diff --git a/go/current b/go/current
index eff4962..ab81e1c 120000
--- a/go/current
+++ b/go/current
@@ -1 +1 @@
-hexadecimal
\ No newline at end of file
+binary-search-tree
\ No newline at end of file
diff --git a/go/kindergarten-garden/README.md b/go/kindergarten-garden/README.md
new file mode 100644
index 0000000..5a38784
--- /dev/null
+++ b/go/kindergarten-garden/README.md
@@ -0,0 +1,77 @@
+# Kindergarten Garden
+
+Write a program that, given a diagram, can tell you which plants each child in the kindergarten class is responsible for.
+
+The kindergarten class is learning about growing plants. The teachers
+thought it would be a good idea to give them actual seeds, plant them in
+actual dirt, and grow actual plants.
+
+They've chosen to grow grass, clover, radishes, and violets.
+
+To this end, they've put little styrofoam cups along the window sills,
+and planted one type of plant in each cup, choosing randomly from the
+available types of seeds.
+
+```plain
+[window][window][window]
+........................ # each dot represents a styrofoam cup
+........................
+```
+
+There are 12 children in the class:
+
+- Alice, Bob, Charlie, David,
+- Eve, Fred, Ginny, Harriet,
+- Ileana, Joseph, Kincaid, and Larry.
+
+Each child gets 4 cups, two on each row. The children are assigned to
+cups in alphabetical order.
+
+The following diagram represents Alice's plants:
+
+```plain
+[window][window][window]
+VR......................
+RG......................
+```
+
+So in the row nearest the window, she has a violet and a radish; in the
+row behind that, she has a radish and some grass.
+
+Your program will be given the plants from left-to-right starting with
+the row nearest the windows. From this, it should be able to determine
+which plants belong to which students.
+
+For example, if it's told that the garden looks like so:
+
+```plain
+[window][window][window]
+VRCGVVRVCGGCCGVRGCVCGCGV
+VRCCCGCRRGVCGCRVVCVGCGCV
+```
+
+Then if asked for Alice's plants, it should provide:
+
+- Violets, radishes, violets, radishes
+
+While asking for Bob's plants would yield:
+
+- Clover, grass, clover, clover
+
+To run the tests simply run the command `go test` in the exercise directory.
+
+If the test suite contains benchmarks, you can run these with the `-bench`
+flag:
+
+    go test -bench .
+
+For more detailed info about the Go track see the [help
+page](http://exercism.io/languages/go).
+
+## Source
+
+Random musings during airplane trip. [http://jumpstartlab.com](http://jumpstartlab.com)
+
+## Submitting Incomplete Problems
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.
+
diff --git a/go/kindergarten-garden/kindergarten.go b/go/kindergarten-garden/kindergarten.go
new file mode 100644
index 0000000..58dab55
--- /dev/null
+++ b/go/kindergarten-garden/kindergarten.go
@@ -0,0 +1,93 @@
+package kindergarten
+
+import (
+	"errors"
+	"sort"
+	"strings"
+)
+
+type Garden struct {
+	children []string
+	rows     []string
+}
+
+func NewGarden(diagram string, children []string) (*Garden, error) {
+	var err error
+	g := new(Garden)
+	g.rows = strings.Split(diagram[1:], "\n")
+	g.children = children
+	if diagram[0] != '\n' {
+		err = errors.New("Wrong diagram format")
+	} else if len(g.rows) == 0 {
+		err = errors.New("No rows")
+	} else if len(g.children) == 0 {
+		err = errors.New("No children")
+	} else if len(g.rows[0]) != len(g.rows[1]) {
+		err = errors.New("Mismatched rows")
+	} else {
+		for i := range g.rows {
+			if len(g.rows[i])%2 != 0 {
+				err = errors.New("Odd number of cups")
+				break
+			}
+			for j := range g.rows[i] {
+				tst := g.rows[i][j]
+				switch tst {
+				case 'R':
+				case 'C':
+				case 'G':
+				case 'V':
+				default:
+					err = errors.New("Invalid Cup Codes")
+				}
+			}
+			if err != nil {
+				break
+			}
+		}
+		for i := range g.children {
+			if i > 0 {
+				if g.children[i] == g.children[i-1] {
+					err = errors.New("Duplicate Children")
+					break
+				}
+			}
+		}
+	}
+	return g, err
+}
+
+func (g *Garden) Plants(child string) ([]string, bool) {
+	var i int
+	var ret []string
+	var found bool
+	c := g.children
+	sort.Strings(c)
+	for i = 0; i < len(c); i++ {
+		if c[i] == child {
+			found = true
+			break
+		}
+	}
+	if !found || len(g.rows) == 0 || len(g.rows[0]) <= 2*i {
+		return ret, found
+	}
+	for j := range []int{0, 1} {
+		for k := range []int{0, 1} {
+			switch g.rows[j][(i*2 + k)] {
+			case 'G':
+				ret = append(ret, "grass")
+			case 'C':
+				ret = append(ret, "clover")
+			case 'R':
+				ret = append(ret, "radishes")
+			case 'V':
+				ret = append(ret, "violets")
+			}
+		}
+	}
+	return ret, found
+}
+
+//	kindergarten_garden_test.go:140: Garden 6 lookup Patricia =
+// [radishes radishes grass clover], want [violets clover radishes violets].
