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NOTE: This software is still in the early stages of development. If you are confused, find some bugs, or just want some help, please file an issue or come talk to us at

Klab is a tool for generating and debugging proofs in the K Framework, tailored for the formal verification of ethereum smart contracts. It includes a succinct specification language for expressing the behavior of ethereum contracts, and an interactive debugger.



See dependency installation instructions here

This project uses the GNU version of getopt and time. OSX and gnu have a complicated relationship but you can run:

export PATH=/usr/local/opt/gnu-getopt/bin:/usr/local/opt/gnu-time/libexec/gnubin:/usr/local/opt/coreutils/libexec/gnubin:$PATH

to make them get along.


Clone the repo and install the latest stable version v0.4.0 with

git clone --branch v0.4.0
cd klab
make deps

OPTIONAL: klab has some optional Haskell components, for which the recommended installation method is nix. If you have nix, you can install the Haskell components with

make deps-haskell

Environment Setup

To make klab available from the terminal, you can either just export the path to the klab executable in bin/, or use:

make link

This installs symlinks globally at /usr/local/bin and /usr/local/libexec (will require sudo on Linux machines). You can also specify a custom directory for installation by doing:

PREFIX=/path/to/custom/prefix make link

The file env will setup the environment for you if sourced from the root directory of the repo.

source ./env

It sets three environment variables:

  • PATH: include the klab executable.
  • KLAB_EVMS_PATH: the EVM semantics to use.

OPTIONAL: If you want to use a different version of K than what the KEVM ships with, you can set:

  • KLAB_K_PATH: override implementation of K.

OPTIONAL: You might also want to add the K tool binaries in evm-semantics/.build/k/k-distribution/bin to your $PATH, if you didn't already have K installed.

OPTIONAL: You can also use nix-shell for a more deterministic environment experience. If you have nix installed, run nix-shell in this repo to start a deterministic shell environment.


To see how klab is used, we can explore the project in examples/SafeAdd:

cd examples/SafeAdd/


The file config.json tells klab where to look for both the specification and the implementation of our contract. In this case, our specification lives in src/, and our implementation lives in dapp/.

Note that this example includes dapp/out/SafeAdd.sol.json compiled from the solidity source. With solc installed, you can compile it yourself:

solc --combined-json=abi,bin,bin-runtime,srcmap,srcmap-runtime,ast dapp/src/SafeAdd.sol > dapp/out/SafeAdd.sol.json


Our goal is to prove that our implementation satisfies our specification. To do so, we'll start by building a set of K modules from our spec:

klab build

This will generate success and failure reachability rules for each act of our specification. We can find the results in the out/specs directory.

Now we're ready to prove each case, for example:

klab prove --dump SafeAdd_add_fail

The --dump flag outputs a log to out/data/<hash>.log, which will be needed later for interactive debugging. We can also do klab prove-all to prove all outstanding claims.

Once the proof is complete, we can explore the generated symbolic execution trace using:

klab debug <hash>

Embedded rules

klab comes with a set of pre-defined K rewrite rules, additional to the ones defined in evm-semantics. They are located in resources/rules.k.tmpl.

Key Bindings

Toggle different views by pressing any of the following keys:

View Commands:

  • t - display the (somewhat) pretty K term.
  • c - display current constraints.
  • k - display <k> cell.
  • b - display behavior tree.
  • s - diaplay source code.
  • e - display evm specific module.
  • m - display memory cell.
  • d - display debug cells (see toggling debug cells below).
  • r - display applied K rule.
  • z - display z3 feedback from attempted rule application.
  • Up/Dn - scroll view up and down.

Navigation Commands:

  • n - step to next opcode
  • p - step to previous opcode
  • Shift+n - step to next k term
  • Shift+p - step to previous k term
  • Ctrl+n - step to next branch point
  • Ctrl+p - step to previous branch point

Toggling Debug Cells:

The following commands are prefixed with : (and are typed at the bottom of the interface). It's possible to toggle the debug cells view for specific cells, which prints out the JSON representation of the given cells. Remember, you must turn on the debug cells view to see these (above).

  • :show ethereum.evm.callState.gas - show the contents of the <gas> cell in the debug cells view.
  • :hide ethereum.evm.callStack.pc - hide the contents of the <pc> cell in the debug cells view.
  • :omit gas pc - omit the contents of the <gas> and <pc> cells in the term view.
  • :unomit pc programBytes - unomit the contents of the <pc> and <programBytes> cells in the term view.

