diff --git a/webcrypto/aes.go b/webcrypto/aes.go
index 5dac06c..63277b1 100644
--- a/webcrypto/aes.go
+++ b/webcrypto/aes.go
@@ -20,7 +20,7 @@ type AESKeyGenParams struct {
 	Algorithm
 
 	// The length, in bits, of the key.
-	Length int64 `json:"length"`
+	Length bitLength `json:"length"`
 }
 
 // newAESKeyGenParams creates a new AESKeyGenParams object, from the
@@ -41,7 +41,7 @@ func newAESKeyGenParams(rt *goja.Runtime, normalized Algorithm, params goja.Valu
 
 	return &AESKeyGenParams{
 		Algorithm: normalized,
-		Length:    algorithmLength,
+		Length:    bitLength(algorithmLength),
 	}, nil
 }
 
@@ -75,7 +75,7 @@ func (akgp *AESKeyGenParams) GenerateKey(
 		return nil, NewError(OperationError, "invalid key length")
 	}
 
-	randomKey := make([]byte, akgp.Length/8)
+	randomKey := make([]byte, akgp.Length.asByteLength())
 	if _, err := rand.Read(randomKey); err != nil {
 		// 4.
 		return nil, NewError(OperationError, "could not generate random key")
@@ -86,7 +86,7 @@ func (akgp *AESKeyGenParams) GenerateKey(
 	key.Type = SecretCryptoKeyType
 	key.Algorithm = AESKeyAlgorithm{
 		Algorithm: akgp.Algorithm,
-		Length:    akgp.Length,
+		Length:    int64(akgp.Length),
 	}
 
 	// 10.
@@ -190,7 +190,7 @@ func (aip *AESImportParams) ImportKey(
 	key := &CryptoKey{
 		Algorithm: AESKeyAlgorithm{
 			Algorithm: aip.Algorithm,
-			Length:    int64(len(keyData) * 8),
+			Length:    int64(byteLength(len(keyData)).asBitLength()),
 		},
 		Type:   SecretCryptoKeyType,
 		handle: keyData,
@@ -443,7 +443,7 @@ type AESGCMParams struct {
 	// in some applications: Appendix C of the specification provides additional guidance here.
 	//
 	// tagLength is optional and defaults to 128 if it is not specified.
-	TagLength int `json:"tagLength"`
+	TagLength bitLength `json:"tagLength"`
 }
 
 // Encrypt encrypts the given plaintext using the AES-GCM algorithm, and returns the ciphertext.
@@ -477,7 +477,7 @@ func (agp *AESGCMParams) Encrypt(plaintext []byte, key CryptoKey) ([]byte, error
 	}
 
 	// 4.
-	var tagLength int
+	var tagLength bitLength
 	if agp.TagLength == 0 {
 		tagLength = 128
 	} else {
@@ -503,7 +503,7 @@ func (agp *AESGCMParams) Encrypt(plaintext []byte, key CryptoKey) ([]byte, error
 		return nil, NewError(OperationError, "could not create cipher")
 	}
 
-	gcm, err := cipher.NewGCMWithTagSize(block, tagLength/8)
+	gcm, err := cipher.NewGCMWithTagSize(block, int(tagLength.asByteLength()))
 	if err != nil {
 		return nil, NewError(ImplementationError, "could not create cipher")
 	}
@@ -526,7 +526,7 @@ func (agp *AESGCMParams) Encrypt(plaintext []byte, key CryptoKey) ([]byte, error
 // [specification]: https://www.w3.org/TR/WebCryptoAPI/#aes-gcm
 func (agp *AESGCMParams) Decrypt(ciphertext []byte, key CryptoKey) ([]byte, error) {
 	// 1.
-	var tagLength int
+	var tagLength bitLength
 	if agp.TagLength == 0 {
 		tagLength = 128
 	} else {
@@ -544,7 +544,7 @@ func (agp *AESGCMParams) Decrypt(ciphertext []byte, key CryptoKey) ([]byte, erro
 	// 2.
 	// Note that we multiply the length of the ciphertext by 8, in order
 	// to get the length in bits.
-	if len(ciphertext)*8 < tagLength {
+	if byteLength(len(ciphertext)).asBitLength() < tagLength {
 		return nil, NewError(OperationError, "ciphertext is too short")
 	}
 
@@ -571,7 +571,7 @@ func (agp *AESGCMParams) Decrypt(ciphertext []byte, key CryptoKey) ([]byte, erro
 		return nil, NewError(OperationError, "could not create AES cipher")
 	}
 
