The Secure Remote Password protocol (SRP) is an augmented password-authenticated key exchange (PAKE) protocol, specifically designed to work around existing patents. Like all PAKE protocols, an eavesdropper or man in the middle cannot obtain enough information to be able to brute-force guess a password or apply a dictionary attack without further interactions with the parties for each guess. Furthermore, being an augmented PAKE protocol, the server does not store password-equivalent data. This means that an attacker who steals the server data cannot masquerade as the client unless they first perform a brute force search for the password. In layman's terms, during SRP (or any other PAKE protocol) authentication, one party (the "client" or "user") demonstrates to another party (the "server") that they know the password, without sending the password itself nor any other information from which the password can be derived. The password never leaves the client and is unknown to the server. Furthermore, the server also needs to know about the password (but not the password itself) in order to instigate the secure connection. This means that the server also authenticates itself to the client which prevents phishing without reliance on the user parsing complex URLs. The only mathematically proven security property of SRP is that it is equivalent to Diffie-Hellman against a passive attacker. Newer PAKEs such as AuCPace and OPAQUE offer stronger guarantees.

Overview

The SRP protocol has a number of desirable properties: it allows a user to authenticate themselves to a server, it is resistant to dictionary attacks mounted by an eavesdropper, and it does not require a trusted third party. It effectively conveys a zero-knowledge password proof from the user to the server. In revision 6 of the protocol only one password can be guessed per connection attempt. One of the interesting properties of the protocol is that even if one or two of the cryptographic primitives it uses are attacked, it is still secure. The SRP protocol has been revised several times, and is currently at revision 6a. The SRP protocol creates a large private key shared between the two parties in a manner similar to Diffie–Hellman key exchange based on the client side having the user password and the server side having a cryptographic verifier derived from the password. The shared public key is derived from two random numbers, one generated by the client, and the other generated by the server, which are unique to the login attempt. In cases where encrypted communications as well as authentication are required, the SRP protocol is more secure than the alternative SSH protocol and faster than using Diffie–Hellman key exchange with signed messages. It is also independent of third parties, unlike Kerberos. The SRP protocol, version 3 is described in RFC 2945. SRP version 6a is also used for strong password authentication in SSL/TLS (in TLS-SRP) and other standards such as EAP and SAML, and is part of IEEE 1363.2 and ISO/IEC 11770-4.

Protocol

The following notation is used in this description of the protocol, version 6: I without affecting any steps required of the host. The standard RFC2945 defines H ( I . Use of I within x avoids a malicious server from being able to learn if two users share the same password. All other variables are defined in terms of these. First, to establish a password p with server Steve, client Carol picks a random salt s, and computes x = H(s, p), v = gx. Steve stores v and s, indexed by I, as Carol's password verifier and salt. Carol must not share x with anybody, and must safely erase it at this step, because it is equivalent to the plaintext password p. This step is completed before the system is used as part of the user registration with Steve. Note that the salt s is shared and exchanged to negotiate a session key later so the value could be chosen by either side but is done by Carol so that she can register I, s and v in a single registration request. The transmission and authentication of the registration request is not covered in SRP. Then to perform a proof of password at a later date the following exchange protocol occurs: Now the two parties have a shared, strong session key K. To complete authentication, they need to prove to each other that their keys match. One possible way is as follows: H(I) . Steve verifies M1. M1 . Carol verifies M2. This method requires guessing more of the shared state to be successful in impersonation than just the key. While most of the additional state is public, private information could safely be added to the inputs to the hash function, like the server private key. Alternatively, in a password-only proof the calculation of K can be skipped and the shared S proven with: B . Steve verifies M1. M1 . Carol verifies M2. When using SRP to negotiate a shared key K which will be immediately used after the negotiation, it is tempting to skip the verification steps of M1 and M2. The server will reject the very first request from the client which it cannot decrypt. This can however be dangerous as demonstrated in the Implementation Pitfalls section below. The two parties also employ the following safeguards:

Example code in Python

Implementation pitfalls

Offline bruteforce attack with server-first messaging in the absence of key verification

If the server sends an encrypted message without waiting for verification from the client then an attacker is able to mount an offline bruteforce attack similar to hash cracking. This can happen if the server sends an encrypted message in the second packet alongside the salt and B or if key verification is skipped and the server (rather than the client) sends the first encrypted message. This is tempting as after the very first packet, the server has every information to compute the shared key K. The attack goes as follow: u = H(A, B); S=Avu; K=H(S) c=ENC(K,m) B = kv + gb and c Carol doesn't know x or v. But given any password p she can compute: xp = H(salt, p) Sp = (B - kgxp)(a + uxp) Kp = H(Sp) Kp is the key that Steve would use if p was the expected password. All values required to compute Kp are either controlled by Carol or known from the first packet from Steve. Carol can now try to guess the password, generate the corresponding key, and attempt to decrypt Steve's encrypted message c to verify the key. As protocol messages tend to be structured, it is assumed that identifying that c was properly decrypted is easy. This allows offline recovery of the password. This attack would not be possible had Steve waited for Carol to prove she was able to compute the correct key before sending an encrypted message. Proper implementations of SRP are not affected by this attack as the attacker would be unable to pass the key verification step.

Offline bruteforce based on timing attack

In 2021 Daniel De Almeida Braga, Pierre-Alain Fouque and Mohamed Sabt published PARASITE, a paper in which they demonstrate practical exploitation of a timing attack over the network. This exploits non-constant implementations of modular exponentiation of big numbers and impacted OpenSSL in particular.

Implementations

History

The SRP project was started in 1997. Two different approaches to fixing a security hole in SRP-1 resulted in SRP-2 and SRP-3. SRP-3 was first published in 1998 in a conference. RFC 2945, which describes SRP-3 with SHA1, was published in 2000. SRP-6, which fixes "two-for-one" guessing and messaging ordering attacks, was published in 2002. SRP-6a appeared in the official "libsrp" in version 2.1.0, dated 2005. SRP-6a is found in standards as: IEEE 1363.2 also includes a description of "SRP5", a variant replacing the discrete logarithm with an elliptic curve contributed by Yongge Wang in 2001. It also describes SRP-3 as found in RFC 2945.

Manual pages

RFCs

Other links