diff --git a/net/rxrpc/ar-internal.h b/net/rxrpc/ar-internal.h
index fda6618df1cc..7bd6f8a66a3e 100644
--- a/net/rxrpc/ar-internal.h
+++ b/net/rxrpc/ar-internal.h
@@ -234,6 +234,11 @@ struct rxrpc_security {
 	int (*init_connection_security)(struct rxrpc_connection *,
 					struct rxrpc_key_token *);
 
+	/* Work out how much data we can store in a packet, given an estimate
+	 * of the amount of data remaining.
+	 */
+	int (*how_much_data)(struct rxrpc_call *, size_t,
+			     size_t *, size_t *, size_t *);
 
 	/* impose security on a packet */
 	int (*secure_packet)(struct rxrpc_call *, struct sk_buff *, size_t);
@@ -467,8 +472,6 @@ struct rxrpc_connection {
 	atomic_t		serial;		/* packet serial number counter */
 	unsigned int		hi_serial;	/* highest serial number received */
 	u32			service_id;	/* Service ID, possibly upgraded */
-	u8			size_align;	/* data size alignment (for security) */
-	u8			security_size;	/* security header size */
 	u8			security_ix;	/* security type */
 	u8			out_clientflag;	/* RXRPC_CLIENT_INITIATED if we are client */
 	u8			bundle_shift;	/* Index into bundle->avail_chans */
diff --git a/net/rxrpc/conn_object.c b/net/rxrpc/conn_object.c
index 8dd1ef25b98f..b2159dbf5412 100644
--- a/net/rxrpc/conn_object.c
+++ b/net/rxrpc/conn_object.c
@@ -49,7 +49,6 @@ struct rxrpc_connection *rxrpc_alloc_connection(gfp_t gfp)
 		conn->security = &rxrpc_no_security;
 		spin_lock_init(&conn->state_lock);
 		conn->debug_id = atomic_inc_return(&rxrpc_debug_id);
-		conn->size_align = 4;
 		conn->idle_timestamp = jiffies;
 	}
 
diff --git a/net/rxrpc/insecure.c b/net/rxrpc/insecure.c
index e06725e21c05..9aae99d67833 100644
--- a/net/rxrpc/insecure.c
+++ b/net/rxrpc/insecure.c
@@ -14,6 +14,17 @@ static int none_init_connection_security(struct rxrpc_connection *conn,
 	return 0;
 }
 
+/*
+ * Work out how much data we can put in an unsecured packet.
+ */
+static int none_how_much_data(struct rxrpc_call *call, size_t remain,
+			       size_t *_buf_size, size_t *_data_size, size_t *_offset)
+{
+	*_buf_size = *_data_size = min_t(size_t, remain, RXRPC_JUMBO_DATALEN);
+	*_offset = 0;
+	return 0;
+}
+
 static int none_secure_packet(struct rxrpc_call *call, struct sk_buff *skb,
 			      size_t data_size)
 {
@@ -81,6 +92,7 @@ const struct rxrpc_security rxrpc_no_security = {
 	.exit				= none_exit,
 	.init_connection_security	= none_init_connection_security,
 	.free_call_crypto		= none_free_call_crypto,
+	.how_much_data			= none_how_much_data,
 	.secure_packet			= none_secure_packet,
 	.verify_packet			= none_verify_packet,
 	.locate_data			= none_locate_data,
diff --git a/net/rxrpc/rxkad.c b/net/rxrpc/rxkad.c
index e5b4bbdd0f34..e2e9e9b0a6d7 100644
--- a/net/rxrpc/rxkad.c
+++ b/net/rxrpc/rxkad.c
@@ -28,6 +28,7 @@
 #define INST_SZ				40	/* size of principal's instance */
 #define REALM_SZ			40	/* size of principal's auth domain */
 #define SNAME_SZ			40	/* size of service name */
+#define RXKAD_ALIGN			8
 
 struct rxkad_level1_hdr {
 	__be32	data_size;	/* true data size (excluding padding) */
@@ -80,7 +81,7 @@ static int rxkad_preparse_server_key(struct key_preparsed_payload *prep)
 
