diff --git a/sys/opencrypto/cryptodev.h b/sys/opencrypto/cryptodev.h
--- a/sys/opencrypto/cryptodev.h
+++ b/sys/opencrypto/cryptodev.h
@@ -95,6 +95,7 @@
 #define	SHA2_256_BLOCK_LEN	64
 #define	SHA2_384_BLOCK_LEN	128
 #define	SHA2_512_BLOCK_LEN	128
+#define	POLY1305_BLOCK_LEN	16
 
 /* HMAC values */
 #define	NULL_HMAC_BLOCK_LEN		64
diff --git a/sys/opencrypto/xform_poly1305.c b/sys/opencrypto/xform_poly1305.c
--- a/sys/opencrypto/xform_poly1305.c
+++ b/sys/opencrypto/xform_poly1305.c
@@ -14,6 +14,7 @@
 
 CTASSERT(POLY1305_KEY_LEN == crypto_onetimeauth_poly1305_KEYBYTES);
 CTASSERT(POLY1305_HASH_LEN == crypto_onetimeauth_poly1305_BYTES);
+CTASSERT(POLY1305_BLOCK_LEN == crypto_onetimeauth_poly1305_BYTES);
 
 static void
 xform_Poly1305_Init(void *polyctx)
@@ -64,7 +65,7 @@
 	.keysize = POLY1305_KEY_LEN,
 	.hashsize = POLY1305_HASH_LEN,
 	.ctxsize = sizeof(struct poly1305_xform_ctx),
-	.blocksize = crypto_onetimeauth_poly1305_BYTES,
+	.blocksize = POLY1305_BLOCK_LEN,
 	.Init = xform_Poly1305_Init,
 	.Setkey = xform_Poly1305_Setkey,
 	.Update = xform_Poly1305_Update,