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

Error message

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

What it means

In FIPS 140-only mode, the des package refuses to construct a TripleDES (3DES) cipher. Although stronger than single DES, 3DES is deprecated and not approved for new FIPS use, so NewTripleDESCipher short-circuits before key-length validation.

Source

Thrown at src/crypto/des/cipher.go:81

	}
	if len(dst) < BlockSize {
		panic("crypto/des: output not full block")
	}
	if alias.InexactOverlap(dst[:BlockSize], src[:BlockSize]) {
		panic("crypto/des: invalid buffer overlap")
	}
	cryptBlock(c.subkeys[:], dst, src, true)
}

// A tripleDESCipher is an instance of TripleDES encryption.
type tripleDESCipher struct {
	cipher1, cipher2, cipher3 desCipher
}

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

	if len(key) != 24 {
		return nil, KeySizeError(len(key))
	}

	c := new(tripleDESCipher)
	c.cipher1.generateSubkeys(key[:8])
	c.cipher2.generateSubkeys(key[8:16])
	c.cipher3.generateSubkeys(key[16:])
	return c, nil
}

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

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

View on GitHub (pinned to b6b368adc5)

Solutions

Migrate to AES (aes.NewCipher) for all new encryption.
If FIPS-only is not required, rebuild the binary without FIPS 140-only enforcement.
For existing 3DES data, decrypt in a non-FIPS context and re-encrypt with AES.

Example fix

// before
block, err := des.NewTripleDESCipher(key24)
// after
block, err := aes.NewCipher(key16)

Defensive patterns

Strategy: validation

Validate before calling

func newBlock(key []byte) (cipher.Block, error) {
    if fipsEnabled() {
        return nil, errors.New("TripleDES is unavailable in FIPS-only mode; configure AES")
    }
    return des.NewTripleDESCipher(key)
}

Try / catch

block, err := des.NewTripleDESCipher(key)
if err != nil && strings.Contains(err.Error(), "FIPS 140-only mode") {
    // migrate to AES or rebuild without FIPS-only mode
}

Prevention

Treat 3DES as deprecated; plan migration to AES.
Gate legacy-crypto paths behind a feature flag so they can be disabled in FIPS builds.
Run a crypto audit when adopting a FIPS toolchain.

When it happens

Trigger: Calling des.NewTripleDESCipher(key) in a binary with FIPS 140-only enforcement enabled.

Common situations: Legacy systems using 3DES (e.g. older TLS/JCE/PKCS interop) moved into a FIPS-regulated runtime; FIPS toolchain build picked up by CI without crypto audit.

Related errors

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/d1e581a7a3f218aa. Report an issue: GitHub.