18 releases (10 breaking)

0.11.0 Sep 20, 2021
0.10.0 Jan 13, 2021
0.9.4 Jan 11, 2021
0.9.3 Jun 8, 2020
0.3.0 Nov 29, 2016

#331 in Network programming

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License: MIT Apache License 2.0 Crates.io Version Github CI Minimum rustc version

TLS Parser

A TLS parser, implemented with the nom parser combinator framework.

The goal of this parser is to implement TLS messages analysis, for example to use rules from a network IDS, for ex during the TLS handshake.

It implements structures and parsing functions for records and messages, but need additional code to handle fragmentation, or to fully inspect messages. Parsing some TLS messages requires to know the previously selected parameters. See the rusticata TLS parser for a full example.

It is written in pure Rust, fast, and makes extensive use of zero-copy. A lot of care is taken to ensure security and safety of this crate, including design (recursion limit, defensive programming), tests, and fuzzing. It also aims to be panic-free.

The code is available on Github and is part of the Rusticata project.

Parsing records

The main parsing functions are located in the tls.rs file. The entry functions are:

  • parse_tls_plaintext: parses a record as plaintext
  • parse_tls_encrypted: read an encrypted record. The parser has no crypto or decryption features, so the content will be left as opaque data.


use tls_parser::parse_tls_plaintext;
use tls_parser::nom::{Err, IResult};

let bytes : &[u8]= include_bytes!("../assets/client_hello_dhe.bin");
// [ 0x16, 0x03, 0x01 ... ];
let res = parse_tls_plaintext(&bytes);
match res {
    Ok((rem,record)) => {
        // rem is the remaining data (not parsed)
        // record is an object of type TlsRecord
    Err(Err::Incomplete(needed)) => {
        eprintln!("Defragmentation required (TLS record)");
    Err(e) => { eprintln!("parse_tls_record_with_header failed: {:?}",e); }

Note that knowing if a record is plaintext or not is the responsibility of the caller.

As reading TLS records may imply defragmenting records, some functions are provided to only read the record as opaque data (which ensures the record is complete and gives the record header) and then reading messages from data.

Here is an example of two-steps parsing:

// [ 0x16, 0x03, 0x01 ... ];
match parse_tls_raw_record(bytes) {
    Ok((rem, ref r)) => {
        match parse_tls_record_with_header(r.data, &r.hdr) {
            Ok((rem2,ref msg_list)) => {
                for msg in msg_list {
                    // msg has type TlsMessage
            Err(Err::Incomplete(needed)) => { eprintln!("incomplete record") }
            Err(_) => { eprintln!("error while parsing record") }
    Err(Err::Incomplete(needed)) => { eprintln!("incomplete record header") }
    Err(_) => { eprintln!("error while parsing record header") }

Some additional work is required if reading packets from the network, to support reassembly of TCP segments and reassembly of TLS records.

For a complete example of a TLS parser supporting defragmentation and states, see the rusticata/src/tls.rs file of the rusticata crate.

State machine

A TLS state machine is provided in tls_states.rs. The state machine is separated from the parsing functions, and is almost independent. It is implemented as a table of transitions, mainly for the handshake phase.

After reading a TLS message using the previous functions, the TLS state can be updated using the tls_state_transition function. If the transition succeeds, it returns Ok(new_state), otherwise it returns Err(error_state).

struct ParseContext {
    state: TlsState,

match tls_state_transition(ctx.state, msg, to_server) {
    Ok(s)  => { ctx.state = s; Ok(()) }
    Err(_) => {
        ctx.state = TlsState::Invalid;
        Err("Invalid state")

Implementation notes

When parsing messages, if a field is an integer corresponding to an enum of known values, it is not parsed as an enum type, but as an integer. While this complicates accesses, it allows to read invalid values and continue parsing (for an IDS, it's better to read values than to get a generic parse error).



