NewCBCDecrypter(iv []byte) cipher.BlockMode
	NewCTR(iv []byte) cipher.Stream
	NewGCM(nonceSize, tagSize int) (cipher.AEAD, error)
}

var _ extraModes = (*aesCipher)(nil)

func NewAESCipher(key []byte) (cipher.Block, error) {
	c := &aesCipher{key: bytes.Clone(key)}
	// Note: 0 is success, contradicting the usual BoringCrypto convention.