diff --git a/README.md b/README.md
index ebd56f2..51c4b35 100644
--- a/README.md
+++ b/README.md
@@ -89,7 +89,7 @@ e.g., to verify the signature of *archive.tar.gz* against
 If the sender wishes to prove to the recipient that they  encrypted
 a file:
 
-   sigtool encrypt -s sender.key to.pub -o archive.tar.gz.enc archive.tar.gz
+    sigtool encrypt -s sender.key to.pub -o archive.tar.gz.enc archive.tar.gz
 
 
 This will create an encrypted file *archive.tar.gz.enc* such that the
@@ -101,7 +101,7 @@ who they expect.
 If the receiver has the public key of the sender, they can verify that
 they indeed sent the file by cryptographically checking the output:
 
-   sigtool decrypt -o archive.tar.gz -v sender.pub to.key archive.tar.gz.enc
+    sigtool decrypt -o archive.tar.gz -v sender.pub to.key archive.tar.gz.enc
 
 Note that the verification is optional and if the `-v` option is not
 used, then decryption will proceed without verifying the sender.
@@ -110,7 +110,7 @@ used, then decryption will proceed without verifying the sender.
 `sigtool` can generate ephemeral keys for encrypting a file such that
 the receiver doesn't need to authenticate the sender:
 
-   sigtool encrypt to.pub -o archive.tar.gz.enc archive.tar.gz
+    sigtool encrypt to.pub -o archive.tar.gz.enc archive.tar.gz
 
 This will create an encrypted file *archive.tar.gz.enc* such that the
 recipient in possession of *to.key* can decrypt it.
diff --git a/crypt.go b/crypt.go
index ebcb23e..236d2dd 100644
--- a/crypt.go
+++ b/crypt.go
@@ -35,17 +35,24 @@ func encrypt(args []string) {
 	var keyfile string
 	var envpw string
 	var pw bool
+	var sblksize string
 
 	fs.StringVarP(&outfile, "outfile", "o", "", "Write the output to file `F`")
 	fs.StringVarP(&keyfile, "sign", "s", "", "Sign using private key `S`")
 	fs.BoolVarP(&pw, "password", "p", false, "Ask for passphrase to decrypt the private key")
 	fs.StringVarP(&envpw, "env-password", "", "", "Use passphrase from environment variable `E`")
+	fs.StringVarP(&sblksize, "block-size", "B", "4M", "Use `S` as the encryption block size")
 
 	err := fs.Parse(args)
 	if err != nil {
 		die("%s", err)
 	}
 
+	blksize, err := utils.ParseSize(sblksize)
+	if err != nil {
+		die("%s", err)
+	}
+
 	var pws, infile string
 
 	if len(envpw) > 0 {
@@ -108,7 +115,7 @@ func encrypt(args []string) {
 		outfd = outf
 	}
 
-	en, err := sign.NewEncryptor(sk)
+	en, err := sign.NewEncryptor(sk, blksize)
 	if err != nil {
 		die("%s", err)
 	}
diff --git a/go.mod b/go.mod
index ef96194..39cbc19 100644
--- a/go.mod
+++ b/go.mod
@@ -4,7 +4,7 @@ go 1.13
 