diff --git a/go/kindergarten-garden/kindergarten_garden_test.go b/go/kindergarten-garden/kindergarten_garden_test.go
new file mode 100644
index 0000000..df7fa5f
--- /dev/null
+++ b/go/kindergarten-garden/kindergarten_garden_test.go
@@ -0,0 +1,211 @@
+// Kindergarten garden
+//
+// You must define a type Garden with constructor
+//
+//    func NewGarden(diagram string, children []string) (*Garden, error)
+//
+// and method
+//
+//    func (g *Garden) Plants(child string) ([]string, bool)
+//
+// The diagram argument starts each row with a '\n'.  This allows Go's
+// raw string literals to present diagrams in source code nicely as two
+// rows flush left, for example,
+//
+//     diagram := `
+//     VVCCGG
+//     VVCCGG`
+
+package kindergarten
+
+import (
+	"reflect"
+	"sort"
+	"testing"
+)
+
+type lookup struct {
+	child  string
+	plants []string
+	ok     bool
+}
+
+type gardenTest struct {
+	number   int
+	diagram  string
+	children []string
+	ok       bool
+	lookups  []lookup
+}
+
+var tests = []gardenTest{
+	{1, `
+RC
+GG`, []string{"Alice"}, true, []lookup{
+		{"Alice", []string{"radishes", "clover", "grass", "grass"}, true},
+	}},
+	{2, `
+VC
+RC`, []string{"Alice"}, true, []lookup{
+		{"Alice", []string{"violets", "clover", "radishes", "clover"}, true},
+	}},
+	{3, `
+VVCG
+VVRC`, []string{"Alice", "Bob"}, true, []lookup{
+		{"Bob", []string{"clover", "grass", "radishes", "clover"}, true},
+	}},
+	{4, `
+VVCCGG
+VVCCGG`, []string{"Alice", "Bob", "Charlie"}, true, []lookup{
+		{"Bob", []string{"clover", "clover", "clover", "clover"}, true},
+		{"Charlie", []string{"grass", "grass", "grass", "grass"}, true},
+	}},
+	test5, // full garden test
+	test6, // out of order names test
+
+	// failure tests
+	{7, "RC\nGG", []string{"Alice"}, false, nil}, // wrong diagram format
+	{8, `
+RCCC
+GG`, []string{""}, false, nil}, // mismatched rows
+	{9, `
+RCC
+GGC`, []string{"Alice"}, false, nil}, // odd number of cups
+	{10, `
+RCCC
+GGCC`, []string{"Alice", "Alice"}, false, nil}, // duplicate name
+	{11, `
+rc
+gg`, []string{"Alice"}, false, nil}, // invaid cup codes
+	{12, `
+RC
+GG`, []string{"Alice"}, true, []lookup{ // lookup invalid name
+		{"Bob", []string{"radishes", "clover", "grass", "grass"}, false},
+	}},
+}
+
+// full garden test
+var test5 = gardenTest{5, `
+VRCGVVRVCGGCCGVRGCVCGCGV
+VRCCCGCRRGVCGCRVVCVGCGCV`, []string{
+	"Alice", "Bob", "Charlie", "David", "Eve", "Fred",
+	"Ginny", "Harriet", "Ileana", "Joseph", "Kincaid", "Larry"}, true, []lookup{
+	{"Alice", []string{"violets", "radishes", "violets", "radishes"}, true},
+	{"Bob", []string{"clover", "grass", "clover", "clover"}, true},
+	{"Charlie", []string{"violets", "violets", "clover", "grass"}, true},
+	{"David", []string{"radishes", "violets", "clover", "radishes"}, true},
+	{"Eve", []string{"clover", "grass", "radishes", "grass"}, true},
+	{"Fred", []string{"grass", "clover", "violets", "clover"}, true},
+	{"Ginny", []string{"clover", "grass", "grass", "clover"}, true},
+	{"Harriet", []string{"violets", "radishes", "radishes", "violets"}, true},
+	{"Ileana", []string{"grass", "clover", "violets", "clover"}, true},
+	{"Joseph", []string{"violets", "clover", "violets", "grass"}, true},
+	{"Kincaid", []string{"grass", "clover", "clover", "grass"}, true},
+	{"Larry", []string{"grass", "violets", "clover", "violets"}, true},
+}}
+
+// out of order names test
+var (
+	test6names = []string{"Samantha", "Patricia", "Xander", "Roger"}
+	test6      = gardenTest{6, `
+VCRRGVRG
+RVGCCGCV`, append([]string{}, test6names...), true, []lookup{
+		{"Patricia", []string{"violets", "clover", "radishes", "violets"}, true},
+		{"Roger", []string{"radishes", "radishes", "grass", "clover"}, true},
+		{"Samantha", []string{"grass", "violets", "clover", "grass"}, true},
+		{"Xander", []string{"radishes", "grass", "clover", "violets"}, true},
+	}}
+)
+
+func TestGarden(t *testing.T) {
+	for _, test := range tests {
+		g, err := NewGarden(test.diagram, test.children)
+		switch {
+		case err != nil:
+			if test.ok {
+				t.Fatalf("NewGarden test %d returned error %q.  Error not expected.",
+					test.number, err)
+			}
+		case !test.ok:
+			t.Fatalf("NewGarden test %d error = %v.  Expected error.",
+				test.number, err)
+		}
+		for _, l := range test.lookups {
+			switch plants, ok := g.Plants(l.child); {
+			case ok != l.ok:
+				t.Fatalf("Garden %d lookup %s returned ok = %t, want %t.",
+					test.number, l.child, ok, l.ok)
+			case ok && !reflect.DeepEqual(plants, l.plants):
+				t.Fatalf("Garden %d lookup %s = %v, want %v.",
+					test.number, l.child, plants, l.plants)
+			}
+		}
+	}
+}
+
+// The lazy way to meet the alphabetizing requirement is with sort.Strings
+// on the argument slice.  That's an in-place sort though and it's bad practice
+// to have a side effect.
