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Les Hazlewood
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README.md
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README.md
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@ -75,7 +75,10 @@ enforcement.
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* [JSON Processor](#json)
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* [Custom JSON Processor](#json-custom)
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* [Jackson ObjectMapper](#json-jackson)
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* [Base64 Codec](#base64)
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* [Base64 Support](#base64)
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* [Base64 in Security Contexts](#base64-security)
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* [Base64 is not Encryption](#base64-not-encryption)
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* [Changing Base64 Characters](#base64-changing-characters)
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* [Custom Base64 Codec](#base64-custom)
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<a name="features"></a>
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@ -1329,6 +1332,106 @@ utility classes.
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* `BASE64` is an RFC 4648 [Base64](https://tools.ietf.org/html/rfc4648#section-4) decoder
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* `BASE64URL` is an RFC 4648 [Base64URL](https://tools.ietf.org/html/rfc4648#section-5) decoder
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<a name="base64-security"></a>
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### Understanding Base64 in Security Contexts
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All cryptographic operations, like encryption and message digest calculations, result in binary data - raw byte arrays.
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Because raw byte arrays cannot be represented natively in JSON, the JWT
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specifications employ the Base64URL encoding scheme to represent these raw byte values in JSON documents or compound
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structures like a JWT.
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This means that the Base64 and Base64URL algorithms take a raw byte array and converts the bytes into a string suitable
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to use in text documents and protocols like HTTP. These algorithms can also convert these strings back
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into the original raw byte arrays for decryption or signature verification as necessary.
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That's nice and convenient, but there are two very important properties of Base64 (and Base64URL) text strings that
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are critical to remember when they are used in security scenarios like with JWTs:
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* [Base64 is not encryption](#base64-not-encryption)
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* [Changing Base64 characters](#base64-changing-characters) **does not automatically invalidate data**.
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<a name="base64-not-encryption"></a>
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#### Base64 is not encryption
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Base64-encoded text is _not_ encrypted.
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While a byte array representation can be converted to text with the Base64 algorithms,
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anyone in the world can take Base64-encoded text, decode it with any standard Base64 decoder, and obtain the
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underlying raw byte array data. No key or secret is required to decode Base64 text - anyone can do it.
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Based on this, when encoding sensitive byte data with Base64 - like a shared or private key - **the resulting
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string NOT is safe to expose publicly**.
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A base64-encoded key is still sensitive information and must
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be kept as secret and as safe as the original thing you got the bytes from (e.g. a Java `PrivateKey` or `SecretKey`
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instance).
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After Base64-encoding data into a string, it is possible to then encrypt the string to keep it safe from prying
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eyes if desired, but this is different. Encryption is not encoding. They are separate concepts.
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<a name="base64-changing-characters"></a>
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#### Changing Base64 Characters
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In an effort to see if signatures or encryption is truly validated correctly, some try to edit a JWT
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string - particularly the Base64-encoded signature part - to see if the edited string fails security validations.
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This conceptually makes sense: change the signature string, you would assume that signature validation would fail.
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_But this doesn't always work. Changing base64 characters is an invalid test_.
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Why?
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Because of the way the Base64 algorithm works, there are multiple Base64 strings that can represent the same raw byte
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array.
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Going into the details of the Base64 algorithm is out of scope for this documentation, but there are many good
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Stackoverflow [answers](https://stackoverflow.com/questions/33663113/multiple-strings-base64-decoded-to-same-byte-array?noredirect=1&lq=1)
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and [JJWT issue comments](https://github.com/jwtk/jjwt/issues/211#issuecomment-283076269) that explain this in detail.
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Here's one [good answer](https://stackoverflow.com/questions/29941270/why-do-base64-decode-produce-same-byte-array-for-different-strings):
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> Remember that Base64 encodes each 8 bit entity into 6 bit chars. The resulting string then needs exactly
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> 11 * 8 / 6 bytes, or 14 2/3 chars. But you can't write partial characters. Only the first 4 bits (or 2/3 of the
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> last char) are significant. The last two bits are not decoded. Thus all of:
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>
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> dGVzdCBzdHJpbmo
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> dGVzdCBzdHJpbmp
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> dGVzdCBzdHJpbmq
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> dGVzdCBzdHJpbmr
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> All decode to the same 11 bytes (116, 101, 115, 116, 32, 115, 116, 114, 105, 110, 106).
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As you can see by the above 4 examples, they all decode to the same exact 11 bytes. So just changing one or two
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characters at the end of a Base64 string may not work and can often result in an invalid test.
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<a name="base64-invalid-characters"></a>
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##### Adding Invalid Characters
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JJWT's default Base64/Base64URL decoders automatically ignore illegal Base64 characters located in the beginning and
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end of an encoded string. Therefore prepending or appending invalid characters like `{` or `]` or similar will also
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not fail JJWT's signature checks either. Why?
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Because such edits - whether changing a trailing character or two, or appending invalid characters - do not actually
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change the _real_ signature, which in cryptographic contexts, is always a byte array. Instead, tests like these
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change a text encoding of the byte array, and as we covered above, they are different things.
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So JJWT 'cares' more about the real byte array and less about its text encoding because that is what actually matters
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in cryptographic operations. In this sense, JJWT follows the [Robustness Principle](https://en.wikipedia.org/wiki/Robustness_principle)
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in being _slightly_ lenient on what is accepted per the rules of Base64, but if anything in the real underlying
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byte array is changed, then yes, JJWT's cryptographic assertions will definitely fail.
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To help understand JJWT's approach, we have to remember why signatures exist. From our documentation above on
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[signing JWTs](#jws):
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> * guarantees it was created by someone we know (it is authentic), as well as
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> * guarantees that no-one has manipulated or changed it after it was created (its integrity is maintained).
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Just prepending or appending invalid text to try to 'trick' the algorithm doesn't change the integrity of the
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underlying claims or signature byte arrays, nor the authenticity of the claims byte array, because those byte
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arrays are still obtained intact.
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Please see [JJWT Issue #518](https://github.com/jwtk/jjwt/issues/518) and its referenced issues and links for more
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information.
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<a name="base64-custom"></a>
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### Custom Base64
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