Validate key format before retrieving from database

Fixes #67

internal/db/paste.go

@@ -4,6 +4,7 @@ import (
 	"crypto/rand"
 	"encoding/base64"
 	"encoding/hex"
+	"net/http"
 	"net/url"
 	"strings"
 	"time"
@@ -53,6 +54,35 @@ const (
 	PasteStateDeleted
 )
 
+// minKeyLen specifies the mimimum length of a paste key.
+const minKeyLen = 4
+
+var (
+	// ErrKeyInvalidChar occurs when a key contains an invalid character.
+	ErrKeyInvalidChar = errors.New("invalid character in key")
+	// ErrKeyInvalidLength occurs when a key embeds a length that is incorrect.
+	ErrKeyInvalidLength = errors.New("key length encoding is incorrect")
+	// ErrPasteDoesNotExist occurs when a key does not exist in the database.
+	ErrPasteDoesNotExist = errors.New("url key not found in the database")
+)
+
+// ErrHTTPStatusCode returns the HTTP status code that should correspond to
+// the provided error.
+// server error, or false if it is not.
+func ErrHTTPStatusCode(err error) int {
+	switch err {
+	case nil:
+		return 0
+	case ErrKeyInvalidChar:
+		return http.StatusNotFound
+	case ErrKeyInvalidLength:
+		return http.StatusNotFound
+	case ErrPasteDoesNotExist:
+		return http.StatusNotFound
+	}
+	return http.StatusInternalServerError
+}
+
 // Base64 encoding and decoding
 var base64Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
 var base64Encoder = base64.RawURLEncoding.WithPadding(base64.NoPadding)
@@ -87,13 +117,45 @@ func (t PasteState) String() string {
 
 // GetPaste retrieves a paste from the database.
 func GetPaste(tx *bolt.Tx, key string) (*Paste, error) {
+	if err := ValidatePasteKey(key); err != nil {
+		return nil, err
+	}
+	return GetPasteNoValidate(tx, key)
+}
+
+// ValidatePasteKey validates the format of the key that has
+func ValidatePasteKey(key string) error {
+	internalLen := minKeyLen
+	countingOnes := true
+	for _, ch := range key {
+		limb := strings.IndexRune(base64Alphabet, ch)
+		if limb == -1 {
+			return ErrKeyInvalidChar
+		}
+		for i := 5; i >= 0 && countingOnes; i-- {
+			if (limb>>uint(i))&0x1 == 0 {
+				countingOnes = false
+				break
+			}
+			internalLen++
+		}
+	}
+	if internalLen != len(key) {
+		return ErrKeyInvalidLength
+	}
+	return nil
+}
+
+// GetPasteNoValidate retrieves a paste from the database without validating
+// the key format first.
+func GetPasteNoValidate(tx *bolt.Tx, key string) (*Paste, error) {
 	pastesBucket := tx.Bucket([]byte(BucketPastes))
 	if pastesBucket == nil {
 		return nil, errors.Errorf("bucket %v does not exist", BucketPastes)
 	}
 	storedBytes := pastesBucket.Get([]byte(key))
 	if storedBytes == nil {
-		return nil, nil
+		return nil, ErrPasteDoesNotExist
 	}
 	return decodePaste(storedBytes)
 }
@@ -214,7 +276,6 @@ func GeneratePasteKey(tx *bolt.Tx, minimumEntropy int) (string, error) {
 // is used to ensure that a new key has at least some amount of guessing
 // entropy.
 func generatePasteKeyInner(epoch, entropy int) (string, error) {
-	minKeyLen := 4
 	entropyEpoch := entropy
 	entropyEpoch -= minKeyLen * 6         // First 4 characters provide 24 bits.
 	entropyEpoch++                        // One bit less because of '0' bit.