Fix OSSH prefix validation

psiphon/common/parameters/parameters.go

@@ -1117,7 +1117,12 @@ func (p *Parameters) Set(
 					return nil, errors.Trace(err)
 				}
 			case transforms.Specs:
-				err := v.Validate()
+				bytemode := false
+				if name == OSSHPrefixSpecs || name == ServerOSSHPrefixSpecs {
+					// Specs for OSSH prefixes use generator in ByteMode.
+					bytemode = true
+				}
+				err := v.Validate(bytemode)
 				if err != nil {
 					if skipOnError {
 						continue

psiphon/common/transforms/transforms.go

@@ -54,16 +54,35 @@ type Specs map[string]Spec
 
 // Validate checks that all entries in a set of Specs is well-formed, with
 // valid regular expressions.
-func (specs Specs) Validate() error {
+func (specs Specs) Validate(bytemode bool) error {
 	seed, err := prng.NewSeed()
 	if err != nil {
 		return errors.Trace(err)
 	}
 	for _, spec := range specs {
-		// Call Apply to compile/validate the regular expressions and generators.
-		_, err := spec.ApplyString(seed, "")
-		if err != nil {
-			return errors.Trace(err)
+		if bytemode {
+
+			args := &regen.GeneratorArgs{
+				RngSource: prng.NewPRNGWithSeed(seed),
+				ByteMode:  true,
+			}
+
+			gen, err := regen.NewGenerator(spec[0][1], args)
+			if err != nil {
+				return errors.Trace(err)
+			}
+
+			_, err = gen.Generate()
+			if err != nil {
+				return errors.Trace(err)
+			}
+
+		} else {
+			// Call Apply to compile/validate the regular expressions and generators.
+			_, err := spec.ApplyString(seed, "")
+			if err != nil {
+				return errors.Trace(err)
+			}
 		}
 	}