[PATCH v3 7/7] crypto: AF_ALG: document the user space interface

[Stephan Mueller]

The extension of the user space interface documentation covers all
aspects of the patchset, including:

        * AEAD cipher interface

        * RNG cipher interface

Signed-off-by: Stephan Mueller <smueller@xxxxxxxxxx>
---
 Documentation/crypto/crypto-API-userspace.txt | 70 ++++++++++++++++++++++++++-
 1 file changed, 68 insertions(+), 2 deletions(-)

diff --git a/Documentation/crypto/crypto-API-userspace.txt b/Documentation/crypto/crypto-API-userspace.txt
index ac619cd..0646034 100644
--- a/Documentation/crypto/crypto-API-userspace.txt
+++ b/Documentation/crypto/crypto-API-userspace.txt
@@ -30,8 +30,9 @@ ciphers are accessible:
 
 	* Symmetric ciphers
 
-Note, AEAD ciphers are currently not supported via the symmetric cipher
-interface.
+	* AEAD ciphers
+
+	* Random number generators
 
 The interface is provided via Netlink using the type AF_ALG. In addition, the
 setsockopt option type is SOL_ALG. In case the user space header files do not
@@ -85,6 +86,7 @@ If a consumer on the other hand wants to maintain the plaintext and the
 ciphertext in different memory locations, all a consumer needs to do is to
 provide different memory pointers for the encryption and decryption operation.
 
+
 Message digest API
 ==================
 
@@ -169,6 +171,69 @@ as large as to hold all blocks of the encrypted or decrypted data. If the output
 data size is smaller, only as many blocks are returned that fit into that
 output buffer size.
 
+
+AEAD cipher API
+===============
+
+The operation is identical to the symmetric cipher API. However, an AEAD
+cipher requires additional information, such as the authentication tag and
+the associated data. This data is to be supplied in addition to the normal
+symmetric cipher data like key and IV discussed for the symmetric ciphers.
+
+During initialization, the struct sockaddr data structure must be filled as
+follows:
+
+struct sockaddr_alg sa = {
+	.salg_family = AF_ALG,
+	.salg_type = "aead", /* this selects the AEAD cipher */
+	.salg_name = "gcm(aes)" /* this is the cipher name */
+};
+
+The discussion about the sendmsg given for the symmetric cipher applies for
+the AEAD interface as well. In addition to the plaintext / ciphertext data and
+the IV, the following data must be supplied with the cmsghdr data structure:
+
+	* The AEAD authentication tag size is set with the flag
+	  ALG_SET_AEAD_AUTHSIZE. The integer value of the authentication tag
+	  size must be provided in the data field of the cmsghdr structure.
+
+	* The AEAD associated data is set with the flag ALG_SET_AEAD_ASSOC.
+	  The data is set the same way as for the IV by supplying the associated
+	  data in the data field of the cmsghdr structure.
+
+The authentication tag itself, however, is handled in a different way to comply
+with the specifics of the kernel crypto API and to avoid copying the
+authentication tag around in memory. The authentication tag is added to the
+memory that immediately follows the ciphertext.
+
+	* When performing an encryption operation, the resulting ciphertext
+	  buffer will hold the tag as follows: ciphertext || tag.  The consumer
+	  must ensure that the ciphertext buffer is large enough to hold the
+	  ciphertext together with the tag of the size set by the consumer using
+	  the ALG_SET_AEAD_AUTHSIZE cmsghdr flag as discussed above.
+
+	* When performing a decryption operation, the initial ciphertext buffer
+	  must hold the tag as follows: ciphertext || tag. The resulting
+	  plaintext has the same size as the ciphertext.
+
+Note: Authentication errors during decryption are marked with a failing
+read/recv system call whose errno is set to EBADMSG.
+
+
+Random number generator API
+===========================
+
+Compared to the symmetric ciphers, the random number generator API is simple:
+it only supports the system calls of read/recv.
+
+The consumer must observe the returned size of the read/recv system calls and
+potentially make subsequent calls if the returned length of random numbers is
+smaller than the expected length.
+
+When initializing a random number generator instance, the AF_ALG interface
+handler ensures that it is appropriately seeded.
+
+
 Setsockopt interface
 ====================
 
@@ -190,6 +255,7 @@ optname:
 
 		- the hash cipher type (keyed message digests)
 
+
 User space API example
 ======================
 
-- 
2.1.0


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