diff --git a/internal/contentenc/offsets.go b/internal/contentenc/offsets.go
index 7487baf..3a0abf3 100644
--- a/internal/contentenc/offsets.go
+++ b/internal/contentenc/offsets.go
@@ -76,20 +76,20 @@ func (be *ContentEnc) CipherSizeToPlainSize(cipherSize uint64) uint64 {
 	return cipherSize - overhead
 }
 
-// PlainSizeToCipherSize calculates the ciphertext size from a plaintext size
+// PlainSizeToCipherSize calculates the ciphertext size from a plaintext size.
 func (be *ContentEnc) PlainSizeToCipherSize(plainSize uint64) uint64 {
 	// Zero-sized files stay zero-sized
 	if plainSize == 0 {
 		return 0
 	}
+	return be.PlainOffToCipherOff(plainSize-1) + 1
+}
 
-	// Block number at last byte
-	blockNo := be.PlainOffToBlockNo(plainSize - 1)
-	blockCount := blockNo + 1
-
-	overhead := be.BlockOverhead()*blockCount + HeaderLen
-
-	return plainSize + overhead
+// PlainOffToCipherOff tells you the highest ciphertext offset that is
+// *guaranteed* to be written/read when you write/read at `plainOff`.
+func (be *ContentEnc) PlainOffToCipherOff(plainOff uint64) uint64 {
+	startOfBlock := be.BlockNoToCipherOff(be.PlainOffToBlockNo(plainOff))
+	return startOfBlock + plainOff%be.PlainBS() + be.BlockOverhead()
 }
 
 // ExplodePlainRange splits a plaintext byte range into (possibly partial) blocks
diff --git a/internal/contentenc/offsets_test.go b/internal/contentenc/offsets_test.go
index 4bc3762..c3b8fcd 100644
--- a/internal/contentenc/offsets_test.go
+++ b/internal/contentenc/offsets_test.go
@@ -15,38 +15,39 @@ func TestSizeToSize(t *testing.T) {
 
 	const rangeMax = 10000
 
-	var c2p [rangeMax]uint64
-	var p2c [rangeMax]uint64
+	// y values in this order:
+	// 0 ... CipherSizeToPlainSize
+	// 1 ... PlainSizeToCipherSize
+	// 2 ... PlainOffToCipherOff
+	var yTable [rangeMax][3]uint64
 
 	// Calculate values
-	for i := range c2p {
-		c2p[i] = ce.CipherSizeToPlainSize(uint64(i))
-		p2c[i] = ce.PlainSizeToCipherSize(uint64(i))
+	for x := range yTable {
+		yTable[x][0] = ce.CipherSizeToPlainSize(uint64(x))
+		yTable[x][1] = ce.PlainSizeToCipherSize(uint64(x))
+		yTable[x][2] = ce.PlainOffToCipherOff(uint64(x))
 	}
 
 	// Print data table
-	fmt.Print("x\tToPlainSize\tToCipherSize\n")
-	for i := range c2p {
-		if i > 1 && i < rangeMax-1 {
+	fmt.Print("x\tCipherSizeToPlainSize\tPlainSizeToCipherSize\tPlainOffToCipherOff\n")
+	for x := range yTable {
+		if x > 1 && x < rangeMax-1 {
 			// If the point before has value-1 and the point after has value+1,
 			// it is not interesting. Don't print it out.
-			if c2p[i] == c2p[i-1]+1 && p2c[i] == p2c[i-1]+1 && c2p[i+1] == c2p[i]+1 && p2c[i+1] == p2c[i]+1 {
+			interesting := false
+			for i := 0; i <= 2; i++ {
+				if yTable[x-1][i]+1 != yTable[x][i] && yTable[x][i]+1 != yTable[x+1][i]+1 {
+					interesting = true
+				}
+				// Monotonicity check
+				if yTable[x][i] < yTable[x-1][i] {
+					t.Errorf("column %d is non-monotonic!", i)
+				}
+			}
+			if !interesting {
 				continue
 			}
 		}
-		fmt.Printf("%d\t%d\t%d\n", i, c2p[i], p2c[i])
-	}
-
-	// Monotonicity check
-	for i := range c2p {
-		if i < 1 {
-			continue
-		}
-		if c2p[i-1] > c2p[i] {
-			t.Errorf("error: c2p is non-monotonic: c2p[%d]=%d c2p[%d]=%d ", i-1, c2p[i-1], i, c2p[i])
-		}
-		if p2c[i-1] > p2c[i] {
-			t.Errorf("error: p2c is non-monotonic: p2c[%d]=%d p2c[%d]=%d ", i-1, p2c[i-1], i, p2c[i])
-		}
+		fmt.Printf("%d\t%d\t%d\t%d\n", x, yTable[x][0], yTable[x][1], yTable[x][2])
 	}
 }