In a January 15 2016 blog post, Vitalik mentions:

Ring signatures are more mathematically involved than simple signatures, but they are quite practical to implement; some sample code for ring signatures on top of Ethereum can be found here.

Here is a snippet:

def verify(msgHash:bytes32, x0:uint256, s:uint256[], Ix:uint256, Iy:uint256, pub_xs:uint256[], pub_ys:bytes): # Number of pubkeys n = len(pub_xs) # Decompress the provided I value Iy = recover_y(Ix, Iy) # Store the list of intermediate values in the "ring" e = array(n + 1) # Set the first value in the ring to that provided in the signature e[0] = [x0, sha3(x0)] i = 1 while i < n + 1: prev_i = (i - 1) % n # Decompress the public key pub_yi = recover_y(pub_xs[i % n], bit(pub_ys, i % n)) # Create the next values in the ring based on the provided s value k1 = ecmul(Gx, Gy, 1, s[prev_i]) k2 = ecmul(pub_xs[i % n], pub_yi, 1, e[prev_i][1]) pub1 = decompose(ecsubtract(k1, k2)) k3 = self.hash_pubkey_to_pubkey([pub_xs[i % n], pub_yi], outitems=2) k4 = ecmul(k3[0], k3[1], 1, s[prev_i]) k5 = ecmul(Ix, Iy, 1, e[prev_i][1]) pub2 = decompose(ecsubtract(k4, k5)) left = sha3([msgHash, pub1[0], pub1[1], pub2[0], pub2[1]]:arr) right = sha3(left) # FOR DEBUGGING # if i >= 1: # log(type=PubkeyLogEvent, pub_xs[i], pub_yi) # log(type=PubkeyLogEvent, pub1[0], pub1[1]) # log(type=PubkeyLogEvent, pub2[0], pub2[1]) # log(type=ValueLogEvent, left) # log(type=ValueLogEvent, right) e[i] = [left, right] i += 1 # Check that the ring is consistent return((e[n][0] == e[0][0] and e[n][1] == e[0][1]):bool)

Note, the top of the file does say "TOTALLY NOT TESTED AND LIKELY BROKEN AT THIS POINT; AWAITING A TEST SUITE".

EDIT by user @bleepbloop: Vitalik has confirmed that this is a very early PoC and not fully functional at present.

Looking at the code again, I think I = Ix, Iy is the 'tag' that makes this scheme linkable, there just isn't yet a function to store and compare the I values that have been previously seen, which would be needed in order to prevent double spending/double withdrawals.