Why does 1Password use RSA with 2048-bit keys?

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afokoue
afokoue
Community Member
edited September 2023 in Lounge

After reading the 1password white paper, it seems that, in case of a security breach in which encrypted vaults and unencrypted user public keys are stolen, the best way for the attacker to access information in stolen vaults is NOT by trying to guest the 128-bit Secret Key and the user master password. Instead, since a vault key is encrypted with the RSA public key of a user who has access to the vault, it is better for the attacker to directly attempt to break the RSA encryption by recovering the private key from the unencrypted public key. 1password uses RSA with 2048-bit keys, which, according to NIST (see Table 2 on page 54), provides only a 112-bit security protection (whereas a strong 80-bit user master password, combined with the 128-bit Secret Key, provides more than 200-bit of security). To get at least the same protection as provided by the 128-bit Secret Key, the size of RSA keys should be at least 3072, which provides a 128-bit security according to NIST.

I understand that increasing the size of the RSA keys will slow down decryption, which is done frequently in the current design. However, the encryption of the vault key with a user public RSA key is only performed to securely share the vault key with only that user. The RSA decryption should really be done only once per user's client (the first time the user's client attempts to access a vault that has been shared with her). After the first successful RSA decryption of an encrypted vault key on the client, the client should cache an encryption of the vault key using AES-256 and the "Account Unlock Key" (AUK) which is derived from the Secret Key and the user master password (note that this type of caching is already routinely done by clients to save time by avoiding to regenerate the secret for authentication, "SRP-𝑥"). Thus, the cost of decrypting a RSA encrypted vault key on a client would be paid only once [1], which means that 1password could certainly afford much larger RSA keys.

My question is then: why not increase the size of RSA keys to at least 3072-bit to provide, in case of a security breach, at least the 128-bit security that 1password advertises (due to the 128-bit Secret Key)? (IMHO, it should be increased to at least 4096-bit)

Best regards

[1] Note: RSA decryption might also be needed for rare account recovery operations.

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