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For the first time, researchers have demonstrated that a large portion of cryptographic keys used to protect data in computer-to-server SSH traffic are vulnerable to complete compromise when naturally occurring computational errors occur while the connection is being established.
Underscoring the importance of their discovery, the researchers used their findings to calculate the private portion of almost 200 unique SSH keys they observed in public Internet scans taken over the past seven years. The researchers suspect keys used in IPsec connections could suffer the same fate. SSH is the cryptographic protocol used in secure shell connections that allows computers to remotely access servers, usually in security-sensitive enterprise environments. IPsec is a protocol used by virtual private networks that route traffic through an encrypted tunnel.
The vulnerability occurs when there are errors during the signature generation that takes place when a client and server are establishing a connection. It affects only keys using the RSA cryptographic algorithm, which the researchers found in roughly a third of the SSH signatures they examined. That translates to roughly 1 billion signatures out of the 3.2 billion signatures examined. Of the roughly 1 billion RSA signatures, about one in a million exposed the private key of the host.
While the percentage is infinitesimally small, the finding is nonetheless surprising for several reasons—most notably because most SSH software in use has deployed a countermeasure for decades that checks for signature faults before sending a signature over the Internet. Another reason for the surprise is that until now, researchers believed that signature faults exposed only RSA keys used in the TLS—or Transport Layer Security—protocol encrypting Web and email connections. They believed SSH traffic was immune from such attacks because passive attackers—meaning adversaries simply observing traffic as it goes by—couldn’t see some of the necessary information when the errors happened.
Passive SSH Key Compromise via Lattices.” It builds on a series of discoveries spanning more than two decades. In 1996 and 1997, researchers published findings that, taken together, concluded that when naturally occurring computational errors resulted in a single faulty RSA signature, an adversary could use it to compute the private portion of the underlying key pair.