 static void rxkad_free_preparse_server_key(struct key_preparsed_payload *prep)
 {
-	
+
 	if (prep->payload.data[0])
 		crypto_free_skcipher(prep->payload.data[0]);
 }
@@ -119,14 +120,8 @@ static int rxkad_init_connection_security(struct rxrpc_connection *conn,
 
 	switch (conn->params.security_level) {
 	case RXRPC_SECURITY_PLAIN:
-		break;
 	case RXRPC_SECURITY_AUTH:
-		conn->size_align = 8;
-		conn->security_size = sizeof(struct rxkad_level1_hdr);
-		break;
 	case RXRPC_SECURITY_ENCRYPT:
-		conn->size_align = 8;
-		conn->security_size = sizeof(struct rxkad_level2_hdr);
 		break;
 	default:
 		ret = -EKEYREJECTED;
@@ -147,6 +142,40 @@ static int rxkad_init_connection_security(struct rxrpc_connection *conn,
 	return ret;
 }
 
+/*
+ * Work out how much data we can put in a packet.
+ */
+static int rxkad_how_much_data(struct rxrpc_call *call, size_t remain,
+			       size_t *_buf_size, size_t *_data_size, size_t *_offset)
+{
+	size_t shdr, buf_size, chunk;
+
+	switch (call->conn->params.security_level) {
+	default:
+		buf_size = chunk = min_t(size_t, remain, RXRPC_JUMBO_DATALEN);
+		shdr = 0;
+		goto out;
+	case RXRPC_SECURITY_AUTH:
+		shdr = sizeof(struct rxkad_level1_hdr);
+		break;
+	case RXRPC_SECURITY_ENCRYPT:
+		shdr = sizeof(struct rxkad_level2_hdr);
+		break;
+	}
+
+	buf_size = round_down(RXRPC_JUMBO_DATALEN, RXKAD_ALIGN);
+
+	chunk = buf_size - shdr;
+	if (remain < chunk)
+		buf_size = round_up(shdr + remain, RXKAD_ALIGN);
+
+out:
+	*_buf_size = buf_size;
+	*_data_size = chunk;
+	*_offset = shdr;
+	return 0;
+}
+
 /*
  * prime the encryption state with the invariant parts of a connection's
  * description
@@ -237,6 +266,7 @@ static int rxkad_secure_packet_auth(const struct rxrpc_call *call,
 	struct rxkad_level1_hdr hdr;
 	struct rxrpc_crypt iv;
 	struct scatterlist sg;
+	size_t pad;
 	u16 check;
 
 	_enter("");
@@ -247,6 +277,12 @@ static int rxkad_secure_packet_auth(const struct rxrpc_call *call,
 	hdr.data_size = htonl(data_size);
 	memcpy(skb->head, &hdr, sizeof(hdr));
 
+	pad = sizeof(struct rxkad_level1_hdr) + data_size;
+	pad = RXKAD_ALIGN - pad;
+	pad &= RXKAD_ALIGN - 1;
+	if (pad)
+		skb_put_zero(skb, pad);
+
 	/* start the encryption afresh */
 	memset(&iv, 0, sizeof(iv));
 
@@ -275,6 +311,7 @@ static int rxkad_secure_packet_encrypt(const struct rxrpc_call *call,
 	struct rxrpc_crypt iv;
 	struct scatterlist sg[16];
 	unsigned int len;
+	size_t pad;
 	u16 check;
 	int err;
 
@@ -288,6 +325,12 @@ static int rxkad_secure_packet_encrypt(const struct rxrpc_call *call,
 	rxkhdr.checksum = 0;
 	memcpy(skb->head, &rxkhdr, sizeof(rxkhdr));
 
