Authenticated Encryption With Associated Data (AEAD) Algorithm Definitions

The AEAD cipher API is used with the ciphers of type CRYPTO_ALG_TYPE_AEAD (listed as type “aead” in /proc/crypto)

The most prominent examples for this type of encryption is GCM and CCM. However, the kernel supports other types of AEAD ciphers which are defined with the following cipher string:

authenc(keyed message digest, block cipher)

For example: authenc(hmac(sha256), cbc(aes))

The example code provided for the symmetric key cipher operation applies here as well. Naturally all skcipher symbols must be exchanged the aead pendants discussed in the following. In addition, for the AEAD operation, the aead_request_set_ad function must be used to set the pointer to the associated data memory location before performing the encryption or decryption operation. In case of an encryption, the associated data memory is filled during the encryption operation. For decryption, the associated data memory must contain data that is used to verify the integrity of the decrypted data. Another deviation from the asynchronous block cipher operation is that the caller should explicitly check for -EBADMSG of the crypto_aead_decrypt. That error indicates an authentication error, i.e. a breach in the integrity of the message. In essence, that -EBADMSG error code is the key bonus an AEAD cipher has over “standard” block chaining modes.

Memory Structure:

The source scatterlist must contain the concatenation of associated data || plaintext or ciphertext.

The destination scatterlist has the same layout, except that the plaintext (resp. ciphertext) will grow (resp. shrink) by the authentication tag size during encryption (resp. decryption).

In-place encryption/decryption is enabled by using the same scatterlist pointer for both the source and destination.

Even in the out-of-place case, space must be reserved in the destination for the associated data, even though it won’t be written to. This makes the in-place and out-of-place cases more consistent. It is permissible for the “destination” associated data to alias the “source” associated data.

As with the other scatterlist crypto APIs, zero-length scatterlist elements are not allowed in the used part of the scatterlist. Thus, if there is no associated data, the first element must point to the plaintext/ciphertext.

To meet the needs of IPsec, a special quirk applies to rfc4106, rfc4309, rfc4543, and rfc7539esp ciphers. For these ciphers, the final ‘ivsize’ bytes of the associated data buffer must contain a second copy of the IV. This is in addition to the copy passed to aead_request_set_crypt(). These two IV copies must not differ; different implementations of the same algorithm may behave differently in that case. Note that the algorithm might not actually treat the IV as associated data; nevertheless the length passed to aead_request_set_ad() must include it.

struct aead_request

AEAD request

Definition

struct aead_request {
  struct crypto_async_request base;
  unsigned int assoclen;
  unsigned int cryptlen;
  u8 *iv;
  struct scatterlist *src;
  struct scatterlist *dst;
  void *__ctx[] ;
};

Members

base

Common attributes for async crypto requests

assoclen

Length in bytes of associated data for authentication

cryptlen

Length of data to be encrypted or decrypted

iv

Initialisation vector

src

Source data

dst

Destination data

__ctx

Start of private context data

struct aead_alg

AEAD cipher definition

Definition

struct aead_alg {
  int (*setkey)(struct crypto_aead *tfm, const u8 *key, unsigned int keylen);
  int (*setauthsize)(struct crypto_aead *tfm, unsigned int authsize);
  int (*encrypt)(struct aead_request *req);
  int (*decrypt)(struct aead_request *req);
  int (*init)(struct crypto_aead *tfm);
  void (*exit)(struct crypto_aead *tfm);
  unsigned int ivsize;
  unsigned int maxauthsize;
  unsigned int chunksize;
  struct crypto_alg base;
};

Members

setkey

see struct skcipher_alg

setauthsize

Set authentication size for the AEAD transformation. This function is used to specify the consumer requested size of the authentication tag to be either generated by the transformation during encryption or the size of the authentication tag to be supplied during the decryption operation. This function is also responsible for checking the authentication tag size for validity.

encrypt

see struct skcipher_alg

decrypt

see struct skcipher_alg

init

Initialize the cryptographic transformation object. This function is used to initialize the cryptographic transformation object. This function is called only once at the instantiation time, right after the transformation context was allocated. In case the cryptographic hardware has some special requirements which need to be handled by software, this function shall check for the precise requirement of the transformation and put any software fallbacks in place.

exit

Deinitialize the cryptographic transformation object. This is a counterpart to init, used to remove various changes set in init.

ivsize

see struct skcipher_alg

maxauthsize

Set the maximum authentication tag size supported by the transformation. A transformation may support smaller tag sizes. As the authentication tag is a message digest to ensure the integrity of the encrypted data, a consumer typically wants the largest authentication tag possible as defined by this variable.

chunksize

see struct skcipher_alg

base

Definition of a generic crypto cipher algorithm.