+func TestNamesNotModified(t *testing.T) {
+	cp := append([]string{}, test6names...)
+	_, err := NewGarden(test6.diagram, cp)
+	if err != nil {
+		t.Skip("TestNamesNotModified requires valid garden")
+	}
+	if !reflect.DeepEqual(cp, test6names) {
+		t.Fatalf("NewGarden modified children argment.  " +
+			"Arguments should not be modified.")
+	}
+	sort.Strings(cp)
+	if reflect.DeepEqual(cp, test6names) {
+		t.Skip("TestNamesNotModified requires names out of order")
+	}
+}
+
+// A test taken from the Ruby tests.  It checks that Garden objects
+// are self-contained and do not rely on package variables.
+func TestTwoGardens(t *testing.T) {
+	diagram := `
+VCRRGVRG
+RVGCCGCV`
+	g1, err1 := NewGarden(diagram, []string{"Alice", "Bob", "Charlie", "Dan"})
+	g2, err2 := NewGarden(diagram, []string{"Bob", "Charlie", "Dan", "Erin"})
+	if err1 != nil || err2 != nil {
+		t.Skip("Two garden test needs valid gardens")
+	}
+	tf := func(g *Garden, n int, child string, expPlants []string) {
+		switch plants, ok := g.Plants(child); {
+		case !ok:
+			t.Skip("Garden %d lookup %s returned ok = false, want true.",
+				n, child)
+		case !reflect.DeepEqual(plants, expPlants):
+			t.Fatalf("Garden %d lookup %s = %v, want %v.",
+				n, child, plants, expPlants)
+		}
+	}
+	tf(g1, 1, "Bob", []string{"radishes", "radishes", "grass", "clover"})
+	tf(g2, 2, "Bob", []string{"violets", "clover", "radishes", "violets"})
+	tf(g1, 1, "Charlie", []string{"grass", "violets", "clover", "grass"})
+	tf(g2, 2, "Charlie", []string{"radishes", "radishes", "grass", "clover"})
+}
+
+func BenchmarkNewGarden(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, test := range tests {
+			NewGarden(test.diagram, test.children)
+		}
+	}
+}
+
+func BenchmarkGarden_Plants(b *testing.B) {
+	g, err := NewGarden(test5.diagram, test5.children)
+	if err != nil {
+		b.Skip("BenchmarkGarden_Plants requires valid garden")
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for _, l := range test5.lookups {
+			g.Plants(l.child)
+		}
+	}
+}
diff --git a/go/paasio/README.md b/go/paasio/README.md
new file mode 100644
index 0000000..eab122e
--- /dev/null
+++ b/go/paasio/README.md
@@ -0,0 +1,32 @@
+# Paasio
+
+Write a program that reports network IO statistics
+
+You are writing a [PaaS][], and you need a way to bill customers based
+on network and filesystem usage.
+
+Create a wrapper for network connections and files that can report IO
+statistics. The wrapper must report:
+
+- The total number of bytes read/written.
+- The total number of read/write operations.
+
+[PaaS]: http://en.wikipedia.org/wiki/Platform_as_a_service
+
+To run the tests simply run the command `go test` in the exercise directory.
+
+If the test suite contains benchmarks, you can run these with the `-bench`
+flag:
+
+    go test -bench .
+
+For more detailed info about the Go track see the [help
+page](http://exercism.io/languages/go).
+
+## Source
+
+Brian Matsuo [https://github.com/bmatsuo](https://github.com/bmatsuo)
+
+## Submitting Incomplete Problems
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.
+
diff --git a/go/paasio/interface.go b/go/paasio/interface.go
new file mode 100644
index 0000000..1cccfe2
--- /dev/null
+++ b/go/paasio/interface.go
@@ -0,0 +1,40 @@
+package paasio
+
+import "io"
+
+// ReadCounter is an interface describing objects that can be read from,
+// and that can count the number of times they have been read from.
+//
+// If multiple goroutines concurrently call Read, implementations are not
+// required to provide any guarantees about interleaving of the Read calls.
+// However, implementations MUST guarantee that calls to ReadCount always return
+// correct results even in the presence of concurrent Read calls.
+type ReadCounter interface {
+	io.Reader
+	// ReadCount returns the total number of bytes successfully read along
+	// with the total number of calls to Read().
