How Does BabaSSL Reduce the 80% Bandwidth of TLS Handshake?

By Chenglong Zhang

BabaSSL is an open-source cryptographic library product. As one of the core components of the Anolis commercial cryptography OS, it is open-source on GitHub. Also,it has been added to the SIG group of the commercial cryptographic software stack of OpenAnolis.

Introduction

With the construction of the 5G network, the development of mobile Internet applications has been accelerated, including short videos, online education, the Internet of Things, and other fields. However, there are still scenes of poor network signals, including underground shopping malls, garages, subways, and other places. In weak network environments caused by network congestion, applications are slowly loaded during use, resulting in poor user experiences. At this time, it is necessary to optimize the weak network environment. One of the means is to find ways to reduce network data transmission.

TLS/SSL is usually used for cryptotransmission to ensure data security. When a client accesses the server in the background, the client performs a TLS handshake with the server first. During the TLS handshake, the server sends a certificate chain for identity authentication, and most of the data transferred during the handshake comes from the certificate.

Is there any way to reduce the bandwidth consumption of TLS handshake? If the certificate can be compressed or even disappear, the data transmission can be reduced. RFC 8879 TLS Certificate Compression aims to solve this problem and provide certificate compression during TLS 1.3 handshake.

BabaSSL is an open-source cryptographic library product. It was open-source on GitHub and added to OpenAnolis. It is based on OpenSSL 1.1 and maintains compatibility. Based on OpenSSL, it implements a series of autonomous and controllable security features, including various Chinese cryptographic algorithms, Chinese cryptographic standards, and a large number of features to improve key security.

BabaSSL supports TLS certificate compression, but OpenSSL does not.

An Introduction to TLS Certificate Compression

If the client supports certificate compression, carry a compress_certificate extension in the ClientHello message, which contains a list of supported compression algorithms.
The server receives ClientHello and finds that the client supports certificate compression. If the server also supports certificate compression and supports the compression method declared by the client, the Certificate message will be compressed using this algorithm.
The server sends a CompressedCertificate message instead of the original Certificate message. CompressedCertificate contains the compression algorithm, the decompressed length, and the compressed Certificate message.
After receiving the CompressedCertificate message, the client uses the algorithm to decompress it. If the decompression is successful, subsequent processing will be carried out. Otherwise, the connection will be closed, and a bad_certificate alert will be sent.

The processing procedure for the server to send a CertificateRequest message and the client to send a CompressedCertificate message is similar to the preceding one.

The compression algorithm defined in the standard:

Algorithm	Value	Reference Standard
zlib	1	RFC1950
brotli	2	RFC7932
zstd	3	RFC8478

In addition to the three algorithms defined in RFC, users can use other algorithms, with values 16384 to 65535 for their use.

Actual TLS Certificate Compression

The open-source BabaSSL cryptographic library supports the TLS certificate compression. You need to enable this feature when building BabaSSL and add enable-cert-compression after configuration.

You can add a certificate compression algorithm when you set SSL_CTX. The following is the example code:

#include <openssl/ssl.h>
#include <zlib.h>

static int zlib_compress(SSL *s,
                         const unsigned char *in, size_t inlen,
                         unsigned char *out, size_t *outlen)
{

    if (out == NULL) {
        *outlen = compressBound(inlen);
        return 1;
    }

    if (compress2(out, outlen, in, inlen, Z_DEFAULT_COMPRESSION) != Z_OK)
        return 0;

    return 1;
}

static int zlib_decompress(SSL *s,
                           const unsigned char *in, size_t inlen,
                           unsigned char *out, size_t outlen)
{
    size_t len = outlen;

    if (uncompress(out, &len, in, inlen) != Z_OK)
        return 0;

    if (len != outlen)
        return 0;

    return 1;
}

int main() {
    const SSL_METHOD *meth = TLS_client_method();
         SSL_CTX *ctx = SSL_CTX_new(meth); 

    /* Configure the certificate and private key... */
    
    /* For example, set the compression algorithm to zlib */
    SSL_CTX_add_cert_compression_alg(ctx, TLSEXT_cert_compression_zlib,
                                                 zlib_compress, zlib_decompress);

         SSL *con = SSL_new(ctx);
    
    /* Handshake... */
    
    return 0;
}

You can also use the s_client and s_server provided by BabaSSL to use the TLS certificate compression feature:

# Server
/opt/babassl/bin/openssl s_server -accept 127.0.0.1:34567 -cert server.crt -key server.key -tls1_3 -cert_comp zlib -www -quiet
 
# Client
/opt/babassl/bin/openssl s_client -connect 127.0.0.1:34567 -tls1_3 -cert_comp zlib -ign_eof -trace

Test the Compression Algorithm and Compression Rate

The server configures the certificate chain, such as CA certificate + intermediate CA + domain name certificate. TLS 1.3 handshake enables certificate compression. Comparing the compression rates of each compression algorithm, the following chart shows the results:

Compression Algorithm	Before Compression (Byte)	After Compression (Byte)	Compression Rate	Remarks
zlib	2666	1959	73.48%	Use default compression level 6
brotli	2666	1889	70.86%	Use default compression level 11
zstd	2666	1951	73.18%	Use default compression level 3
zstd + dictionary	2666	18	0.68%	Dictionary based on Certificate messages

Some compression algorithms support setting dictionaries, such as brotli and zstd. The dictionary content can be calculated in advance, embedded in the client and server, and used when compressing and decompressing, so the certificate chain can disappear. For example, when the zstd + dictionary is used in the preceding table, the Certificate message of 2666 bytes before compression becomes 18 bytes after compression.

If the certificate compression feature is enabled, the transfer during handshake can be reduced, especially when using dictionaries, such as zstd + dictionaries.

Disable certificate compression, handshake co-transmission: 3331 bytes

Open certificate compression: handshake co-transmission: 698 bytes

Compression Rate: 698/3331 * 100% = 20.95%. The handshake bandwidth is reduced by nearly 80%.

Summary

The certificate is not needed when a TLS session is reused, so certificate compression can optimize the complete handshake.

Client authentication is enabled on the server in the scenario of mutual authentication. If the client and server enable the TLS certificate compression, the compression effect is clear, which can save more than 80% bandwidth in the TLS handshake. BabaSSL will support Compact TLS, the pocket version of TLS 1.3, which occupies the minimum bandwidth while maintaining the protocol isomorphism. You are welcome to use BabaSSL.

BabaSSL website: https://github.com/BabaSSL/BabaSSL

About The Author

Chenglong Zhang is a technical expert of Ant Group, one of the core developers of BabaSSL project, a contributer to multiple open source projects, including OpenSSL, Tengine, etc. And he is also one of the members of SIG of the ShangMi cryptographic software stack of OpenAnolis.

Community

How Does BabaSSL Reduce the 80% Bandwidth of TLS Handshake?

Introduction

An Introduction to TLS Certificate Compression

Actual TLS Certificate Compression

Test the Compression Algorithm and Compression Rate

Summary

About The Author

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