Validate byte-preserving record fields as base64 strings

internal/publish/record.go

@@ -3,6 +3,7 @@ package publish
 import (
 	"context"
 	"crypto/rand"
+	"encoding/base64"
 	"encoding/hex"
 	"encoding/json"
 	"errors"
@@ -52,8 +53,15 @@ func LoadEvidence(path string) (Evidence, error) {
 		}
 		for _, field := range evidenceFields(&evidence) {
 			if raw, ok := fields[field.name]; ok {
-				var bytes []byte
-				if err := json.Unmarshal(raw, &bytes); err != nil {
+				var encoded *string
+				if err := json.Unmarshal(raw, &encoded); err != nil {
+					return Evidence{}, &RecoveryError{Kind: "record_format", RecordPath: path, cause: fmt.Errorf("decode record field %s bytes %q: %w", field.name, path, err)}
+				}
+				if encoded == nil {
+					return Evidence{}, &RecoveryError{Kind: "record_format", RecordPath: path, cause: fmt.Errorf("record field %s bytes %q must be a base64 string", field.name, path)}
+				}
+				bytes, err := base64.StdEncoding.DecodeString(*encoded)
+				if err != nil {
 					return Evidence{}, &RecoveryError{Kind: "record_format", RecordPath: path, cause: fmt.Errorf("decode record field %s bytes %q: %w", field.name, path, err)}
 				}
 				*field.value = string(bytes)