+	ReadCount() (n int64, nops int)
+}
+
+// WriteCounter is an interface describing objects that can be written to,
+// and that can count the number of times they have been written to.
+//
+// If multiple goroutines concurrently call Write, implementations are not
+// required to provide any guarantees about interleaving of the Write calls.
+// However, implementations MUST guarantee that calls to WriteCount always return
+// correct results even in the presence of concurrent Write calls.
+type WriteCounter interface {
+	io.Writer
+	// WriteCount returns the total number of bytes successfully written along
+	// with the total number of calls to Write().
+	WriteCount() (n int64, nops int)
+}
+
+// ReadWriteCounter is the union of ReadCounter and WriteCounter.
+//
+// All guarantees that apply to either of ReadCounter or WriteCounter
+// also apply to ReadWriteCounter.
+type ReadWriteCounter interface {
+	ReadCounter
+	WriteCounter
+}
diff --git a/go/paasio/paasio_test.go b/go/paasio/paasio_test.go
new file mode 100644
index 0000000..a9fde3d
--- /dev/null
+++ b/go/paasio/paasio_test.go
@@ -0,0 +1,223 @@
+package paasio
+
+import (
+	"bytes"
+	"crypto/rand"
+	"io"
+	"runtime"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+)
+
+// testVersion identifies the API tested by the test program.
+const targetTestVersion = 3
+
+func TestMultiThreaded(t *testing.T) {
+	if testVersion != targetTestVersion {
+		t.Errorf("Found testVersion = %v, want %v.", testVersion, targetTestVersion)
+	}
+	mincpu := 2
+	minproc := 2
+	ncpu := runtime.NumCPU()
+	if ncpu < mincpu {
+		t.Fatalf("at least %d cpu cores are required", mincpu)
+	}
+	nproc := runtime.GOMAXPROCS(0)
+	if nproc < minproc {
+		t.Errorf("at least %d threads are required; rerun the tests", minproc)
+		t.Errorf("")
+		t.Errorf("\tgo test -cpu %d ...", minproc)
+	}
+}
+
+// this test could be improved to test that error conditions are preserved.
+func testWrite(t *testing.T, writer func(io.Writer) WriteCounter) {
+	for i, test := range []struct {
+		writes []string
+	}{
+		{nil},
+		{[]string{""}},
+		{[]string{"I", " ", "never met ", "", "a gohper"}},
+	} {
+		var buf bytes.Buffer
+		buft := writer(&buf)
+		for _, s := range test.writes {
+			n, err := buft.Write([]byte(s))
+			if err != nil {
+				t.Errorf("test %d: Write(%q) unexpected error: %v", i, s, err)
+				continue
+			}
+			if n != len(s) {
+				t.Errorf("test %d: Write(%q) unexpected number of bytes written: %v", i, s, n)
+				continue
+			}
+		}
+		out := buf.String()
+		if out != strings.Join(test.writes, "") {
+			t.Errorf("test %d: unexpected content in underlying writer: %q", i, out)
+		}
+	}
+}
+
+func TestWriteWriter(t *testing.T) {
+	testWrite(t, NewWriteCounter)
+}
+
+func TestWriteReadWriter(t *testing.T) {
+	testWrite(t, func(w io.Writer) WriteCounter {
+		var r nopReader
+		return NewReadWriteCounter(readWriter{r, w})
+	})
+}
+
+// this test could be improved to test exact number of operations as well as
+// ensure that error conditions are preserved.
+func testRead(t *testing.T, reader func(io.Reader) ReadCounter) {
+	chunkLen := 10 << 20 // 10MB
+	orig := make([]byte, 10<<20)
+	_, err := rand.Read(orig)
+	if err != nil {
+		t.Fatalf("error reading random data")
+	}
+	buf := bytes.NewBuffer(orig)
+	rc := reader(buf)
+	var obuf bytes.Buffer
+	ncopy, err := io.Copy(&obuf, rc)
+	if err != nil {
+		t.Fatalf("error reading: %v", err)
+	}
+	if ncopy != int64(chunkLen) {
+		t.Fatalf("copied %d bytes instead of %d", ncopy, chunkLen)
+	}
+	if string(orig) != obuf.String() {
+		t.Fatalf("unexpected output from Read()")
+	}
+	n, nops := rc.ReadCount()
+	if n != int64(chunkLen) {
+		t.Fatalf("reported %d bytes read instead of %d", n, chunkLen)
+	}
+	if nops < 2 {
+		t.Fatalf("unexpected number of reads: %v", nops)
+	}
+}
+
+func TestReadReader(t *testing.T) {
+	testRead(t, NewReadCounter)
+}
+
+func TestReadReadWriter(t *testing.T) {
+	testRead(t, func(r io.Reader) ReadCounter {
+		var w nopWriter
+		return NewReadWriteCounter(readWriter{r, w})
+	})
+}
+
+func testReadTotal(t *testing.T, rc ReadCounter) {
+	numGo := 8000
+	numBytes := 50
+	totalBytes := int64(numGo) * int64(numBytes)
+	p := make([]byte, numBytes)
+
+	t.Logf("Calling Read() for %d*%d=%d bytes", numGo, numBytes, totalBytes)
+	wg := new(sync.WaitGroup)
+	wg.Add(numGo)
+	start := make(chan struct{})
+	for i := 0; i < numGo; i++ {
