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Yubico Security Keys with a Crypto Flaw

Wow, is this an embarrassing bug:

Yubico is recalling a line of security keys used by the U.S. government due to a firmware flaw. The company issued a security advisory today that warned of an issue in YubiKey FIPS Series devices with firmware versions 4.4.2 and 4.4.4 that reduced the randomness of the cryptographic keys it generates. The security keys are used by thousands of federal employees on a daily basis, letting them securely log-on to their devices by issuing one-time passwords.

The problem in question occurs after the security key powers up. According to Yubico, a bug keeps “some predictable content” inside the device’s data buffer that could impact the randomness of the keys generated. Security keys with ECDSA signatures are in particular danger. A total of 80 of the 256 bits generated by the key remain static, meaning an attacker who gains access to several signatures could recreate the private key.

Boing Boing post.

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On Security Tokens

Mark Risher of Google extols the virtues of security keys:

I’ll say it again for the people in the back: with Security Keys, instead of the *user* needing to verify the site, the *site* has to prove itself to the key. Good security these days is about human factors; we have to take the onus off of the user as much as we can.

Furthermore, this “proof” from the site to the key is only permitted over close physical proximity (like USB, NFC, or Bluetooth). Unless the phisher is in the same room as the victim, they can’t gain access to the second factor.

This is why I keep using words like “transformative,” “revolutionary,” and “lit” (not so much anymore): SKs basically shrink your threat model from “anyone anywhere in the world who knows your password” to “people in the room with you right now.” Huge!

Cory Doctorow makes a critical point, that the system is only as good as its backup system:

I agree, but there’s an important caveat. Security keys usually have fallback mechanisms — some way to attach a new key to your account for when you lose or destroy your old key. These mechanisms may also rely on security keys, but chances are that they don’t (and somewhere down the line, there’s probably a fallback mechanism that uses SMS, or Google Authenticator, or an email confirmation loop, or a password, or an administrator who can be sweet talked by a social engineer).

So while the insight that traditional 2FA is really “something you know and something else you know, albeit only very recently,” security keys are “Something you know and something you have, which someone else can have, if they know something you know.”

And just because there are vulnerabilities in cell phone-based two-factor authentication systems doesn’t mean that they are useless. They’re still much better than traditional password-only authentication systems.

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Quantum Tokens for Digital Signatures

This paper wins “best abstract” award: “Quantum Tokens for Digital Signatures,” by Shalev Ben David and Or Sattath:

Abstract: The fisherman caught a quantum fish. “Fisherman, please let me go,” begged the fish, “and I will grant you three wishes.” The fisherman agreed. The fish gave the fisherman a quantum computer, three quantum signing tokens and his classical public key.

The fish explained: “to sign your three wishes, use the tokenized signature scheme on this quantum computer, then show your valid signature to the king, who owes me a favor.”

The fisherman used one of the signing tokens to sign the document “give me a castle!” and rushed to the palace. The king executed the classical verification algorithm using the fish’s public key, and since it was valid, the king complied.

The fisherman’s wife wanted to sign ten wishes using their two remaining signing tokens. The fisherman did not want to cheat, and secretly sailed to meet the fish. “Fish, my wife wants to sign ten more wishes.”

But the fish was not worried: “I have learned quantum cryptography following the previous story (The Fisherman and His Wife by the brothers Grimm). The quantum tokens are consumed during the signing. Your polynomial wife cannot even sign four wishes using the three signing tokens I gave you.”

“How does it work?” wondered the fisherman.

“Have you heard of quantum money? These are quantum states which can be easily verified but are hard to copy. This tokenized quantum signature scheme extends Aaronson and Christiano’s quantum money scheme, which is why the signing tokens cannot be copied.”

“Does your scheme have additional fancy properties?” the fisherman asked.

“Yes, the scheme has other security guarantees: revocability, testability and everlasting security. Furthermore, if you’re at the sea and your quantum phone has only classical reception, you can use this scheme to transfer the value of the quantum money to shore,” said the fish, and swam his way.

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