Available klab Commands

  • klab build - builds a set of K reachability claims in out/specs based on the spec, lemmas and source code as specified in the projects config.json.
  • klab prove <hash> [--dump] - executes a K reachability claim specified as a hash to the K prover. If the --dump flag is present, the proof can be explored using klab debug.
  • klab prove-all - builds and executes all proof objects in the project directory.
  • klab debug <hash> - opens up the cli proof explorer of a particular proof execution. See key bindings above.
  • klab focus <hash> - focus on a hash, allowing you to leave out it as an argument to other commands.
  • klab hash - prints the hash of the focused proof
  • klab get-gas <hash> - Traverses the execution trace of a proof object to fetch its gas usage, put in out/gas/<hash>gas.k.
  • klab solve-gas <hash> - Constructs the gas condition necessary for an execution to succeed.
  • klab evm <hash> - Shows opcodes and source code side-by-side (useful for extracting pc values).
  • klab status <hash> - Shows whether a proof has been run, and whether it was accepted or rejected.
  • klab status-js <hash> - Shows the behaviour tree for an executed proof.
  • klab fetch <url> - Fetches the execution trace of a proof object at the url, preparing it for local debugging.
  • klab compress <hash> - compresses an execution trace so you can share it with a friend (or enemy).
  • klab storage <contractName> - Guesses what the storage layout of a given contract is
  • klab report - Generates a html report of the current project state in out/report/index.html.
  • klab help - Generates this view


The config.json file is used to configure klab.

Here's an example:

  "name": "k-dss",
  "url": "",
  "src": {
    "specification": "./src/",
    "smt_prelude": "./src/",
    "rules": [
    "dirty_rules": [
  "implementations": {
    "Vat": {
      "src": "src/vat.sol"
    "Vow": {
      "src": "src/vow.sol"
  "timeouts": {
    "Vat_grab_pass_rough": "16h",
  "memory" : {
    "Vat_frob-diff-nonzero_fail_rough": "25G",
  "dapp_root": "./dss",
  "solc_output_path": "out/dapp.sol.json",
  "host": ""



By default, klab-prove sets a timeout of 1 day. This can be changed by passing the --timeout flag a value of the format [0-9]+[dhms].

klab-prove-all defaults to a per-proof timeout of 200m. This can be changed by setting timeouts to a different value in config.json, as shown above.


By default, both klab-prove and klab-prove-all run the JVM with a maximum heap size of 10GB.

This can be changed by setting the K_OPTS environment variable to something like --Xmx4G. Refer to the JVM docs for more information.

klab-prove-all also reads the config.json file, and the maximum heap size can be changed with the memory key, as shown above.


In rough specs, the amount of gas available defaults to 3,000,000. This can be changed using the gas header.

Once a pass_rough spec has been proven, the gas used for each execution path is combined into a single expression, which is the upper gas bound for the stronger pass spec.

Zsh completions

There are automatic tab completions for zsh that can be installed by adding the following to your .zshrc:

# completions for klab
fpath=(~/dapphub/klab/resources/zsh $fpath)
autoload -U compinit


Outdated npm

You might have problems due to an outdated npm, in that case try updating it with:

npm install npm@latest -g
npm install -g n
n stable

KLab server requesting files at incorrect directory

What it looks like:

$ klab server

18.07.30 14-46-50: exec dfc688db4cc98b5de315bdfaa2512b84d14c3aaf3e58581ae728247097ff300d/
18.07.30 14-47-32: out Debugg: dfc688db4cc98b5de315bdfaa2512b84d14c3aaf3e58581ae728247097ff300d

throw err;

Error: ENOENT: no such file or directory, open '/tmp/klab/b042c99687ae5018744dc96107032b291e4a91f1ab38a6286b2aff9a78056665/abstract-semantics.k'
at Object.openSync (fs.js:443:3)
at Object.readFileSync (fs.js:348:35)
at getFileExcerpt (/home/dev/src/klab/lib/rule.js:5:4)
at Object.parseRule (/home/dev/src/klab/lib/rule.js:21:16)
at Object.getblob (/home/dev/src/klab/lib/driver/dbDriver.js:49:19)
at (/home/dev/src/klab/lib/driver/dbDriver.js:113:56)
at Stream._n (/home/dev/src/klab/node_modules/xstream/index.js:797:18)
at /home/dev/src/klab/node_modules/@cycle/run/lib/cjs/index.js:57:61
at process._tickCallback (internal/process/next_tick.js:61:11)
[1] [dev@arch-ehildenb klab]% klab server
throw err;

Notice how it's requesting abstract-semantics.k from proof-hash b042... but we're actually running proof-hash dfc6.... This is a problem with how K caches compiled definitions, and must be fixed upstream.

To fix this, run:

make clean && make deps

This will remove and recompile the KEVM semantics.


All contributions to this repository are licensed under AGPL-3.0. Authors:

  • Denis Erfurt
  • Martin Lundfall
  • Everett Hildenbrandt
  • Lev Livnev