disallow upper characters (/A-F/) in hex-encoded portion

algorithm.go

@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"hash"
 	"io"
+	"regexp"
 )
 
 // Algorithm identifies and implementation of a digester by an identifier.
@@ -28,9 +29,9 @@ type Algorithm string
 
 // supported digest types
 const (
-	SHA256 Algorithm = "sha256" // sha256 with hex encoding
-	SHA384 Algorithm = "sha384" // sha384 with hex encoding
-	SHA512 Algorithm = "sha512" // sha512 with hex encoding
+	SHA256 Algorithm = "sha256" // sha256 with hex encoding (lower case only)
+	SHA384 Algorithm = "sha384" // sha384 with hex encoding (lower case only)
+	SHA512 Algorithm = "sha512" // sha512 with hex encoding (lower case only)
 
 	// Canonical is the primary digest algorithm used with the distribution
 	// project. Other digests may be used but this one is the primary storage
@@ -50,6 +51,14 @@ var (
 		SHA384: crypto.SHA384,
 		SHA512: crypto.SHA512,
 	}
+
+	// anchoredEncodedRegexps contains anchored regular expressions for hex-encoded digests.
+	// Note that /A-F/ disallowed.
+	anchoredEncodedRegexps = map[Algorithm]*regexp.Regexp{
+		SHA256: regexp.MustCompile(`^[a-f0-9]{64}$`),
+		SHA384: regexp.MustCompile(`^[a-f0-9]{96}$`),
+		SHA512: regexp.MustCompile(`^[a-f0-9]{128}$`),
+	}
 )
 
 // Available returns true if the digest type is available for use. If this
@@ -164,3 +173,20 @@ func (a Algorithm) FromBytes(p []byte) Digest {
 func (a Algorithm) FromString(s string) Digest {
 	return a.FromBytes([]byte(s))
 }
+
+// Validate validates the encoded portion string
+func (a Algorithm) Validate(encoded string) error {
+	r, ok := anchoredEncodedRegexps[a]
+	if !ok {
+		return ErrDigestUnsupported
+	}
+	// Digests much always be hex-encoded, ensuring that their hex portion will
+	// always be size*2
+	if a.Size()*2 != len(encoded) {
+		return ErrDigestInvalidLength
+	}
+	if r.MatchString(encoded) {
+		return nil
+	}
+	return ErrDigestInvalidFormat
+}

digest.go

@@ -101,26 +101,18 @@ func FromString(s string) Digest {
 // error if not.
 func (d Digest) Validate() error {
 	s := string(d)
-
 	i := strings.Index(s, ":")
-
-	// validate i then run through regexp
-	if i < 0 || i+1 == len(s) || !DigestRegexpAnchored.MatchString(s) {
+	if i <= 0 || i+1 == len(s) {
 		return ErrDigestInvalidFormat
 	}
-
-	algorithm := Algorithm(s[:i])
+	algorithm, encoded := Algorithm(s[:i]), s[i+1:]
 	if !algorithm.Available() {
+		if !DigestRegexpAnchored.MatchString(s) {
+			return ErrDigestInvalidFormat
+		}
 		return ErrDigestUnsupported
 	}
-
-	// Digests much always be hex-encoded, ensuring that their hex portion will
-	// always be size*2
-	if algorithm.Size()*2 != len(s[i+1:]) {
-		return ErrDigestInvalidLength
-	}
-
-	return nil
+	return algorithm.Validate(encoded)
 }
 
 // Algorithm returns the algorithm portion of the digest. This will panic if

digest_test.go

@@ -93,6 +93,10 @@ func TestParseDigest(t *testing.T) {
 			encoded:   "LCa0a2j_xo_5m0U8HTBBNBNCLXBkg7-g-YpeiGJm564",
 			err:       ErrDigestUnsupported,
 		},
+		{
+			input: "sha256:E58FCF7418D4390DEC8E8FB69D88C06EC07039D651FEDD3AA72AF9972E7D046B",
+			err:   ErrDigestInvalidFormat,
+		},
 	} {
 		digest, err := Parse(testcase.input)
 		if err != testcase.err {