AEGIS-256X Update function is similar to the AEGIS-256 Update function but absorbs R (128 * D) bits at once. M is 128 * D bits instead of 128 bits and is split into 128-bit blocks, each of them updating a different AEGIS-256 state. Steps:

The Init Function The Init function initializes a vector of D AEGIS-256 states with the same key and nonce but a different context ctx[i]. The context is added to the state before every update. Steps:

The Absorb Function The Absorb function is similar to the AEGIS-256 Absorb function but absorbs R bits instead of 128 bits. Steps:

The Enc Function The Enc function is similar to the AEGIS-256 Enc function but encrypts R bits instead of 128 bits. Steps:

The Dec Function The Dec function is similar to the AEGIS-256 Dec function but decrypts R bits instead of 128 bits. Steps:

The DecPartial Function The DecPartial function is similar to the AEGIS-256 DecPartial function but decrypts up to R bits instead of 128 bits. Steps:

The Finalize Function The Finalize function finalizes every AEGIS-256 instance and combines the resulting authentication tags using the bitwise exclusive OR operation. Steps:

Implementation Considerations AEGIS-128X and AEGIS-256X with a degree of 1 are identical to AEGIS-128L and AEGIS-256, respectively. This property can be used to reduce the size of a generic implementation. In AEGIS-128X, V can be represented as eight 256-bit registers (when D = 2) or eight 512-bit registers (when D = 4). In AEGIS-256X, V can be represented as six 256-bit registers (when D = 2) or six 512-bit registers (when D = 4). With this representation, loops over 0..D in the above pseudocode can be replaced by vector instructions.

Operational Considerations The AEGIS parallel modes are specialized and can only improve performance on specific CPUs. The degrees of parallelism implementations are encouraged to support are 2 (for CPUs with 256-bit registers) and 4 (for CPUs with 512-bit registers). The resulting algorithms are called AEGIS-128X2, AEGIS-128X4, AEGIS-256X2, and AEGIS-256X4. The following table summarizes how many bits are processed in parallel (rate), the memory requirements (state size), and the minimum vector register size a CPU should support for optimal performance.

Algorithm	Rate (bits)	Optimal Register Size	State Size (bits)
AEGIS-128L	256	128 bits	1024
AEGIS-128X2	512	256 bits	2048
AEGIS-128X4	1024	512 bits	4096
AEGIS-256	128	128 bits	768
AEGIS-256X2	256	256 bits	1536
AEGIS-256X4	512	512 bits	3072

Note that architectures with smaller vector registers but with many registers and large pipelines may still benefit from the parallel modes. Protocols SHOULD opt for a parallel mode only when all the involved parties agree on a specific variant. AEGIS-128L and AEGIS-256 SHOULD remain the default choices. Implementations MAY choose not to include the parallel AEGIS modes.

Encoding (ct, tag) Tuples Applications MAY keep the ciphertext and the authentication tag in distinct structures or encode both as a single string. In the latter case, the tag MUST immediately follow the ciphertext:

AEGIS as a Stream Cipher All AEGIS variants can also be used as stream ciphers. The Stream function expands a key and an optional nonce into a variable-length keystream. Inputs:

• len: the length of the keystream to generate in bits.
• key: the AEGIS key.
• nonce: the AEGIS nonce. If unspecified, it is set to N_MAX zero bytes.

Outputs:

• stream: the keystream.

Steps: This is equivalent to encrypting a len all-zero bits message without associated data and discarding the authentication tag. Instead of relying on the generic Encrypt function, implementations can omit the Finalize function. After initialization, the Update function is called with constant parameters, allowing further optimizations.

AEGIS as a Message Authentication Code All AEGIS variants can be used to construct a Message Authentication Code (MAC). For all the variants, the Mac function takes a key, a nonce, and data as input and produces a 128- or 256-bit tag as output. Security:

• This is the only function that allows the reuse of (key, nonce) pairs with different inputs.
• AEGIS-based MAC functions MUST NOT be used as hash functions: if the key is known, inputs causing state collisions can easily be crafted.
• Unlike hash-based MACs, tags MUST NOT be used for key derivation as there is no guarantee that they are uniformly random.

Inputs:

• data: the input data to authenticate (length MUST be less than or equal to A_MAX).
• key: the secret key.
• nonce: the public nonce.

Outputs:

• tag: the authentication tag.

