protected function encrypt(
        Stream $plaintext,
        array $cipherOptions,
        MaterialsProvider $provider,
        MetadataEnvelope $envelope
    ) {
        $materialsDescription = $provider->getMaterialsDescription();

        $cipherOptions = array_intersect_key(
            $cipherOptions,
            self::$allowedOptions
        );

        if (empty($cipherOptions['Cipher'])) {
            throw new \InvalidArgumentException('An encryption cipher must be'
                . ' specified in the "cipher_options".');
        }

        if (!self::isSupportedCipher($cipherOptions['Cipher'])) {
            throw new \InvalidArgumentException('The cipher requested is not'
                . ' supported by the SDK.');
        }

        if (empty($cipherOptions['KeySize'])) {
            $cipherOptions['KeySize'] = 256;
        }
        if (!is_int($cipherOptions['KeySize'])) {
            throw new \InvalidArgumentException('The cipher "KeySize" must be'
                . ' an integer.');
        }

        if (!MaterialsProvider::isSupportedKeySize(
            $cipherOptions['KeySize']
        )) {
            throw new \InvalidArgumentException('The cipher "KeySize" requested'
                . ' is not supported by AES (128, 192, or 256).');
        }

        $cipherOptions['Iv'] = $provider->generateIv(
            $this->getCipherOpenSslName(
                $cipherOptions['Cipher'],
                $cipherOptions['KeySize']
            )
        );

        $cek = $provider->generateCek($cipherOptions['KeySize']);

        list($encryptingStream, $aesName) = $this->getEncryptingStream(
            $plaintext,
            $cek,
            $cipherOptions
        );

        // Populate envelope data
        $envelope[MetadataEnvelope::CONTENT_KEY_V2_HEADER] =
            $provider->encryptCek(
                $cek,
                $materialsDescription
            );
        unset($cek);

        $envelope[MetadataEnvelope::IV_HEADER] =
            base64_encode($cipherOptions['Iv']);
        $envelope[MetadataEnvelope::KEY_WRAP_ALGORITHM_HEADER] =
            $provider->getWrapAlgorithmName();
        $envelope[MetadataEnvelope::CONTENT_CRYPTO_SCHEME_HEADER] = $aesName;
        $envelope[MetadataEnvelope::UNENCRYPTED_CONTENT_LENGTH_HEADER] =
            strlen($plaintext);
        $envelope[MetadataEnvelope::UNENCRYPTED_CONTENT_MD5_HEADER] =
            base64_encode(md5($plaintext));
        $envelope[MetadataEnvelope::MATERIALS_DESCRIPTION_HEADER] =
            json_encode($materialsDescription);
        if (!empty($cipherOptions['Tag'])) {
            $envelope[MetadataEnvelope::CRYPTO_TAG_LENGTH_HEADER] =
                strlen($cipherOptions['Tag']) * 8;
        }

        return $encryptingStream;
    }