crypto/des: use of DES is not allowed in FIPS 140-only mode

Error message

crypto/des: use of DES is not allowed in FIPS 140-only mode

What it means

In FIPS 140-only mode (fips140only.Enforced() == true), the des package refuses to construct a DES cipher because DES is not a FIPS-approved algorithm (its 56-bit key is insecure). The check runs before any key-length validation, so it short-circuits regardless of the key supplied.

Source

Thrown at src/crypto/des/cipher.go:33

// The DES block size in bytes.
const BlockSize = 8

type KeySizeError int

func (k KeySizeError) Error() string {
	return "crypto/des: invalid key size " + strconv.Itoa(int(k))
}

// desCipher is an instance of DES encryption.
type desCipher struct {
	subkeys [16]uint64
}

// NewCipher creates and returns a new [cipher.Block].
func NewCipher(key []byte) (cipher.Block, error) {
	if fips140only.Enforced() {
		return nil, errors.New("crypto/des: use of DES is not allowed in FIPS 140-only mode")
	}

	if len(key) != 8 {
		return nil, KeySizeError(len(key))
	}

	c := new(desCipher)
	c.generateSubkeys(key)
	return c, nil
}

func (c *desCipher) BlockSize() int { return BlockSize }

func (c *desCipher) Encrypt(dst, src []byte) {
	if len(src) < BlockSize {
		panic("crypto/des: input not full block")
	}
	if len(dst) < BlockSize {

View on GitHub (pinned to b6b368adc5)

Solutions

Replace DES with AES: aes.NewCipher with a 16/24/32-byte key.
If FIPS-only mode is not actually required for this binary, rebuild without FIPS 140-only enforcement.
For legacy DES-encrypted data, decrypt it in a non-FIPS build and re-encrypt with AES before operating under FIPS mode.

Example fix

// before
block, err := des.NewCipher(key)
// after
block, err := aes.NewCipher(key) // 16, 24, or 32 byte key

Defensive patterns

Strategy: validation

Validate before calling

// Detect FIPS-only mode up front and refuse DES with a clear message:
func newBlock(key []byte) (cipher.Block, error) {
    // des.NewCipher already enforces this; gate your config instead.
    if fipsEnabled() {
        return nil, errors.New("DES is unavailable in FIPS-only mode; configure AES")
    }
    return des.NewCipher(key)
}

Try / catch

block, err := des.NewCipher(key)
if err != nil {
    if strings.Contains(err.Error(), "FIPS 140-only mode") {
        // Switch to AES or disable FIPS-only mode.
    }
    return err
}

Prevention

Audit all crypto usage before enabling FIPS 140-only mode.
Prefer AES everywhere; reserve DES only for legacy-data migration outside FIPS mode.
Make the FIPS mode of the build explicit in deployment docs and CI.

When it happens

Trigger: Calling des.NewCipher(key) in a binary built/run with FIPS 140-only enforcement enabled (e.g. a fips140-enabled Go toolchain build).

Common situations: Moving a legacy DES-based application into a FIPS-regulated deployment; building with FIPS toolchain settings without auditing crypto usage; compliance environments (government, finance, healthcare) that mandate FIPS-only.

Related errors

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/e4969646153f9275. Report an issue: GitHub.