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Soter KDF

Soter KDF #

Soter KDF is a key derivation function used by Themis in symmetric cryptosystems to derive encryption keys from user-provided key material.

KDF enables secure usage of symmetric keys of arbitrary length, regardless of the technical requirements of particular encryption algorithms. It also reduces effective key reuse by incorporating additional context data into derived keys.

However, note that Soter KDF still requires strong keys as input. It is also not suitable for passphases. Themis makes use of alternative KDFs (such as PBKDF2) in contexts where encryption keys need to be derived from passphrases.

Soter KDF is similar in construction to ZRTP KDF defined by RFC 6189, section 4.5.1.

KDF(KI, Label, Context…, L) = truncate(KDF*, L)

KDF* = HMAC-SHA-256(KI, 0x00000001 || Label || 0x00 || Context…)

where the inputs are

  • KI is the input symmetric key provided by the user
  • Label indicates purpose of key derivation, a byte string
  • Context is a list of additional “context data” bytes, such as a nonce pertaining to this particular derivation
  • L is the length of derived key in bytes

The input key KI is used to key the HMAC function (HMAC-SHA-256 in case of Soter KDF), and the rest of the parameters are concatenated (||) and hashed by the HMAC. Its output is then truncated to the desired length L.

It is not possible to derive keys longer than the HMAC output. That is 32 bytes in case of HMAC-SHA-256 used by Soter KDF.

Key differences between Soter KDF and ZRTP KDF:

  • the input key KI may be omitted, in which case it is derived from Label and Contexts
  • the counter i has fixed value of 0x00000001
  • the Context data is a list, not a single byte string
  • the length L is not included into the hashed string

If the input key KI is not provided by the user, an implicit key KI* is derived in the following manner:

KI* = truncate(Label, 32) ⊕ truncate(Context, 32) ⊕ ⋯

The implicit key is 32 bytes long. If the Label or any of the Contexts is longer, they are truncated to 32 bytes, and if they are shorter, they are padded with zeros before being added up with XOR (⊕).

Reference implementation #

On GitHub you can find a reference implementation of Soter KDF in Go.

Example #

Here is a code snippet in Go using the reference implementation to produce test vectors:

package main

import (
	"encoding/hex"
	"fmt"

	"github.com/cossacklabs/themis/docs/reference-implementation/soter"
)

func main() {
	inputKey, _ := hex.DecodeString(
		"4e6f68365577616568696564316b696a6f74686168326f506f68306565517565",
	)
	label := "Example key derivation"
	context := [][]byte{[]byte("2020-12-20"), []byte("11:18:24")}

	var outputKey []byte

	fmt.Println("With explicit input key:")
	// d5f5be45fd6eab6dcbf93c21c3d2d1e3e888fa20ef38f2f4a121c196382342dd
	outputKey = soter.SoterKDF(inputKey, label, 32, context...)
	fmt.Println(hex.EncodeToString(outputKey))
	fmt.Println()

	fmt.Println("With implicit input key:")
	// cf9846b8026c5b76a0641aa85f4152ff02c15ad45b726c6e578be52afdfd6930
	outputKey = soter.SoterKDF(nil, label, 32, context...)
	fmt.Println(hex.EncodeToString(outputKey))
}