  • Upgrade to nom 7
  • Add ClientHello trait for common CH attributes
  • Get extensions in DTLS ClientHello
  • Add example/helper program to list/query ciphersuite information
  • Update ciphersuites file
  • Added parsing for SignedCertificateTimestamp list
  • Re-export nom
  • Add support for no_std

Thanks: @JackLiar, @xonatius


  • Upgrade to nom 6
  • Remove all macro-base parsers (use functions, and nom-derive when possible)
  • Add support for DTLS (Handshake)
  • Add functions to parse extensions expected in Client/Server Hello


  • In ServerHello, an empty SNI extension can be sent (RFC 6066)


  • Fix error in state machine (wrong Client Certificate direction)


  • Upgrade to phf 0.8
  • Upgrade cookie-factory to 0.3.0


  • Mark cookie-factory as optional (only used for serialization)


  • Upgrade to nom 5
  • Rustfmt


  • Set edition to 2018
  • Check heartbeat message length (subtraction could underflow)
  • Add more checks for record length (RFC compliance, not for parser safety)


  • Add support for record size limit extension
  • Add support for encrypted server name (eSNI) extension
  • State machine: use direction and support TLS 1.3 0-RTT
  • State machine: add new state to indicate connection is closed (after fatal alert)
  • Use TlsVersion type for SSL record version
  • Update doc, and use cargo sync-readme


  • Improve state machine, handle resumption failure, and non-fatal alerts
  • Improve handling of Signature/Hash algorithms, and display
  • Update ciphersuites to 2019-03-19


  • Convert most enums to newtypes
    • warning: this is a breaking change
  • Update dependencies and remove unused crates
  • Update ciphersuites to 2019-01-23


  • Upgrade to nom 4.0
    • warning: this is a breaking change
  • Fix wrong extension ID for padding and signed timestamp
  • Rewrite parse_cipher_suites and parse_compressions_algs to be faster
  • Update ciphersuites to 2018-08-13


Here is a non-exhaustive list of RFCs this parser is based on:

  • RFC 2246: The TLS Protocol Version 1.0
  • RFC 4346: The Transport Layer Security (TLS) Protocol Version 1.1
  • RFC 4366: Transport Layer Security (TLS) Extensions
  • RFC 4492: Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)
  • RFC 4507: Transport Layer Security (TLS) Session Resumption without Server-Side State
  • RFC 5077: Transport Layer Security (TLS) Session Resumption without Server-Side State
  • RFC 5246: The Transport Layer Security (TLS) Protocol Version 1.2
  • RFC 5430: Suite B Profile for Transport Layer Security (TLS)
  • RFC 5746: Transport Layer Security (TLS) Renegotiation Indication Extension
  • RFC 6066: Transport Layer Security (TLS) Extensions: Extension Definitions
  • RFC 6520: Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) Heartbeat Extension
  • RFC 6961: The Transport Layer Security (TLS) Multiple Certificate Status Request Extension
  • RFC 7027: Elliptic Curve Cryptography (ECC) Brainpool Curves for Transport Layer Security (TLS)
  • RFC 7301: Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension
  • RFC 7366: Encrypt-then-MAC for Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)
  • RFC 7627: Transport Layer Security (TLS) Session Hash and Extended Master Secret Extension
  • RFC 7919: Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for Transport Layer Security (TLS)
  • RFC 8422: Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS) Versions 1.2 and Earlier
  • RFC 8446: The Transport Layer Security (TLS) Protocol Version 1.3
  • draft-agl-tls-nextprotoneg-03: Transport Layer Security (TLS) Next Protocol Negotiation Extension

FAQ and limitations

Can the parser decrypt a TLS session if I provide the master secret ?

No, it's not implemented

Does the parser support TLS compression ?

No. Note that most TLS implementations disabled it after the FREAK attack, so while detecting compression in ServerHello is possible in tls-parser, it should probably be interpreted as an alert.

Where are located the TLS CipherSuites ?

They are built when running cargo build.

To ease updating the list from the IANA TLS parameters, a script is provided (scripts/extract-iana-ciphers.py). This script will download and pre-parse the list from IANA, and produce a file containing all ciphersuites names and parameters.

During the build, build.rs parses this file and produces a static, read-only hash table of all known ciphers and their properties.


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


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