AEGISMAC-128L AEGISMAC-128L refers to the Mac function based on the building blocks of AEGIS-128L. Steps:

AEGISMAC-256 AEGISMAC-256 refers to the Mac function based on the building blocks of AEGIS-256. Steps:

AEGISMAC-128X AEGISMAC-128X is based on the building blocks of AEGIS-128X but replaces the Finalize function with a dedicated FinalizeMac function.

The Mac Function Steps:

The FinalizeMac Function The FinalizeMac function computes a 128- or 256-bit tag that authenticates the input data. It finalizes all the instances, absorbs the resulting tags into the first state, and computes the final tag using that single state, as done in AEGIS-128L. Steps: 1: # Absorb tags into state 0; other states are not used anymore for v in Split(tags, 256): x0, x1 = Split(v, 128) Absorb(ZeroPad(x0, R / 2) || ZeroPad(x1, R / 2)) u = LE64(D) || LE64(tag_len_bits) t = ZeroPad(V[2,0] ^ u, R) Repeat(7, Update(t, t)) if tag_len_bits == 128: tag = V[0,0] ^ V[1,0] ^ V[2,0] ^ V[3,0] ^ V[4,0] ^ V[5,0] ^ V[6,0] else: # 256 bits t0 = V[0,0] ^ V[1,0] ^ V[2,0] ^ V[3,0] t1 = V[4,0] ^ V[5,0] ^ V[6,0] ^ V[7,0] tag = t0 || t1 return tag ]]>

AEGISMAC-256X AEGISMAC-256X is based on the building blocks of AEGIS-256X but replaces the Finalize function with a dedicated FinalizeMac function.

The Mac Function Steps:

The FinalizeMac Function The FinalizeMac function computes a 128- or 256-bit tag that authenticates the input data. It finalizes all the instances, absorbs the resulting tags into the first state, and computes the final tag using that single state, as done in AEGIS-256. 1: # Absorb tags into state 0; other states are not used anymore for v in Split(tags, 128): Absorb(ZeroPad(v, R)) u = LE64(D) || LE64(tag_len_bits) t = ZeroPad(V[3,0] ^ u, R) Repeat(7, Update(t)) if tag_len_bits == 128: tag = V[0,0] ^ V[1,0] ^ V[2,0] ^ V[3,0] ^ V[4,0] ^ V[5,0] else: # 256 bits t0 = V[0,0] ^ V[1,0] ^ V[2,0] t1 = V[3,0] ^ V[4,0] ^ V[5,0] tag = t0 || t1 return tag ]]>

Implementation Status This note is to be removed before publishing as an RFC. Multiple implementations of the schemes described in this document have been developed and verified for interoperability. A comprehensive list of known implementations and integrations can be found at , which includes reference implementations closely aligned with the pseudocode provided in this document.

Security Considerations

Usage Guidelines

Key and Nonce Selection All AEGIS variants MUST be used in a nonce-respecting setting: for a given key, a nonce MUST only be used once, even with different tag lengths. Failure to do so would immediately reveal the bitwise difference between two messages. Every key MUST be randomly chosen from a uniform distribution. The nonce MAY be public or predictable. It can be a counter, the output of a permutation, or a generator with a long period. With AEGIS-128L and AEGIS-128X, random nonces can safely encrypt up to 2⁴⁸ messages using the same key with negligible (~2^-33, to align with NIST guidelines) collision probability. With AEGIS-256 and AEGIS-256X, random nonces can be used with no practical limits.

Committing Security An authentication tag may verify under multiple keys, nonces, or associated data, but AEGIS is assumed to be key committing in the receiver-binding game. This mitigates common attacks when used with low-entropy keys such as passwords. Finding distinct keys and/or nonces that successfully verify the same (ad, ct, tag) tuple is expected to require ~2⁶⁴ attempts with a 128-bit authentication tag and ~2¹²⁸ attempts with a 256-bit tag. AEGIS is fully committing in the restricted setting where an adversary cannot control the associated data. As shown in , with the ability to alter the associated data, it is possible to efficiently find multiple keys that will verify the same authenticated ciphertext. Protocols mandating a fully committing scheme without that restriction can provide the associated data as input to a cryptographic hash function and use the output as the ad parameter of the Encrypt and Decrypt functions. The selected hash function must ensure a minimum of 128-bit collision and preimage resistance. An instance of such a function is SHA-256 . Alternatively, the associated data can be fed into a collision-resistant KDF, such as HKDF , via the info input to derive the key parameter. The ad parameter can then be left empty. Note that the salt input MUST NOT be used since large salts get hashed, which affects commitment. Furthermore, this requires values concatenated to form the info input to be unambiguously encoded, like by appending their lengths.