Description

All fields except ivsize is mandatory and must be filled.

Authenticated Encryption With Associated Data (AEAD) Cipher API

struct crypto_aead *crypto_alloc_aead(const char *alg_name, u32 type, u32 mask)

allocate AEAD cipher handle

Parameters

const char *alg_name

is the cra_name / name or cra_driver_name / driver name of the AEAD cipher

u32 type

specifies the type of the cipher

u32 mask

specifies the mask for the cipher

Description

Allocate a cipher handle for an AEAD. The returned struct crypto_aead is the cipher handle that is required for any subsequent API invocation for that AEAD.

Return

allocated cipher handle in case of success; IS_ERR() is true in case

of an error, PTR_ERR() returns the error code.

void crypto_free_aead(struct crypto_aead *tfm)

zeroize and free aead handle

Parameters

struct crypto_aead *tfm

cipher handle to be freed

Description

If tfm is a NULL or error pointer, this function does nothing.

unsigned int crypto_aead_ivsize(struct crypto_aead *tfm)

obtain IV size

Parameters

struct crypto_aead *tfm

cipher handle

Description

The size of the IV for the aead referenced by the cipher handle is returned. This IV size may be zero if the cipher does not need an IV.

Return

IV size in bytes

unsigned int crypto_aead_authsize(struct crypto_aead *tfm)

obtain maximum authentication data size

Parameters

struct crypto_aead *tfm

cipher handle

Description

The maximum size of the authentication data for the AEAD cipher referenced by the AEAD cipher handle is returned. The authentication data size may be zero if the cipher implements a hard-coded maximum.

The authentication data may also be known as “tag value”.

Return

authentication data size / tag size in bytes

unsigned int crypto_aead_blocksize(struct crypto_aead *tfm)

obtain block size of cipher

Parameters

struct crypto_aead *tfm

cipher handle

Description

The block size for the AEAD referenced with the cipher handle is returned. The caller may use that information to allocate appropriate memory for the data returned by the encryption or decryption operation

Return

block size of cipher

int crypto_aead_setkey(struct crypto_aead *tfm, const u8 *key, unsigned int keylen)

set key for cipher

Parameters

struct crypto_aead *tfm

cipher handle

const u8 *key

buffer holding the key

unsigned int keylen

length of the key in bytes

Description

The caller provided key is set for the AEAD referenced by the cipher handle.

Note, the key length determines the cipher type. Many block ciphers implement different cipher modes depending on the key size, such as AES-128 vs AES-192 vs. AES-256. When providing a 16 byte key for an AES cipher handle, AES-128 is performed.

Return

0 if the setting of the key was successful; < 0 if an error occurred

int crypto_aead_setauthsize(struct crypto_aead *tfm, unsigned int authsize)

set authentication data size

Parameters

struct crypto_aead *tfm

cipher handle

unsigned int authsize

size of the authentication data / tag in bytes

Description

Set the authentication data size / tag size. AEAD requires an authentication tag (or MAC) in addition to the associated data.

Return

0 if the setting of the key was successful; < 0 if an error occurred

int crypto_aead_encrypt(struct aead_request *req)

encrypt plaintext

Parameters

struct aead_request *req

reference to the aead_request handle that holds all information needed to perform the cipher operation

Description

Encrypt plaintext data using the aead_request handle. That data structure and how it is filled with data is discussed with the aead_request_* functions.

IMPORTANT NOTE The encryption operation creates the authentication data /

tag. That data is concatenated with the created ciphertext. The ciphertext memory size is therefore the given number of block cipher blocks + the size defined by the crypto_aead_setauthsize invocation. The caller must ensure that sufficient memory is available for the ciphertext and the authentication tag.

Return

0 if the cipher operation was successful; < 0 if an error occurred

int crypto_aead_decrypt(struct aead_request *req)

decrypt ciphertext

Parameters

struct aead_request *req

reference to the aead_request handle that holds all information needed to perform the cipher operation

Description

Decrypt ciphertext data using the aead_request handle. That data structure and how it is filled with data is discussed with the aead_request_* functions.

IMPORTANT NOTE The caller must concatenate the ciphertext followed by the

authentication data / tag. That authentication data / tag must have the size defined by the crypto_aead_setauthsize invocation.

