crypto: doc - AEAD API documentation

The API function calls exported by the kernel crypto API for AEAD
ciphers to be used by consumers are documented.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/linux/crypto.h

@@ -1174,18 +1174,66 @@ static inline void ablkcipher_request_set_crypt(
 	req->info = iv;
 }
 
+/**
+ * DOC: Authenticated Encryption With Associated Data (AEAD) Cipher API
+ *
+ * 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 asynchronous block cipher operation
+ * applies here as well. Naturally all *ablkcipher* symbols must be exchanged
+ * the *aead* pendants discussed in the following. In addtion, for the AEAD
+ * operation, the aead_request_set_assoc 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.
+ */
+
 static inline struct crypto_aead *__crypto_aead_cast(struct crypto_tfm *tfm)
 {
 	return (struct crypto_aead *)tfm;
 }
 
+/**
+ * crypto_alloc_aead() - allocate AEAD cipher handle
+ * @alg_name: is the cra_name / name or cra_driver_name / driver name of the
+ *	     AEAD cipher
+ * @type: specifies the type of the cipher
+ * @mask: specifies the mask for the cipher
+ *
+ * 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.
+ */
 struct crypto_aead *crypto_alloc_aead(const char *alg_name, u32 type, u32 mask);
 
 static inline struct crypto_tfm *crypto_aead_tfm(struct crypto_aead *tfm)
 {
 	return &tfm->base;
 }
 
+/**
+ * crypto_free_aead() - zeroize and free aead handle
+ * @tfm: cipher handle to be freed
+ */
 static inline void crypto_free_aead(struct crypto_aead *tfm)
 {
 	crypto_free_tfm(crypto_aead_tfm(tfm));
@@ -1196,16 +1244,47 @@ static inline struct aead_tfm *crypto_aead_crt(struct crypto_aead *tfm)
 	return &crypto_aead_tfm(tfm)->crt_aead;
 }
 
+/**
+ * crypto_aead_ivsize() - obtain IV size
+ * @tfm: cipher handle
+ *
+ * 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
+ */
 static inline unsigned int crypto_aead_ivsize(struct crypto_aead *tfm)
 {
 	return crypto_aead_crt(tfm)->ivsize;
 }
 
+/**
+ * crypto_aead_authsize() - obtain maximum authentication data size
+ * @tfm: cipher handle
+ *
+ * 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
+ */
 static inline unsigned int crypto_aead_authsize(struct crypto_aead *tfm)
 {
 	return crypto_aead_crt(tfm)->authsize;
 }
 
+/**
+ * crypto_aead_blocksize() - obtain block size of cipher
+ * @tfm: cipher handle
+ *
+ * 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
+ */
 static inline unsigned int crypto_aead_blocksize(struct crypto_aead *tfm)
 {
 	return crypto_tfm_alg_blocksize(crypto_aead_tfm(tfm));
@@ -1231,6 +1310,22 @@ static inline void crypto_aead_clear_flags(struct crypto_aead *tfm, u32 flags)
 	crypto_tfm_clear_flags(crypto_aead_tfm(tfm), flags);
 }
 
+/**
+ * crypto_aead_setkey() - set key for cipher
+ * @tfm: cipher handle
+ * @key: buffer holding the key
+ * @keylen: length of the key in bytes
+ *
+ * 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
+ */
 static inline int crypto_aead_setkey(struct crypto_aead *tfm, const u8 *key,
 				     unsigned int keylen)
 {
@@ -1239,34 +1334,122 @@ static inline int crypto_aead_setkey(struct crypto_aead *tfm, const u8 *key,
 	return crt->setkey(crt->base, key, keylen);
 }
 
+/**
+ * crypto_aead_setauthsize() - set authentication data size
+ * @tfm: cipher handle
+ * @authsize: size of the authentication data / tag in bytes
+ *
+ * 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_setauthsize(struct crypto_aead *tfm, unsigned int authsize);
 
 static inline struct crypto_aead *crypto_aead_reqtfm(struct aead_request *req)
 {
 	return __crypto_aead_cast(req->base.tfm);
 }
 
+/**
+ * crypto_aead_encrypt() - encrypt plaintext
+ * @req: reference to the aead_request handle that holds all information
+ *	 needed to perform the cipher operation
+ *
+ * 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
+ */
 static inline int crypto_aead_encrypt(struct aead_request *req)
 {
 	return crypto_aead_crt(crypto_aead_reqtfm(req))->encrypt(req);
 }
 