 require (
 	github.com/gogo/protobuf v1.3.1
-	github.com/opencoff/go-utils v0.3.0
+	github.com/opencoff/go-utils v0.4.0
 	github.com/opencoff/pflag v0.3.3
 	golang.org/x/crypto v0.0.0-20191002192127-34f69633bfdc
 	gopkg.in/yaml.v2 v2.2.4
diff --git a/go.sum b/go.sum
index e318adb..aff4d00 100644
--- a/go.sum
+++ b/go.sum
@@ -4,6 +4,8 @@ github.com/kisielk/errcheck v1.2.0/go.mod h1:/BMXB+zMLi60iA8Vv6Ksmxu/1UDYcXs4uQL
 github.com/kisielk/gotool v1.0.0/go.mod h1:XhKaO+MFFWcvkIS/tQcRk01m1F5IRFswLeQ+oQHNcck=
 github.com/opencoff/go-utils v0.3.0 h1:/TQXjf50o3GSB9MItog5L8Gf4GWJ4B5+rmqjB4g2RZQ=
 github.com/opencoff/go-utils v0.3.0/go.mod h1:c+7QUAiCCHcNH6OGvsZ0fviG7cgse8Y3ucg+xy7sGXM=
+github.com/opencoff/go-utils v0.4.0 h1:pu08Om//u2+YGvLkHa2CyL6eI+/1J0bXih1Z6nuITp8=
+github.com/opencoff/go-utils v0.4.0/go.mod h1:c+7QUAiCCHcNH6OGvsZ0fviG7cgse8Y3ucg+xy7sGXM=
 github.com/opencoff/pflag v0.3.3 h1:yohZkwYGPkB34WXvUQzU5GyLhImnjfePDARUaE8me3U=
 github.com/opencoff/pflag v0.3.3/go.mod h1:mTLzGGUGda1Av3d34iAJlh0JIlRxmFZtmc6qoWPspK0=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
diff --git a/sign/crypt_test.go b/sign/crypt_test.go
new file mode 100644
index 0000000..b65d18d
--- /dev/null
+++ b/sign/crypt_test.go
@@ -0,0 +1,218 @@
+// crypt_test.go -- Test harness for encrypt/decrypt bits
+//
+// (c) 2016 Sudhi Herle <sudhi@herle.net>
+//
+// Licensing Terms: GPLv2
+//
+// If you need a commercial license for this work, please contact
+// the author.
+//
+// This software does not come with any express or implied
+// warranty; it is provided "as is". No claim  is made to its
+// suitability for any purpose.
+
+package sign
+
+import (
+	"bytes"
+	"crypto/rand"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"testing"
+)
+
+// one sender, one receiver no verification of sender
+func TestSimple(t *testing.T) {
+	assert := newAsserter(t)
+
+	receiver, err := NewKeypair()
+	assert(err == nil, "receiver keypair gen failed: %s", err)
+
+	// cleartext
+	buf := make([]byte, 64*1024)
+	for i := 0; i < len(buf); i++ {
+		buf[i] = byte(i & 0xff)
+	}
+
+	ee, err := NewEncryptor(nil, 4096)
+	assert(err == nil, "encryptor create fail: %s", err)
+
+	err = ee.AddRecipient(&receiver.Pub)
+	assert(err == nil, "can't add recipient: %s", err)
+
+	rd := bytes.NewBuffer(buf)
+	wr := bytes.Buffer{}
+
+	err = ee.Encrypt(rd, &wr)
+	assert(err == nil, "encrypt fail: %s", err)
+
+	rd = bytes.NewBuffer(wr.Bytes())
+
+	dd, err := NewDecryptor(rd)
+	assert(err == nil, "decryptor create fail: %s", err)
+
+	err = dd.SetPrivateKey(&receiver.Sec, nil)
+	assert(err == nil, "decryptor can't add SK: %s", err)
+
+	wr = bytes.Buffer{}
+	err = dd.Decrypt(&wr)
+	assert(err == nil, "decrypt fail: %s", err)
+
+	b := wr.Bytes()
+	assert(len(b) == len(buf), "decrypt length mismatch: exp %d, saw %d", len(buf), len(b))
+
+	assert(byteEq(b, buf), "decrypt content mismatch")
+}
+
+// test corrupted header or corrupted input
+func TestCorrupted(t *testing.T) {
+	assert := newAsserter(t)
+
+	receiver, err := NewKeypair()
+	assert(err == nil, "receiver keypair gen failed: %s", err)
+
+	// cleartext
+	buf := make([]byte, 64*1024)
+	for i := 0; i < len(buf); i++ {
+		buf[i] = byte(i & 0xff)
+	}
+
+	ee, err := NewEncryptor(nil, 4096)
+	assert(err == nil, "encryptor create fail: %s", err)
+
+	err = ee.AddRecipient(&receiver.Pub)
+	assert(err == nil, "can't add recipient: %s", err)
+
+	rd := bytes.NewBuffer(buf)
+	wr := bytes.Buffer{}
+
+	err = ee.Encrypt(rd, &wr)
+	assert(err == nil, "encrypt fail: %s", err)
+
