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add support for the hybrid mode of Argon2 for passphrases
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Passphrase support currently hardcodes the use of argon2i, which is
the data-independent version of Argon2. Whilst this has better
side-channel resistance than data-dependent versions, it requires
more iterations to protect against time-memory tradeoff attacks.

Cryptsetup recently switch to argon2id by default for passphrase
hashing, which is the hybrid version that provides a good balance
between side-channel resistance and time-memory tradeoff.

This introduces support for selecting between the 2 versions for
passphrase support.

There are other versions of Argon2, such as the data-dependent version
which sacrifices side-channel resistance for better resistance against
time-memory tradeoff attacks, but this isn't supported by the argon2 go
package.
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@chrisccoulson
chrisccoulson committed Mar 15, 2024
1 parent c1302a8 commit f88aa4f
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Showing 8 changed files with 373 additions and 137 deletions.
115 changes: 84 additions & 31 deletions argon2.go
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Original file line number Diff line number Diff line change
Expand Up @@ -21,6 +21,8 @@ package secboot

import (
"errors"
"fmt"
"math"
"runtime"
"sync"
"time"
Expand Down Expand Up @@ -63,9 +65,26 @@ func argon2KDF() Argon2KDF {
return argon2Impl
}

// Argon2Options specifies parameters for the Argon2 KDF used by cryptsetup
// and for passphrase support.
// Argon2Mode describes the Argon2 mode to use.
type Argon2Mode string

const (
// Argon2Default is used by Argon2Options to select the default
// Argon2 mode, which is currently Argon2id.
Argon2Default Argon2Mode = ""

// Argon2i is the data-independent mode of Argon2.
Argon2i Argon2Mode = "argon2i"

// Argon2id is the hybrid mode of Argon2.
Argon2id Argon2Mode = "argon2id"
)

// Argon2Options specifies parameters for the Argon2 KDF used for passphrase support.
type Argon2Options struct {
// Mode specifies the KDF mode to use.
Mode Argon2Mode

// MemoryKiB specifies the maximum memory cost in KiB when ForceIterations
// is zero. In this case, it will be capped at 4GiB or half of the available
// memory, whichever is less. If ForceIterations is not zero, then this is
Expand All @@ -90,43 +109,71 @@ type Argon2Options struct {
Parallel uint8
}

func (o *Argon2Options) deriveCostParams(keyLen int) (*Argon2CostParams, error) {
func (o *Argon2Options) kdfParams(keyLen uint32) (*kdfParams, error) {
switch o.Mode {
case Argon2Default, Argon2i, Argon2id:
// ok
default:
return nil, errors.New("invalid argon2 mode")
}

mode := o.Mode
if mode == Argon2Default {
mode = Argon2id
}

switch {
case o.ForceIterations > 0:
threads := runtimeNumCPU()
if threads > 4 {
threads = 4
// The non-benchmarked path. Ensure that ForceIterations
// and MemoryKiB fit into an int32 so that it always fits
// into an int
switch {
case o.ForceIterations > math.MaxInt32:
return nil, fmt.Errorf("invalid iterations count %d", o.ForceIterations)
case o.MemoryKiB > math.MaxInt32:
return nil, fmt.Errorf("invalid memory cost %dKiB", o.MemoryKiB)
}
params := &Argon2CostParams{
Time: o.ForceIterations,
MemoryKiB: 1 * 1024 * 1024,
Threads: uint8(threads)}

defaultThreads := runtimeNumCPU()
if defaultThreads > 4 {
// limit the default threads to 4
defaultThreads = 4
}

params := &kdfParams{
Type: string(mode),
Time: int(o.ForceIterations), // no limit to the time cost.
Memory: 1 * 1024 * 1024, // the default memory cost is 1GiB.
CPUs: defaultThreads, // the default number of threads is min(4,nr_of_cpus).
}
if o.MemoryKiB != 0 {
params.MemoryKiB = o.MemoryKiB
// no limit to the memory cost.
params.Memory = int(o.MemoryKiB)
}
if o.Parallel != 0 {
params.Threads = o.Parallel
// no limit to the threads if set explicitly.
params.CPUs = int(o.Parallel)
}

return params, nil
default:
benchmarkParams := &argon2.BenchmarkParams{
MaxMemoryCostKiB: 1 * 1024 * 1024,
TargetDuration: 2 * time.Second}
MaxMemoryCostKiB: 1 * 1024 * 1024, // the default maximum memory cost is 1GiB.
TargetDuration: 2 * time.Second, // the default target duration is 2s.
}

if o.MemoryKiB != 0 {
benchmarkParams.MaxMemoryCostKiB = o.MemoryKiB
benchmarkParams.MaxMemoryCostKiB = o.MemoryKiB // this is capped to 4GiB by internal/argon2.
}
if o.TargetDuration != 0 {
benchmarkParams.TargetDuration = o.TargetDuration
}
if o.Parallel != 0 {
benchmarkParams.Threads = o.Parallel
benchmarkParams.Threads = o.Parallel // this is capped to 4 by internal/argon2.
}

