The Secure Remote Password Protocol Isn't Bad

Blizzard Entertainment has been receiving a lot of flak recently for using the Secure Remote Password protocol for password authentication in their service because SRP doesn’t provide the same level of protection against offline attacks that one-way key derivation and password hash functions like PBKDF2, bcrypt, and scrypt do.

I applaud them. Well done, Blizzard. You’ve done more to protect your users than most other companies that handle user passwords. It is great to see a company employ real safeguards like SRP and two-factor authentication (which you introduced long before it was cool.)

All of the recent criticism of Blizzard’s design decisions kind of misses the point. SRP was designed to prevent eavesdropping attacks (by never transmitting the password over the wire,) not dictionary attacks against the password verifiers (the kind of digests that are stored on the server side.) This is akin to blaming Diffie-Hellman key exchange for the fact that DES is easy to break, since the SRP authors never made any claim that the verifiers were resistant to dictionary attacks.

Blizzard absolutely made the right choice by choosing not to transmit passwords over the wire. The people who are suggesting that they throw out SRP on the client side for a KDF on the server side seem to completely miss that this would only switch out one security vulnerability with another. A better solution would be to employ a one-way key derivation function on the client side, store the salt on the server side (so any client can produce the same digest for the same account, even if it’s on another machine,) and then transmit the verification “digest” (or proof of it) in a non-revealing/non-reusable way (if the traffic is snooped, or the verifiers are compromised,) the latter being precisely what SRP does.

The above would provide more protection against password compromise than the password authentication used by virtually all web applications and almost all desktop clients. It seems strange to me to criticize Blizzard so aggressively for not doing both when nobody else does.