@CanIgnoreReturnValue
    public Builder setPrivateExponent(SecretBigInteger d) {
      this.d = Optional.of(d);
      return this;
    }
Sets the private exponent d.

<p>See https://datatracker.ietf.org/doc/html/rfc7518#section-6.3.2.1.

<p>This is required.
java_src/src/main/java/com/google/crypto/tink/jwt/JwtRsaSsaPssPrivateKey.java