+	rb := wr.Bytes()
+	n := len(rb)
+
+	for i := 0; i < n; i++ {
+		j := randint() % n
+		rb[j] = byte(randint() & 0xff)
+	}
+
+	rd = bytes.NewBuffer(rb)
+	dd, err := NewDecryptor(rd)
+	assert(err != nil, "decryptor works on bad input")
+	assert(dd == nil, "decryptor not nil for bad input")
+}
+
+// one sender, one receiver with verification of sender
+func TestSenderVerified(t *testing.T) {
+	assert := newAsserter(t)
+
+	sender, err := NewKeypair()
+	assert(err == nil, "sender keypair gen failed: %s", err)
+
+	receiver, err := NewKeypair()
+	assert(err == nil, "receiver keypair gen failed: %s", err)
+
+	// cleartext
+	buf := make([]byte, 64*1024)
+	for i := 0; i < len(buf); i++ {
+		buf[i] = byte(i & 0xff)
+	}
+
+	ee, err := NewEncryptor(&sender.Sec, 4096)
+	assert(err == nil, "encryptor create fail: %s", err)
+
+	err = ee.AddRecipient(&receiver.Pub)
+	assert(err == nil, "can't add recipient: %s", err)
+
+	rd := bytes.NewBuffer(buf)
+	wr := bytes.Buffer{}
+
+	err = ee.Encrypt(rd, &wr)
+	assert(err == nil, "encrypt fail: %s", err)
+
+	rd = bytes.NewBuffer(wr.Bytes())
+
+	dd, err := NewDecryptor(rd)
+	assert(err == nil, "decryptor create fail: %s", err)
+
+	err = dd.SetPrivateKey(&receiver.Sec, &sender.Pub)
+	assert(err == nil, "decryptor can't add SK: %s", err)
+
+	wr = bytes.Buffer{}
+	err = dd.Decrypt(&wr)
+	assert(err == nil, "decrypt fail: %s", err)
+
+	b := wr.Bytes()
+	assert(len(b) == len(buf), "decrypt length mismatch: exp %d, saw %d", len(buf), len(b))
+
+	assert(byteEq(b, buf), "decrypt content mismatch")
+}
+
+// one sender, multiple receivers, each decrypting the blob
+func TestMultiReceiver(t *testing.T) {
+	assert := newAsserter(t)
+
+	sender, err := NewKeypair()
+	assert(err == nil, "sender keypair gen failed: %s", err)
+
+	// cleartext
+	buf := make([]byte, 64*1024)
+	for i := 0; i < len(buf); i++ {
+		buf[i] = byte(i & 0xff)
+	}
+
+	ee, err := NewEncryptor(&sender.Sec, 4096)
+	assert(err == nil, "encryptor create fail: %s", err)
+
+	n := 4
+	rx := make([]*Keypair, n)
+	for i := 0; i < n; i++ {
+		r, err := NewKeypair()
+		assert(err == nil, "can't make receiver key %d: %s", i, err)
+		rx[i] = r
+
+		err = ee.AddRecipient(&r.Pub)
+		assert(err == nil, "can't add recipient %d: %s", i, err)
+	}
+
+	rd := bytes.NewBuffer(buf)
+	wr := bytes.Buffer{}
+
+	err = ee.Encrypt(rd, &wr)
+	assert(err == nil, "encrypt fail: %s", err)
+
+	encBytes := wr.Bytes()
+	for i := 0; i < n; i++ {
+		rd = bytes.NewBuffer(encBytes)
+
+		dd, err := NewDecryptor(rd)
+		assert(err == nil, "decryptor %d create fail: %s", i, err)
+
+		err = dd.SetPrivateKey(&rx[i].Sec, &sender.Pub)
+		assert(err == nil, "decryptor can't add SK %d: %s", i, err)
+
+		wr = bytes.Buffer{}
+		err = dd.Decrypt(&wr)
+		assert(err == nil, "decrypt %d fail: %s", i, err)
+
+		b := wr.Bytes()
+		assert(len(b) == len(buf), "decrypt %d length mismatch: exp %d, saw %d", i, len(buf), len(b))
+
+		assert(byteEq(b, buf), "decrypt %d content mismatch", i)
+	}
+}
+
+func randint() int {
+	var b [4]byte
+
+	_, err := io.ReadFull(rand.Reader, b[:])
+	if err != nil {
+		panic(fmt.Sprintf("can't read 4 rand bytes: %s", err))
+	}
+
+	u := binary.BigEndian.Uint32(b[:])
+
+	return int(u & 0x7fffffff)
+}
diff --git a/sign/encrypt.go b/sign/encrypt.go
index a421287..4615ee1 100644
--- a/sign/encrypt.go
+++ b/sign/encrypt.go
@@ -1,4 +1,4 @@
-// cipher.go -- Ed25519 based encrypt/decrypt
+// encrypt.go -- Ed25519 based encrypt/decrypt
 //
 // (c) 2016 Sudhi Herle <sudhi@herle.net>
 //
@@ -31,6 +31,7 @@ import (
 