Multi-User Security AEGIS nonces match the size of the key. AEGIS-128L and AEGIS-128X feature 128-bit nonces, offering an extra 32 bits compared to the commonly used AEADs in IETF protocols at the time of writing. The AEGIS-256 and AEGIS-256X variants provide even larger nonces. With 192 random bits, 64 bits remain available to optionally encode additional information. In all these variants, unused nonce bits can encode a key identifier, enhancing multi-user security. If every key has a unique identifier, multi-target attacks do not provide any advantage over single-target attacks.

Implementation Security If tag verification fails, the unverified plaintext and computed authentication tag MUST NOT be released. As shown in , even a partial leak of the plaintext without verification facilitates chosen ciphertext attacks. The security of AEGIS against timing and physical attacks is limited by the implementation of the underlying AESRound function. Failure to implement AESRound in a fashion safe against timing and physical attacks, such as differential power analysis, timing analysis, or fault injection attacks, may lead to leakage of secret key material or state information. The exact mitigations required for timing and physical attacks depend on the threat model in question. Regardless of the variant, the key and nonce are only required by the Init function; other functions only depend on the resulting state. Therefore, implementations can overwrite ephemeral keys with zeros right after the last Update call of the initialization function.

Security Guarantees AEGIS-256 offers 256-bit security against plaintext and state recovery, whereas AEGIS-128L offers 128-bit security. Under the assumption that the secret key is unknown to the attacker, all AEGIS variants offer at least 128-bit security against forgery attacks. Encrypting the same message with the same key and nonce but different associated data generates distinct ciphertexts that do not reveal any additional information about the message. However, (key, nonce) pairs MUST NOT be reused, even if the associated data differs. AEGIS has been shown to have reforgeability resilience in . Without the ability to set the associated data, a successful forgery does not increase the probability of subsequent forgeries. AEGIS-128X and AEGIS-256X share the same security properties and requirements as AEGIS-128L and AEGIS-256, respectively. In particular, the security level and usage limits remain the same . AEGIS is considered secure against guess-and-determine attacks aimed at recovering the state from observed ciphertexts. This resilience extends to quantum adversaries operating within the Q1 model, where the attacker has access to a quantum computer but is restricted to classical (non-quantum) communications with the systems under attack. In this model, quantum attacks offer no practical advantage in decrypting previously recorded ciphertexts or in recovering the encryption key. This document extends the original specification by introducing optional support for 256-bit authentication tags, which are constructed similarly to the 128-bit tags. As shown in , with 256-bit tags, all AEGIS variants achieve more than 128-bit security against forgery by differential attacks. Security analyses of AEGIS can be found in , , , , , , , , , , and .

IANA Considerations IANA has assigned the following identifiers in the AEAD Algorithms Registry: AEGIS entries in the AEAD Algorithms Registry

Algorithm Name	ID
AEAD_AEGIS128L	32
AEAD_AEGIS256	33
AEAD_AEGIS128X2	34
AEAD_AEGIS128X4	35
AEAD_AEGIS256X2	36
AEAD_AEGIS256X4	37

IANA is requested to update the references of these entries to refer to the final version of this document.