Return

0 if the cipher operation was successful; -EBADMSG: The AEAD

cipher operation performs the authentication of the data during the decryption operation. Therefore, the function returns this error if the authentication of the ciphertext was unsuccessful (i.e. the integrity of the ciphertext or the associated data was violated); < 0 if an error occurred.

Asynchronous AEAD Request Handle

The aead_request data structure contains all pointers to data required for the AEAD cipher operation. This includes the cipher handle (which can be used by multiple aead_request instances), pointer to plaintext and ciphertext, asynchronous callback function, etc. It acts as a handle to the aead_request_* API calls in a similar way as AEAD handle to the crypto_aead_* API calls.

unsigned int crypto_aead_reqsize(struct crypto_aead *tfm)

obtain size of the request data structure

Parameters

struct crypto_aead *tfm

cipher handle

Return

number of bytes

void aead_request_set_tfm(struct aead_request *req, struct crypto_aead *tfm)

update cipher handle reference in request

Parameters

struct aead_request *req

request handle to be modified

struct crypto_aead *tfm

cipher handle that shall be added to the request handle

Description

Allow the caller to replace the existing aead handle in the request data structure with a different one.

struct aead_request *aead_request_alloc(struct crypto_aead *tfm, gfp_t gfp)

allocate request data structure

Parameters

struct crypto_aead *tfm

cipher handle to be registered with the request

gfp_t gfp

memory allocation flag that is handed to kmalloc by the API call.

Description

Allocate the request data structure that must be used with the AEAD encrypt and decrypt API calls. During the allocation, the provided aead handle is registered in the request data structure.

Return

allocated request handle in case of success, or NULL if out of memory

void aead_request_free(struct aead_request *req)

zeroize and free request data structure

Parameters

struct aead_request *req

request data structure cipher handle to be freed

void aead_request_set_callback(struct aead_request *req, u32 flags, crypto_completion_t compl, void *data)

set asynchronous callback function

Parameters

struct aead_request *req

request handle

u32 flags

specify zero or an ORing of the flags CRYPTO_TFM_REQ_MAY_BACKLOG the request queue may back log and increase the wait queue beyond the initial maximum size; CRYPTO_TFM_REQ_MAY_SLEEP the request processing may sleep

crypto_completion_t compl

callback function pointer to be registered with the request handle

void *data

The data pointer refers to memory that is not used by the kernel crypto API, but provided to the callback function for it to use. Here, the caller can provide a reference to memory the callback function can operate on. As the callback function is invoked asynchronously to the related functionality, it may need to access data structures of the related functionality which can be referenced using this pointer. The callback function can access the memory via the “data” field in the crypto_async_request data structure provided to the callback function.

Description

Setting the callback function that is triggered once the cipher operation completes

The callback function is registered with the aead_request handle and must comply with the following template:

void callback_function(struct crypto_async_request *req, int error)

void aead_request_set_crypt(struct aead_request *req, struct scatterlist *src, struct scatterlist *dst, unsigned int cryptlen, u8 *iv)

set data buffers

Parameters

struct aead_request *req

request handle

struct scatterlist *src

source scatter / gather list

struct scatterlist *dst

destination scatter / gather list

unsigned int cryptlen

number of bytes to process from src

u8 *iv

IV for the cipher operation which must comply with the IV size defined by crypto_aead_ivsize()

Description

Setting the source data and destination data scatter / gather lists which hold the associated data concatenated with the plaintext or ciphertext. See below for the authentication tag.

For encryption, the source is treated as the plaintext and the destination is the ciphertext. For a decryption operation, the use is reversed - the source is the ciphertext and the destination is the plaintext.

The memory structure for cipher operation has the following structure:

• AEAD encryption input: assoc data || plaintext

• AEAD encryption output: assoc data || ciphertext || auth tag

• AEAD decryption input: assoc data || ciphertext || auth tag

• AEAD decryption output: assoc data || plaintext

Albeit the kernel requires the presence of the AAD buffer, however, the kernel does not fill the AAD buffer in the output case. If the caller wants to have that data buffer filled, the caller must either use an in-place cipher operation (i.e. same memory location for input/output memory location).

void aead_request_set_ad(struct aead_request *req, unsigned int assoclen)

set associated data information

Parameters

struct aead_request *req

request handle

unsigned int assoclen

number of bytes in associated data

Description

Setting the AD information. This function sets the length of the associated data.