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Update README.md to include HMAC key length recommendations #447

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Update README.md to include HMAC key length recommendations #447

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8 changes: 7 additions & 1 deletion README.md
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Expand Up @@ -70,6 +70,12 @@ Algorithm algorithmRS = Algorithm.RSA256(publicKey, privateKey);

> Note: How you obtain or read keys is not in the scope of this library. For an example of how you might implement this, see [this gist](https://gist.github.com/lbalmaceda/9a0c7890c2965826c04119dcfb1a5469).

##### HMAC Key Length and Security

When using a Hash-based Message Authenticaton Code e.g. HS256 or HS512, in order to comply with the strict requirements of the JSON Web Algorithms (JWA) specification (RFC7518), you **must** use a secret key which has the same bit length as the size of the output hash - to avoid weakening the security strength of the authentication code (see NIST recomendations NIST SP 800-117). For example, when using HMAC256, the secret key length must be a minimum of 256 bits (larger key sizes do not increase the security strength further).
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@panva panva Oct 24, 2020 •
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When using a Hash-based Message Authenticaton Code e.g. HS256 or HS512, in order to comply with the strict requirements of the JSON Web Algorithms (JWA) specification (RFC7518), you **must** use a secret key which has the same bit length as the size of the output hash - to avoid weakening the security strength of the authentication code (see NIST recomendations NIST SP 800-117). For example, when using HMAC256, the secret key length must be a minimum of 256 bits (larger key sizes do not increase the security strength further).
When using a Hash-based Message Authenticaton Code e.g. HS256 or HS512, in order to comply with the strict requirements of the JSON Web Algorithms (JWA) specification (RFC7518), you **must** use a secret key which has the same (or larger) bit length as the size of the output hash - to avoid weakening the security strength of the authentication code (see NIST recomendations NIST SP 800-117). For example, when using HMAC256, the secret key length must be a minimum of 256 bits.


Note, the secret key string is converted to bytes assuming a UTF-8 encoding, you must therefore gurantee the results of that conversion produce the required bit length. For example, if only ASCII characters where used in the secret key, each character would convert to a single byte (8 bits), which means for HS256 you would need a mimimum of a 32 character secret.
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is there no affordance for passing non-utf8 secrets? As in - providing the byte array directly?

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I think the API for the Algorithm class supports byte arrays directly, yes. You could perhaps remove the last paragraph if you wanted.

However, I still think it is probably wise to let people know what happens if they do use a String input to the Algorithm API. Because I can not see there is an option to tell it that the String is actually a binary to text representation e.g. base64.


#### Using a KeyProvider:

By using a `KeyProvider` you can change in runtime the key used either to verify the token signature or to sign a new token for RSA or ECDSA algorithms. This is achieved by implementing either `RSAKeyProvider` or `ECDSAKeyProvider` methods:
Expand Down Expand Up @@ -435,4 +441,4 @@ If you have found a bug or if you have a feature request, please report them at
This project is licensed under the MIT license. See the [LICENSE](LICENSE) file for more info.


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