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→Crypto Notes
An HKDF-based stream cipher is used to protect the response for getToken2(), and the request for resetAccount(). HKDF is used to create a number of random bytes equal to the length of the message, then these are XORed with the plaintext to produce the ciphertext. An HMAC is then computed from the ciphertext, to protect the integrity of the message.
HKDF, like all KDFsany KDF, is defined to produce output that is indistinguishable from random data ("The HKDF Scheme", http://eprint.iacr.org/2010/264.pdf , by Hugo Krawczyk, section 3). XORing a plaintext with a random keystream to produce ciphertext is a simple and secure approach to data encryption, epitomized by AES-CTR or a stream cipher (http://cr.yp.to/snuffle/design.pdf). HKDF is not the fastest way to generate such a keystream, but it is safe, easy to specify, and easy to implement (just HMAC and XOR).
Each keystream must be unique. SRP is defined to produce a random session key for each run (as long as at least one of the sides provides a random ephemeral key). We define resetToken to be a single-use randomly-generated value. Hence our two HKDF-XOR keystreams will be unique.
A slightly more-traditional alternative would be to use AES-CTR (with the same HMAC-SHA256 used here), with a randomly-generated IV. This is equally secure, but requires implementors to obtain an AES library (with CTR mode, which does not seem to be universal). An even more traditional technique would be AES-CBC, which introduces the need for padding and a way to specify the length of the plaintext. The additional specification complexity, plus the library load, leads me to prefer HKDF+XOR.