+	pad = sizeof(struct rxkad_level2_hdr) + data_size;
+	pad = RXKAD_ALIGN - pad;
+	pad &= RXKAD_ALIGN - 1;
+	if (pad)
+		skb_put_zero(skb, pad);
+
 	/* encrypt from the session key */
 	token = call->conn->params.key->payload.data[0];
 	memcpy(&iv, token->kad->session_key, sizeof(iv));
@@ -303,8 +346,7 @@ static int rxkad_secure_packet_encrypt(const struct rxrpc_call *call,
 	if (skb_shinfo(skb)->nr_frags > 16)
 		goto out;
 
-	len = data_size + call->conn->size_align - 1;
-	len &= ~(call->conn->size_align - 1);
+	len = round_up(data_size, RXKAD_ALIGN);
 
 	sg_init_table(sg, ARRAY_SIZE(sg));
 	err = skb_to_sgvec(skb, sg, 8, len);
@@ -1353,6 +1395,7 @@ const struct rxrpc_security rxkad = {
 	.free_preparse_server_key	= rxkad_free_preparse_server_key,
 	.destroy_server_key		= rxkad_destroy_server_key,
 	.init_connection_security	= rxkad_init_connection_security,
+	.how_much_data			= rxkad_how_much_data,
 	.secure_packet			= rxkad_secure_packet,
 	.verify_packet			= rxkad_verify_packet,
 	.free_call_crypto		= rxkad_free_call_crypto,
diff --git a/net/rxrpc/sendmsg.c b/net/rxrpc/sendmsg.c
index 367654a558c2..af8ad6c30b9f 100644
--- a/net/rxrpc/sendmsg.c
+++ b/net/rxrpc/sendmsg.c
@@ -327,7 +327,7 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
 			rxrpc_send_ack_packet(call, false, NULL);
 
 		if (!skb) {
-			size_t size, chunk, max, space;
+			size_t remain, bufsize, chunk, offset;
 
 			_debug("alloc");
 
@@ -342,24 +342,21 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
 					goto maybe_error;
 			}
 
-			max = RXRPC_JUMBO_DATALEN;
-			max -= call->conn->security_size;
-			max &= ~(call->conn->size_align - 1UL);
-
-			chunk = max;
-			if (chunk > msg_data_left(msg) && !more)
-				chunk = msg_data_left(msg);
-
-			space = chunk + call->conn->size_align;
-			space &= ~(call->conn->size_align - 1UL);
-
-			size = space + call->conn->security_size;
+			/* Work out the maximum size of a packet.  Assume that
+			 * the security header is going to be in the padded
+			 * region (enc blocksize), but the trailer is not.
+			 */
+			remain = more ? INT_MAX : msg_data_left(msg);
+			ret = call->conn->security->how_much_data(call, remain,
+								  &bufsize, &chunk, &offset);
+			if (ret < 0)
+				goto maybe_error;
 
-			_debug("SIZE: %zu/%zu/%zu", chunk, space, size);
+			_debug("SIZE: %zu/%zu @%zu", chunk, bufsize, offset);
 
 			/* create a buffer that we can retain until it's ACK'd */
 			skb = sock_alloc_send_skb(
-				sk, size, msg->msg_flags & MSG_DONTWAIT, &ret);
+				sk, bufsize, msg->msg_flags & MSG_DONTWAIT, &ret);
 			if (!skb)
 				goto maybe_error;
 
@@ -371,8 +368,7 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
 
 			ASSERTCMP(skb->mark, ==, 0);
 
-			_debug("HS: %u", call->conn->security_size);
-			__skb_put(skb, call->conn->security_size);
+			__skb_put(skb, offset);
 
 			sp->remain = chunk;
 			if (sp->remain > skb_tailroom(skb))
@@ -421,17 +417,6 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
 		    (msg_data_left(msg) == 0 && !more)) {
 			struct rxrpc_connection *conn = call->conn;
 			uint32_t seq;
-			size_t pad;
-
-			/* pad out if we're using security */
-			if (conn->security_ix) {
-				pad = conn->security_size + skb->mark;
-				pad = conn->size_align - pad;
-				pad &= conn->size_align - 1;
-				_debug("pad %zu", pad);
-				if (pad)
-					skb_put_zero(skb, pad);
-			}
 
 			seq = call->tx_top + 1;
 
