code: private static byte[] decodeBase64Digest(String base64Digest) {
          String expectedPrefix = "sha1-";
          if (!base64Digest.startsWith(expectedPrefix)) {
              throw new IllegalArgumentException(base64Digest + " did not start with " +
                      expectedPrefix);
          }
          base64Digest = base64Digest.replaceFirst(expectedPrefix, "");
          byte[] bytes = new byte[0];
          try {
              bytes = Base64.decode(base64Digest);
          } catch (IOException e) {
              throw new IllegalArgumentException(e);
          }
          return bytes;
      }
docs: Decode base64'd getDigest into a byte array that is suitable for use
  as a blob key.
  
  @param base64Digest string with base64'd getDigest, with leading "sha1-" attached.
  eg, "sha1-LKJ32423JK..."
  @return a byte[] blob key