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 definedwith 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 operationapplies here as well. Naturally allskcipher symbols must be exchangedtheaead pendants discussed in the following. In addition, for the AEADoperation, the aead_request_set_ad function must be used to set thepointer to the associated data memory location before performing theencryption or decryption operation. In case of an encryption, the associateddata memory is filled during the encryption operation. For decryption, theassociated data memory must contain data that is used to verify the integrityof the decrypted data. Another deviation from the asynchronous block cipheroperation is that the caller should explicitly check for -EBADMSG of thecrypto_aead_decrypt. That error indicates an authentication error, i.e.a breach in the integrity of the message. In essence, that -EBADMSG errorcode is the key bonus an AEAD cipher has over “standard” block chainingmodes.

Memory Structure:

The source scatterlist must contain the concatenation ofassociated 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 sizeduring encryption (resp. decryption).

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

Even in the out-of-place case, space must be reserved in the destination forthe associated data, even though it won’t be written to. This makes thein-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 elementsare not allowed in the used part of the scatterlist. Thus, if there is noassociated 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’ bytesof the associated data buffer must contain a second copy of the IV. This isin addition to the copy passed toaead_request_set_crypt(). These two IVcopies must not differ; different implementations of the same algorithm maybehave differently in that case. Note that the algorithm might not actuallytreat the IV as associated data; nevertheless the length passed toaead_request_set_ad() must include it.

structaead_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

structaead_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. Thisfunction is used to specify the consumer requested size of theauthentication tag to be either generated by the transformationduring encryption or the size of the authentication tag to besupplied during the decryption operation. This function is alsoresponsible for checking the authentication tag size forvalidity.

encrypt

see struct skcipher_alg

decrypt

see struct skcipher_alg

init

Initialize the cryptographic transformation object. This functionis used to initialize the cryptographic transformation object.This function is called only once at the instantiation time, rightafter the transformation context was allocated. In case thecryptographic hardware has some special requirements which need tobe handled by software, this function shall check for the preciserequirement of the transformation and put any software fallbacksin place.

exit

Deinitialize the cryptographic transformation object. This is acounterpart toinit, used to remove various changes set ininit.

ivsize

see struct skcipher_alg

maxauthsize

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

chunksize

see struct skcipher_alg

base

Definition of a generic crypto cipher algorithm.

Description

All fields exceptivsize 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

constchar*alg_name

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

u32type

specifies the type of the cipher

u32mask

specifies the mask for the cipher

Description

Allocate a cipher handle for an AEAD. The returned structcrypto_aead is the cipher handle that is required for any subsequentAPI 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.

voidcrypto_free_aead(struct crypto_aead * tfm)¶

zeroize and free aead handle

Parameters

structcrypto_aead*tfm

cipher handle to be freed

unsigned intcrypto_aead_ivsize(struct crypto_aead * tfm)¶

obtain IV size

Parameters

structcrypto_aead*tfm

cipher handle

Description

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

Return

IV size in bytes

unsigned intcrypto_aead_authsize(struct crypto_aead * tfm)¶

obtain maximum authentication data size

Parameters

structcrypto_aead*tfm

cipher handle

Description

The maximum size of the authentication data for the AEAD cipher referencedby the AEAD cipher handle is returned. The authentication data size may bezero 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 intcrypto_aead_blocksize(struct crypto_aead * tfm)¶

obtain block size of cipher

Parameters

structcrypto_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 thedata returned by the encryption or decryption operation

Return

block size of cipher

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

set key for cipher

Parameters

structcrypto_aead*tfm

cipher handle

constu8*key

buffer holding the key

unsignedintkeylen

length of the key in bytes

Description

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

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

Return

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

intcrypto_aead_setauthsize(struct crypto_aead * tfm, unsigned int authsize)¶

set authentication data size

Parameters

structcrypto_aead*tfm

cipher handle

unsignedintauthsize

size of the authentication data / tag in bytes

Description

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

Return

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

intcrypto_aead_encrypt(structaead_request * req)¶

encrypt plaintext

Parameters

structaead_request*req

reference to the aead_request handle that holds all informationneeded to perform the cipher operation

Description

Encrypt plaintext data using the aead_request handle. That data structureand 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 ofblock cipher blocks + the size defined by thecrypto_aead_setauthsize invocation. The caller must ensurethat sufficient memory is available for the ciphertext andthe authentication tag.