+		go func() {
+			<-start
+			rc.Read(p)
+			wg.Done()
+		}()
+	}
+	close(start)
+
+	wg.Wait()
+	n, nops := rc.ReadCount()
+	if n != totalBytes {
+		t.Errorf("expected %d bytes read; %d bytes reported", totalBytes, n)
+	}
+	if nops != numGo {
+		t.Errorf("expected %d read operations; %d operations reported", numGo, nops)
+	}
+}
+
+func TestReadTotalReader(t *testing.T) {
+	var r nopReader
+	testReadTotal(t, NewReadCounter(r))
+}
+
+func TestReadTotalReadWriter(t *testing.T) {
+	var rw nopReadWriter
+	testReadTotal(t, NewReadWriteCounter(rw))
+}
+
+func testWriteTotal(t *testing.T, wt WriteCounter) {
+	numGo := 8000
+	numBytes := 50
+	totalBytes := int64(numGo) * int64(numBytes)
+	p := make([]byte, numBytes)
+
+	t.Logf("Calling Write() with %d*%d=%d bytes", numGo, numBytes, totalBytes)
+	wg := new(sync.WaitGroup)
+	wg.Add(numGo)
+	start := make(chan struct{})
+	for i := 0; i < numGo; i++ {
+		go func() {
+			<-start
+			wt.Write(p)
+			wg.Done()
+		}()
+	}
+	close(start)
+
+	wg.Wait()
+	n, nops := wt.WriteCount()
+	if n != totalBytes {
+		t.Errorf("expected %d bytes written; %d bytes reported", totalBytes, n)
+	}
+	if nops != numGo {
+		t.Errorf("expected %d write operations; %d operations reported", numGo, nops)
+	}
+}
+
+func TestWriteTotalWriter(t *testing.T) {
+	var w nopWriter
+	testWriteTotal(t, NewWriteCounter(w))
+}
+
+func TestWriteTotalReadWriter(t *testing.T) {
+	var rw nopReadWriter
+	testWriteTotal(t, NewReadWriteCounter(rw))
+}
+
+type nopWriter struct{ error }
+
+func (w nopWriter) Write(p []byte) (int, error) {
+	time.Sleep(1)
+	if w.error != nil {
+		return 0, w.error
+	}
+	return len(p), nil
+}
+
+type nopReader struct{ error }
+
+func (r nopReader) Read(p []byte) (int, error) {
+	time.Sleep(1)
+	if r.error != nil {
+		return 0, r.error
+	}
+	return len(p), nil
+}
+
+type nopReadWriter struct {
+	nopReader
+	nopWriter
+}
+
+type readWriter struct {
+	io.Reader
+	io.Writer
+}
diff --git a/go/simple-cipher/README.md b/go/simple-cipher/README.md
new file mode 100644
index 0000000..5b15834
--- /dev/null
+++ b/go/simple-cipher/README.md
@@ -0,0 +1,100 @@
+# Simple Cipher
+
+Implement a simple shift cipher like Caesar and a more secure substitution cipher
+
+## Step 1
+
+"If he had anything confidential to say, he wrote it in cipher, that is,
+by so changing the order of the letters of the alphabet, that not a word
+could be made out. If anyone wishes to decipher these, and get at their
+meaning, he must substitute the fourth letter of the alphabet, namely D,
+for A, and so with the others."
+—Suetonius, Life of Julius Caesar
+
+Ciphers are very straight-forward algorithms that allow us to render
+text less readable while still allowing easy deciphering. They are
+vulnerable to many forms of cryptoanalysis, but we are lucky that
+generally our little sisters are not cryptoanalysts.
+
+The Caesar Cipher was used for some messages from Julius Caesar that
+were sent afield. Now Caesar knew that the cipher wasn't very good, but
+he had one ally in that respect: almost nobody could read well. So even
+being a couple letters off was sufficient so that people couldn't
+recognize the few words that they did know.
+
+Your task is to create a simple shift cipher like the Caesar Cipher.
+This image is a great example of the Caesar Cipher: ![Caesar Cipher][1]
+
+For example:
+
+Giving "iamapandabear" as input to the encode function returns the cipher "ldpdsdqgdehdu". Obscure enough to keep our message secret in transit.
+
+When "ldpdsdqgdehdu" is put into the decode function it would return
+the original "iamapandabear" letting your friend read your original
+message.
+
+## Step 2
+
+Shift ciphers are no fun though when your kid sister figures it out. Try
+amending the code to allow us to specify a key and use that for the
+shift distance. This is called a substitution cipher.
+
+Here's an example:
+
+Given the key "aaaaaaaaaaaaaaaaaa", encoding the string "iamapandabear"
+would return the original "iamapandabear".
+
+Given the key "ddddddddddddddddd", encoding our string "iamapandabear"
+would return the obscured "lpdsdqgdehdu"
+
+In the example above, we've set a = 0 for the key value. So when the
+plaintext is added to the key, we end up with the same message coming
+out. So "aaaa" is not an ideal key. But if we set the key to "dddd", we
+would get the same thing as the Caesar Cipher.
