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age-keygen-deterministic

This is a very simple Rust CLI tool which deterministically derives a single (or multiple) age encryption secret keys from a passphrase. I use this because I use age for long-term backup and want to be able to restore my backup files even if my house burns down and I lost all copies of the secret key - but still remember the passphrase.

Disclaimer

I am not a professional cryptographer and have not done any review of the third-party dependencies. Use at your own risk.

The output of this utility should be piped directly into age-keygen -y and only the public key should be stored permanently. If you run this utility repeatedly inside scripts or pipe the secret key directly to age -i- -d ..., then you are most likely doing something stupid and will leak your passphrase.

Compile

This utility is written in Rust. Just clone this repository and then run cargo build.

Usage

USAGE:
    age-keygen-deterministic [OPTIONS]

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

OPTIONS:
    -c, --count <count>      optional number of secret keys which should be created [default: 1]
    -o, --offset <offset>    optional u64 offset for index of keys [default: 0]

If you only need a single age key, just use it without any command line arguments. If you want to generate more than one secret key, then you can simply use the --count argument.

Example run

If you use the passphrase example-passphrase-do-not-use! and want to generate the deterministic secret keys #2, #3 and #4, then just run the commands below:

$ cargo build --release
$ ./target/release/age-keygen-deterministic -c 3 -o 2
Enter passphrase: 
# secret key 2 below
AGE-SECRET-KEY-1RSWAHJR48AWPN6HHTVVGXN7X3X0YWWA7TM7H22T7TF35EZPPVHHQ7WYGRZ
# secret key 3 below
AGE-SECRET-KEY-144T9ZKX0HK6CMMGYEN6WPN82Q4K9LVR376NUJF33HKVAQ70TXMHSPV96MY
# secret key 4 below
AGE-SECRET-KEY-1FMPVFDE9WD8CSTNS4J3QRNQ5VRTFE8973FVJ2JANT56HEPZTKA4SQZZ84R

The public key for any of those private keys can be obtained by piping the secret key to age-keygen -y.

$ echo AGE-SECRET-KEY-1VZ3CREDN87LLHYDVS6FK36EZEVWNZGGFFSWZDN7DL0J04WG723MQCZUS9Q | age-keygen -y
age1z568mysf7kulsml0rxt6vxp3h26hjmgcmdpz8x6dfh0zlazspquqqawzn4

Cryptography

Please note that you should specify at least 256 bits of entropy. This program will accept a passphrase with only 16 characters but in general you should use longer ones.

The passphrase is stretched with Argon2id to a 64 byte master key. Afterwards, secret keys are generated by incrementing a u64 counter i and using HMAC_SHA256(master_key, i) for the age secret keys. Each 32 byte HMAC-SHA256 output is converted to Bech32 format and written to stdout.

Please note that there are some cryptographic limitations which this toy project does not care about:

  • This program does not attempt to clear any secrets from RAM before it exits.
  • This program does not make any guarantees about side-channel resistance.
  • No clamping of the X25519 key is done - but as far as I know, age-keygen doesn't do it either.