params, err := argon2.Benchmark(benchmarkParams, func(params *argon2.CostParams) (time.Duration, error) {
return argon2KDF().Time(&Argon2CostParams{
return argon2KDF().Time(mode, &Argon2CostParams{
Time: params.Time,
MemoryKiB: params.MemoryKiB,
Threads: params.Threads})
Expand All @@ -135,10 +182,12 @@ func (o *Argon2Options) deriveCostParams(keyLen int) (*Argon2CostParams, error)
return nil, xerrors.Errorf("cannot benchmark KDF: %w", err)
}

return &Argon2CostParams{
Time: params.Time,
MemoryKiB: params.MemoryKiB,
Threads: params.Threads}, nil
o = &Argon2Options{
Mode: mode,
MemoryKiB: params.MemoryKiB,
ForceIterations: params.Time,
Parallel: params.Threads}
return o.kdfParams(keyLen)
}
}

Expand Down Expand Up @@ -168,18 +217,20 @@ func (p *Argon2CostParams) internalParams() *argon2.CostParams {
// to delegate execution to a short-lived utility process where required.
type Argon2KDF interface {
// Derive derives a key of the specified length in bytes, from the supplied
// passphrase and salt and using the supplied cost parameters.
Derive(passphrase string, salt []byte, params *Argon2CostParams, keyLen uint32) ([]byte, error)
// passphrase and salt and using the supplied mode and cost parameters.
Derive(passphrase string, salt []byte, mode Argon2Mode, params *Argon2CostParams, keyLen uint32) ([]byte, error)

// Time measures the amount of time the KDF takes to execute with the
// specified cost parameters.
Time(params *Argon2CostParams) (time.Duration, error)
// specified cost parameters and mode.
Time(mode Argon2Mode, params *Argon2CostParams) (time.Duration, error)
}

type inProcessArgon2KDFImpl struct{}

func (_ inProcessArgon2KDFImpl) Derive(passphrase string, salt []byte, params *Argon2CostParams, keyLen uint32) ([]byte, error) {
func (_ inProcessArgon2KDFImpl) Derive(passphrase string, salt []byte, mode Argon2Mode, params *Argon2CostParams, keyLen uint32) ([]byte, error) {
switch {
case mode != Argon2i && mode != Argon2id:
return nil, errors.New("invalid mode")
case params == nil:
return nil, errors.New("nil params")
case params.Time == 0:
Expand All @@ -188,11 +239,13 @@ func (_ inProcessArgon2KDFImpl) Derive(passphrase string, salt []byte, params *A
return nil, errors.New("invalid number of threads")
}

return argon2.Key(passphrase, salt, params.internalParams(), keyLen), nil
return argon2.Key(passphrase, salt, argon2.Mode(mode), params.internalParams(), keyLen), nil
}

func (_ inProcessArgon2KDFImpl) Time(params *Argon2CostParams) (time.Duration, error) {
func (_ inProcessArgon2KDFImpl) Time(mode Argon2Mode, params *Argon2CostParams) (time.Duration, error) {
switch {
case mode != Argon2i && mode != Argon2id:
return 0, errors.New("invalid mode")
case params == nil:
return 0, errors.New("nil params")
case params.Time == 0:
Expand All @@ -201,7 +254,7 @@ func (_ inProcessArgon2KDFImpl) Time(params *Argon2CostParams) (time.Duration, e
return 0, errors.New("invalid number of threads")
}

return argon2.KeyDuration(params.internalParams()), nil
return argon2.KeyDuration(argon2.Mode(mode), params.internalParams()), nil
}

// InProcessArgon2KDF is the in-process implementation of the Argon2 KDF. This
Expand All @@ -212,10 +265,10 @@ var InProcessArgon2KDF = inProcessArgon2KDFImpl{}

type nullArgon2KDFImpl struct{}

func (_ nullArgon2KDFImpl) Derive(passphrase string, salt []byte, params *Argon2CostParams, keyLen uint32) ([]byte, error) {
func (_ nullArgon2KDFImpl) Derive(passphrase string, salt []byte, mode Argon2Mode, params *Argon2CostParams, keyLen uint32) ([]byte, error) {
return nil, errors.New("no argon2 KDF: please call secboot.SetArgon2KDF")
}

func (_ nullArgon2KDFImpl) Time(params *Argon2CostParams) (time.Duration, error) {
func (_ nullArgon2KDFImpl) Time(mode Argon2Mode, params *Argon2CostParams) (time.Duration, error) {
return 0, errors.New("no argon2 KDF: please call secboot.SetArgon2KDF")
}
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