Return

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

intcrypto_aead_decrypt(structaead_request * req)¶

decrypt ciphertext

Parameters

structaead_request*req

reference to the aead_request handle that holds all informationneeded to perform the cipher operation

Description

Decrypt ciphertext data using the aead_request handle. That data structureand 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 / tagmust have the size defined by the crypto_aead_setauthsizeinvocation.

Return

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

cipher operation performs the authentication of the data during thedecryption operation. Therefore, the function returns this error ifthe authentication of the ciphertext was unsuccessful (i.e. theintegrity 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 forthe AEAD cipher operation. This includes the cipher handle (which can beused by multiple aead_request instances), pointer to plaintext andciphertext, asynchronous callback function, etc. It acts as a handle to theaead_request_* API calls in a similar way as AEAD handle to thecrypto_aead_* API calls.

unsigned intcrypto_aead_reqsize(struct crypto_aead * tfm)¶

obtain size of the request data structure

Parameters

structcrypto_aead*tfm

cipher handle

Return

number of bytes

voidaead_request_set_tfm(structaead_request * req, struct crypto_aead * tfm)¶

update cipher handle reference in request

Parameters

structaead_request*req

request handle to be modified

structcrypto_aead*tfm

cipher handle that shall be added to the request handle

Description

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

structaead_request *aead_request_alloc(struct crypto_aead * tfm, gfp_t gfp)¶

allocate request data structure

Parameters

structcrypto_aead*tfm

cipher handle to be registered with the request

gfp_tgfp

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

Description

Allocate the request data structure that must be used with the AEADencrypt and decrypt API calls. During the allocation, the provided aeadhandle is registered in the request data structure.

Return

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

voidaead_request_free(structaead_request * req)¶

zeroize and free request data structure

Parameters

structaead_request*req

request data structure cipher handle to be freed

voidaead_request_set_callback(structaead_request * req, u32 flags, crypto_completion_t compl, void * data)¶

set asynchronous callback function

Parameters

structaead_request*req

request handle

u32flags

specify zero or an ORing of the flagsCRYPTO_TFM_REQ_MAY_BACKLOG the request queue may back log andincrease the wait queue beyond the initial maximum size;CRYPTO_TFM_REQ_MAY_SLEEP the request processing may sleep

crypto_completion_tcompl

callback function pointer to be registered with the request handle

void*data

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

Description

Setting the callback function that is triggered once the cipher operationcompletes

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

void callback_function(struct crypto_async_request *req, int error)

voidaead_request_set_crypt(structaead_request * req, struct scatterlist * src, struct scatterlist * dst, unsigned int cryptlen, u8 * iv)¶

set data buffers

Parameters

structaead_request*req

request handle

structscatterlist*src

source scatter / gather list

structscatterlist*dst

destination scatter / gather list

unsignedintcryptlen

number of bytes to process fromsrc

u8*iv

IV for the cipher operation which must comply with the IV size definedbycrypto_aead_ivsize()

Description

Setting the source data and destination data scatter / gather lists whichhold the associated data concatenated with the plaintext or ciphertext. Seebelow for the authentication tag.

For encryption, the source is treated as the plaintext and thedestination is the ciphertext. For a decryption operation, the use isreversed - 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 || cipherntext || 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 thecaller wants to have that data buffer filled, the caller must eitheruse an in-place cipher operation (i.e. same memory location forinput/output memory location).

voidaead_request_set_ad(structaead_request * req, unsigned int assoclen)¶

set associated data information

Parameters

structaead_request*req

request handle

unsignedintassoclen

number of bytes in associated data

Description

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