 // Encryption chunk size = 4MB
 const chunkSize int = 4 * 1048576
+const maxChunkSize int = 16 * 1048576
 
 const _Magic = "SigTool"
 const _MagicLen = len(_Magic)
@@ -51,11 +52,19 @@ type Encryptor struct {
 // Create a new Encryption context and use the optional private key 'sk' for
 // signing any recipient keys. If 'sk' is nil, then ephmeral Curve25519 keys
 // are generated and used with recipient's public key.
-func NewEncryptor(sk *PrivateKey) (*Encryptor, error) {
+func NewEncryptor(sk *PrivateKey, blksize uint64) (*Encryptor, error) {
+
+	if blksize >= uint64(maxChunkSize) {
+		return nil, fmt.Errorf("encrypt: Blocksize is too large (max 16M)")
+	}
+
+	if blksize == 0 {
+		blksize = uint64(chunkSize)
+	}
 
 	e := &Encryptor{
 		Header: Header{
-			ChunkSize: uint32(chunkSize),
+			ChunkSize: uint32(blksize),
 			Salt:      make([]byte, _AEADNonceLen),
 		},
 
@@ -282,7 +291,7 @@ func NewDecryptor(rd io.Reader) (*Decryptor, error) {
 		return nil, fmt.Errorf("decrypt: decode error: %s", err)
 	}
 
-	if d.ChunkSize == 0 || d.ChunkSize > (16*1048576) {
+	if d.ChunkSize == 0 || d.ChunkSize >= uint32(maxChunkSize) {
 		return nil, fmt.Errorf("decrypt: invalid chunkSize %d", d.ChunkSize)
 	}
 
@@ -397,7 +406,7 @@ func (d *Decryptor) decrypt(i int) ([]byte, error) {
 	chunklen := int(binary.BigEndian.Uint32(b[:4]))
 
 	// Sanity check - in case of corrupt header
-	if chunklen > (d.ae.Overhead()+chunkSize) {
+	if chunklen > (d.ae.Overhead() + chunkSize) {
 		return nil, fmt.Errorf("decrypt: chunksize is too large (%d)", chunklen)
 	}
 
diff --git a/version b/version
index 0c62199..9e11b32 100644
--- a/version
+++ b/version
@@ -1 +1 @@
-0.2.1
+0.3.1