+/**
+ * crypto_aead_decrypt() - decrypt ciphertext
+ * @req: reference to the ablkcipher_request handle that holds all information
+ *	 needed to perform the cipher operation
+ *
+ * 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.
+ */
 static inline int crypto_aead_decrypt(struct aead_request *req)
 {
 	return crypto_aead_crt(crypto_aead_reqtfm(req))->decrypt(req);
 }
 
+/**
+ * DOC: 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.
+ */
+
+/**
+ * crypto_aead_reqsize() - obtain size of the request data structure
+ * @tfm: cipher handle
+ *
+ * Return: number of bytes
+ */
 static inline unsigned int crypto_aead_reqsize(struct crypto_aead *tfm)
 {
 	return crypto_aead_crt(tfm)->reqsize;
 }
 
+/**
+ * aead_request_set_tfm() - update cipher handle reference in request
+ * @req: request handle to be modified
+ * @tfm: cipher handle that shall be added to the request handle
+ *
+ * Allow the caller to replace the existing aead handle in the request
+ * data structure with a different one.
+ */
 static inline void aead_request_set_tfm(struct aead_request *req,
 					struct crypto_aead *tfm)
 {
 	req->base.tfm = crypto_aead_tfm(crypto_aead_crt(tfm)->base);
 }
 
+/**
+ * aead_request_alloc() - allocate request data structure
+ * @tfm: cipher handle to be registered with the request
+ * @gfp: memory allocation flag that is handed to kmalloc by the API call.
+ *
+ * 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; IS_ERR() is true in case
+ *	   of an error, PTR_ERR() returns the error code.
+ */
 static inline struct aead_request *aead_request_alloc(struct crypto_aead *tfm,
 						      gfp_t gfp)
 {
@@ -1280,11 +1463,40 @@ static inline struct aead_request *aead_request_alloc(struct crypto_aead *tfm,
 	return req;
 }
 
+/**
+ * aead_request_free() - zeroize and free request data structure
+ * @req: request data structure cipher handle to be freed
+ */
 static inline void aead_request_free(struct aead_request *req)
 {
 	kzfree(req);
 }
 
+/**
+ * aead_request_set_callback() - set asynchronous callback function
+ * @req: request handle
+ * @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
+ * @compl: callback function pointer to be registered with the request handle
+ * @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.
+ *
+ * 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)
+ */
 static inline void aead_request_set_callback(struct aead_request *req,
 					     u32 flags,
 					     crypto_completion_t compl,
@@ -1295,6 +1507,36 @@ static inline void aead_request_set_callback(struct aead_request *req,
 	req->base.flags = flags;
 }
 
+/**
+ * aead_request_set_crypt - set data buffers
+ * @req: request handle
+ * @src: source scatter / gather list
+ * @dst: destination scatter / gather list
+ * @cryptlen: number of bytes to process from @src
+ * @iv: IV for the cipher operation which must comply with the IV size defined
+ *      by crypto_aead_ivsize()
+ *
+ * Setting the source data and destination data scatter / gather lists.
+ *
+ * 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.
+ *
+ * IMPORTANT NOTE AEAD requires an authentication tag (MAC). For decryption,
+ *		  the caller must concatenate the ciphertext followed by the
+ *		  authentication tag and provide the entire data stream to the
+ *		  decryption operation (i.e. the data length used for the
+ *		  initialization of the scatterlist and the data length for the
+ *		  decryption operation is identical). For encryption, however,
+ *		  the authentication tag is created while encrypting the data.
+ *		  The destination buffer must hold sufficient space for the
+ *		  ciphertext and the authentication tag while the encryption
+ *		  invocation must only point to the plaintext data size. The
+ *		  following code snippet illustrates the memory usage
+ *		  buffer = kmalloc(ptbuflen + (enc ? authsize : 0));
+ *		  sg_init_one(&sg, buffer, ptbuflen + (enc ? authsize : 0));
+ *		  aead_request_set_crypt(req, &sg, &sg, ptbuflen, iv);
+ */
 static inline void aead_request_set_crypt(struct aead_request *req,
 					  struct scatterlist *src,
 					  struct scatterlist *dst,
@@ -1306,6 +1548,15 @@ static inline void aead_request_set_crypt(struct aead_request *req,
 	req->iv = iv;
 }
 
+/**
+ * aead_request_set_assoc() - set the associated data scatter / gather list
+ * @req: request handle
+ * @assoc: associated data scatter / gather list
+ * @assoclen: number of bytes to process from @assoc
+ *
+ * For encryption, the memory is filled with the associated data. For
+ * decryption, the memory must point to the associated data.
+ */
 static inline void aead_request_set_assoc(struct aead_request *req,
 					  struct scatterlist *assoc,
 					  unsigned int assoclen)