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In cryptography, we often write as "AES has 128-bit key" and "AES provides 128 bits of security"
Is there any difference between writing 128-bit and 128 bits? What is the correct choice?
Example; From Lindell & Katz's book;
Often, we can use a compound hyphenated adjective as an alternative to a longer phrase meaning the same thing. The correct choice may depend on the level of the writing - more technical readers may understand at once what a '128-bit key' is, especially if the idea has been explained already.
A hundred-tonne load
A load of one hundred tonnes
A 128-bit key
A key with 128 bits of security
| Has Hyphen | Is Singular | Type of Word | |
|---|---|---|---|
| x-bit | yes | yes | adjective |
| 1 bit | - | yes | noun |
| y bits | - | - | noun |
x is any number
y is any number except 1
The chart above is basically true for most countable nouns in English in general. This includes nouns like "dog", "cat", "house", "car", "person", etc. In your question, you gave two examples:
AES has [a] 128-bit key - Here, "128-bit" is used like an adjective. That is the reason it does not have an "s" (despite being a number other than 1), but does have a hyphen.
AES provides 128 bits of security - Here, "128 bits" is used like a noun. That is the reason it does have an "s", but does not have a hyphen.
The hyphenated phrase “128-bit” is, all together, an adjective. The hyphen indicates that it is being used in this manner. As an adjective, it modifies the noun “key.” These kinds of hyphenated adjectival phrases are very common, often with units of measure (“128-bit,” “12-inch,” “100-gram,” “5-hour,” and so on), and they always use the singular form of the unit in their construction.
Meanwhile, in “128 bits,” we have an adjective—the number “128”—and a noun, “bits.” Because the number is not 1, the noun is plural, hence “bits” and not “bit.”
Since I did mention that the above construction is common with units of measure, I should note that when you use the International System of Units (SI) abbreviations (m, g, s, etc.), those aren’t pluralized. This is an SI rule, though, not an English-language one—in fact, you see that traditional English units often do pluralize even unit abbreviation, as in “5 lbs.” for “5 pounds” vs. “1 lb.” for “1 pound.” But we don’t do this with the hyphenated construction, for instance “a 5-lb. dog.”
Which to use depends on context and style. For instance, if you were differentiating between two different things of a given measure, like a “5-lb. dog” vs. a “5-lb. cat,” the adjectival-phrase version, as I’ve used here, is much easier to read and write. Otherwise you’d have to say something like “the dog that is 5 lbs.” vs. “the cat that is 5 lbs.,” which is very unwieldy. In other cases, it’s easier to not do that, for example, “we have two keys, one with 128 bits and another with 256.” But it largely comes down to style and what exactly you’re trying to say, which will sound or read better.
The correct choice is nearly always 'bit', not 'bits', and you would include the hyphen.
In your example of "128-bit security", '128-bit' is a compound noun acting as an adjective, effectively describing the level of encryption technology providing the security.
A comparable example would be "a 12-inch pizza". We never say "12 inches of pizza". You could say "this pizza is 12 inches in diameter" and it would be grammatically correct, but overly complex.
As an adjective it is hyphenated:
As a noun it is not hyphenated:
In titles and in advertising it is often ignored.
A very simple explanation:
128-bit is together an adjective that describes a characteristic of something, for example:
"We use 128-bit encryption."
... where "128-bit" is the adjective describing the noun "encryption".
"128 bits" is a number followed by a noun. It together describes the number (amount) of bits present, for example:
"There are 128 bits available".
... where "128" is a number specifying how many "bits", that is a noun, is present.
Therefore, both sentences in Lindell & Katz's book are correct.
