[crypto] Move AES_convert_key() hack into axtls_aes.c

Although the nature of the hack is essentially unchanged, this allows
us to remove the hardcoded assumption in tls.c that the RX cipher is
AES.

src/crypto/axtls_aes.c

 #include <gpxe/crypto.h>
 #include <gpxe/aes.h>
 
+struct aes_cbc_context {
+	AES_CTX ctx;
+	int decrypting;
+};
+
 static int aes_cbc_setkey ( void *ctx, const void *key, size_t keylen ) {
-	AES_CTX *aesctx = ctx;
+	struct aes_cbc_context *aesctx = ctx;
 	AES_MODE mode;
 
 	switch ( keylen ) {
 		return -EINVAL;
 	}
 
-	AES_set_key ( aesctx, key, aesctx->iv, mode );
+	AES_set_key ( &aesctx->ctx, key, aesctx->ctx.iv, mode );
+
+	aesctx->decrypting = 0;
+
 	return 0;
 }
 
 static void aes_cbc_setiv ( void *ctx, const void *iv ) {
-	AES_CTX *aesctx = ctx;
+	struct aes_cbc_context *aesctx = ctx;
 
-	memcpy ( aesctx->iv, iv, sizeof ( aesctx->iv ) );
+	memcpy ( aesctx->ctx.iv, iv, sizeof ( aesctx->ctx.iv ) );
 }
 
 static void aes_cbc_encrypt ( void *ctx, const void *data, void *dst,
 			      size_t len ) {
-	AES_CTX *aesctx = ctx;
+	struct aes_cbc_context *aesctx = ctx;
 
-	AES_cbc_encrypt ( aesctx, data, dst, len );
+	if ( aesctx->decrypting )
+		assert ( 0 );
+
+	AES_cbc_encrypt ( &aesctx->ctx, data, dst, len );
 }
 
 static void aes_cbc_decrypt ( void *ctx, const void *data, void *dst,
 			      size_t len ) {
-	AES_CTX *aesctx = ctx;
+	struct aes_cbc_context *aesctx = ctx;
+
+	if ( ! aesctx->decrypting ) {
+		AES_convert_key ( &aesctx->ctx );
+		aesctx->decrypting = 1;
+	}
 
-	AES_cbc_decrypt ( aesctx, data, dst, len );
+	AES_cbc_decrypt ( &aesctx->ctx, data, dst, len );
 }
 
 struct crypto_algorithm aes_cbc_algorithm = {
 	.name		= "aes_cbc",
-	.ctxsize	= sizeof ( AES_CTX ),
+	.ctxsize	= sizeof ( struct aes_cbc_context ),
 	.blocksize	= 16,
 	.setkey		= aes_cbc_setkey,
 	.setiv		= aes_cbc_setiv,

src/net/tls.c

 		       tls, strerror ( rc ) );
 		return rc;
 	}
-
-	/* FIXME: AES needs to be fixed to not require this */
-	AES_convert_key ( rx_cipherspec->cipher_ctx );
-
 	DBGC ( tls, "TLS %p RX key:\n", tls );
 	DBGC_HD ( tls, key, key_size );
 	key += key_size;