@Override
  public byte[] encrypt(final byte[] plaintext, final byte[] contextInfo)
      throws GeneralSecurityException {
    byte[] ciphertext = noPrefixEncrypt(plaintext, contextInfo);
    if (outputPrefix.length == 0) {
      return ciphertext;
    }
    return Bytes.concat(outputPrefix, ciphertext);
  }
Encrypts {@code plaintext} using {@code contextInfo} as <b>info</b>-parameter of the underlying
HKDF.

@return resulting ciphertext.
java_src/src/main/java/com/google/crypto/tink/subtle/EciesAeadHkdfHybridEncrypt.java