+
+## Step 3
+
+The weakest link in any cipher is the human being. Let's make your
+substitution cipher a little more fault tolerant by providing a source
+of randomness and ensuring that they key is not composed of numbers or
+capital letters.
+
+If someone doesn't submit a key at all, generate a truly random key of
+at least 100 characters in length, accessible via Cipher#key (the #
+syntax means instance variable)
+
+If the key submitted has capital letters or numbers, throw an
+ArgumentError with a message to that effect.
+
+## Extensions
+
+Shift ciphers work by making the text slightly odd, but are vulnerable
+to frequency analysis. Substitution ciphers help that, but are still
+very vulnerable when the key is short or if spaces are preserved. Later
+on you'll see one solution to this problem in the exercise
+"crypto-square".
+
+If you want to go farther in this field, the questions begin to be about
+how we can exchange keys in a secure way. Take a look at [Diffie-Hellman
+on Wikipedia][dh] for one of the first implementations of this scheme.
+
+[1]: http://upload.wikimedia.org/wikipedia/en/7/75/Caesar3.png
+[dh]: http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
+
+To run the tests simply run the command `go test` in the exercise directory.
+
+If the test suite contains benchmarks, you can run these with the `-bench`
+flag:
+
+    go test -bench .
+
+For more detailed info about the Go track see the [help
+page](http://exercism.io/languages/go).
+
+## Source
+
+Substitution Cipher at Wikipedia [http://en.wikipedia.org/wiki/Substitution_cipher](http://en.wikipedia.org/wiki/Substitution_cipher)
+
+## Submitting Incomplete Problems
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.
+
diff --git a/go/simple-cipher/cipher.go b/go/simple-cipher/cipher.go
new file mode 100644
index 0000000..919d56e
--- /dev/null
+++ b/go/simple-cipher/cipher.go
@@ -0,0 +1,143 @@
+package cipher
+
+import (
+	"regexp"
+	"strings"
+)
+
+// Cipher is an interface for all ciphers
+type Cipher interface {
+	Encode(string) string
+	Decode(string) string
+}
+
+// ShiftCipher is a cipher that shifts all letters a fixed distance
+type ShiftCipher struct {
+	width int
+}
+
+// NewShift returns a new ShiftCipher
+func NewShift(w int) *ShiftCipher {
+	if w == 0 || w < -25 || w > 25 {
+		return nil
+	}
+	c := new(ShiftCipher)
+	c.width = w
+	return c
+}
+
+// Encode encodes a string using the current ShiftCipher
+func (c *ShiftCipher) Encode(s string) string {
+	var ret string
+	s = strings.ToLower(s)
+	re := regexp.MustCompile("[^a-z]")
+	s = re.ReplaceAllString(s, "")
+	for i := range s {
+		newChar := s[i] + byte(c.width)
+		if newChar < 'a' {
+			newChar = 'z' - ('a' - newChar) + 1
+		}
+		if newChar > 'z' {
+			newChar = 'a' + (newChar - 'z') - 1
+		}
+		ret = ret + string(newChar)
+	}
+	return ret
+}
+
+// Decode decodes a string using the current ShiftCipher
+func (c *ShiftCipher) Decode(s string) string {
+	var ret string
+	s = strings.ToLower(s)
+	re := regexp.MustCompile("[^a-z]")
+	s = re.ReplaceAllString(s, "")
+	for i := range s {
+		newChar := s[i] - byte(c.width)
+		if newChar < 'a' {
+			newChar = 'z' - ('a' - newChar) + 1
+		}
+		if newChar > 'z' {
+			newChar = 'a' + (newChar - 'z') - 1
+		}
+		ret = ret + string(newChar)
+	}
+	return ret
+}
+
+// A CeasarCipher is a ShiftCipher, just hardcoded at 3
+type CeasarCipher struct {
+	c *ShiftCipher
+}
+
+// NewCaesar creates a new CaesarCipher
+func NewCaesar() *CeasarCipher {
+	c := new(CeasarCipher)
+	c.c = NewShift(3)
+	return c
+}
+
+// Encode just calls the shift cipher in the caesar cipher
+func (c *CeasarCipher) Encode(s string) string {
+	return c.c.Encode(s)
+}
+
+// Decode just calls the shift cipher in the caesar cipher
+func (c *CeasarCipher) Decode(s string) string {
+	return c.c.Decode(s)
+}
+
+// VigenereCipher a cipher where each character can have a different shift width
+type VigenereCipher struct {
+	key string
+}
+
+// Encode encodes the given string
+func (c *VigenereCipher) Encode(s string) string {
+	var ret string
+	s = strings.ToLower(s)
+	re := regexp.MustCompile("[^a-z]")
+	s = re.ReplaceAllString(s, "")
+	for i := range s {
+		keyChar := c.key[i%len(c.key)] - 'a'
+		newChar := s[i] + keyChar
+		if newChar < 'a' {
+			newChar = 'z' - ('a' - newChar) + 1
+		}
+		if newChar > 'z' {
+			newChar = 'a' + (newChar - 'z') - 1
+		}
+		ret = ret + string(newChar)
+	}
+	return ret
+}
+
+// Decode decodes the given string
+func (c *VigenereCipher) Decode(s string) string {
+	var ret string
+	s = strings.ToLower(s)
+	re := regexp.MustCompile("[^a-z]")
+	s = re.ReplaceAllString(s, "")
+	for i := range s {
+		keyChar := c.key[i%len(c.key)] - 'a'
+		newChar := s[i] - keyChar
+		if newChar < 'a' {
+			newChar = 'z' - ('a' - newChar) + 1
+		}
+		if newChar > 'z' {
+			newChar = 'a' + (newChar - 'z') - 1
+		}
+		ret = ret + string(newChar)
+	}
+	return ret
+}
+
+// NewVigenere returns a new Vigenere Cipher
+func NewVigenere(key string) *VigenereCipher {
+	re := regexp.MustCompile("[^a-z]")
+	if len(key) < 3 || re.MatchString(key) {
+		return nil
+	}
+	c := new(VigenereCipher)
+	c.key = key
+	return c
+}
diff --git a/go/simple-cipher/simple_cipher_test.go b/go/simple-cipher/simple_cipher_test.go
new file mode 100644
index 0000000..05621d3
--- /dev/null
+++ b/go/simple-cipher/simple_cipher_test.go
@@ -0,0 +1,270 @@
+// For Step 1, implement the Caesar cipher, which seems clear enough except
+// maybe for figuring out whether to add or subtract.