-	gcm, err := cipher.NewGCMWithTagSize(block, tagLength/8)
+	gcm, err := cipher.NewGCMWithTagSize(block, int(tagLength.asByteLength()))
 	if err != nil {
 		return nil, NewError(OperationError, "could not create GCM cipher")
 	}
diff --git a/webcrypto/hmac.go b/webcrypto/hmac.go
index cb2322c..1c93ad8 100644
--- a/webcrypto/hmac.go
+++ b/webcrypto/hmac.go
@@ -98,20 +98,23 @@ func (hkgp *HMACKeyGenParams) GenerateKey(
 	// part of the normalized algorithm, and as accessing the runtime from the
 	// callback below could lead to a race condition.
 	if !hkgp.Length.Valid {
+		var length bitLength
 		switch hkgp.Hash.Name {
 		case SHA1:
-			hkgp.Length = null.IntFrom(sha1.BlockSize * 8)
+			length = byteLength(sha1.BlockSize).asBitLength()
 		case SHA256:
-			hkgp.Length = null.IntFrom(sha256.BlockSize * 8)
+			length = byteLength(sha256.BlockSize).asBitLength()
 		case SHA384:
-			hkgp.Length = null.IntFrom(sha512.BlockSize * 8)
+			length = byteLength(sha512.BlockSize).asBitLength()
 		case SHA512:
-			hkgp.Length = null.IntFrom(sha512.BlockSize * 8)
+			length = byteLength(sha512.BlockSize).asBitLength()
 		default:
 			// This case should never happen, as the normalization algorithm
 			// should have thrown an error if the hash algorithm is invalid.
 			return nil, NewError(ImplementationError, "invalid hash algorithm: "+hkgp.Hash.Name)
 		}
+
+		hkgp.Length = null.IntFrom(int64(length))
 	}
 
 	if hkgp.Length.Int64 == 0 {
@@ -119,7 +122,7 @@ func (hkgp *HMACKeyGenParams) GenerateKey(
 	}
 
 	// 3.
-	randomKey := make([]byte, hkgp.Length.Int64/8)
+	randomKey := make([]byte, bitLength(hkgp.Length.Int64).asByteLength())
 	if _, err := rand.Read(randomKey); err != nil {
 		// 4.
 		return nil, NewError(OperationError, "failed to generate random key; reason:  "+err.Error())
@@ -285,28 +288,20 @@ func (hip *HMACImportParams) ImportKey(
 	}
 
 	// 5. 6.
-	length := int64(len(keyData) * 8)
+	length := byteLength(len(keyData)).asBitLength()
 	if length == 0 {
 		return nil, NewError(DataError, "key length cannot be 0")
 	}
 
 	// 7.
-	if hip.Length.Valid {
-		if hip.Length.Int64 > length {
-			return nil, NewError(DataError, "key length cannot be greater than the length of the imported data")
-		}
-
-		if hip.Length.Int64 < length {
-			return nil, NewError(DataError, "key length cannot be less than the length of the imported data")
-		}
-
-		length = hip.Length.Int64
+	if hip.Length.Valid && hip.Length.Int64 != int64(length) {
+		return nil, NewError(DataError, "key length cannot be different from the length of the imported data")
 	}
 
 	// 8.
 	key := CryptoKey{
 		Type:   SecretCryptoKeyType,
-		handle: keyData[:length/8],
+		handle: keyData[:length.asByteLength()],
 	}
 
 	// 9.
@@ -316,7 +311,7 @@ func (hip *HMACImportParams) ImportKey(
 	algorithm.Name = HMAC
 
 	// 11.
-	algorithm.Length = length
+	algorithm.Length = int64(length)
 
 	// 12.
 	algorithm.Hash = hash
diff --git a/webcrypto/types.go b/webcrypto/types.go
index a2c3a97..73161d7 100644
--- a/webcrypto/types.go
+++ b/webcrypto/types.go
@@ -6,6 +6,22 @@ import (
 	"github.com/dop251/goja"
 )
 
+// bitLength is a type alias for the length of a bits collection.
+type bitLength int
+
+// asByteLength returns the length of the bits collection in bytes.
+func (b bitLength) asByteLength() byteLength {
+	return byteLength(b) / 8
+}
+
+// byteLength is a type alias for the length of a byte slice.
+type byteLength int
+
+// asBitLength returns the length of the byte slice in bits.
+func (b byteLength) asBitLength() bitLength {
+	return bitLength(b) * 8
+}
+
 // ToBytes tries to return a byte slice from compatible types.
 func ToBytes(data interface{}) ([]byte, error) {
 	switch dt := data.(type) {