References Normative References National Institute of Standards and Technology

US Gaithersburg

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document defines algorithms for Authenticated Encryption with Associated Data (AEAD), and defines a uniform interface and a registry for such algorithms. The interface and registry can be used as an application-independent set of cryptoalgorithm suites. This approach provides advantages in efficiency and security, and promotes the reuse of crypto implementations. [STANDARDS-TRACK] Federal Information Processing Standard, FIPS This document specifies a simple Hashed Message Authentication Code (HMAC)-based key derivation function (HKDF), which can be used as a building block in various protocols and applications. The key derivation function (KDF) is intended to support a wide range of applications and requirements, and is conservative in its use of cryptographic hash functions. This document is not an Internet Standards Track specification; it is published for informational purposes. Informative References Nanyang Technological University KU Leuven Indraprastha Institute of Information Technology Delhi; University of Hyogo University of Hyogo Indian Statistical Institute Kolkata University of Hyogo University of Hyogo; Indian Institute of Technology Bhilai Journal of Cryptographic Engineering, 2024 Université de Lorraine, CNRS, Inria, LORIA Université de Rennes, CNRS, Inria, IRISA Cryptology ePrint Archive, Paper 2023/1306 Fastly Inc. Cryptology ePrint Archive, Paper 2023/523 Graz University of Technology Graz University of Technology Graz University of Technology IACR Transactions on Symmetric Cryptology, 2019(4), pp. 348–368 Beuth Hochschule für Technik Berlin Bauhaus-Universität Weimar Bauhaus-Universität Weimar Bauhaus-Universität Weimar Cryptology ePrint Archive, Paper 2017/332 University of Hyogo University of Hyogo Cryptology ePrint Archive, Paper 2023/1495 State Key Laboratory of Cryptology State Key Laboratory of Information Security, Institute of Information Engineering, Chinese Academy of Sciences; School of Cyber Security, University of Chinese Academy of Sciences State Key Laboratory of Cryptology The Computer Journal, vol 65, 2022(8), pp. 2221–2230 Cornell Tech Cornell Tech Cornell Tech 30th USENIX Security Symposium (USENIX Security 21), pp. 195–212 East China Normal University; University of Hyogo University of Hyogo; National Institute of Information and Communications Technology; PRESTO, Japan Science and Technology Agency University of Applied Sciences and Arts Northwestern Switzerland University of Hyogo IACR Transactions on Symmetric Cryptology, 2021(2), pp. 104–139 ANSSI Selected Areas in Cryptography. SAC 2014. Lecture Notes in Computer Science, vol 8781, pp. 290–305 University of Hyogo Mitsubishi Electric Corporation University of Hyogo Advances in Information and Computer Security. IWSEC 2024. Lecture Notes in Computer Science, vol 14977 University of Hyogo University of Hyogo University of Hyogo University of Hyogo; National Institute of Information and Communications Technology IET Information Security, vol 17, 2023(3), pp. 458-467 commit 8e289c40 EPFL EPFL Applied Cryptography and Network Security. ACNS 2018. Lecture Notes in Computer Science, vol 10892, pp. 476–494

Test Vectors The following test vectors are also available in JSON format at . In this format, byte strings are represented as JSON strings containing their hexadecimal encoding.

AESRound Test Vector

AEGIS-128L Test Vectors

Update Test Vector

Test Vector 1

Test Vector 2

Test Vector 3

Test Vector 4

Test Vector 5

Test Vector 6 This test MUST return a “verification failed” error.

Test Vector 7 This test MUST return a “verification failed” error.

Test Vector 8 This test MUST return a “verification failed” error.

Test Vector 9 This test MUST return a “verification failed” error.

AEGIS-256 Test Vectors

Update Test Vector

Test Vector 1

Test Vector 2

Test Vector 3

Test Vector 4

Test Vector 5

Test Vector 6 This test MUST return a “verification failed” error.

Test Vector 7 This test MUST return a “verification failed” error.

Test Vector 8 This test MUST return a “verification failed” error.

Test Vector 9 This test MUST return a “verification failed” error.

AEGIS-128X2 Test Vectors

Initial State After initialization:

Test Vector 1

Test Vector 2

AEGIS-128X4 Test Vectors

Initial State After initialization:

Test Vector 1

Test Vector 2

AEGIS-256X2 Test Vectors

Initial State After initialization:

Test Vector 1

Test Vector 2

AEGIS-256X4 Test Vectors

Initial State After initialization:

Test Vector 1

Test Vector 2

AEGISMAC Test Vectors

AEGISMAC-128L Test Vector

AEGISMAC-128X2 Test Vector

AEGISMAC-128X4 Test Vector

AEGISMAC-256 Test Vector

AEGISMAC-256X2 Test Vector

AEGISMAC-256X4 Test Vector

Acknowledgments The AEGIS family of authenticated encryption algorithms was invented by Hongjun Wu and Bart Preneel. The state update function leverages the AES permutation invented by Joan Daemen and Vincent Rijmen. They also authored the Pelican MAC, which partly motivated the design of the AEGIS MAC. We would like to thank the following individuals for their contributions:

• Eric Lagergren, Daniel Bleichenbacher, and Conrad Ludgate for catching invalid test vectors, and Daniel Bleichenbacher for many helpful suggestions.
• Soatok Dreamseeker for his early review of the draft and for suggesting the addition of negative test vectors.
• John Preuß Mattsson for his review of the draft and for suggesting how AEGIS should be used in the context of DTLS and QUIC.
• Bart Mennink and Charlotte Lefevre, as well as Takanori Isobe and Mostafizar Rahman, for investigating the committing security of the schemes specified in this document.
• Scott Fluhrer for his review of the draft as a member of the CFRG Crypto Review Panel.
• Yawning Angel, Chris Barber, and Neil Madden for their review of the draft.
• Gilles Van Assche for reviewing the draft and providing insightful comments on the implications of nonce reuse in AEGIS-128X and AEGIS-256X.