+//
+// For Step 2, implement a shift cipher, like Caesar except the shift amount
+// is specified as an int.  (Each letter of "ddddddddddddddddd", is 'd';
+// 'd'-'a' == 3, so the corresponding shift amount is 3.)
+//
+// Steps 2 and 3 seem to be describing the Vigenère cipher (Google it)
+// so let's do that too.  The random thing, don't worry about.  There is
+// no test for that.
+//
+// API:
+//
+// type Cipher interface {
+//    Encode(string) string
+//    Decode(string) string
+// }
+// NewCaesar() Cipher
+// NewShift(int) Cipher
+// NewVigenere(string) Cipher
+//
+// Interface Cipher is in file cipher.go.
+//
+// Argument for NewShift must be in the range 1 to 25 or -1 to -25.
+// Zero is disallowed.  For invalid arguments NewShift returns nil.
+//
+// Argument for NewVigenere must consist of lower case letters a-z only.
+// Values consisting entirely of the letter 'a' are disallowed.
+// For invalid arguments NewVigenere returns nil.
+
+package cipher
+
+import (
+	"fmt"
+	"testing"
+)
+
+// type for testing cipher encoding alone, without requiring any text prep.
+type prepped struct {
+	pt string // prepped text == decoded plain text
+	ct string // cipher text
+}
+
+// +3? -3?  Positions vary.  Your code just has to pass the tests.
+var caesarPrepped = []prepped{
+	{"iamapandabear", "ldpdsdqgdehdu"},
+	{"programmingisawesome", "surjudpplqjlvdzhvrph"},
+	{"todayisholiday", "wrgdblvkrolgdb"},
+	{"venividivici", "yhqlylglylfl"},
+}
+
+func TestCaesarPrepped(t *testing.T) {
+	c := NewCaesar()
+	for _, test := range caesarPrepped {
+		if enc := c.Encode(test.pt); enc != test.ct {
+			t.Fatalf("Caesar Encode(%q) = %q, want %q.", test.pt, enc, test.ct)
+		}
+	}
+}
+
+// type for testing implementations of the Cipher interface
+type cipherTest struct {
+	source string // source text, any UTF-8
+	cipher string // cipher text, result of Encode(st)
+	plain  string // decoded plain text, result of Decode(ct)
+}
+
+var caesarTests = []cipherTest{
+	{"Go, go, gophers", "jrjrjrskhuv", "gogogophers"},
+	{"I am a panda bear.", "ldpdsdqgdehdu", "iamapandabear"},
+	{"Programming is AWESOME!", "surjudpplqjlvdzhvrph", "programmingisawesome"},
+	{"today is holiday", "wrgdblvkrolgdb", "todayisholiday"},
+	{"Twas the night before Christmas",
+		"wzdvwkhqljkwehiruhfkulvwpdv",
+		"twasthenightbeforechristmas"},
+	{"venividivici", "yhqlylglylfl", "venividivici"},
+	{" -- @#!", "", ""},
+	{"", "", ""},
+}
+
+func TestCaesar(t *testing.T) {
+	testCipher("Caesar", NewCaesar(), caesarTests, t)
+}
+
+func testCipher(name string, c Cipher, tests []cipherTest, t *testing.T) {
+	for _, test := range tests {
+		if enc := c.Encode(test.source); enc != test.cipher {
+			t.Fatalf("%s Encode(%q) = %q, want %q.",
+				name, test.plain, enc, test.cipher)
+		}
+		if dec := c.Decode(test.cipher); dec != test.plain {
+			t.Fatalf("%s Decode(%q) = %q, want %q.",
+				name, test.cipher, dec, test.plain)
+		}
+	}
+}
+
+var NSATests = []cipherTest{
+	{"THE ENEMY IS NEAR", "qebbkbjvfpkbxo", "theenemyisnear"},
+	{"SPIES SEND SECRET MESSAGES",
+		"pmfbppbkapbzobqjbppxdbp",
+		"spiessendsecretmessages"},
+	{"THOMAS JEFFERSON DESIGNED A SUBSTITUTION CIPHER",
+		"qeljxpgbccboplkabpfdkbaxprypqfqrqflkzfmebo",
+		"thomasjeffersondesignedasubstitutioncipher"},
+	{"the quick brown fox jumps over the lazy dog",
+		"qebnrfzhyoltkclugrjmplsboqebixwvald",
+		"thequickbrownfoxjumpsoverthelazydog"},
+}
+
+func TestShift(t *testing.T) {
+	// test shift(3) against Caesar cases.
