diff --git a/lib/libzfs/libzfs_sendrecv.c b/lib/libzfs/libzfs_sendrecv.c
index 988a85b2da..8b5f26a8c0 100644
--- a/lib/libzfs/libzfs_sendrecv.c
+++ b/lib/libzfs/libzfs_sendrecv.c
@@ -48,7 +48,6 @@
 #include "zfs_prop.h"
 #include "zfs_fletcher.h"
 #include "libzfs_impl.h"
-#include <sha2.h>
 #include <sys/zio_checksum.h>
 #include <sys/ddt.h>
 
@@ -341,12 +340,11 @@ cksummer(void *arg)
 			if (ZIO_CHECKSUM_EQUAL(drrw->drr_key.ddk_cksum,
 			    zero_cksum) ||
 			    !DRR_IS_DEDUP_CAPABLE(drrw->drr_checksumflags)) {
-				SHA256_CTX	ctx;
-				zio_cksum_t	tmpsha256;
+				zio_cksum_t tmpsha256;
+
+				zio_checksum_SHA256(buf,
+				    drrw->drr_length, &tmpsha256);
 
-				SHA256Init(&ctx);
-				SHA256Update(&ctx, buf, drrw->drr_length);
-				SHA256Final(&tmpsha256, &ctx);
 				drrw->drr_key.ddk_cksum.zc_word[0] =
 				    BE_64(tmpsha256.zc_word[0]);
 				drrw->drr_key.ddk_cksum.zc_word[1] =
diff --git a/module/zfs/sha256.c b/module/zfs/sha256.c
index f515be6bb3..cf9dd8fcba 100644
--- a/module/zfs/sha256.c
+++ b/module/zfs/sha256.c
@@ -19,32 +19,109 @@
  * CDDL HEADER END
  */
 /*
- * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
+ * Copyright 2007 Sun Microsystems, Inc.  All rights reserved.
  * Use is subject to license terms.
  */
+
 #include <sys/zfs_context.h>
 #include <sys/zio.h>
-#include <sys/sha2.h>
+#include <sys/zio_checksum.h>
+
+/*
+ * SHA-256 checksum, as specified in FIPS 180-3, available at:
+ * http://csrc.nist.gov/publications/PubsFIPS.html
+ *
+ * This is a very compact implementation of SHA-256.
+ * It is designed to be simple and portable, not to be fast.
+ */
+
+/*
+ * The literal definitions of Ch() and Maj() according to FIPS 180-3 are:
+ *
+ * 	Ch(x, y, z)     (x & y) ^ (~x & z)
+ * 	Maj(x, y, z)    (x & y) ^ (x & z) ^ (y & z)
+ *
+ * We use equivalent logical reductions here that require one less op.
+ */
+#define	Ch(x, y, z)	((z) ^ ((x) & ((y) ^ (z))))
+#define	Maj(x, y, z)	(((x) & (y)) ^ ((z) & ((x) ^ (y))))
+#define	Rot32(x, s)	(((x) >> s) | ((x) << (32 - s)))
+#define	SIGMA0(x)	(Rot32(x, 2) ^ Rot32(x, 13) ^ Rot32(x, 22))
+#define	SIGMA1(x)	(Rot32(x, 6) ^ Rot32(x, 11) ^ Rot32(x, 25))
+#define	sigma0(x)	(Rot32(x, 7) ^ Rot32(x, 18) ^ ((x) >> 3))
+#define	sigma1(x)	(Rot32(x, 17) ^ Rot32(x, 19) ^ ((x) >> 10))
+
+static const uint32_t SHA256_K[64] = {
+	0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5,
+	0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
+	0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3,
+	0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
+	0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc,
+	0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
+	0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7,
+	0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
+	0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13,
+	0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
+	0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3,
+	0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
+	0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5,
+	0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
+	0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208,
+	0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2
+};
+
+static void
+SHA256Transform(uint32_t *H, const uint8_t *cp)
+{
+	uint32_t a, b, c, d, e, f, g, h, t, T1, T2, W[64];
+
+	for (t = 0; t < 16; t++, cp += 4)
+		W[t] = (cp[0] << 24) | (cp[1] << 16) | (cp[2] << 8) | cp[3];
+
+	for (t = 16; t < 64; t++)
+		W[t] = sigma1(W[t - 2]) + W[t - 7] +
+		    sigma0(W[t - 15]) + W[t - 16];
+
+	a = H[0]; b = H[1]; c = H[2]; d = H[3];
+	e = H[4]; f = H[5]; g = H[6]; h = H[7];
+
+	for (t = 0; t < 64; t++) {
+		T1 = h + SIGMA1(e) + Ch(e, f, g) + SHA256_K[t] + W[t];
+		T2 = SIGMA0(a) + Maj(a, b, c);
+		h = g; g = f; f = e; e = d + T1;
+		d = c; c = b; b = a; a = T1 + T2;
+	}
+
+	H[0] += a; H[1] += b; H[2] += c; H[3] += d;
+	H[4] += e; H[5] += f; H[6] += g; H[7] += h;
+}
 
 void
 zio_checksum_SHA256(const void *buf, uint64_t size, zio_cksum_t *zcp)
 {
-	SHA2_CTX ctx;
-	zio_cksum_t tmp;
+	uint32_t H[8] = { 0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a,
+	    0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19 };
+	uint8_t pad[128];
+	int i, padsize;
 
-	SHA2Init(SHA256, &ctx);
-	SHA2Update(&ctx, buf, size);
-	SHA2Final(&tmp, &ctx);
+	for (i = 0; i < (size & ~63ULL); i += 64)
+		SHA256Transform(H, (uint8_t *)buf + i);
 
-	/*
-	 * A prior implementation of this function had a
-	 * private SHA256 implementation always wrote things out in
-	 * Big Endian and there wasn't a byteswap variant of it.
-	 * To preseve on disk compatibility we need to force that
-	 * behaviour.
-	 */
-	zcp->zc_word[0] = BE_64(tmp.zc_word[0]);
-	zcp->zc_word[1] = BE_64(tmp.zc_word[1]);
-	zcp->zc_word[2] = BE_64(tmp.zc_word[2]);
-	zcp->zc_word[3] = BE_64(tmp.zc_word[3]);
+	for (padsize = 0; i < size; i++)
+		pad[padsize++] = *((uint8_t *)buf + i);
+
+	for (pad[padsize++] = 0x80; (padsize & 63) != 56; padsize++)
+		pad[padsize] = 0;
+
+	for (i = 56; i >= 0; i -= 8)
+		pad[padsize++] = (size << 3) >> i;
+
+	for (i = 0; i < padsize; i += 64)
+		SHA256Transform(H, pad + i);
+
+	ZIO_SET_CHECKSUM(zcp,
+	    (uint64_t)H[0] << 32 | H[1],
+	    (uint64_t)H[2] << 32 | H[3],
+	    (uint64_t)H[4] << 32 | H[5],
+	    (uint64_t)H[6] << 32 | H[7]);
 }