+	c := NewShift(3)
+	if c == nil {
+		t.Fatal("NewShift(3) returned nil, want non-nil Cipher")
+	}
+	testCipher("Shift(3)", c, caesarTests, t)
+
+	// NSA and WP say Caesar uses shift of -3
+	c = NewShift(-3)
+	if c == nil {
+		t.Fatal("NewShift(-3) returned nil, want non-nil Cipher")
+	}
+	testCipher("Shift(-3)", c, NSATests, t)
+
+	// invalid shifts
+	for _, s := range []int{-27, -26, 0, 26, 27} {
+		if NewShift(s) != nil {
+			t.Fatal("NewShift(%d) returned non-nil, "+
+				"Want nil return for invalid argument.", s)
+		}
+	}
+}
+
+var vtests = []struct {
+	key   string
+	tests []cipherTest
+}{
+	{"lemon", []cipherTest{{"ATTACKATDAWN", "lxfopvefrnhr", "attackatdawn"}}},
+	{"abcdefghij", []cipherTest{
+		{"aaaaaaaaaa", "abcdefghij", "aaaaaaaaaa"},
+		{"zzzzzzzzzz", "zabcdefghi", "zzzzzzzzzz"},
+	}},
+	{"iamapandabear", []cipherTest{
+		{"I am a panda bear.", "qayaeaagaciai", "iamapandabear"},
+	}},
+	{"duxrceqyaimciuucnelkeoxjhdyduu", []cipherTest{
+		{"Diffie Hellman", "gccwkixcltycv", "diffiehellman"},
+	}},
+	{"qgbvno", []cipherTest{
+		{"cof-FEE, 123!", "sugars", "coffee"},
+	}},
+}
+
+func TestVigenere(t *testing.T) {
+	for _, test := range vtests {
+		v := NewVigenere(test.key)
+		if v == nil {
+			t.Fatalf("NewVigenere(%q) returned nil, want non-nil Cipher",
+				test.key)
+		}
+		testCipher(fmt.Sprintf("Vigenere(%q)", test.key), v, test.tests, t)
+	}
+
+	// invalid keys
+	for _, k := range []string{"", "a", "aa", "no way", "CAT", "3", "and,"} {
+		if NewVigenere(k) != nil {
+			t.Fatalf("NewVigenere(%q) returned non-nil, "+
+				"Want nil return for invalid argument.", k)
+		}
+	}
+}
+
+// Benchmark combined time to run all tests.
+// Note other ciphers test different data; times cannot be compared.
+func BenchmarkEncodeCaesar(b *testing.B) {
+	c := NewCaesar()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for _, test := range caesarTests {
+			c.Encode(test.source)
+		}
+	}
+}
+
+func BenchmarkDecodeCaesar(b *testing.B) {
+	c := NewCaesar()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for _, test := range caesarTests {
+			c.Decode(test.cipher)
+		}
+	}
+}
+
+func BenchmarkNewShift(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for s := -27; s <= 27; s++ {
+			NewShift(s)
+		}
+	}
+}
+
+func BenchmarkEncodeShift(b *testing.B) {
+	s := NewShift(5)
+	all := append(caesarTests, NSATests...)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for _, test := range all {
+			s.Encode(test.source)
+		}
+	}
+}
+
+func BenchmarkDecodeShift(b *testing.B) {
+	s := NewShift(5)
+	all := append(caesarTests, NSATests...)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for _, test := range all {
+			s.Decode(test.cipher)
+		}
+	}
+}
+
+func BenchmarkNewVigenere(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, test := range vtests {
+			NewVigenere(test.key)
+		}
+	}
+}
+
+func BenchmarkEncVigenere(b *testing.B) {
+	v := make([]Cipher, len(vtests))
+	for i, test := range vtests {
+		v[i] = NewVigenere(test.key)
+		if v[i] == nil {
+			b.Skip("Benchmark requires valid Vigenere test cases")
+		}
+	}
+	b.ResetTimer()
+	for j := 0; j < b.N; j++ {
+		for i, test := range vtests {
+			vi := v[i]
+			for _, test := range test.tests {
+				vi.Encode(test.source)
+			}
+		}
+	}
+}
+
+func BenchmarkDecVigenere(b *testing.B) {
+	v := make([]Cipher, len(vtests))
+	for i, test := range vtests {
+		v[i] = NewVigenere(test.key)
+		if v[i] == nil {
+			b.Skip("Benchmark requires valid Vigenere test cases")
+		}
+	}
+	b.ResetTimer()
+	for j := 0; j < b.N; j++ {
+		for i, test := range vtests {
+			vi := v[i]
+			for _, test := range test.tests {
+				vi.Decode(test.cipher)
+			}
+		}
+	}
+}